CONTROL VALVES

Rejath Jacob Thomas

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CONTROL VALVE - An Introduction

Ø A Control Valve is the most commonly used final control element used to regulate fluid flow in a process. In a process, normally it is the only controllable element residing in the loop.

Ø This is a device used to modulate flow of process fluid in pipe lines by creating a variable area in the flow path.

Ø The flow path is varied with respect to the control signal received from the controller towards the required flow modulation.

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CONTROL VALVE - ISA Definition

International Society of Automation -ISA (previously known as Instrument Society of America) defines the control valve as a power operated device that modulates the fluid flow rate in a process control system

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CONTROL VALVE - In a process control loop

Representation of a typical process control loop

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CONTROL VALVE - PARTS

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PARTS OF CONTROL VALVE

ØValve body assembly

• Body • Bonnet• Bolting• Seat ring• Plug• Stem• Cage• Packing • Seal ring• Gasket and shim

ØActuator assembly

Pressure Retaining Components

Trim Materials

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VALVE BODY

Main fluid boundary and pressure containing component

• End connections that allows installation in the pipeline

• Provision for attaching the bonnet

It Includes

• Provision for securing internal parts

BODY

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BONNET• Also a major pressure containing component and

fluid boundary• Bolted or threaded into valve body• Bonnet locates and guides the valve stem and it

includes a bore for packing (packing box)• Also includes yoke boss or some other means

mounting an actuator

BONNET

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TRIM

• Trim refers to all internal parts of a valve which are in flowing contact with the controlled fluid [also referred to as “Wetted” parts]

• It includes the valve plug, stem, cage, seat ring, plug sealing rings, gaskets and packing parts

Trim Parts

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TRIM COMPONENTS

ØPlug Provides throttling control and shutoff in globe valves

Ø Seat ringSeat rings in globe valves work with the valve plug to arrest leakage.

ØCageProvides plug guidance and flow characterization

ØStemConnects the plug to the actuator

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PARTS OF CONTROL VALVE - Contd

Ø Stem ConnectorA Clamp in two pieces to connect the actuator stem to the valve plug stem.

ØYokeThe connecting structure / part between the Valve Actuator and the bonnet assembly.

STEM CONNECTOR

YOKE

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• Bonnet gasket : Between Body and bonnet

• Seat ring gasket : Between seat ring and body

ØGaskets Provides sealing between mating surfaces.

Ø Packing Prevents leakage of the fluid through the valve stem

• Packing is compressed to form a tight seal between the packing box wall and the valve plug stem

PARTS OF CONTROL VALVE - Contd

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ACTUATOR

It the part of the valve which generates the motive power. It is the mechanism which operates the valve plug, in correspondence to the control signal received from the controller.

ØActuators Types:

• PneumaticSpring DiaphragmPiston Cylinder

• Electrical

• Hydraulic

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VALVE ACCESSORIES

ØPOSITIONER

•Pneumatic

•Electro-pneumatic

•Conventional•SMART – HART•Foundation Field bus

ØPOSITION SWITCHES

•Inductive

•Mechanical Cam Operated

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VALVE ACCESSORIES – Cont . . .

ØSOLENOID VALVE

•Latching type

•Manual Reset

ØVOLUME BOOSTERS

ØAIR LOCK RELAY

ØAIR FILTER REFULATOR

ØAIR VOLUME TANK

ØHANDWHEEL

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ØCapacity : It is the ability of the valve to allow fluid flow through it under stated condition.

ØCv –Valve Coefficient : It is the unit of measurement to define the capacity of a valve. The number of US Gallons of water at 60degrees F that will pass through a valve with a drop of one pound per square inch in one minute.

ØVALVE Kv : Quantity of water in M3/Hr. at temperature between 5 to 40 degrees C that will flow through the valve at a specified travel with a pressure drop of 1 Bar.•Kv = 0.856Cv

ØRated CV : Value of Cv at full open position.

VALVE TERMINOLOGIES

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Rangeability of a Control Valve is defined as the ratio of the maximum controllable flow to the minimum controllable flow. The Inherent rangeability of a valve is the ratio of the maximum and minimum flow under constant pressure differential . Hence it is expressed as Cv Max divided by Cv min.

TERMINOLOGIES Cont . .

ØLeakage:

ØRangeability:

Quantity of fluid passing through an assembled valve when the valve is in the closed position under stated closure forces withpressure differential and pressure specified. [ref to ANSI B-16 –104-1976]

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Flow characteristics when constant pressure drop is maintained across the valve.

TERMINOLOGIES Cont . .

Flow characteristics when pressure drop across the valve varies as dictated by flow and related conditions in the system in which the valve is installed.

