Avoid Condensation-Induced Transient Pressure Waves

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GAS PROCESSING DEVELOPMENTS SPECIALREPORT

HYDROCARBON PROCESSING JANUARY 2011 I

41

Avoid condensation-induced

transient pressure wavesCase studies give an indication as to probable causes for water hammer

G. MANI, BP Canada Energy Co., Calgary, Alberta

Condensation-induced water hammer is a commonly occur-ring phenomenon in steam and condensate systems. Insteam/condensate systems, condensation-induced water

hammer occurs when steam bubbles come in contact with, or areencapsulated in, subcooled condensate and then loses heat andcondenses rapidly. This creates a low-pressure zone into whichcondensate moves rapidly. The resultant collision creates a pres-sure wave that reverberates through the body of condensate. Thepressure developed from the collision can be derived from themomentum balance and can be written as follows:

P = ρV 2

where:P = Pressureρ = Liquid density V = Condensate velocity The velocity can be as high as sonic velocity, i.e., the velocity of

sound in the condensate. For steam/condensate systems, this tran-sient pressure can be high enough to create catastrophic damageto piping. The velocity and consequently the pressure developedis a function of the following:

• Steam pressure• Condensate temperature• Bubble size• Quantity of noncondensable gasesDirectionally, higher steam pressure,

lower condensate temperature and higherbubble size favor development of higher-pressure transients. The noncondensablegases reduce the transient pressure due to

their presence in the low pressure zone.For hydrocarbon systems, condensation

induced transient pressure waves similarto water hammer in steam systems do notoccur often. This is because the fluids aregenerally multicomponent hydrocarbonmixtures and the sudden collapse of bubblesdoes not occur. In addition, flow regimes where vapor bubbles can be enclosed in sub-cooled liquid are generally avoided in thedesign. However, in one plant, there weretwo cases of condensation-induced transientpressure wave problems in relatively pure

component service (~97% propane) that will be described in greater detail.

Case 1: Propane rundown system. The propane fraction-ation and rundown system is illustrated in Fig. 1. There are fourdepropanizers in the plant. The feed to the depropanizers is natu-

ral gas liquid (NGL) that contains propane, iso-butane, n-butaneand condensate or light naphtha. High-purity propane (~97%) isthe overhead product of these depropanizers. Three depropanizershave air-fin coolers and one has a water cooler in the overhead con-denser service. The overhead pressure in the columns is controlledby withdrawing the propane product from the overhead. In other words, it is a flooded condenser pressure control where the levelin the condenser is varied indirectly to control the pressure. Inaddition, the columns are equipped with variable pressure-controllogic to take advantage of varying ambient temperature duringthe day. The variable pressure control increases the pressure inthe morning and reduces it in the evening with varying ambienttemperature. The control logic maintains a minimum subcool-

ing to avoid vapor formation in the reflux drum and for properfunctioning of the pressure control system. However, when theambient temperature increases fast, the control system might fail

PIC

PI

PIC

PIC

150 to 250 psig

Air cooler

Depropanizer

Refluxdrum

Reflux pump

Boosterpump

PCV To flare

Filter

coalescers

Propane drier

Propanetreaters

Storagebullets8 nos.

Changing leveland temperature

leading to changingpressures

To flare

Set pressure320 psig


135 to 200 psig

To flare



150 to 250 psig

Air cooler

Depropanizer

Refluxdrum

Reflux pump

Boosterpump

PCV

150 to 250 psig

Air cooler

Depropanizer

Refluxdrum

Reflux pump

Boosterpump

PCV

Propane rundown system.FIG. 1



GAS PROCESSING DEVELOPMENTSSPECIALREPORT

42 I JANUARY 2011 HYDROCARBON PROCESSING

to maintain the subcooling required and operator action wouldbe necessary to reduce the feed to the columns.

During a capacity-expansion project, booster pumps wereadded to increase the propane production rate. Downstream ofthe pumps, there are filter coalescers to remove water carried overfrom the reflux drum. These are followed by driers and treatersthat remove water and H2S from propane, respectively. The driers

and treaters are molecular sieve beds that are taken through vari-ous cycles of operation such as absorption, draining, depressuring,regeneration, cooling and liquid filling. The switch between thevarious cycles is automated and air-operated isolation valves areused for this purpose.

