Pipe Line Dewatering(Ch6)

8/13/2019 Pipe Line Dewatering(Ch6)

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Pipe Line Dewatering, Cleaning, and Drying*

Newly constructed pipe lines are typically hydrostatically tested, using water as the test

medium, upon completion of construction. Older lines already in service may be re-testedeither to confirm an earlier test or to qualify the line for a higher operating pressure. Once

the hydrostatic testing has been completed, it is necessary to remove the water from the

line and place the line in service. Dewatering can be a simple process or, if the procedureis not properly planned, a difficult one. Pipe lines used to transport crude oil and/or

refined products will probably only require removal of the test water before the line is

placed in service. f the pipe line will be used to transport materials which must meet aspecified dryness requirement, the pipe line will need to be dewatered, cleaned, and

dried. Pipe lines used to transport natural gas will need some drying, depending on the

operating pressure and the location of the line, to prevent the formation of hydrates. Other

pipe lines may require drying to protect the pipe from internal corrosion caused by theformation of corrosive acids, such a carbonic acid in the case of carbon dio!ide pipe

lines.

" #ased on a paper entitled $%he &tate of the 'rt of Drying, (arge Diameter )as Pipelinesafter *ydrotest,$ by +arvin D. Powers, Pipeline Dehydrators, nc., *ouston, %e!as.

Dewatering

Dewatering is considered to commence with the running of the first pig after hydrostatic

testing is completed and begins with the insertion of a displacer, commonly referred to as

a pig, in the pipe line. %he dewatering pig may be pushed through the pine line withcrude oil or other petroleum product if no drying is required. f the pipe line is to be

cleaned and/or dried, the pig will be pushed by either compressed air or gas. n either

case, proper precautions must be taen to be sure the test water is properly disposed ofand that any required water discharge permits are obtained ahead of the dewatering

operation.

&everal types of pigs may be used for the dewatering phase and pig selection should

depend upon the design characteristics of the pipe line and the degree of cleaning, if any,that is desired. deally, the pig will form a perfect seal with the inner periphery of the

pipe and will not allow any of the material behind the pig to lea past the pig and

commingle with the test water ahead of the pig.

Normally, the pig will move through the pipe line without difficulty. *owever, there aremany opportunities for problems to develop, causing the pig to stic or even disintegrate.

&ticing may be caused by any or a combination of the following things

• Pig is incorrect length and cannot negotiate chec valves, tees, and bends



• Pig is too large for heavy wall sections of the line

• Pipe line may be equipped with reduced opening valves

• !cess debris or construction material may be left in the pipe line

• 'ir/gas may bypass the pig and create an air loc condition and,

• Other unnown reasons.

'ir locs are more liely to occur in hilly country than in flat land. 'ir locs occur when

the accumulated static heads are greater than the available displacing pressure. n somecases, a pressure greater than the ma!imum allowable pipe line pressure would be

required to overcome the air loc. 0sually, the air loc occurs where air/gas has bypassed

the pig and the downhill legs of the pipe line are filled with water. %he air/gas can get in

front of the dewatering pig in several ways such as

• Poor filling techniques

• Poor dewatering procedure such as draining water from the line at low points

• 'ir/gas bypassing the pig because the pig is too small for the pipe line

•

'ir/gas bypassing the pig before it is launched• 'ir/gas bypassing the pig in a fitting such as in a tee, or a steel shaft pig in a short

radius bend and,

• Other unnown reasons.

1hen an air loc condition occurs, it is necessary to either increase the displacing

pressure, or remove air/gas through e!isting vents or other connections at high points infront of the pig.

2or these reasons, it is important that the dewatering process be carefully planned,

especially for pipe lines located in hilly country.