•Inherent characteristics:

ØValve Characteristics:

•Installed Characteristics:

It is a relation between flow through the valve & percentage rated travel as the latter is varied from 0-100%.

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Linear Rotary

Each type of valve has its special generic features, which may, in a given situation, be either an advantage or a disadvantage

ØDepending on the motion of the plug

CONTROL VALVE - CLASSIFICATION

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LINEAR VALVES ROTARY VALVES

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Control Valve

Rotary Motion

Linear Motion

Butterfly

Eccentric Plug

Ball

Swing Through

Lined

Eccentric

Segmented

Full

V-Notch

Globe

Diaphragm

Pinch or Clamp

Globe

Angle

3-Way

Single Seated

Double Seated

Split body

CONTROL VALVE - CLASSIFICATION

ØDepending on the motion of the plug

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SINGLE SEATED GLOBE VALVE

Sizes : 1/2" to 16" Higher sizes available on request

To control large range of process parameters, specially for oil and gas production, power generation, chemical, petrochemical, fertilizer, pharmaceutical, processed food and other process industries.

Application :

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SINGLE SEATED GLOBE VALVE

Features :

Ø Leak proof packing are available on request

Ø Characteristics - Equal percentage, Linear and Quick open

Ø Can be supplied with wide range of interchangeable trimsizes

Ø Large variety of trim design varying from contoured trims tosingle stage ported and multiple stage low noise anticavitation trims

Ø High rangeability using low flow trims

Ø Tight shut off permissible

Ø Streamlined flow passage and excellent flow controlrangeability

Ø High powered actuator, field reversible

Ø High flow capacity and high gain

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DOUBLE SEATED GLOBE VALVE

Sizes : 11/2" to 18" (40 - 450 mm) Higher sizes available on request

For high flow and high pressure servicesApplication :

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DOUBLE SEATED GLOBE VALVE

Features :

Ø High Flow Capacity Minimum differential plug areas to reduce actuator force requirement

Ø Optional anti - cavitation / low noise trim option for high pressure drop application

Ø Characteristics - Equal percentage, Linear, Quick open

Ø Leak proof packing are available on request

Ø Heavy duty top and bottom guided construction

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ANGLE VALVE

Sizes : 1/2" to 6"

Specially for very high pressure drop and also used where the fluids contain solid particles.

Application :

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Features :

DOUBLE SEATED GLOBE VALVE

u High Flow Capacity

u Tight shut off

u High rangeability

u Body proportioned to withstand high pipe stresses without distortion

uWide range of interchangeable trim styles and sizesavailable

u Adaptability to specialist duties i.e. venturi outlet, low noiseetc.

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THREE WAY VALVE

Sizes : 1/2" to 12“ Higher sizes available on request

Definition : is ideal for blending two separate flows, by having two inlets and a common outlet, or dividing a flow into two proportional parts by having one common inlet and two outlets.

For high temperature fluid and particularly for textile industries, to mix or divert fluids

Application :

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THREE WAY VALVE

Features :

u High Flow Capacity

uLeakage capability better than ANSI class IV & consistentwith normal single seat valve

u High rangeability

u Leak proof packing are available on request

u Valve is used for mixing or diverting services depending on customers requirement

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THREE WAY VALVE

MIXING SERVICE DIVERTING SERVICE

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BELLOWS SEALED VALVE

Definition : Available as additional feature to give a leak proof packing in the single seated, double seated and three way valves.

For hazardous and corrosive fluids.

Application :

Features :u Bellows are of stainless steel to withstand high pressure & temperature.

uPTFE bellows available for low pressures on request.

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BUTTERFLY VALVE

Sizes : 2" to 36"

Steel, Sugar and textile industries and specially for fluid containing slurry particle. Used for on-off as well as control applications.

Application :

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BUTTERFLY VALVEFeatures :

uWater style connection Concentric design as standard, offset design for special application

uMetal to metal or Sleeved type construction

u Choice of various rubber or PTFE sleeves

u Tight shut off with rubber sleeves

u Minimum obstruction for flowing media

u High Flow Capacity

u Simple & Light construction

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BALL VALVE

Sizes : 1/2" to 8" (15 - 200 mm)

Used in paper, Pulp and textile industries

Application :

Features :

uTight shut off Minimum obstruction for the flowing media

u High Flow Capacity

u 'O' Ball & 'V' Notch construction available

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DIAPHRAGM VALVE

Sizes : 1/2" to 14"

For water treatment plant, caustic soda plant, chemical plants, fertilizers, specially for corrosive fluid.

