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ENGINEERING STANDARDS

PRESSURE SAFETY DEVICES DESIGN GUIDELINE – SELECTION

OF RELIEF DEVICE

EG-PRO-0107

The information contained in this document is confidential and proprietary to NOVA Chemicals and

may not be disclosed in whole or in part to any other individual or organization without the prior written consent of NOVA Chemicals.

Use of this information for any purpose other than that authorized by NOVA Chemicals is strictly prohibited.

Rev. #: 00

Original Issue Date: 15.11.2011 Rev. Date: dd.mm.yyyy

ENGINEERING STANDARDS REVISION REGISTER

Standard Number: EG-PRO-0107

Standard Name: Pressure Safety Devices Design Guideline – Selection of Relief Device

Rev No.

Rev to Section

No. Description Date

Reviewed By

Approved By

NOVA Chemicals ENGINEERING STANDARDS Eng. Std. #: EG-PRO-0107

Pressure Safety Devices Design Guideline

– Selection of Relief Device

Page No.: 3 of 60

Initial Issue Date: Rev. No.: 00

15.11.2011 Rev. Date: dd.mm.yyyy

TABLE OF CONTENTS

Page No.

1. GENERAL ........................................................................................................................... 5

2. OPERATIONAL CHARACTERISTICS ................................................................................ 5 2.1 Set Pressure and MAWP ......................................................................................... 5 2.2 Accumulation............................................................................................................ 6 2.3 Overpressure ........................................................................................................... 6 2.4 Backpressure ........................................................................................................... 7 2.5 Blowdown ................................................................................................................. 7 2.6 Simmer..................................................................................................................... 8 2.7 Chatter ..................................................................................................................... 8 2.8 General Operation of a Conventional Safety Valve .................................................. 9

3. PRESSURE RELIEF VALVES .......................................................................................... 13 3.1 Conventional Valves............................................................................................... 13 3.2 Balanced Valves .................................................................................................... 14 3.3 Pilot-operated Valves ............................................................................................. 15 3.4 Multiple Devices ..................................................................................................... 19

4. RUPTURE DISKS ............................................................................................................. 23 4.1 Application ............................................................................................................. 23 4.2 Classification .......................................................................................................... 24 4.3 How to Specify Rupture Disks ................................................................................ 25

5. PIN ACTUATED DEVICES................................................................................................ 35 5.1 General .................................................................................................................. 35 5.2 Buckling Pin Devices .............................................................................................. 36 5.3 Breaking Pin Devices ............................................................................................. 37 5.4 Buckling Pin Device versus Rupture Disc ............................................................... 37 5.5 Breaking Pin Device versus Rupture Disk .............................................................. 38 5.6 Examples ............................................................................................................... 38

6. TANK VENTING RELIEF DEVICES .................................................................................. 41 6.1 Conservation Vents ................................................................................................ 41 6.2 Pilot – Operated Vent Valves ................................................................................. 42 6.3 Emergency Vents ................................................................................................... 42 6.4 Manufacturers ........................................................................................................ 43 6.5 Explosion Prevention .............................................................................................. 43

7. VACUUM RELIEF DEVICES AND OPEN VENTS ............................................................ 47 7.1 Vacuum Relief Vents .............................................................................................. 47 7.2 Open Vents ............................................................................................................ 47

8. TEMPERATURE SAFETY RELIEF DEVICES .................................................................. 47




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9. MATERIAL SELECTION ................................................................................................... 49

10. BASIS FOR SELECTION CHECKLISTS .......................................................................... 49

11. APPENDIX A: ASME BPVC[5] SECTION I VS. SECTION VIII STEAM SAFETY VALVES52 11.1 ASME BPVC[5] Section I Safety Valves .................................................................. 52 11.2 ASME BPVC[5] Section VIII Safety Relief Valves .................................................... 52

12. APPENDIX B: HOW BALANCED BELLOWS VALVES WORK ...................................... 55 12.1 Bellows PSV with Non-Compromised Bellows and a Plugged Bonnet .................... 56 12.2 Bellows PSV with Ruptured Bellows ....................................................................... 57 12.3 Set Pressure and the ASME Code[5] ....................................................................... 58

13. APPENDIX C: EXPERIENCES AT THE CORUNNA SITE ............................................... 58 13.1 PSV-2146 ............................................................................................................... 58 13.2 PSV-1003 ............................................................................................................... 58 13.3 PSV-2600 ............................................................................................................... 59 13.4 PSV-8150A/B ......................................................................................................... 59

14. REFERENCES .................................................................................................................. 60




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1. GENERAL

This section provides an overview of the different types of pressure safety devices, operational characteristics and guidance in the selection of the type of pressure safety device to be used for a given service. A pressure safety device must be capable of operating at all times, especially during a power failure when system controls are nonfunctional. The sole source of power for the pressure safety device is therefore the process fluid. Once a condition occurs that causes the pressure in a system or vessel to increase to an unsafe level, the safety device may be the only safeguard remaining to prevent a loss of containment due to breach of mechanical integrity e.g.: vessel or piping rupture, gasket blowouts, etc.. Since reliability is directly related to the complexity of the device, it is important that the device design be as simple as possible. The pressure safety device must open at a predetermined pressure, flow a rated capacity at a specified pressure and, if desired, close when the system pressure has returned to a safe level. Pressure safety devices must be fabricated from materials compatible with the process fluid and the relieving conditions. Pressure safety devices must also be designed to operate in a consistently smooth and stable manner with a variety of fluids and fluid phases.

The selection of pressure safety device type is covered extensively in API STD 520[1]

, 8th

Edition, December 2008 „Sizing, Selection and Installation of Pressure-relieving Devices in Refineries, Part I – Sizing and Selection‟.

2. OPERATIONAL CHARACTERISTICS

The operational characteristics of the different types of PSD‟s are discussed at length in API

STD 520[1]

, Section 4. A comprehensive list of terms is included in API STD 520[1]

, Section 3.

Several of these terms are critical to the understanding of how pressure safety devices operate and how they are specified. These are discussed in further detail below, followed by a description of the operation of a typical conventional safety valve.

2.1 Set Pressure and MAWP

Set pressure is the inlet gauge pressure at which the pressure safety device is set to open under service conditions. In operating terms, the set pressure is the value of increasing inlet static pressure at which a pressure safety valve displays the operational characteristics defined by opening pressure or popping pressure. Opening pressure is the value of increasing inlet static pressure at which there is a measurable lift of the disc or at which discharge of the fluid becomes continuous. Popping pressure is the value of increasing inlet static pressure at which the disk moves in the opening direction at a faster rate as compared with corresponding movement at higher or lower pressures. It applies only to safety valves in compressible fluid service. In general, opening pressure is the set pressure for a liquid relief valve and popping pressure is the set pressure for a gas or vapor safety valve.




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In design terms, the set pressure can only be less than or equal to the Maximum Allowable Working Pressure (MAWP) of the protected equipment. The MAWP is the maximum permissible gauge pressure at the top of a vessel in its normal operating position at the designated coincidental temperature specified for that pressure. The MAWP is normally greater than the design pressure and is calculated by the vessel manufacturer once the vessel detailed design is complete. During the design stage of a typical project, the MAWP may not be available at the time the PSD systems are designed and hence the process designer will commonly select a set pressure equal to the design pressure of the vessel or system.

2.2 Accumulation

Accumulation is the pressure increase over the MAWP of a vessel, usually expressed as a percentage of MAWP. An allowable accumulation of pressure is necessary because of the operating characteristics of a spring loaded pressure safety device. At the safety device set pressure the valve disc begins to lift, but does not pass the rated flow capacity. As the pressure in the vessel continues to rise there is a further „accumulation‟ of pressure until the maximum allowable accumulation is reached, at which point the valve disc is at full lift and the valve passes its rated capacity.

Allowable accumulation is discussed in detail in API STD 520[1]

, Section 5.4 „Relieving

Pressure. For non-fire cases, maximum allowable accumulation is 10% for a single device installation and 16% for multiple device installations. For fire cases, the maximum allowable accumulation is 21% for both single and multiple device installations.

2.3 Overpressure

Overpressure is the pressure difference between sizing (relieving) pressure and set pressure, expressed as a percentage of set pressure. Relieving pressure is the pressure at the inlet to the relieving device during an overpressure condition and is equal to the set pressure plus the overpressure. It is also the sizing pressure, i.e.: the pressure specified in the calculation of a safety device orifice area. All direct spring operated pressure safety valves should have an overpressure of at least 10% or 3 psi (21 kPa), whichever is greater, to design for fully open conditions. Low pressure vents require more, as much as 100% overpressure to be fully open. If the relieving device set pressure is equal to the MAWP, then overpressure is equal to the allowable accumulation. If the relieving device set pressure is less than the MAWP, then overpressure is greater than the allowable accumulation.




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2.4 Backpressure

Backpressure is the pressure that exists at the outlet of a PSD as a result of the pressure in the discharge system and is the sum of the superimposed and built-up back pressures. Superimposed backpressure is the pressure that exists at the outlet of a PSD at the time the device is required to operate. Superimposed backpressure is composed of constant superimposed backpressure and variable superimposed backpressure. Constant superimposed backpressure is the normal pressure in the discharge system when no PSD‟s are relieving. For example, flare systems may have a small positive backpressure due to nitrogen purging. Variable superimposed backpressure may be caused by one or more other safety devices venting into a common header. Built-up back pressure is the increase in pressure at the outlet of a PSD that develops as a result of flow after the PSD opens. It is determined by the relieving flow rate and the design of the discharge piping. Total backpressure is the sum of the superimposed backpressure and the built-up backpressure. Total Backpressure = Constant Superimposed Backpressure + Variable Superimposed Backpressure + Built-Up Backpressure.