Cleaning Pipe Lines

t is believed there are no written specifications to define the degree of cleaning for a pipeline since there is a question of the definition of a clean pipe line and furthermore, a

method for measuring the cleanliness has not been developed. t is nown, however, that

cleaning a pipe line does at least four good things. 3leaning the pipe line will

• mprove flow efficiency because of a smoother pipe wall

• 4educe product contamination and formation of hydrates

• 4educe abrasive damage to pipe line appurtenances such as valves and

instruments

• 2acilitate pipe line drying

nternal cleaning of the pipe line may be accomplished by any of or a combination of thefollowing methods



• 4unning a brush pig with air, gas, or liquid

• nternal sand blasting

• 3hemical cleaning

• Purging with air or gas followed by a liquid flush

Brush Pig Run With Gas

One brush pig pushed by gas or air will not usually remove a significant amount of small

debris from the pipe line, particularly if the debris is wet. *owever, the scratching action

of a good brush pig will more evenly distribute the loose debris and leave a smoother pipe wall, which will improve pipe line flow efficiency.

Brush Pig Run With Liquid

' brush pig displaced by water is a very efficient means for removing debris from the

pipe line provided certain precautions are taen. !perience has shown that a pig velocitygreater than three feet per second is desired. ' pig, such as the Power #rush Pig, that

allows some of the liquid to bypass the pig to eep the bristles clean and move the loose

debris into suspension in the liquid in front of the pig is also desirable. %he debris must be free of any sticy material such as oil, grease, or paraffin.

t is not necessary that the pipe line be completely filled with liquid. &hort slugs of liquid

batched between two pigs is usually preferable as long as there is enough water to eep

the pigs apart and maintain a velocity greater than three fee per second. &hort slugs will

minimi5e the water/debris disposal problems and will allow for higher velocities withlower pressures.

Disposal of the dirty liquid may present a problem, depending upon the location. 's

indicated earlier, environmental aspects should be investigated thoroughly during the pre- planning phase. 0sually, rust will settle out of still water in about one hour, leaving an

almost clear liquid. f the dumping of semi-clear water is acceptable, then the problem

may be solved by the use of holding tans.

%he debris-holding ability of the propelling fluid is proportional to the viscosity/densityof the propelling fluid. %he greater the viscosity/density, the higher the holding ability. t

has been reported that gels have been developed and used that are supposedly moreeffective than water.

Internal Sand Blasting



nternal sand blasting is an e!tremely effective method for cleaning pipe lines. %his

method was e!tensively used when natural gas was ine!pensive and there were no

environmental controls. &ome short, small-diameter pipe lines are still being internallysand blasted by using air or nitrogen to propel the sand. %his procedure, if properly

e!ecuted, will leave the pipe line dry upon completion.

'fter the pipe line has been dewatered, clay is blown into short sections of the pipe line

with a dray gas to absorb the remaining water. %he correct type of sand is then blownthrough the dust dry section at a high velocity to remove the clay, rust, and mill scale.

3aution must be taen to ensure that short sections are cleaned each time so that high

velocities may be maintained that large quantities of dry air or gas are available tosustain the high velocities that the pipe line is dust dry that all sand has been removed

from the pipe line after the sand blasting is complete and that the pipe line facilities will

not be damaged by the high velocity sand.

Chemical Cleaning

3leaning with chemicals is typically used for smaller diameter lines. ' properly planned

chemical cleaning operation can produce a very clean pipe line. 3hemical cleaning will

remove rust from any internal pits. 3hemical cleaning is accomplished by pushing a batch of hydrochloric acid and water for rust removal, a neutrali5er, and a passivator, all

separated by pigs, through the pipe line. %his method will leave a bright metal finish on

the interior of the pipe line. f a pipe line is being converted for a different service, suchas a crude oil line being converted to chemical service, a batch of detergent can be added

behind the cleaning process to remove any traces of the crude oil or other product.

%ypically, chemical cleaning is more involved than other cleaning processes in that somespecial equipment will be required to safely handle the chemicals. Disposing of the spentmaterial will also present a problem. 3are must be e!ercised in the selection of pigs to be

sure they will not be consumed by the cleaning solution. Personnel safety must be

carefully considered when choosing this method of pipe line cleaning.

Purging with gas %his method is seldom used for pipe line cleaning since the amount ofdebris that will be removed is small. ' high gas velocity is required.