Application :

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DIAPHRAGM VALVEFeatures :uCastings standardized to IS 210 FG grade cast Iron

u Better sealing and longer diaphragm life with the help of weir design

u Stronger bonnet design to take care of higher thrust andsupport to the diaphragm

u Diaphragm construction design with exclusively reinforcedfabric

u Fail safe assembly on auto valves totally protects processoperation in the event of electrical power or air supply failure

u Available in various linings to handle different fluids

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VALVE CLASSIFICATION - ACTUATOR TYPE

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Ø SCREWED

VALVE CLASSIFICATION – END CONNECTION

Ø WELDED END

Ø FLANGED END

Ø WAFER TYPEØ WELDED END

Ø WELDED END

• BUTT WELDED• SOCKET WELDED

• RAISED FACE, RF• FLAT FACE, FF• RING TYPE JOINT, RTJ

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End connection – Screwed end

• Popular for small control valves because of their economy and ease of installation

• Not available for bodies above 2 inch in size

• Aligning the threaded in valve with pipelines needs extreme careand mismatch may lead to damage to thread in valve

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End connection – Welded end

• For high pressure and high temperature applications• Valve body is directly welded into the pipeline• Leak tight at all pressure and temperature• Economical• Socket weld ends are common for smaller valve sizes and butt

weld ends are standard for sizes 2-1/2 inch and larger valve bodies• Precise alignment of valve body with pipe line is mandatory for

welded end connection

Butt Weld Socket Weld

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Butt welded end

Butt Weld end Connection

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End connection – Raised Face (RF)

• Most common end connection• Flange has circular, raised faces with concentric circular grooves

for good gasket sealing and resistance to gasket blowout• Gasket only covers the raised face portion of the flange• As line bolting is tightened, high stresses are produced at the

perimeter of each flange• Not available for brittle/Soft material like Cast Iron, Bronze etc.

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End connection – Flat Face (FF)

• Allows full face contact with gasket clamped between the flanges• Full face contact has the advantage of minimizing flange stresses

caused by forces of line bolting• This type of flanges are common in cast iron bodies which are

brittle and would crack if raised face flanges were used. Flat face flanges are also common in brass and aluminum bodies

• These materials are soft and would bend if raised face flanges were used

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End connection – Ring type joint (RTJ)

• Well suited for extremely high pressure applications but generally not used at high temperature

• Mating grooves in the body and pipe flanges retain a solid metalseal ring

• As the line bolting is tightened the ring deforms and conforms to the grooves

• Pressure assisted seal; since as the pressure increases, the ring pushes against the grooves

• Soft iron or Monel is generally used as gasket material

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End connection – Separable flange

• Flanges are manufactured independently of the valve body

• Split rings are required to secure the flange to the body and totransmit the bolting force to the gasket and mating connection

• If the body material is expensive separable flanges may result in economy since flanges can be made of an economical material like carbon steel

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End connection – Wafer style

• Generally called as Wafer style or flangeless

• Includes a raised-face contact surface in the body casting

• Long line bolting sandwiches the body and gasket between the pipe flanges

• This is more economical body style since it requires less material for construction

• Ball and butterfly valves are commonly available with this body style

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Shutoff class

ØImportance of control valve shutoff

• To maintain proper operation of a process

– If a control valve leaks excessively in closed position the leakage may affect the downstream processes & end product quality

• To avoid potential damage to the valve in certain applications

– If a valve in boiler feed water application leaks, the high velocity streams that leaks across the seat can erode critical seating surfaces

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Shutoff class

ØThe ability of a control valve to prevent flow from the inlet side to the outlet side of the valve in the fully closed condition.

ØANSI/FCI 70-2 came up with different shutoff classes to standardize leakage across control valve

ØTo provide a range of shutoff options linear valves have different type of plug and seat construction

ØRotary valves have various seal configuration and materials to offer a range of shutoff

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Shutoff class

ANSI/FCI Leakage Classes

ANSI/FCI 70-2 Test Medium Pressure and temperatureClass IIClass IIIClass IVClass V Water Service DP at 10 to 52deg C

Port dia.Bubbles per

Min.mL per Min.

1 1 0.151 - 1/2 2 0.30

2 3 0.452 - 1/2 4 0.60

3 6 0.904 11 1.706 27 4.008 45 6.75

0.0005ml/min/psid/in. port dia

Class VI

Service DP or 50 PSID whichever is lower at 10 to 52deg C

Water / Air

Service DP or 50 PSID whichever is lower at 10 to 52deg C

Air

Maximum Leakage0.5% valve capacity at full travel0.1% valve capacity at full travel0.01% valve capacity at full travel

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Flow Characteristic

Ø Relationship between flow through the valve and the plug/disc travel.

Flow Characteristics

• Quick opening-Shut off & Fast flow

• Linear-Liquid level control

• Equal percentage-Pressure & Flow control

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Why Characterization?

ØTo compensate the process gain changes so that loop gain remains independent of load.