2.5 Blowdown

Blowdown is the difference between the set pressure and the closing pressure of a pressure safety device, expressed as a percentage of the set pressure or in pressure units. The closing pressure is the value of decreasing inlet static pressure at which the valve disc reestablishes contact with the seat or at which lift becomes zero. The reason why the closing pressure of a pressure safety valve is lower than the set pressure is related to basic valve design and operating characteristics. This is explained in Section 2.8, General Operation of a Conventional Safety Valve. The reclosing pressure is about 7% less than the set pressure (i.e.: 7% blowdown) for compressible fluids and 15% to 30% for incompressible fluids. It is important for the process designer to understand blowdown characteristics when specifying maximum operating pressure (not design pressure) relative to set pressure. Note that ASME Code UG-131[2] indicates that for compressible fluids and PSVs with adjustable blowdown, blowdown must not exceed 5% of set P or 3 psi whichever is greater. For incompressible fluids and PSVs having a non-adjustable blowdown, the blowdown must be noted on PSV. When assessing existing installations, please also be aware of the possibility of adjusting blowdown to compensate for increase PSD inlet losses as discussed in NOVA Engineering Guideline EG-PRO-0109[3] Pressure Safety Devices Design Guideline – Installation, Section 1.1.




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2.6 Simmer

Simmer is the audible or visible escape of compressible fluid between the seat and disc of a pressure relief valve which may occur at an inlet static pressure below the set pressure prior to opening. At a pressure below the set pressure, typically 93% to 98% of set pressure, some slight leakage, called „simmer‟ may occur between the valve seat and disc. This is due to the progressively decreasing net closing force, spring pressure plus superimposed back pressure minus valve inlet pressure, acting on the disc. It is important for the process designer to understand simmering characteristics when specifying maximum operating pressure relative to set pressure. Chattering and fluttering (see below) can cause galling of the PSV, which could prevent the valve from opening.

2.7 Chatter

Chatter is the rapidly (in the order of magnitude of 100 to 200 cycles per minutes) alternating opening and closing of a pressure relief valve. The vibration may result in misalignment and leakage when the valve returns to the normal closed position, or, if it continues for a sufficient period, in mechanical failure of valve internals or associated pipe fittings such that the valve can no longer provide adequate protection. Chattering may occur in both liquid and vapor service pressure relief valves. The principal causes of chattering are:

Oversized valve including valve handling widely differing rates

Excessive inlet pressure drop

Excessive built-up back pressure

Note that „flutter‟ is similar to chatter except that the disc does not contact the seat of the valve during cycling.

2.7.1 Oversized Valve

Pop action pressure safety valves in vapor service open at the set point by the action of static pressure on the valve disc and move to the fully open position at only a small overpressure. Typically, a flow through the valve equal to at least 25% of its capacity is necessary to keep the disc in the open position. At lower rates, the flow is insufficient to keep the valve open against the action of the spring and it returns to the closed position, only to reopen immediately, since the static pressure in the system still exceeds the set pressure. Chattering results from continuous cycling in this manner. Liquid service pressure relief valves are characterized by progressively increased lift with increasing inlet pressure, rather than the pop action of vapor service valves. Liquid service valves are therefore less liable to chattering at low relieving rates and they exhibit stable operation down to about 10% of design flow.




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2.7.2 Excessive Inlet Pressure Drop

A pressure relief valve starts to open at its set pressure, but once a flow is established through the valve, the pressure acting on the disc is reduced by an amount equal to the pressure drop through the inlet pipe and fittings. If this pressure drop is sufficiently large, the valve inlet pressure may fall below the reseating pressure (i.e.: blowdown), causing it to close, only to reopen immediately as the static pressure is still above the set pressure. Chattering results from the rapid repetition of this cycle. To avoid this mechanism of chattering, the inlet piping pressure drop (including entrance loss, piping and isolation valve pressure drop) should be no more than 3% of the set pressure at relieving conditions (There are some exceptions to the 3% rule, please refer to NOVA Engineering Guideline EG-PRO-0109[3] Pressure Safety Devices Design Guideline – Installation, Section 1.1 for more information).

2.7.3 Excessive Built-Up Back Pressure

Built-up back pressure resulting from discharge flow through the outlet system of a conventional pressure relief valve results in a force on the valve disc tending to return it to the closed position. If the forces are sufficiently large, it may cause the valve to close, only to reopen immediately when the effect of built-up back pressure is removed. Chattering results from the rapid repetition of this cycle. To prevent chattering from this mechanism, conventional pressure relief valve discharge systems should be designed for a maximum built-up back pressure of 10% of set pressure at relieving conditions. For more information on back pressure limits, see NOVA Engineering Guideline EG-PRO-0109[3] Pressure Safety Devices Design Guideline – Installation, Section 1.2 and Section 3.

2.8 General Operation of a Conventional Safety Valve

Although it is important to have a clear understanding of the definitions used in describing and specifying PSD‟s, it is also important to understand how these terms relate to the actual operation of a PSD as it performs its function of overpressure protection. A detailed description of the operation of the different types of PSD‟s is given in API STD 520[1], Section 4. A brief description of the operation of a conventional safety valve is given below. The construction of a basic spring-loaded safety relief valve is shown in Figure 2.1. The valve consists of a valve inlet or nozzle mounted on the pressurized system, a disk held against the nozzle to prevent flow under normal operating conditions, a spring to hold the disc closed and a body and bonnet to contain the operating elements. The spring load is adjustable, within a limited range, to vary the pressure at which the valve will open.




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Figure 2.1 – Conventional Direct Spring Safety Valve

When the system pressure, P1, reaches the opening pressure, the force acting on the exposed disc area equals the force of the spring; the disc will lift and allow fluid to flow out through the valve. When the system pressure returns to the closing pressure (set pressure minus blowdown pressure), the valve will return to the closed position. When a pressure relief valve begins to lift, the spring force increases. Therefore system pressure must increase if lift is to continue. For this reason, pressure relief valves are permitted an overpressure allowance to reach full lift. The allowable overpressure is generally 10% for valves on unfired systems. This margin is relatively small and some means must be provided to assist in the lift effort. Most safety valves therefore have a secondary control chamber, or huddling chamber, to enhance lift. As the disc begins to lift, fluid enters the huddling chamber, exposing a larger area of the disc to system pressure. This causes an incremental change in force that is larger than the increase in spring force and causes the valve to open at a rapid rate. At the same time, the direction of the fluid flow is reversed and the momentum effect resulting from the change in flow direction further enhances lift. These effects combine to allow the valve to achieve maximum lift and maximum flow within the allowable overpressure limits. Because there is a larger disk area exposed to system pressure after the valve achieves lift, the valve will not close until the system pressure has been reduced to a value below set pressure. The design of the huddling chamber determines where the closing point will occur.




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The difference between the set pressure and the closing point pressure is called blowdown and is usually expressed as a percentage of set pressure. Blowdown is discussed in Section 2.5. The design of the huddling chamber impacts both the lift effort and the blowdown. If the design maximizes lift effort, then blowdown will be long. If the design objective is to minimize blowdown, then the lift effort will be diminished. Many pressure relief valves are therefore equipped with a nozzle ring, called the blowdown ring, that can be adjusted to vary the geometry of the control chamber to meet a particular system operating requirement. This type of valve is called a variable or adjustable blowdown valve. As shown in Figure 2.1 the force of fluid pressure acting on the inlet side of the disk (P1) at set pressure will be balanced by the force of the spring plus the back pressure (P2) that exists on the outlet side of the valve. If the superimposed back pressure at the valve outlet varies while the valve is closed, the valve set pressure will change. If the total back pressure varies while the valve is open and flowing, valve lift and flow rate will be affected. Care must be taken in the design and application of safety relief valves to compensate for these variations. Compensation for constant superimposed back pressure can be provided by adjusting the spring force. When superimposed back pressure is variable, a balanced bellows or pilot operated valve should be considered. The operation and characteristics of a conventional safety valve are shown diagrammatically in Figure 2.2.




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Figure 2.2 – Characteristics Of A Typical Safety Relief Valve (from API STD 520[1]

)




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3. PRESSURE RELIEF VALVES

Section 4.2 of API STD 520[1]

, Part 1 describes the characteristics of each type of pressure

relief valve. It should be consulted to gain basic knowledge of the different types of valves. This guideline will only highlight the main characteristics of each type of valve and provide a comparison.

3.1 Conventional Valves

Conventional valves are spring-operated valves. It is a simple and reliable pressure device. Because of its design, its functionality depends on the backpressure. In this case, the Cold Differential Test Pressure (CDTP) must account for the constant superimposed backpressure so that the equipment is protected at all times. Figure 3.1 provides a schematic of the different elements of a conventional valve. Note that there is an opening between the PSV discharge and the bonnet. When the PSV opens, the bonnet will see the same pressure and fluid type as the discharge line. This is the reason why the bonnet of conventional valves is plugged (to avoid hydrocarbon relieving to atmosphere) and why conventional valves are affected by backpressure. Conventional valves are suitable for systems that operate at pressures less than 90% of set pressure. Vendors actually recommend operation at a minimum of 10% or 25 psig, whichever is greater, below the set pressure. For steam applications, it is recommended to operate at pressures less than 80% of set pressure. Conventional valves are suitable when the variable backpressure does not exceed 10% of set pressure. Variable backpressure includes superimposed AND built-up backpressure. Any change in backpressure will directly affect the PSV opening pressure. For vapors and gases, when the total backpressure is higher than about 55% of the relieving pressure (absolute units), a de-rating coefficient is applied to the PRV capacity because of the force applied to the disc that will limit the lift of the disc. PRV manufacturers must be contacted to obtain data specific to the PSV model. See NOVA Engineering Guidelines EG-PRO-0108[4] Pressure Safety Devices Design Guideline – Sizing of Relief Device for sizing details. The shape of the disc / disc holder defines the huddling chamber and is different for compressible and non-compressible fluids. The huddling chamber allows the PSV to fully open rather than „simmer‟ when the pressure is just above the set pressure. Care should be taken to discuss operating conditions and all possible scenarios so that the valve is built with the appropriate trim. For example, a PSV is installed in a liquid filled system where all scenarios relieve liquid except for the fire case, which is the sizing case. If the valve specification was only based on the datasheet showing fire as the size determining case, the PSV trim specified would likely be a vapor trim. However, a liquid trim may be more appropriate because it would handle all the more probable liquid scenarios.




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Conventional valves in steam applications should be manufactured with an open bonnet (exposed spring) to ensure proper spring cooling. This helps with valve stability (to avoid set pressure drifting). When an exposed spring PSV opens, there is no pressure built up in the bonnet and the PSV can handle a larger amount of backpressure without affecting its relieving capacity. PSVs manufactured for ASME[5], Section I, of the code are of the conventional type and have a specific design. In certain cases (very high pressure steam), it may be useful to install a valve manufactured according to ASME[5], Section I, in a location where Section VIII applies. The PSV will be at full lift faster and will reclose sooner. However, there are some issues as well, especially with piping. Section 11, Appendix A provides information on the operating and manufacturing differences between BPVC[5], Section I and BPVC[5], Section VIII valves. A quick discussion is also offered on inlet piping pressure loss.