Pipe Line Drying



Pipe lines used to transport petrochemicals such as propylene and ethylene must be dried

in order for the delivered product to meet moisture specifications. Natural gas pipe lines

are usually dried to a lesser e!tent to prevent the formation of hydrates. t is not unusualfor a petrochemical line to be dried to a dew point of -6782. ' typical dew point for a

propylene pipe line will be -9782. ' carbon dio!ide pipe line might be typically dried to a

-:782 dew point. %he natural gas industry specifies dryness in pounds of water permillion standard cubic feet of gas. %he table at the end of this section will enable you to

convert from one method to the other.

Dew point, by definition, is the temperature at which water vapor begins to condense out

of a gas at atmospheric pressure. 2or e!ample, at atmospheric pressure, water vapor begins to condense out of a gas that has a moisture content of seven pounds of water per

million standard cubic feet at -;<82. %herefore, a gas that has a moisture content of seven

pounds of water per million standard cubic feet has a dew point of -;<82.

'ir at =9782 with a humidity of >77? has a dew point of =9782 and holds appro!imately

>,@A; pounds of water B>A7 gallonsC per million standard cubic feet.

%he most common methods for drying pipe lines are as follows

• Drying with super dry air

• Drying with methanol

• Drying with inert gas such as nitrogen

• nternal sand blasting

• Drying with the medium to be transported

• acuum drying

'll of these methods may be applied to pipe line drying depending on the particular lineand amount of dryness required. No single method can be considered ideal for allsituations. +any times, a combination of two or more methods will be used to achieve a

dry pipe line at the least cost. %he first three methods are probably the most economical

and technically feasible for most pipe line drying applications.

#efore any type of drying operation commences, it will be necessary to clean the line

using one of the previously described processes. f the rust and mill scale are not removed

from the pipe wall, moisture will remain trapped and will bleed out over a long period of

time. ' pipe line can be dried without cleaning however, the cost and time required will be great.

n drying with super dry air , soft form pigs pushed by dry air are used to absorb any free

water remaining in the pipe line after dewatering. 'fter the line is dust dry, wire brush

pigs are run to remove any water bearing debris from the pipe wall. %he wire brush pigsare then followed by soft foam pigs to absorb the loosened debris. Near the end of this

phase, the pigs may be weighed prior to insertion and after removal to monitor the

amount of debris that is being removed. %he first pigs run through the line will naturallyweigh much more than their clean weight. 's the cleaning and drying progress, the pig



weights will approach their original weight. Pig color will also give some indication of

how the debris removal is proceeding.

Dew-point readings will need to be made to determine when the line has been dried to thespecified dew point.

t will be necessary to give special consideration to laterals, by-passes, and valve body

cavities as any free water trapped here could affect the final dew point readings.

Drying with super dry air provides internal corrosion protection if the line is to remain

out of service for some period of time before it is placed in service.

Methanol drying relies on the hydroscopic effect of the methanol. 'ny remaining

moisture in the line will be absorbed by batches of methanol pushed through the line with

either gas or dry air. Pigs are used to separate the methanol batches form the displaying

medium.> +ethanol drying usually requires fewer pig runs and, consequently, less line

cleaning is accomplished. Pure methanol is e!pensive and sometimes a <E?methanol/water mi! is used. &ince the methanol mi! contains water, some water will be

left in the pipe line. &ome of the methanol will vapori5e in the pipe line and will beabsorbed by the displacing medium. %oward the end of the line, the moisture content of

the methanol will increase, which in turn reduces the amount of water that it can absorb.

f the pressure used in the drying operation is too high, hydrate formation can occur,usually at the far end of the line. f natural gas is being used to push the methanol

batches, it will probably be necessary to flare some of the line fill volume to be sure that

no methanol impurities are contained in the gas.>

+ethanol run with a dry gas will absorb most of the water and facilitate the vapori5ation

of the remaining water. &oft swabs run through a line with a dry purge gas will acceleratethe evaporation of remaining methanol/water solution.

'lso of great concern with the methanol method of drying is the fact that e!plosive

mi!tures can easily be formed, whether gas or air is used to displace the methanol batches. t may be desirable to use an inert gas such as nitrogen to buffer the methanol

batch from the air or gas used to displace the methanol batch. *owever, many pipe lines

have been dried without the use of nitrogen buffers with no adverse results. %heair/methanol mi!ture is also highly poisonous and corrosive.>

Plans will need to be made for proper disposal of the spent methanol.

f internal corrosion protection is desired, then another drying method should be

considered. f the pipe line is to be used to transport sour gas, the methanol drying

method should be carefully evaluated before it is used.