ØTo provide uniform control loop stability over the expected range of operations.

ØTo match the valve gain to the system gain for optimum performance.

Characteristics methods:

• Trim shaping

• Positioner characterization

• Controller characterization

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Valve Characteristics

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Quick opening

• Inherent flow characteristic with high flow at initial % of travel.

• High and constant gain at initial half of its travel.

• Decreasing gain at travel above 70%

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Equal percentage

• Equal increments of change in valve travel produce equal percentage changes in the existing flow coefficient

• Provides low valve gain at low percentage of valve travel, and high valve gain at higher percentage of valve travel

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Linear Characteristics

• Flow is a linear function of travel

• Gain remains constant throughout the travel i.e 1

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Cage Shapes

Quick opening Linear Equal percentage

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Plug Shapes

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Plug Shapes

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Plug Shapes

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Inherent Characteristics

• Relationship between flow rate and valve travel, under constant pressure drop conditions

• Inherent valve characteristics can be Quick opening, Linear, Equal percentage

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Inherent Characteristics

uFlow at constant pressure drop

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Installed Characteristics

• Relationship between flow rate and valve travel, under actual process conditions

• Installed flow characteristics is process dependent where pressure drop changes with process variable

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Installed flow Characteristics

uFlow at varying pressure drops

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Valve Gain

Ratio of change in flow to change in stem travel

Valve Gain =

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Guidelines for selection of Valve Characteristic

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Non Linear Process

• At Low flow, change in input signal will cause large change in flow-rate.

• At high flows, the change in input signal will cause very small change in flow-rate

• Impossible to tune controller for stability as the controllability and the operating range sometimes is out of boundaries

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Compensation- for Non linear process

• To provide linear gain over entire system, in a non linear process, equal percentage characteristic is chosen.

• The Equal Percent’s low gain compensates for high gain at low flow

• Similarly, the Equal Percent’s high gain Compensates for low gain at high flow

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Application guidelines

LinearConstant dPBack Pressure

Equal PercentMax dP>5X Min dP

LinearMax dP<5X Min dPLiquid Level

LinearSet point changes

Equal PercentLoad changesFlow

Equal PercentMostTemperature

Equal PercentGas , Large volumeMax dP > 5X Min dP

LinearGas , Large volumeMax dP < 5X Min dP

Equal PercentGas, Small volume

Equal PercentLiquid processPressure

GUIDELINESAPPLICATIONPROCESS

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Body/Bonnet material – carbon steel body

ØMost commonly used carbon steel bodies are WCC and LCC

ØWCC – Weldable Carbon steel Cast

• Preferable to killed carbon steel

• Above 427 deg C graphitization can occur

• Can be used between -29 to 427 deg

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Body/Bonnet material – carbon steel body

ØLCC – Low temperature carbon steel cast

• Quenched and tempered in order to meet impact requirement

• Same chemical composition as WCC

• Can be used between -46 to 343 deg C

• Impact tested for confirming suitability

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Body/Bonnet material – Alloy steels

ØAddition of Chromium and molybdenum enhances the resistance to graphitization and tempering at elevated temperature

• WC6 : -29 to 593 deg C

• WC9 : -29 to 593 deg C

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Body/Bonnet material – Stainless steel

ØFor increased high temperature capability and for increased pressure retaining capability, Alloys with still more chromium and molybdenum are specified

ØTo avoid sensitization (formation of carbides) carbon content iskept low in stainless steel

ØCF8M, CF3M, CG8M, CF8C

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P&ID symbols of various control valves per ISA S5.1

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Applicable codes and standardsØ ASME B16.34 - valves - flanged, threaded and welding ends

Ø ASME B1.20.1 - Pipe threads

Ø ANSI FCI 70-2 - Control valve seat leakage

Ø ISA 71.01 - Environmental conditions for process measurement & control

systems: Temperature and Humidity

Ø ISA 75.01 - Flow equation for sizing control valves

Ø ISA 75.03 - Face to face dimensions for flanges globe-style control valve bodies

Ø ISA 75.04 - Face to face dimensions for flangeless control valves

Ø ISA 75.17 - Control valves aerodynamic noise prediction

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Applicable codes and standards

Ø NEMA ICS 6 - Enclosures for Industrial controls and systems

Ø IEC 529 - Specification for degree of protection provided by enclosure

Ø NFPA 70 - National electric code

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Major vendors

Valve Assembly

Ø Masoneilan Dresser

Ø Fisher

Ø Flowserve (Valtek)

Ø Tyco

Ø Instrumentation Limited

Ø Metso

Actuators= EIM

= Bettis

= Keystone

= Automax

= Rotork

= Limitork

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Questions?

Thank You !