3.2 Balanced Valves

Balanced valves (Figure 3.2 and Figure 3.3) are also spring-operated valves in which bellows or a piston have been added to isolate the upper section of the disc with an area equal to that of the nozzle area.

3.2.1 General

Balanced PSVs are used when the superimposed backpressure is variable or when the built-up backpressure is higher than 10% of set pressure. Balanced PSVs are also used with corrosive or toxic fluids, with fluid that leaves deposits or that can cause scaling. Compared to a conventional valve, balanced valves prevent the fluid and the pressure from entering the bonnet of the valve. The bonnet of balanced PSVs must be vented to atmosphere so that the valve is balanced, i.e.: the pressure in the bonnet is atmospheric pressure that allows the opening pressure of the PSV to remain constant even when the backpressure varies. Backpressure does affect the capacity of balanced PSVs because it affects the lift of the disc. If the total backpressure is higher than 45% to 55 % (vapors and gas) and 20% (liquid) of the set pressure (gauge values), a de-rating factor applies. It is advised to consult the manufacturer to obtain charts for de-rating factors. See also NOVA Engineering Guideline EG-PRO-0108[4] Pressure Safety Devices Design Guideline – Sizing of Relief Device for sizing details. Balanced PSVs are not recommended when the total backpressure is higher than 60% to 80% of the set pressure because of the extremely low de-rating factor. Vendors‟ charts will not provide any data at those high backpressures.




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3.2.3 Specific to Bellows PSVs

The venting of the bonnet also allows early detection of bellows damage. If the bellows are damaged, the relief valve will not function properly: the set pressure will be affected by backpressure and the valve might not fully lift when required. Section 12, Appendix B describes the theory behind the bellows functionality and explains what happens if the bonnet is not vented to atmosphere. The outer pressure rating of bellows is usually available in PSV vendors‟ catalogues. If the total backpressure (constant and variable) is higher than the pressure rating, the bellows can be damaged. The pressure rating depends on the PSV orifice size and flange rating and is provided at 100°F (38°C), therefore a correction factor must be applied if the relieving temperature is higher than 100°F (38°C). The higher the temperature, the weaker the bellows. Annex C of API STD 526[6] provides a graph to evaluate the correction factor for different bellows metallurgies and for temperatures up to 1000°F. Examples of pressure rating of bellows for Farris products at 100°F:

E orifice 1 inch inlet flange and 2 inch outlet flange, model 26EB11: 230 psig (15.8 barg)

N orifice 4 inch inlet flange and 6 inch outlet flange, model 26NB11: 80 psig (5.5 barg) Bellows are less capable to withstand inner tensile stress compared to outer compression stress. Therefore, the air compression taking place in the bonnet when the PSV lifts and the bonnet is not vented to atmosphere is sufficient to cause premature bellows failure, as shown in Figure 3.4.

3.3 Pilot-operated Relief Valves (PORVs)

Pilot-operated relief valves (PORV) function differently from spring-operated valves. PORVs contain a main valve with a piston or a diaphragm operated disc and a pilot valve, which is either a small spring or diaphragm operated valve. The pilot is the pressure sensing element and commands the opening of the main valve using the process pressure. The area of the piston (Ap) is exactly 30% larger than the disc (Ai). Under normal operating conditions, the pilot allows the process pressure into the piston chamber (or dome). The balance of forces (same pressure, different areas) keeps the main valve closed (Figure 3.5). When the process pressure reaches the set pressure, the pilot evacuates the pressure in the piston chamber and the main valve opens. PORVs are not affected by backpressure the way spring-operated valves are.

The set pressure does not depend on backpressure.

Because there is no spring force to overcome, there is no reduction in disc lift.

For gas and vapor service, a de-rating factor applies because the reduction in pressure gradient across the valve reduces the flow. The de-rating factor applies when the total backpressure reaches 60% of the set pressure (absolute values). See NOVA

Engineering Guideline EG-PRO-0108[4]

Pressure Safety Devices Design Guideline –

Sizing of Relief Device for sizing details.




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Pilot-operated valves are NOT allowed in Section I of the ASME BPVC Code[5]. Pilot-operated valves come in two main types: snap action and modulating.

3.3.1 Snap acting valves. See Figure 3.6 and Figure 3.7.

Snap acting valves pop open like a conventional valve. This is an advantage when freezing is possible (low temperature systems or where auto-refrigeration is possible as the pressure is let down at the outlet of the PSV). The piston chamber (or dome) is vented to atmosphere (or a safe location with no backpressure) so that the set point is not affected by the backpressure (like a conventional valve). Full lift occurs at the set pressure. The blowdown can be adjusted and can be as low as 3%. Attention must be given as a low blowdown could cause the valve to chatter (inlet pressure losses must be lower than blowdown). Snap acting valves are not recommended in liquid service.

3.3.2 Modulating Valves. See Figure 3.8.

Modulating valves have a modulating pilot that makes the main valve open as much as required (more like a control valve). This system prevents chattering as the main valve will only open as much as necessary. Lift begins at set pressure and then proportionally increases up to 4% to 6% overpressure after which it will open fully. Thus the modulating action is valid only for the lower end of the capacity. The pilot vent can be routed to the PSV discharge (to avoid release to atmosphere of the dome volume each time the PSV opens). Care must be taken in the compatibility of the internals with the process side and whatever can be present in the discharge of the PSV. The blowdown is fixed between 3% and 6%.

3.3.3 Pilot Types

The pilot can be of two types: flowing and non-flowing. As long as the PSV is closed, whether the pilot is flowing or non-flowing, there is no process flow through the pilot.

Flowing pilots allow the process stream to flow through as long as the PSV is open and relieving. The flowing pilot type is preferred in steam service because it allows the pilot to warm up and prevent sudden condensation in the pilot, which could cause instabilities (condensation creates loss of pressure, which would lead to closing the valve).




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Non-flowing pilots will evacuate the process fluid contained in the dome volume only as the PSV opens. Then, there is no flow from the process into the pilot and the dome until the PSV closes. For a snap-acting valve, this action takes place at once. For a modulating pilot, the release of the dome pressure will be gradual, depending on how much the main valve opens.

Non-flowing pilots minimize the volume of process fluid through the pilot and thus the amount of impurities potentially accumulating in the dome, in the pilot or in the sensing line. Therefore non-flowing pilots are usually preferred. Pilot-operated valves are recommended as follows:

For two-phase flow.

When the operating pressure is close to the set pressure. A PORV remains tightly shut up to 95% to 97% of set pressure.

When a short blowdown is recommended.

When the inlet pressure losses are high, there is the possibility to install remote sensing lines, e.g.: at the process vessel. For existing systems with high inlet pressure losses that could cause the valve to chatter, a PORV with a remote sensing line ensures that the sensed pressure is independent of the inlet pressure losses and prevents chattering.

Operating temperature does not affect the opening pressure.

Snap acting PORVs are recommended for cryogenic service or when the rate of pressure rise is high.

Modulating PORVs are recommended when there is a large difference in required area between two likely scenarios.

Pilot-operated valves can be equipped with a number of accessories that improve operability and maintenance.

Field test connection: Used to verify / change the set pressure without removing the PSV from the equipment. The field test connection can also be used to close the valve (see Section 13, Appendix C, particularly the experience of PSV-1003).

Lift indicator: device that can be linked to a field alarm or DCS to indicate when the PSV opens.

Pilot lift lever: device used to test the operation of the valve. A pilot lift lever must be installed on air, hot water (over 60°C/140°F) and steam systems unless Code Case 2203 has been claimed. If Code Case 2203 is used, the PSV must be stamped accordingly.




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Backflow preventer: to be used when the back pressure is expected to be higher than the operating pressure. See Figure 3.9.

Pilot filters: To be used when dirty service is expected.

Pressure spike snubber: Pressure dampener. Items to consider when choosing a PORV:

When the operating pressure is low compared to the set pressure, the closing force is low and there could be some leakage.

If the backpressure is higher than the operating pressure (vacuum operation for example), the backpressure would tend to open the PSV. This is why a back flow preventer is used to make sure there is always a closing pressure on the dome.

If the backpressure is higher than the set pressure, then the PSV would tend not to open. Discussion with the manufacturer is recommended when the backpressure can reach 60% to 65% of the set pressure. This is very important for PSVs with a low set pressure.

Snap acting PORV must vent the volume of the dome to atmosphere (or a safe location) so that the set point is not affected by the backpressure (like a conventional valve). The volume depends on the PSV size (from 0.865 inches3 to 218 inches3) and there will be a release each time the PSV opens.

A pilot-operated valve has soft goods internal parts. They must be compatible with the process. Some of the chemicals to watch for are: highly aromatic content, hydrogen sulfide, caustic and amines.

Because of the small diameter of the sensing line, PORVs are not recommended in polymerization systems where the sensing line can quickly be plugged with polymers.

Pilot-operated valve response time slows down considerably with very viscous fluids since the fluid must go through a relatively small passage within the pilot.

A PORV is smaller in body size than a spring-operated valve because it does not have a bonnet for the spring. This is particularly true of large valves. In terms of cost, for a small to medium size valve, pilot-operated valves have a similar cost to bellows valves, which are 50% to 80% more costly than a conventional valve. However, for large valves, the cost of a PORV can be significantly lower.




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3.4 Multiple Devices

When multiple devices are installed, care should be taken in setting the set pressures. The

ASME BPVC[5]

specifies limits on staggered pressure levels (See NOVA Engineering

Guideline EG-PRO-0104[7]

, Pressure Safety Devices Design Guideline – General, Section 4.).

However, if the set pressure staggering is not wide enough, there is a high risk that PSVs will

lift at the same time because the ASME BPVC[5]

does authorize a 3% tolerance on the set

pressure above 70 psi (2 psi if set pressure is lower than 70 psi) out of the factory. This could cause chattering and damage to the PSVs. For example, staggering the set pressures at 500 psig and 505 psig will not guarantee that a single valve will lift, even for small relieving rates.