Drying with nitrogen is accomplished in much the same manner as when using super dry

air. Nitrogen drying will cost more than super dry air by a factor of appro!imately A7?.

'lso, super dry air is plentiful and non-polluting.



Internal sand blasting will leave the pipe line clean and dry. %he procedure for internal

sand blasting is described earlier in this section.

Drying with natural gas requires large volumes of gas. %his method is slow and not veryeffective unless the line is thoroughly cleaned by one of the cleaning processes

previously described. f, however, the gas being used to dry with can be blended withanother dry gas stream and sold or used, then this is an economical method for pipe line

drying. %he cost of the gas that will be used to purge the line during the drying processshould be weighed against the cost of using super dry air. %he #tu value of a volume of

natural gas is appro!imately fifteen times greater than that of the fuel required to produce

an equal volume of super dry air.

Vacuum drying is a slow process and all free water should be removed from the pipe line

before drying begins. %his method appears to be used infrequently, and perhaps only

offshore.

f the pipe line has been properly cleaned by the water slug method using brush pigs runwith liquid, drying can be accomplished by running soft foam pigs with dry air or gas to

remove any free water left in the pipe line. %his will usually produce a pipe line dry

enough for natural gas operations. f additional drying is desired, it can be accomplished

by using methanol or super dry air.

Re!erence

" #$pp, %$isture C$ntent $! &ir



Pipe Line Industry, October ><6>, Fanuary ><6E.



C$mmissi$ning Petr$chemical Pipe Lines

Pipe lines carrying a variety of gaseous and volatile liquid products are e!tensively

employed in todayGs industry as a safe and efficient means of transportation. New pipe

lines must be placed into service initially Bi.e., commissionedC. and older linesoccasionally are taen out of service in order to perform some desired maintenance and

are then recommissioned. 'mong the reasons for taing a pipe line out of service Bi.e.,decommissioningC are hydrostatic testing to recertify or upgrade the pipe lineGs ability to

be used at higher operating pressures performance of construction wor on the pipe line

a change in the product transported by the pipe line. Pipe lines, or sections of a pipe linemay need to be relocated because of highway wor, the necessity to deepen a canal, or

because of increases in the population surrounding the pipe line. t may also be necessary

to replace valves, fittings or damaged section of the pipe line or add a new connection to

service a customer or supplier.

%ypically, decommissioning and recommissioning a petrochemical pipe line will includethe steps of decommissioning by removing the product from the pipe line, and flaring any

remaining residual product. 'ny necessary construction upgrade, or cleaning of the pipeline can then be performed. 0sually the pipe line is then filled with water for hydrostatic

testing. 'fter pressure testing, the water is removed, the pipe line is cleaned and dried to

a specific low dewpoint Bto avoid the problems of water contamination of the productC

and the pipe line is inerted with nitrogen for recommissioning. 4efer to the section onPipe (ine Drying for drying techniques.

n order to recommission the pipe line, the nitrogen must be displaced by the desired

petrochemical. #efore returning to service, product purity must be established and the

line safely filled to operating pressure. %he terms GcommissioningG and GrecommissioningGare used interchangeably to refer to a process whereby a first inerting gas, normally

nitrogen in a pipe line is replaced with the desired product at the desired purity and

pressure.

&afety and economics are two primary concerns for any proposed pipe line operation.

%hus, a commissioning process which brings the purity and pressure of potentially

e!plosive products such as ethylene or propylene up to specification quicly, but at the

ris of damage to the pipe line or reduced safety to operating personnel, is not acceptable.&imilarly, a process which uses large quantities of product to push nitrogen from a line

results in wasted product/nitrogen mi!tures that must be flared or otherwise disposed of

would also be unacceptable. mprovement over nown methods requires consideration ofits safety and economic benefits Bconsidering both the cost of wasted product and the cost

attributed to the time the line must remain out of serviceC as well as its ability to bring the

line bac into service with product at desired pressures and purities. B>C

%he main problems posed by recommissioning are B>C the fast and economical purging ofnitrogen so as to obtain uncontaminated products in the line, and B@C possible damage to

the pipe line due to cold temperatures to which it may be subHected during the process.



%he latter can be a problem when the pipe line is to be recommissioned with a product at

a pressure significantly higher than that of the nitrogen inerted line. %he pressure drop of

a petrochemical product entering a pipe line can cause large drops in temperature and,consequently, potential damage to the carbon steel pipe line.

Due to the demands of the e!panding petrochemical industry, and the aging of the present pipe line system, the need for decommissioning, repairing, cleaning, drying and

recommissioning pipe lines is increased. )iven the value of todayGs petrochemical products, a recommissioning procedure, which brings the product purity to acceptable

levels as quicly as possible, is desirable. 2urthermore, processes and equipment which

reduce the possibility for damage to the pipe line due to cold temperature conditionswhich can occur during the recommissioning process are also highly desirable.B@C

Due to the unusual properties of ethylene, decommissioning and recommissioning

ethylene pipe lines require special consideration. thylene is a colorless flammable

gaseous unsaturated hydrocarbon obtained by the pyrolysis of petroleum hydrocarbons.

%he relative density of ethylene is 7.<E6E Bair I >C and its molecular weight is @6.7A:.%he molecular weight of air is @6,<E: and the molecular weight of nitrogen is @6.7>;.

'thylene Dec$mp$siti$n

thylene is subHect to thermal decomposition Bself propagating reaction 5one, e!plosionor flameC under certain circumstances. %hermal decomposition occurs when the

temperature of a substance is raised above the value needed to cause it to begin to self-

heat because of a change in molecular structure at a rate high enough to result incombustion. 'n e!ternal source of ignition is not required to precipitate this process nor

is a source of o!ygen needed. %hermal decomposition is usually initiated by a source of

heat from events such as sudden compression, e!ternal source of heat, or the thermalrunaway of a heater.B:C *eaters are sometimes used in ethylene measurement stations tomaintain the ethylene at an optimum temperature for measurement.

%he ethylene is routinely handled under conditions where decomposition may be

initiated. %he point at which decomposition begins is dependent upon system parameters

such as line or vessel si5e, operating pressure, and operating temperatures. )enerally,ethylene decomposition is initiated in pipe line facilities by a rapid rise in temperature

associated with sudden compression in the presence of a diatomic gas such as nitrogen or

o!ygen, e!posure of the pipe to flame, possibly hot tapping operations.

%he autoignition temperature B'%C for ethylene is <>:82 at atmospheric pressure.

B;C

e!perience of other suggests that at >,777 to >,A77 psig, the '% could drop to as low a

:778 to 9782. B;,:C 's the concentration of o!ygen increases, the decomposition initiation

temperature decreases.B:C 2rom this, it may be concluded that a mi!ture of low o!ygenconcentration and ethylene could be very ha5ardous. 2or this reason, ethylene pipelines,

which have been evacuated for maintenance or repairs, are usually filled with nitrogen to

displace o!ygen from the pipe. 3ompressing nitrogen with ethylene may result in highnitrogen temperatures - especially if the compression is rapid. 'lso, when the pressure of



the ethylene source is high, refrigeration across the throttling valve used to control the

ethylene may result in e!tremely low temperatures and could cause the valve to fail due

to e!cessively low temperature.

3ommissioning an ethylene pipe line with warm ethylene B=/-9782C at a low pressure B=/-

A7 psigC is quic, easy, ine!pensive and very safe when the ethylene is heated before it isadmitted to the pipe line being commissioned.

' patented process used by Pipeline Dehydrators can be used to safely commissionethylene pipelines. %he process uses a shell and tube heat e!changer to heat the high

pressure ethylene before the pressure is reduced to the low pressure desired for line

commissioning.

%he methods currently used to commission ethylene pipelines are potentially ha5ardous,e!pensive and time consuming. %he two most severe problems are potential damage to

the carbon steel in the pipe line due to the cold temperature of the e!panded ethylene and

contamination of the ethylene from the nitrogen used to inert the pipe line duringcommissioning.