Figure 3.1 Conventional Valve (Farris)




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Figure 3.2 – Bellow Valve (Farris)

Figure 3.3 – Schematic of a Balanced Piston and Balanced Bellows Valve.




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Figure 3.4 – A Bellows found Damaged. The Bonnet Vent was Blocked

Figure 3.5 – Schematic of a Pilot-operated Relief Valve (Farris)

A i

A p




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Figure 3.6 – Snap Acting Valve Closed (Farris)

Figure 3.7 – Snap Acting Valve Open

Figure 3.8 – Modulating PORV

(Note the two vents on the pilot. The lower vent is the one evacuating the pressure from the dome. It can be routed to the PSV discharge). (API)




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Figure 3.9 – Snap Acting Valve with Backflow Preventer (API)

4. RUPTURE DISKS (RD)

4.1 Application

A rupture disk (RD) is a metallic or graphite membrane that is held between flanges and that is designed and manufactured to burst at a specified pressure and corresponding temperature. As one kind of safety relief devices they are generally installed on pressure vessels alone or combined with pressure relief valves.

4.1.1 Rupture disk as stand-alone PSD

Since it is a non-reclosing device, once the rupture disk bursts, the contents in the system will be relieved continuously until the internal pressure balances with the outlet pressure. When rupture disks are used as stand-alone pressure relief devices:

Relieving fluids are typically non-toxic, non-hazardous and inexpensive.

When the process fluids are highly viscous or contain solids or can freeze when undergoing rapid depressurization, rupture disks are used as a substitute for pressure relief valves.

As a fast-reacting relief device, a rupture disk is considered as an option when there is a potential for tube rupture or runaway reaction and a relief valve will not react fast enough to prevent catastrophic failures.




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4.1.2 Combination of rupture disk and pressure relief valve

In applications when a relief valve discharges to the atmosphere, a rupture disk upstream of the relief valve can reduce the emissions to the environment.

When a system contains toxic, noxious, or very costly fluids where a relief valve leak is a concern, a rupture disk can be installed in combination with a pressure relief valve to ensure a positive seal.

When the fluids on either side of a relief valve are corrosive, a rupture disk made of corrosion resistant material can be installed in combination with the relief valve for protection and thus the cost of purchasing expensive alloy valves is avoided.

4.2 Classification

Rupture disks can be classified as fragmenting disks or non-fragmenting disks. Depending on the reacting direction, metal disks can also be classified as forward acting or reverse acting.

4.2.1 Fragmenting Disks

Fragmenting reflects a low cost RD design and construction. When the disk bursts it could eject fragments downstream. In general, graphite disks or pre-bulged solid metal (non-scored) disks are fragmenting model disks. This model should be avoided for a rupture disk installed upstream of a pressure relief valve. If a disk fragment becomes lodged in a relief valve, it can prevent the valve from closing properly and damage the seat of the valve. Fragmenting rupture disks should also be avoided when it is important not to contaminate the process with pieces of rupture disk or when there is a chance that personnel or property could be injured or damaged by fragments that result from a burst disk.

4.2.2 Non Fragmenting Disks

With the development of rupture disk technology, fragmentation is eliminated by the use of a scored pattern on high performance metal disks. Please note that rupture disks should always be vented to a safe location even when a non-fragmenting disk is used. The high speed discharge flow (even without fragments from a rupture disk) can still be dangerous to plant personnel and property.




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4.2.3 Forward acting disks

A forward acting or so called „tension loaded‟ RD is installed into a system such that the process pressure is on the concave side of the formed crown as shown in Figure 4.1.

Figure 4.1 – Forward Acting (Tension Loaded) Disk with Angled Seat

4.2.4 Reverse Acting Disks

A reverse acting or so called „compression loaded‟ RD is installed into a system such that the process pressure is on the convex side of the formed crown as shown in Figure 4.2. Most reverse acting disks are capable of 90% operating ratio and perform better in cyclic / pulsating duties.

Figure 4.2 – Reverse Acting (Compression Loaded) Disk with Flat Seat

4.3 How to Specify Rupture Disks

4.3.1 Specified Burst pressure and Stamped Burst Pressure

When specifying a rupture disk, the first subject is the burst pressure. The maximum allowable stamped burst pressure is the MAWP of the vessel as ruled by ASME UG-134[5], Section VIII, Division 1 paragraphs. When a process engineer specifies the burst pressure of a rupture disk as the MAWP of a vessel, can it be expected the manufacturer will make a disk that will burst at the specified burst pressure? Will the stamped burst pressure (or so called „rated burst pressure‟ marked on the nameplate of the rupture disk) be equivalent to the specified burst pressure? Not necessarily, because the manufacturer applies a factor called the „Manufacturing Range‟ on the specified burst pressure.




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ASME indicates the manufacturing range of a rupture disk as: "a range of pressure within which the marked burst pressure must fall to be acceptable for a particular requirement as agreed upon between the rupture disk manufacturer and the user or his agent (UG-127 Foot Note 46).” This means that the manufacturing ranges are predetermined, allowable deviations from the specified burst pressure, within which the stamped burst pressure may fall and still be considered acceptable to the manufacturer and user. Manufacturing ranges can be found in the catalog by product type. Each disk style has its own table of manufacturing ranges

published by the vendor.

Two examples are as follows:

Designer A specified a rupture disk burst pressure as 100 psig (= MAWP of the vessel) with a manufacturing range of “-5% to +5%”

Designer B specified a rupture disk burst pressure as 100 psig (= MAWP of the vessel) with a request for ±0% manufacturing range

What can be expected when the disk is delivered with a stamped burst pressure?

For case A, this order of disks could be produced with a stamped burst pressure anywhere from 95 psig to 105 psig, and would be considered „good parts‟ within the range. The stamped burst pressure of the rupture disks in the same lot is the average burst pressure of all the destructive tests (e.g.: burst tests) performed at the manufacturing factory. Therefore, every disk in the same lot would be stamped with the same burst pressure.

For case B, the stamped burst pressure will be 100 psig with no deviation since a zero manufacturing range is specified. This requires that the average burst pressure of the burst tests must equal the specified burst pressure.

Remember, the stamped burst pressure must never exceed the vessels MAWP (assumes the rupture disk is the primary relief device). For case A, the specification of the rupture disk is not appropriate because the delivered rupture disk may have a burst pressure of 5 psig greater than the MAWP. It is a violation of the ASME code. There are a number of ways to correct this problem:

(1) Specify a burst pressure below the vessels MAWP (or design pressure) by taking into account the positive manufacturing range.

A correction to Case A would be specifying the burst pressure as 95.2 psig (= 100 psig / 105%). Then the stamped burst pressure can be anywhere between 90.4 psig to 100 psig which does not violate the code.

(2) Specify a burst pressure with ±0% manufacturing range.




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4.3.2 Burst Tolerance

This is the same as permitting the „Set Pressure Tolerance‟ on PSVs (see ASME UG-126[5] in Section VIII). The Code also allows a small deviation between the Stamped Burst Pressure and the Actual Burst Pressure on rupture disks (UG-127). The allowable deviation is called „Burst Tolerance‟ which shall not exceed ±2 psi (±15 kPa) for Stamped Burst Pressure up to and including 40 psi (300 kPa) and ±5% for Stamped Burst Pressure above 40 psi (300 kPa). The combination of the „Manufacturing Range‟ and the „Burst Tolerance‟ defines the burst pressure limits. For Case A above (assume the specified burst pressure is corrected to 95.2 psig with ±5% manufacturing range) and Case B (specified burst pressure 100 psig with ±0% manufacturing range) in the foregoing context, the burst pressure limits are shown in Figure 4.3 and Figure 4.4:

Figure 4.3 – Burst Pressure Limits for Case A

Figure 4.4 – Burst Pressure Limits for Case B

The important thing to be aware of is that the Manufacturing Range is applied to the Specified Burst Pressure while the Burst Tolerance is applied to the Stamped Burst Pressure. Unlike the Stamped Burst Pressure, which by code cannot exceed the equipment MAWP, the upper limit of the burst pressure can exceed the equipment MAWP if it is caused by the Burst Tolerance. When taking the lower limit of the Burst Pressure into consideration, the process engineer must be aware that the maximum operating pressure in the process should not exceed the lower burst limit. In addition, the rupture disk manufacturing industry introduced another important factor, called „Operating Ratio‟ explained further below.

4.3.3 Operating Ratio (OR)

The „Operating Ratio‟ is defined as the ratio of the maximum allowable operating pressure to the stamped burst pressure. The manufacturers use it to place a sufficient margin to protect against premature bursting of the rupture disks. It means when the system maximum operating pressure < [Operating Ratio * Stamped Burst Pressure], the risk of premature bursting of the rupture disk will be mitigated.




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Selecting a proper Operating Ratio (OR) is a key step when specifying a rupture disk. For example, a system with a maximum operating pressure of 80 PSIG. There are two rupture disk models available from a qualified vendor, one is a forward acting type of disk with an operating ratio of 80%, the other is a reverse acting type with an operating ratio of 90%. The results of applying the different operating ratios to Case A and Case B from Section 0 are shown as comparisons of the two cases in Figure 4.5, Figure 4.6, Figure 4.7, and Figure 4.8 following.

Figure 4.5 – Case A with 80% Operating Ratio

Figure 4.6 – Case A with 90% Operating Ratio

Figure 4.7 – Case B with 80% Operating Ratio

Figure 4.8 – Case B with 90% Operating Ratio




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When comparing the options listed in Figure 4.5, Figure 4.6, Figure 4.7 and Figure 4.8, you will find:

Figure 4.5 illustrates the “maximum operating pressure > [OR *Lowest Stamped Burst Pressure]“ which is a failure case (rupture disk will experience premature burst at maximum operating pressure)

Figures Figure 4.6 and Figure 4.7 illustrate a scenario when the maximum system operating pressure is too close (or equal) to the [OR *Lowest Stamped Burst Pressure] which is not a good practice (the material will fatigue and can eventually lose its integrity).

Figure 4.8 illustrates the “maximum operating pressure < [OR *Lowest Stamped Burst Pressure]“ which is the best option for this application

As a summary, the rules of thumb given below should be considered when specifying the burst pressure and selecting disk models with various manufacturing ranges and operating ratios:

(1) For applications where a rupture disk is the only relief device or is used as primary relief device in a system with multiple relief devices:

a. Specify the burst pressure appropriately to ensure the stamped burst pressure on the disk shall not exceed MAWP of the pressure vessel. If superimposed back pressure exists, increase the burst pressure to compensate for the back pressure.

b. Select a narrow manufacturing range. Then check if the upper / lower limit of burst pressure is acceptable. When necessary, specify zero manufacturing range to avoid unwanted nuisance burst or code violation.

c. Investigate the maximum system operating pressure and select a disk model with proper operating ratio to mitigate the risk of premature failure. Reverse acting disks can normally provide a higher operating ratio (90%) compared to other models. However, the reverse acting RD with knife blades should be avoided since the disk would not open at burst pressure if the blades are dull.

(2) For applications where multiple relief devices are installed and a rupture disk is used as secondary relief device:

a. Specify the burst pressure appropriately to ensure the stamped burst pressure on the disk shall not exceed 105% of the MAWP for overpressure scenarios other than fire exposure; The stamped burst pressure on the disk shall not exceed 110% of the MAWP for overpressure scenario caused by fire exposure.

b. and c. above are also applicable.




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(3) For applications where a rupture disk is installed in combination with a pressure relief

valve:

a. Code requires the opening of the valve shall be coincident with the bursting of the rupture disk (ASME UG-127[5], Section VIII, Division I). Therefore,

API STD 520[1]

recommends the burst pressure on the rupture disk and the set

pressure on the pressure relief valve should be the same. For a disk installed at the outlet of the valve, ensure the stamped burst pressure

on the disk does not exceed the back pressure limitation for the style of the pressure relief valve in use (e.g.: 10% of the relief valve set pressure for conventional style, 30% to 50% for balanced bellows style).

b. and c above are also applicable.

4.3.4 Burst Temperature

The Burst Temperature is most commonly referred as the temperature coincident with the burst pressure at relieving conditions. It is specified by the process engineer by conducting physical property calculations at relieving conditions. Extra caution is required if the overpressure event could occur in a very short period of time. For instance, during a runaway reaction, the process fluid temperature rises very fast with pressure and the disk material may not have sufficient time to come to equilibrium with the higher process relieving temperature. In this case, the normal process temperature should be specified as burst temperature as a conservative but safe practice. If the rupture disk is exposed to ambient conditions, 72°F (22°C) burst temperature is commonly specified.

4.3.5 Material Selection

The metal materials to be used are stainless steel, nickel, aluminum, copper, silver, platinum, palladium, Monel, Inconel, Hastelloy, etc. The selection of the metal is based on corrosion, temperature and rupture pressure.

a. Corrosion Resistance As the metal disk is thin and the stresses are relatively high, no corrosion allowance is allowed. Consequently, the disk material shall be more corrosion resistant than the metal from which the pressure vessel is constructed. For this reason, carbon steel is not acceptable as a rupture disk material. Please be aware, some metals are limited to specific applications, e.g.: stainless steel is prone to failure with chloride attack, etc. As an alternative, coatings and linings of Teflon, vinyl resins or other liners can be applied to provide additional protection against corrosive media. Due to maximum temperature limitations, the liner materials usually lower the temperature ratings of the metal disks.




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b. Pressure Rating

At the same style and size, some metal disks can withstand higher pressure than others. For instance, 316SS, Inconel and Hastelloy disks usually have higher pressure ratings than nickel or Monel disks. The Min / Max pressure ratings at ambient temperature (72°F or 22°C) and temperature limitations of the different metal materials are published and obtainable from the manufacturers. Repeated pressure pulsing in the system will impact the service life of the rupture disks. The metal will fatigue by pressure variation and be stressed to a point where the disk can rupture below its original design rating. Therefore, it is important to provide a sufficient margin between the system operating pressure and the rupture pressure of a disk as emphasized in previously. If acceptable in the application, graphite disks with vacuum support (end of pipe application) or composite material rupture disks can be considered for use where pressure cycles exist. Reverse acting rupture disks normally have higher cycle life when compared to forward acting styles. Process engineers can consult vendors or manufacturers for assistance when addressing pressure cycles and temperature cycles.

c. Temperature Impact As the temperature of the disk changes, the strength of the metal material, and hence the burst pressure, will also change. Figure 4.9 shows the comparison of different materials for forward-acting rupture disks (burst pressure is rated at an ambient temperature of 72°F).

Figure 4.9 – Temperature Effect on Metals (courtesy of Continental Disk Corporation)




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The comparison in the diagram illustrates how different materials have different sensitivities to temperature. Particularly, Inconel is less affected by temperature changes above 250°F and more durable when dealing with temperature cycles. In addition, manufacturers recommend reverse acting disks since they are less affected by temperature than forward acting rupture disks.

4.3.6 Sufficient Opening

Applications of rupture disks in liquid-filled systems should be carefully evaluated to assure that the design of the rupture disks and the dynamic energy of the system on which it is installed will result in sufficient opening of the rupture disk to provide full discharge flow. Contact the manufacturer for assistance with liquid service applications and information about the sufficient opening of a specific style of disk.

4.3.7 Highly Viscous or Polymerization Application

Special rupture disk designs are available for highly viscous fluids, where the flow is directed across the rupture disk inlet to avoid product build-up that may otherwise adversely affect rupture disk performance. The disk manufacturers shall be consulted for details in these applications. For instance, FIKE Poly-SD rupture disks are specially designed for polymerization processes.

4.3.8 Rupture Disk Specification Guideline

As stated in API STD 520[1]

Part 1 Section 4.3.6.2.3 “rupture disk selection is an iterative and

sometimes complex process”. Failure to select a proper rupture disk for an application can result in unnecessary down time and maintenance cost due to nuisance burst. In a worst case scenario, an improperly selected disk can fail to open during an overpressure scenario, resulting in a catastrophic failure of a pressure vessel. With so many RD designs and models available in the market place, the structured process that follows in Figure 4.10 provides a step-by-step guideline for specifying and selecting proper rupture disks for your application.




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.

Step 1. Gas or Liquid Service

• Verify RD is for gas service only or can be used in liquid, gas, and multi-phase application

• RD for gas service requires stored energy in compressed gas to facilitate the opening. The stored energy is not available in liquid since liquid is incompressible. This means a "gas-only" disk may not open in liquid service and can result in a safety incident.

Step 2 : Fragmenting or Non-fragmenting

• Graphite disks and most solid metal disks are fragmenting.

• Fragmenting designs should be avoided if the RD is installed at the inlet of a Pressure Relief Valve, to avoid contaminating the process with pieces of rupture disk or when there is a chance that personnel or property could be injured or damaged by fragments that result from a burst disk.

Step 3: RD Operating Ratio

• Must satisfy: Maximum operating pressure < (RD burst pressure * operating ratio) to prevent premature failure

• Applications with cyclic or pulsing pressure should not operate too close to operating ratio to avoid fatigue of the RD

Step 4: RD Manufacturing Range

• RD Stamped BP (Burst Pressure) can be anywhere between RD specified BP *(-MR% ) to RD specified BP *(+MR%)

• If RD sepcified burst pressure= system MAWP, no possitive MR% allowed

• If the margin between operating pressure and RD burst pressure is tight, select a RD with MR of ±0%




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Step 5: Burst Temperature and Materials

• Different Materials have different sensitivities to temperature; for cyclic temperature applications, materials insensitive to temperature variation should be selected.

• Normally, to calibrate the burst pressure rating at elevated or cold temperatures, manufacturers apply a temperature correction factor to pressure ratings at 72oF.

• In critical applications, if exact burst pressure and burst temperature are desired (i.e. reactor), request an"Oven Test" be done by the manufacturer with additional cost.

Step 6: Vacuum Support

• Determine if the rupture disk will see vacuum during its service life (all conditions including startup/shutdow/regeneration, etc).

• Most forward acting (tension-loaded) designs apply vacuum supports at bottom to help the disks withstand vacuum.

• Reverse acting (compression-loaded) designs normally can withstand full vacuum without the need for vacuum support.

Step 7: Disk Holder

• Angular seat or Flat seat Insertion types are common and fit between standard ANSI flanges.

• Bolted type, union type or screw type safety head assemblies are desired at special applications.

• DO NOT OVERTORQUE RD ASSEMBLY. Expecially for insert type angular seat RDs, the angular disk surface helps seal the disk with light torque. Excessive tightening may damage the disk with nicks, dents or scratches that contribute to leaking or premature burst.




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Figure 4.10 – Process for Rupture Disk Selection

5. PIN ACTUATED DEVICES

5.1 General

Pin actuated devices are non-reclosing relief devices that use an external pin linked to the disk of the valve. Pin actuated devices may be loaded in tension (breaking pins) or in compression (buckling pins).

ASME BPVC[5]

Section I does not allow pin-actuated devices.

ASME BPVC[5]

Section VIII UG-127(b) allows breaking pin devices ONLY in combination with

a primary relief device (such as a PSV). However, the use of breaking pin devices in single relief device applications is authorized in Code Case 2487.

ASME BPVC[5]

Section VIII allows the use of a buckling pin device (or rupture pin device) as

the primary pressure relief device under Code Case 2091-3. The use of a breaking pin device or buckling pin device as a standalone relief pressure device should be confirmed by the local jurisdiction (e.g.: ABSA or TSSA). For more information on piston type buckling and breaking pin devices, refer to API STD

520[1]

, Part I, Section 4.4. Buckling pin devices are available from Rupture Pin Technology.

Step 8: Minimum/Maximum Pressure Rating and Temperature Rating

• Check if the minimum/maximum pressure rating and temperature rating of the selected disk design and disk materials meets the process requirement

• Low burst pressure may not be possible with corrosion resistant exotic materials.

Step 9: ASME UD Stamp and Certificate of Compliance (C of C)

• UD stamping and ASME Burst Test C of C are required for novel rupture disk installations in accordance with the updated ASME Section VIII, Division 1




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Disk style buckling pin devices such as offset shaft butterfly valves are not described in API. This type of device can be used to segregate multiple flare systems as described in Section 5.6 and is available from BS&B.