'n ethylene pipe line that is being commissioned has usually been dried to a -9782 dew

point or more and inerted with nitrogen. %he nitrogen pressure left on the line for

commissioning varies from >7 to >,777 psig depending on the pressure of the ethylenesource and the commissioning procedure used and the pipe line ownerGs preference.

%he source of ethylene used for commissioning an ethylene pipe line is usually over <77

psig and may be up to @,@77 psig.

f the pressure of the ethylene source used for commissioning is high and the nitrogen pressure in the pipe line being commissioned is low, the resulting temperature of theethylene, due to the temperature drop, may be well below the -@782 design temperature of

most carbon steel pipe lines.

%he carbon steel in some pipelines is subHect to becoming brittle at temperatures below-@782 and may fail at a very low internal pressure because of the low temperature.

't low pressures the very cold ethylene will be much more dense than nitrogen, thereby

producing an e!tensive product interface during commissioning or trapping pocets of

the less dense nitrogen at each high place in the pipe line.

n this case, the amount of ethylene burned to produce product purity may be e!pensive,time consuming, create environmental problems in addition to subHecting the steel in the

pipe line to sub-design temperatures.

f the pressure of the ethylene source used for commissioning is high, and the nitrogen

pressure in the pipe line being commissioned is also high, cold temperatures will not beencountered. *owever, even at warm temperatures, high-pressure ethylene is still much



more dense than nitrogen at the same pressure. %herefore, the e!tensive product interface

and the trapped nitrogen at the high points will still occur, resulting in an e!pensive and

time consuming purification process plus the cost of the large volume of nitrogen. 'lso,there is a large inventory of ethylene in a pipe line at high pressure. f a pipe line blow-

down is required because of a lea, lac of purity or for whatever reason, it will be a

maHor product loss and e!pense.

%he Pipeline Dehydrators system utili5es a tube and shell heat e!changer and a water-glycol mi! that is used as the heat transfer fluid. ' pump forces the heat transfer fluid

through the fired heater and then through the shell side of the e!changer. %he ethylene

passes through the tubes of the heat e!changer is safely and properly warmed by the heattransfer fluid without entering the direct fired heater, thereby eliminating the possibility

of an ethylene decomposition due to heater thermal runaway.

%he heater will produce ethylene at ground temperature B=/>9782C at =/-A7 psig

corresponding to the conditions of the nitrogen in the pipe line. 0nder these conditions,

ethylene and nitrogen have basically the same density therefore, the product interfaceduring commissioning will be insignificant, no nitrogen will be trapped at the high points

in the pipe line, the steel in the pipe line will not be subHected to cold temperatures,

purification will be simple, short and ine!pensive and the ethylene inventory in the pipeline will be minimal in case a pipe line blow-down is required. %he savings on nitrogen

will also be significant.



%wo pressure drops will be taen on the ethylene, one on each side of the tube-shell heat

e!changer. %he pressure will be dropped to appro!imately E77 psig prior to the ethylene

entering the heat e!changer. %he E77 psig ethylene will be warmed to appro!imately>:782. %hen a second pressure drop will occur allowing the pressure and temperature of

the ethylene to equali5e with the pressure and temperature of the nitrogen in the pipe line

B=/-A7 psig J =/-9782C. 0nder these conditions, P(DGs E million #%0 per hour heatercan handle up to :A,777 lbs. of ethylene per hour. %his high flow rate will normally be

used only during the pressuring up phase of the process and not used during the purification process. %he warm ethylene at low pressure, traveling about >7 mph in the

small pipe lines that are typically used to transport ethylene, will produce e!cellentresults during the purification phase of the process.

%he minimum ethylene temperature encountered in this commissioning process is well

above 782, and the ma!imum temperature is >E782. %his is a very safe temperature rangeto handle ethylene in carbon steel piping systems.

Purity can be established at A7 psig or less, with very little loss of product, and the system

can be checed for leas with ethylene in the pipe line while the pressure is low the dollar

value of ethylene in the pipe line is still small.