5.2 Buckling Pin Devices

Buckling pin devices are also called rupture pin devices. Buckling pin devices uses an external pin linked to the disc of the valve. When the set pressure is reached, the pin fails and the valve opens. The pin diameter, length and material dictate its „rupture‟ pressure. The pin will buckle precisely at set pressure allowing instantaneous full bore relief.

a) The Rupture Pin Valve has only one moving part, the piston or the disk (in butterfly valves).

b) The pin has two stable conditions, straight or buckled – providing an excellent visual indicator when the device has lifted.

c) The device can be made to work as „conventional‟ (i.e.: affected by backpressure. See Figure 5.) or can be balanced (i.e.: not impacted by back pressure. See Figure 5.2).

d) The pin is not affected by pulsation. It will not fail due to fatigue. e) The pin is very stable and maximum operating pressure can be 95% of set pressure. f) The tolerance on a buckling pin device‟s set pressure can be as high as 5% (for PSVs,

it is 3%), but is normally lower. Consult with the manufacturer if a specific tolerance is required.

g) Because the pin is external to the process, it is not affected by process temperature. However the range of ambient conditions must be accounted for when specifying the pin.

h) The pin is not affected by the process chemical composition. i) Buckling pin devices have an extremely fast opening time. They will open about ten

times faster than a rupture disk. j) The Rupture Pin Valve is custom made to solve individual problems. Sizes can go

from ⅛ of an inch to 36 inches and pressure ranges from inches of water column to 35,000 psi.

k) Valves can be installed in any direction. However the manufacturer must know in advance the orientation as it impacts the design.

l) Buckling pin devices uses soft seats such as O-rings. O-ring seals shall be appropriate for process composition, temperature and pressure.

m) If the application is vacuum service and / or backpressure exists, discuss with the valve manufacturer.




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5.3 Breaking Pin Devices

Shear pin or breaking pin valves are designed to shear a pin, in the horizontal position, that sits on a piston with sharp edges (See Figure 5.2). When the pressure reaches the set pressure, the pin is sheared.

a) Pin will be damaged by pulsations. b) Piston edges must remain sharp. If not, the set pressure will be impacted. Time will dull

piston edges designed for shearing. This type of pin device suffers the same problems as reverse-acting rupture discs.

c) Breaking pin devices are designed to operate at a specified differential pressure. If pressure builds up downstream of the device, the set pressure will increase by the same amount. Like rupture disks, the space between the breaking pin device and the PRV shall be provided with equipment to detect any pressure build-up.

5.4 Buckling Pin Device versus Rupture Disc

Table 5.1 shows how buckling pin devices can answer some of the rupture disk‟s major drawbacks.

Rupture Disk Buckling Pin

Disks fatigues and usually fails early Rupture Pins cannot fatigue. Buckles at set point

May not know if disk has failed External pin shows that the device has lifted

Disks are in contact with process fluid, which can be corrosive

Pins are external to the process fluid

Downstream fragmentation is expected No downstream debris possible

Working close to set point is impossible Maximum operating pressure to 95% of set point. (Zero leakage at set point)

Senses differential pressure only Can sense upstream pressure only or differential pressure only

Costly storage and handling required Pins can be stored at the valve

Costly time is required to change disks Pin changing can be done with one person in minutes – even for 30 inch valves

Manufacturing tolerance varies widely (Could be +/- 10%). Tolerance must be requested before manufacturing. The closer the tolerance, the more expensive

Zero manufacturing tolerance

Table 5.1 – Comparison of Rupture Disks and Buckling Pin Devices




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5.5 Breaking Pin Device versus Rupture Disk

Most of the advantages of the buckling pin over the rupture disk as presented in Table 5.1 are also valid for breaking pin devices except:

The piston used for shearing the pin can be dulled with time, by the process fluid or by pulsation.

The breaking pin device does not have external signs that the pin is broken.

The breaking pin device can only sense differential pressure.

5.6 Examples

There are two applications in Ethylene 3 (Manufacturing West) where buckling pin pressure relief devices are in use. These are:

(1) The cracked-gas stream from the Quench tower overhead. Each device is 18 inches inlet nominal diameter and is equivalent to seventeen 8T10 conventional pressure relief valves. Those devices are installed horizontally. NOVA experience would recommend that piston type rupture pin valves not be oriented with the piston in a horizontal direction. Testing has shown that the frictional loads on the valves account for approximately 30% of the force required to open the valves and the high variability in friction load significantly impacts rupture pressure accuracy.

(2) The flare header in Ethylene 3 has two flare stacks and the second flare stack is

isolated from the first by a butterfly control valve. The butterfly valve type buckling-pin device is installed in a bypass around this control valve in order to relieve excess flare header pressure during any unintended closure of the control valve. This device has opened once during a plant upset in 2000 because the flare control valve did not open fast enough. Tuning has been changed and the device has not opened since.




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Figure 5.2 – Buckling Pin Device

This type senses differential pressure (impacted by backpressure). Reference: API




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Figure 5.1 – Buckling Pin Device with Balanced Piston

This type of device senses process pressure. (NOTE: the reference for this drawing is the ‘Rupture Pin technology’ Powerpoint presentation available on their web site:

www.rupturepin.com)




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Figure 5.2 – Breaking Pin Device

(The pin is horizontal, the vertical piston has sharp edges) (NOTE: the reference for this drawing is the ‘Rupture Pin Technology’ Powerpoint presentation available on their website:

www.rupturepin.com)

6. TANK VENTING RELIEF DEVICES

6.1 Conservation Vents

Atmospheric and low-pressure storage tanks are typically equipped with a combination pressure / vacuum vent called a conservation vent which is also called a direct-acting vent or a breather vent. These vents have separate pressure relief and vacuum relief assemblies that are combined in a common housing and are flange mounted onto a connection flange on the tank roof. These vents are designed to relieve a large volume of vapor or allow a large volume of air to enter the tank at very small differential pressures. See Figure 6.1 and Figure 6.2. Conservation vents are modulating devices with flow starting when the inlet pressure exceeds a specified set pressure. Unlike safety valves, these devices do not „pop‟ open at the set pressure.




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Conservation vents can be weight or spring-loaded. Each of the pressure or vacuum vents includes a machined seat that is closed by a moveable pallet assembly. The pallet assembly is designed to provide a specific amount of closing force. The amount of closing force determines the set point of the vent. The closing force is created either by metal plates that mount on the pallet assembly and provide the total weight required for a particular set point or by a spring that is attached to the pallet. Pressure or vacuum from within the tank that acts in opposition to the pallet closing force causes the pallet to lift off its seat thereby relieving the tank pressure or vacuum. The vents shall be in communication with the tank vapor space and not sealed off by the liquid contents of the tank. The vents can be specified for local discharge or equipped with an outlet flange connection that can be piped to a safe venting location or to a vapor processing location. In addition to protecting the tank from excessive pressure and vacuum, vents also conserve product and minimize tank emissions. The vents open only when necessary to relieve pressure or vacuum. At all other times, the vents are closed and no opening connects the tank‟s vapor space to the atmosphere.

6.2 Pilot – Operated Vent Valves

Another type of pressure or vacuum vent valve used on atmospheric and low-pressure storage tanks is the pilot-operated vent valve. A pilot-operated vent valve for pressure relief uses the tank pressure, not weights or a spring, to keep the vent valve closed. The main seat is held closed by tank pressure acting on a large diaphragm. A pilot-operated vent valve for vacuum relief uses atmospheric pressure to keep the seat closed with the main seat being held closed by the pressure differential across the seat. See Figure 6.3. Pilot-operated vent valves are less frequently used than conservation vents. The advantage of pilot-operated valves is that they achieve full lift at or below 10% overpressure. This lift characteristic permits overpressure protection to be accomplished with smaller or fewer venting devices. In addition, relative to conservation vent valves, pilot-operated vent valves can have a tank-operating pressure closer to the set pressure.

6.3 Emergency Vents

Emergency relief venting is required to provide emergency relief capacity beyond that furnished by the normal operating conservation vent on the tank. The emergency pressure relief valve protects the tank against rupture or explosion that can result from excessive internal pressure caused by fire exposure or other emergency over-pressure events. It is good practice to supply separate vents for normal breathing and emergency discharges. Discharges from emergency vents are not provided with outlet connections for collection of vented material. Emergency pressure relief vents are hinged cover type and flanged to fit standard API type tank connections (they do not fit ANSI flanges).




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6.5 Manufacturers

The major manufacturers of pressure / vacuum vents are Anderson-Greenwood, Groth, Protectoseal, Shand & Jurs and VAREC. Check for site preferences before selecting a manufacturer.

6.6 Explosion Prevention

According to NOVA LPS 6.5[8]

, „Atmospheric and Low Pressure Storage Tanks‟, Section 6.5.4

(k), where the tank contents can generate a flammable mixture in the vapor space, a floating

roof or inert gas blanket design shall be selected. NOVA LPS 6.5[8]

, Section 6.5.5 (e) also

states that flame arrestors should be installed where the potential for flammable atmosphere exists and inerting is not practical.

API STD 2000[9]

, Section 4.5.2 (c) states that a flame arrestor in an open vent line or on the

inlet to the pressure / vacuum valve is an effective method to reduce the risk of flame transmission. However the use of a flame arrestor within the tank‟s relief path introduces the risk of tank damage from overpressure or vacuum due to plugging if the flame arrestor is not properly maintained. Also, the use of a flame arrestor increases the pressure drop of the venting system. The manufacturer should be consulted for assessing the magnitude of these effects as well as for the selection of the correct flame arrestor for the service.

NOVA LPS 6.5[8]

, Section 6.5.4 (h) states that blowdown (vent) lines shall be protected from

plugging by one of the following,

Facilities that allow inspection and cleaning to be conducted at the necessary frequency to ascertain the desired reliability of the system

Redundant systems

In case of possible polymerization of a monomer, adding stabilizers or preventing the formation of polymerization initiators by adding an inert sweep gas




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Figure 6.1 – Pressure Vacuum and Emergency Vents




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Figure 6.2 – Examples of Pressure and Vacuum Vents




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Figure 6.3 – Example of Pilot-operated Pressure Vacuum Vent




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7. VACUUM RELIEF DEVICES AND OPEN VENTS

7.1 Vacuum Relief Vents

Vacuum relief vents can be purchased separately from combination pressure-vacuum vents. Their design and function is the same as described above for pressure-vacuum vents. Also, the same vendor list as in the above section applies to vacuum vents.