'fter purity has been established at =/-A7 psig and potential lea points have been chec,the maHor remaining effort will be to warm enough ethylene to bring the pipe line up to

appro!imately E77 psig. #eyond this point, the ethylene can be throttled into the pipe line

without heating because the cooling due to pressure drop will no longer be great enoughto cause damage to the pipe line and equipment. During this pressure-up phase of the

commissioning operation, the ethylene does not have to be heated to =9782 temperature



and the pipe line pressure will be increasing both resulting in a reduction in the amount

of heat required per pound of ethylene. %herefore, for this phase of the commissioning

operation, P(DGs new ethylene heater will handle considerable more than :A,777 lbs. ofethylene per hour.

%his system provides e!cellent conditions for commissioning an ethylene pipe linequicly, ine!pensively and safely.

%his heater can also be utili5ed to commission other product pipe lines where coldtemperatures are encountered due to pressure drops such as with propylene, ethane,

propane and carbon dio!ide.B@C

&c(n$wledgement) %he ditor wishes to than +arvin D. Powers - Pipeline

Dehydrators for permission to use material from paper on Ethylene Heater ForCommissioning Pipelines.

(iterature 3ited> 0& Patent No. A,7@9, 6:@ Ful. @, ><<> Powers@ Powers, +arvin D. G Ethylene Heater for Commissioning PipelinesG A-6< PipelineDehydrators; *ilado, 3arlos F. and 3lar, &tanley 1. G utoignition !emperatures of "rganic

ChemicalsG 3hemical ngineering &eptember :, ><9@, p9E.: +cKay, 2.2., 1orrell, ).4., %hornton, #.3., and (ewis, *.(. G How !o a#oid

Decomposition In Ethylene PipelinesG, Pipe (ine ndustry 'ugust ><96, p 67.

acuum Drying

%he vacuum drying process is shown graphically in 2igure > and consists of threeseparate phases. 3orrosion is generally inhibited at relative humidity B4.*.C levels below

;7? but in the presence of hydroscoic dirt Bpresent in millscaleC corrosion can occur at

4.*. levels of @7?. %herefore, systems should be thoroughly drained and then vacuumdried to lower than @7? 4.*.

%arget Dewpoint BD.P.C

4.*. I 1ater vapor pressure / &aturated apor Pressure B&PC

for a constant temperature. 'ssume the lowest average temperature of the system is 7o3Bduring the winter monthsC which gives

&P I 7.E KPa'

%he vacuum level required for water vapor at @7? 4.*. is

7.@ ! &P I 7.@ ! 7.E I 7.>@ PKa'



%his corresponds to a dewpoint of ->683. ' safety allowance should be provided for

some desorption from the pipe walls and therefore a dewpoint of-@783 would be used.

Phase I + 'acuati$n

During this phase, the pressure in the pipe line is reduced to a level where the ambienttemperature of the pipe line will cause the free water to boil and change to water vapor.

%his pressure level corresponds to the saturated vapor pressure of the free water in the

pipe line which is dependent upon the ambient temperature of the pipe line.

%he appro!imate pressure value is calculated in advance but is easily recogni5ed on site by a fall in the rate of pressure reduction, which is noted from the plot of pressure against

time.

't some convenient point in time a $lea test$ is carried out by stopping the vacuum

equipment and observing the pressure, usually for a period of : hours. 'ny $air-in$ leas

on flanges, fittings, or hoses are rectified at this time, although leas are not a commonoccurrence.

Phase - + 'ap$rati$n

Once the saturated vapor pressure has been reached then evaporation of the free waterinto water vapor will commence. During this phase, the vacuum equipment is carefully

controlled to maintain the pressure at a constant level until all the free water has been

converted into water vapor. %his phase may tae several days to complete depending onB>C the amount of water to be evaporated B@C the si5e of the vacuum equipment and B;C

the ambient temperature of the pipe line. %he end of the evaporation phase will be

observed on site by a noticeable decrease in pressure.

't this time it is prudent to carry out a $soa test$ to ensure that all the free water has infact evaporated. %he vacuum equipment is temporarily isolated from the pipe line,

usually for a period of >@ hours, and a careful note made of the pressure. f all free water

has evaporated then the pressure will remain constant and the final drying phase can becommenced.