7.2 Open Vents

A vent that opens directly or indirectly to the atmosphere may be used as the sole pressure

relief device on a tank. In accordance with NOVA LPS 6.5[8]

as explained in Section 6.6,

Explosion Protection, open vents should only be used on tanks that do not contain a flammable vapor space. In situations where a tank contains a flammable vapor space and the installation of a floating roof or inerting is not practical then an open vent with a flame arrestor should be used. Also, concerns with the potential for vent line plugging should be addressed

per NOVA LPS 6.5[8]

as listed in Section 6.6 of this document.

In areas with strict fugitive emission regulations, open vents may not be acceptable in some services.

8. TEMPERATURE SAFETY RELIEF DEVICES

Overpressure by thermal (hydraulic) expansion is discussed in API STD 521[10]

, „Pressure-

relieving and Depressuring Systems‟, Section 5.14. The requirements for overpressure protection in the event of thermal expansion in liquid filled systems are discussed in Section 20

of NOVA Engineering Guideline EG-PRO-0105[11]


– Overpressure Causes and Assumptions and the calculation of required relief capacity is

discussed in Section 2.15 of NOVA Engineering Guideline EG-PRO-0106[12]

Pressure Safety

Devices Design Guideline – Calculation of Required Relief Capacity. Before selecting a thermal relief valve, alternative methods of pressure relief should be considered such as:

Add an open bypass around one of the system block valves. This is only suitable if

backflow around the bypass is acceptable.

Add a check valve around one of the system block valves. This is only suitable if

leakage around the closed check valve is acceptable during normal operation.

For thermal expansion due to ambient heating, a gas filled expansion chamber located

on top of the equipment could be used. The simplest design is a single expansion

chamber with an inert gas pad. The gas must be replaced periodically, since the gas

will dissolve in the liquid over time. Alternatively, a chamber equipped with a bladder

or bellows could be used, where the gas pad is not in contact with the liquid. This

option might be considered in the case of hazardous liquids where discharge to a

closed system through a thermal relief valve is not feasible or is too expensive.

Implement procedures to ensure that liquid is drained from the pipe or equipment

before it is blocked in.

If the system is rarely blocked in, consider car sealing open one or more of the isolation

valves.




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For most thermal relief applications, a valve size of ¾ inch x 1 inch is adequate. The valve market offers special models of relief valves for this application. These valves have smaller orifices (typically 0.06 square inch) than the smallest common relief valve size (D orifice, 0.11 square inch). Typically, these valves can relieve around 20 gallons/minute (76 liters/minute) for a set pressure of 150 psig (1034 kPag) and a viscosity of 1 cP (0.001 Pa.s).

An article published in Chemical Engineering Progress[13]

gives a useful rule of thumb for

assessing the adequacy of this thermal relief valve size for piping applications: “Pressure relief valves with an orifice size of 0.06 sq in. can be used in applications for which the total length of the protected system is up to about 10,000 ft. For longer distances, pipe diameters much bigger than 4 in. or compounds with higher thermal expansion coefficients, a quick check is recommended.” This rule is based on calculation of maximum lengths of 4 inch pipe for which a 0.06 square inch orifice is adequate for a heat input of 50W/feet (about 7 times the typical input provided by electrical tracing and equivalent to summer sunlight radiation in the southern US). For the following cases, a sizing check should be performed before assuming a ¾ inch x 1 inch valve is sufficient:

1. Long, large diameter piping runs.

2. Pressure vessels which can operate liquid full and are not already equipped with a

relief valve. Nearly all pressure vessels have a relief valve, unless exempted by an

ASME Case Code. Thermal relief is usually the smallest overpressure contingency, so

if a vessel does have a relief valve it will almost certainly be adequate for a thermal

relief case.

3. Heat exchanger systems in which the cold side is not protected from overpressure by

an existing PRD.

4. Systems handling liquids with atmospheric boiling points below the calculated relieving

temperature. The relief device may discharge flashing liquid and require a larger relief

valve than ½ inch.

To avoid inadvertent and nuisance releases, thermal expansion PRV‟s should be specified with a set point as high as possible above operating pressure. The set pressure must not exceed the design pressure of the equipment being protected. When determining the design pressure, the design rating of all the items included in the blocked-in system must be considered. The thermal relief set pressure must never exceed the maximum design pressure of the weakest component in the system. If the PRV discharges to a closed system, the effects of back pressure must be considered, such that the system design pressure is not exceeded.




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9. MATERIAL SELECTION

Materials in which a PRV is fabricated must be compatible with the process fluid and process

temperature. API STD 526[6]

Section 8 provides minimum requirements for body, bonnet and

spring materials. NOVA Engineering Standards MATERIALS series provides guidelines for metallic material selection for a variety of services. Soft goods used in PRV such as seals, O-rings and gaskets shall be compatible with the fluid nature. In particular, information on the fouling potential, contaminants, corrosive characteristics and solid content of the fluid(s) to which the PRV is exposed at any time of the operation cycle is required to make an adequate choice of materials. Soft goods shall be compatible with all process conditions to which the PRV could be exposed. This includes normal operating conditions, process upset, startup and shutdown, potential regeneration, cleaning and relieving conditions. A NOVA Materials Specialist shall be consulted for all material selection. Typically, vendors‟ only offers advice and are not accountable for material selection.

10. BASIS FOR SELECTION CHECKLISTS

Tables Table 10.1, Table 10.2, Table 10.3 and Table 10.4 give a brief description of the advantages and limitations of each type of PRD. They are not intended to be an exhaustive list but provide a rapid view of the main characteristics of each type of valve.

Advantages Limitations

Large chemical compatibility Seat leakage when simmering

High temperature compatibility Sensitive to variable backpressure

Widely used (parts widely available) Sensitive to inlet pressure losses

Set pressure depends on backpressure

Operate at or below 90% of set pressure

Table 10.1 – Conventional PSV


Very large chemical compatibility (guide and spring are protected)

Seat leakage when simmering

High temperature compatibility Bellows life limitation

It is a conventional valve with the addition of the bellows (parts availability)

Sensitive to inlet pressure losses

Set pressure independent of backpressure Potential of flammable or toxic material to atmosphere when bellows rupture

Operate at or below 90% of set pressure

Table 10.2 – Balanced Bellows PSV




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Smaller valves at higher pressure and larger orifice sizes

Not recommended for polymerizing service or fouling service

No seat leakage Soft goods must be compatible with process conditions and chemicals

Can operate as high as 95% of set pressure with no leakage

Not acceptable for BPVC section I applications

Availability of modulating type valve More complex and less used valve (parts are not available as easily)

Possibility of remote sensing (less sensitive to inlet pressure losses)

Temperature limitations (low and high)

No effect of backpressure on lift

Number of accessories available (lift sensing device, field test connection)

Table 10.3 – Pilot-operated PSV


Very rapid opening time Non-reclosing

Minimum space required Susceptible to pressure spikes

Exotic metallurgy available (chemical compatibility)

Large burst pressure tolerance (burst pressure is not necessarily very accurate)

No leakage if disc is intact Disc will fatigue

Table 10.4 – Rupture Disks (Dtandalone)

Figure 10.1 provides a guideline on criteria that can be used to choose a pressure relief device. This is a general guideline and exceptions will always exist. Sections 3 to 5 also provide information on advantages and limitations of each type of PRD.




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Figure 10.1 – Guideline to Choose Pressure Relief Device

All No

No

2-Yes 3-Yes

Any Yes

No Yes

Yes Is the pressure rise

too rapid for a PRV?

No

Is the process fluid toxic or aggressive? Is the process fluid likely to foul or freeze?

Rupture Disc (RD) or

pin-actuated device

Any Yes

All No

Rupture disc

Can a single PRV meet the

required relieving rate?

Is loss of content after

rupture acceptable?

RD alone RD and PRV

in series

Yes No

Single PRV Multiple PRVs

in parallel

Is required relieving area very large? Is the operating pressure ≥ 90% of set pressure? Is the inlet pressure drop already high?

Pilot-operated

valve

Spring-

operated valve

1. Is operating or relieving temperature ≤ -40F?

2. Is operating temperature ≥ 450F?

3. Is relieving temperature ≥ 450F?

Snap acting valve.

1-Yes

Engineering study

required to determine if

PORV can be used.

PORV not

recommended because

of soft goods.

Is venting of the dome volume to atmosphere accepted?

Consider

spring-operated

valve

Snap

acting

valve

Yes

Is backpressure

≥ 10% of set

pressure?

No

Yes

Conventional

valve

Balanced

valve




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11. APPENDIX A: ASME BPVC[5]

SECTION I VERSUS SECTION VIII STEAM SAFETY VALVES

11.1 ASME BPVC[5]

Section I Safety Valves

Used only for steam (or other gas / vapor) service

Designed to open to 100% with minimal overpressure (3% maximum) and is characterized by a rapid popping action. See Figure 11.1.

Blowdown mandated by code – maximum 4% (4 psi for set pressure 100 psig and below ). See Figure 11.1.

Figure 11.1 – Section I Safety Valves

11.2 ASME BPVC[5]

Section VIII Safety Relief Valves

Used in steam, other gas or liquid service, with modifications for steam

100% open at 10% overpressure – see Figure 11.2.

Designed to open with a rapid popping action (like a Safety Valve) or in proportion to the increase in pressure over the opening pressure (like a Relief Valve)

Blowdown not mandated, but typically less than 10%. See Figure 11.2.




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Figure 11.2 – Section VIII Safety Valves

They look similar from the outside:

Figure 11.3 – ASME I (Farris series 4200)

Figure 11.4 – ASME VIII (Farris series 2600S)




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But are different on the inside:

Figure 11.5 – Interior of ASME I and ASME VIII Valves

ASME BPVC[5] Section VIII requires that the inlet pressure drop be less than 3% of the set pressure. Because the blowdown of a Section I valve is less than 4%, if a Section I valve is installed where Section VIII applies, the inlet piping shall be kept to minimum so that the inlet pressure loss is much less than 3% of set pressure. Otherwise, the valve will chatter because it will reach its re-closing pressure as soon as the PSV opens.

Typical Section I Safety Valve Typical Section VIII Safety Relief Valve

Open Bonnet Open or Closed BonnetOpen bonnet keeps hot steam

Adjustable Lift Stop Fixed Lift StopSome models provide an adjustable lift stop to tune the Grooved Disc Holder Solid Disc HolderA grooved or notched disc holder reduces the contact area with

Dual Adj. Rings Single Adj. RingDual adjusting rings allow for better control of valve opening and closing.