.igure 0

Dewp$int /emperature s Saturated ap$ur Pressure

What is dryness1

%he dryness of a pipe line is measured in terms of dewpoint, which is the temperature at

which mist or dew will begin to form. ' convenient method of measuring dewpoint is to

use an instrument called a mirror hygrometer where the water vapor is passed across a polished surface which is slowly cooled until dew forms. %he temperature at which the

dew forms is the dewpoint of the water vapor and is normally e!pressed in degreescentigrade. %he drier the air, the lower the temperature at which dew will form.

n terms of a pipe line being vacuum dried, the lower the pressure in the pipe line, the

lower the dewpoint will be. 2or e!ample, at a pressure level of 7.@E Pa ', the equivalent

dewpoint of the pipe line would be ->783. f the pressure were further reduced to 7.>7:Pa ', then the dewpoint would be -@783.

2or gas pipe lines a dewpoint level of -@783 is generally considered to be adequate and

the 7.>7 Pa ' bar pressure level required to achieve this dewpoint is readily attainableusing the portable vacuum equipment previously described.

2or e!ample, consider a >77-mile-long ;E-inch-diameter pipe line which, prior to drying,

contained >7,777 gallons of water as a film on the inside surface of the pipe. 't a

dewpoint of ->783 the quantity remaining would be reduced to only :E.A gallons and at-@783 to ><.9 gallons. 'lso, this water would not be free water but rather water vapor and

it would only revert bac to free water if the ambient temperature of the pipe line were



further reduced. %his water vapor can subsequently be removed from the pipe line during

the purging operation.

%he relations between pressure and dewpoint are shown in 2igure ; and %able >.

Pr$ing the dryness

mmediately following on form the final drying phase, a dry gas purge using atmospheric

air or nitrogen is carried out to prove the dryness of the pipe line. t is possible, under

certain circumstances, for a small amount of free water to still remain in the pipe line.

0sually this water will have turned to ice due to the chilling effect of the vacuum drying process and may not be apparent during the final drying phase or soa test.

/a2le "

Water ap$r Pressure /a2le

/

3C

SP 4(Pa &5

4m2 6 "7+"5

ap$r Density

4gm m05 -A7 7.77;< B7.7;< mbC 7.7;6

-:A 7.7>@6 7.>><

-;A 7.7@@; 7.@7;

-;7 7.7;76 7.;;<

-@A 7.7E;@ 7.AA@

-@7 7.>7:; 7.66:

->A 7.>EA@ >.;69

->7 7.@A<9 @.>;<-A 7.:7>A ;.@:E

7 7.E>76 :.6:9

A 7.69>< E.9<9

>7 >.@@97 <.;<<

>A >.97:7 >@.6;7

@7 @.;;97 >9.;77

@A ;.>E97 @;.7A7

;7 :.@:;7 ;7.;67



Nitrogen or atmospheric air is allowed to enter the pipe line through a valve at the end

remote from the vacuum equipment, until the pressure has risen to the &P equivalent of

the target dewpoint.

Once this pressure level has been reached, the vacuum equipment is started and that

pressure level maintained. %his has the effect of drawing gas through the pipe line undervacuum at a relatively constant dewpoint equal to the final dewpoint required.

't some point in time the purge gas, now under vacuum at a dewpoint of say, -@783, will

reach the vacuum equipment and be pulled through it. %he dewpoint at both ends of the pipe line is carefully monitored and compared. f there is not free water remaining in the

pipe line then the dewpoint at the vacuum equipment end will be the seam as the

dewpoint at the remote end. *owever, if there is any free water present then the dry air



passing through the pipe line under vacuum will absorb the water hygroscopically. %he

dry gas purge operation must then continue to remove the remaining free water until the

dewpoint at both ends are equal, at which time purging is discontinued. %he pipe line hasnow been vacuum dried to the required dewpoint level and the dryness proved.

Purging and c$mmissi$ning

Once the dryness has been attained and proved, the pipe line is ready for commissioning.

t is possible to introduce the gas directly into the vacuum or to relieve the vacuum usingdry nitrogen.

4eprinted with permission Nowsco 1ell &ervice

Pipe Line Dewatering(Ch6)

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Transcript of Pipe Line Dewatering(Ch6)