Temp. Equalizing Disc Standard DiscA temperature equalizing disc

Farris Farris




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12. APPENDIX B: HOW BALANCED BELLOWS VALVES WORK

“The operation of a conventional spring-loaded pressure relief valve is based on a force balance. The spring-load is preset to equal the force exerted on the closed disc by the inlet fluid when the system pressure is at the set pressure of the valve. When the inlet pressure is below the set pressure, the disc remains seated on the nozzle in the closed position. When the inlet pressure exceeds the set pressure, the pressure force on the disc overcomes the spring force and the valve opens.” (API STD 520[1] Part I, 8th Edition)

The formulas listed below are based on Figure 12.1 where,

AB is the effective Bellows Area, inches2 AD is the Disk Area, inches2 AN is the Nozzle Seat Area, inches2 FS is the Spring Force, lbf PV is the Vessel Gauge Pressure, psig PB is the Superimposed Backpressure, psig PS is the Set Pressure, psig

The following terms have been added to aid in demonstrating the effect of a plugged bonnet vent:

PBonnet is the Pressure inside the bonnet ABonnet is the Area of the bonnet

Figure 12.1: Schematic of a Balanced Bellows PSV (API STD 520

[1] Part I, 8

th Edition)




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The balanced force equation for a bellows PSV is as follows, with the downward force equaling the upward force:

(Down Force) (Up Force) (12.1)

“In a balanced-bellows pressure relief valve, a bellows is attached to the disc holder with a pressure area, AB, approximately equal to the seating area of the disc, AN. This isolates an area on the disc, approximately equal to the disc seat area, from the back pressure. (…) With the addition of a bellows, therefore, the set pressure of the pressure relief valve will remain

constant in spite of variations in back pressure.” (API STD 520[1]

Part I, 8th Edition)

Therefore NB AA , and equation (12.1) becomes:

(12.2)

For proper operation of a bellows PSV, the bonnet vent is unplugged and PBonnet is equal to atmospheric pressure (or 0 psig). Therefore the force associated with the bonnet term is

cancelled and the typical spring force equation of ))(( Nss APF is obtained.

This is not the case when the bonnet vent is plugged, as there will be an additional force acting down on the disc, as described in equation (12.2), thus affecting the set pressure. The extent of the change in set pressure will be dependent on the PSV and which of the following scenarios exist, a non-compromised bellows or a ruptured bellows.

12.1 Bellows PSV with Non-Compromised Bellows and a Plugged Bonnet

With a non-compromised bellows pressure safety valve, BonnetP atmospheric pressure BP ,

and the pressure in the bonnet can be described by the equation of state for real gases

(12.3)

Where:

z is the Gas Compressibility Factor n is the Number of Moles (of the gas) R is the Gas Constant T is the Temperature V is the Volume

))(())(( NozzleSetNozzleDiskBack APAAP sBellowsDiskBackBonnetBonnet FAAPAP ))(())((

))(())(( NozzleSetsBonnetBonnet APFAP

V

znRTPBonnet




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In equation (12.3), n and R will remain constant. Also, the volume of the bonnet is fixed since the bonnets of bellows PSV‟s are designed to have a tight seal so that the process fluid will not be in contact with the spring. (This was confirmed with Farris). Assuming that the gas trapped in the bonnet is air, z does not change significantly. Therefore as the temperature increases, the bonnet pressure will increase. The change in bonnet pressure due to the fluctuations in ambient temperature is slight. The main concern arises when the temperature increases greatly from ambient temperature, which occurs when the valve is traced and insulated and/or in hot service. For example, increasing the temperature from 25°C to 200°C will result in an 8-psi rise in bonnet pressure (for air).

12.2 Bellows PSV with Ruptured Bellows

When the bellows is ruptured, the bonnet is exposed to the backpressure ( BackBonnet PP ) and

equation (12.2) becomes:

))(())(( NozzleSetsBonnetBack APFAP . (12.4)

The amount of superimposed backpressure experienced by a pressure safety valve is dependent on several factors including:

The discharge location of the PSV

The type of equipment being protected and whether that piece of equipment is involved in a global scenario

If the PSV discharges back into the process (i.e.: a vessel or a tank), there will be a constant backpressure superimposed by the normal operating pressure of the process. As there are normal fluctuations of the process pressure, so too will the backpressure vary. If the discharge location is the flare header, there are two different situations that occur. During normal operation, the backpressure from the flare header is assumed to be a constant 5 psig. The variation of backpressure is due to the occurrence of a global scenario. A global scenario is when an event occurs, such as a power failure or a cooling water failure, which affects several systems simultaneously. Then there will be numerous PSV‟s relieving into the flare header at the same time causing an increase in the flare header pressure. The variable backpressure could range anywhere from 5 psig to 30 psig according to information available from dynamic flare model studies.




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12.3 Set Pressure and the ASME Code[5]

The ASME code[5]

stipulates that the set pressure of a PSV shall not exceed the Maximum

Allowable Working Pressure (MAWP) of the equipment being protected. In other words, if the PSV opens at a pressure higher than the MAWP, the ASME code is being violated. (ASME

Section VIII, UG-125-3c)[5]

In order to be in violation of the ASME code, the set pressure of the plugged Bellows PSV must be at the MAWP of the equipment being protected and the various conditions described above met when a relieving scenario occurs. It is at that time that the opening pressure of the PSV will exceed the MAWP.

13. APPENDIX C: EXPERIENCES AT THE CORUNNA SITE

13.1 PSV-2146

This bellows PSV is located on the Boiler Feed Water system. The PSV has experienced multiple bellows failures. The PSV is located downstream of a local pressure controller in an intermittent service (to heater for decoke). The operating pressure is 455 psig, the set pressure is 550 psig. The operating pressure upstream of the control valve is about 700 psig. The first thought was that the valve was chattering. Upon inspection of the failed bellows, it was discovered that the valve was overpressuring. A pressure survey showed that the local controller would not close when the line was not in service – this is normal since it was trying to control at 450 psig. However the pressure would reach about 200 psig when the line was isolated, so the controller would open. With no forward flow, the pressure would become the pressure upstream of the controller, i.e.: 650 to 700 psig, and then the PSV would open. The procedure was changed to isolate the line upstream of the control valve when not in use. There have been no issues since this change was made. This shows that bellows failure are not always due to chattering.

13.2 PSV-1003

This PSV is located in the Crude unit on the desalter. Three bellows PSVs were replaced with one pilot-operated valve. The new modulating PORV failed wide open on startup as the sensing line filter plugged, diverting all the forward flow to the PSV (because of its very large size). The PSV would not reclose although the operating pressure had returned to normal operating levels. The reason for this is that there was not enough pressure gradient between the inlet and outlet of the PSV as the PSV relieves to process. The PSV was closed using the field test connection. It was later discovered that the PSV had opened and functioned properly at least 6 or 7 times before during the same startup, but the filter was not removed or cleaned. The startup procedure was changed to avoid pressure spikes due to pump startups. This shows that fouling was accounted for in the design and dual filters were requested. However, the design failed to recognize that once the PSV had fully opened, it would not reclose on its own (due to the large flow and the low pressure drop across the valve) and operations failed to recognize that the PSV had opened several times before, fouling the filter.




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13.3 PSV-2600

The original PSV is a bellows PSV in an ethylene refrigeration system. There was a plan to change this valve to a PORV because of multiple bellows failures. Operating pressure is 3 psig and operating temperature is -100C, set pressure is 120 psig. Low temperature calls for a snap acting valve. Snap acting valve domes must be vented to atmosphere. So there would be an ethylene release each time the PSV opens, i.e.: deliberate hydrocarbon atmospheric venting by design. Low temperature also calls for Teflon as soft goods materials for seat, Teflon is not so „soft‟ in low temperatures, i.e.: leaks can occur. It was decided to keep the bellows PSV and to vent the bonnet to a safe location. The vent can be isolated in case of bellows failure, so that the PSV can be safely isolated.

13.4 PSV-8150A/B

This conventional PSV is located on a high pressure steam system (500 psig). A letdown station control valve failed open, letting very high pressure (1500 psig) steam into the 500 psig steam header and causing a pressure surge. PSV-8150B opened but failed to reseat, PSV-8150A opened and shut closed so hard that it failed the pop test during maintenance. The header pressure was lost and the plant was tripped. Failure of the PSV is partially attributed to galling, due to higher temperature than designed for. It was also discovered that the PSV set pressure was 540 psig with an operating pressure at 500 psig. The PSV had been operating at 93% of set pressure.




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14. REFERENCES

[1] API STD 520 8th Edition (2008), Part 1 “Sizing and Selection of Pressure Relieving Devices in Refineries”

[2] ASME Code UG-131 – Certification of Capacity of PRV‟s – Actual Flow Tests [3] NOVA Engineering Guideline EG-PRO-0109 – Pressure Safety Devices Design

Guideline – Installation [4] NOVA Engineering Guideline EG-PRO-0108 – Pressure Safety Devices Design

Guideline – Sizing of Relief Device, Calculation of Required Relief Capacity [5] ASME Boiler Pressure Vessel Code, Section I and VIII [6] API STD 526 – Flanged Steel Pressure Relief Valves [7] NOVA Engineering Guideline EG-PRO-0104 – Pressure Safety Devices Design

Guideline – General [8] NOVA LPS 6.5 – Atmospheric and Low Pressure Storage Tanks [9] API STD 2000 – Venting Atmospheric and Low Pressure Storage Tanks [10] API STD 521. FIFTH EDITION, JANUARY 2007 (INCLUDES ERRATA JUNE 2007).

ADDENDUM, MAY 2008. (Identical to ANSI STANDARD 521 and ISO 23251), Pressure-relieving and Depressuring Systems

[11] NOVA Engineering Guideline EG-PRO-0105 – Pressure Safety Devices Design

Guideline – Overpressure Causes and Assumptions [12] NOVA Engineering Guideline EG-PRO-0106 – Pressure Safety Devices Design

Guideline – Calculation of Required Relief Capacity [13] Fabio Bravo and Brent D. Beatty “Decide Whether To Use Thermal Relief Valves” –

Chemical Engineering Progress, pages 35 to 38 (December 1993)

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