The propane rundown is finally routed to storage bullets. Thebullets are intermediate storage tanks where propane is routedto underground caverns for long-term storage, then to loadingtrucks and rail wagons. The bullets also act as intermediate stor-age for propane that comes from the underground caverns forloading trucks and rail wagons.

Problem. Relief valves are located on the filter coalescers and on

the line, set at 320 psig. During summer, while the storage bulletsand the columns are operated at higher pressures, these relief valves were chattering, causing damage to the valve seats. The relief valveslocated on treaters and driers that have a similar set pressure didnot have any incidents of chattering or valve damage. The maxi-mum pressure recorded by the pressure indicator in the controlroom was only around 250 psig. Tell-tale gauges (local gaugesthat record maximum pressure) installed close the relief valvesalso recorded only a maximum of 250 psig. Even after repeatedrepairs and calibration of the relief valves, the problem persisted.

Actions taken. Drier/treater controls and relief valves.• Drier/treater controls. Switching the driers and treaters

between different cycles of operation is done using automatedvalves. It was suspected that the premature closing of the valves and/

or errors in logic might be causing flow restrictions. The switchinglogic was reviewed and revised to avoid any potential problems. Also, the closing of the automatic valves were checked by operatorsto ensure that they were not getting stuck or prematurely closing.

• Relief valves. Relief valves were replaced by pilot-operatedvalves with the following objectives:

• Relieving close to the set pressure

• The effect of inlet line pressure drop was eliminated by tak-ing pressure signals close to vessels and the main line.

• Slower closure of the valves in the event of relieving.The steps taken described previously did not make much

improvement in the situation. Hence, it was decided to conducta theoretical investigation of other causes of overpressure.

Theoretical investigation. The storage bullets act as inter-mediate storage where levels and temperatures can fluctuate.Therefore, a pressure transmitter was installed on the bulletsto monitor the pressure continuously. The rundown system was modeled using simulation software. With monitoring thebullet pressure, it became evident that the propane in the bul-

let was not always at equilibrium, and, due to level changes,considerable transient pressure changes were occurring. Someof the lowest pressures recorded in the bullets were used in thesimulation for checking two-phase conditions. From the model,the following conclusions were reached:

• While the pressure in the bullets was lower than the vaporpressure, considerable vaporization was occurring in the rundownsystem (~15%)

• Some parts of the rundown lines were in the slug flow regime.• Due to a reduction in elevation, the vapor was partly col-

lapsing at a few locations.From these conclusions, it was surmised that the collapsing

of the vapor due to elevation changes might be causing transient

shock waves. A phenomenon similar to condensation-induced water hammer in steam systems was suspected to be occurring due

to different causes.Unfortunately, most of the dynamic sim-

ulation programs are not capable of simulat-ing this type of transient pressure waves.Hence, only a qualitative analysis from thesteady-state simulation results was made.

Solution. A backpressure controller wasinstalled close to the storage bullets to iso-late the rundown system from the pressurevariations in the bullet (Fig.2). Backpressure

higher than the expected vapor pressure wasmaintained and this eliminated the reliefvalve chattering problem.

Case 2: Propane overhead system.The system in this case is the overheads ofthe depropanizers, previously described. Theoverhead system is shown in Fig 3. There arerelief valves on the column overhead and thereflux drums with set pressure of 280 psig.

Problem description. Of the four reliefvalves on reflux drums, one used to chatter

and cause frequent damage and another onereceived occasional damage. Both of these

PIC

PI

PIC

PIC

PIC

150 to 250 psig

Air cooler

Depropanizer

Refluxdrum

Reflux pump

Boosterpump

PCV To flare

Filtercoalescers

Propane drier

Propanetreaters

Storagebullets8 nos.

Modificationimplemented

Changing leveland temperature

leading to changingpressures

To flare



135 to 200 psig

To flare


150 to 250 psig

Air cooler

Depropanizer

Refluxdrum

Reflux pump

Boosterpump

PCV

150 to 250 psig

Air cooler

Depropanizer

Refluxdrum

Reflux pump

Boosterpump

PCV


Modified propane rundown system.FIG. 2



GAS PROCESSING DEVELOPMENTS SPECIALREPORT

HYDROCARBON PROCESSING JANUARY 2011 I

43

depropanizers have air coolers in the overhead condenser service.One of the propanizers with air coolers and one with a water coolerdid not have this problem.

Analysis. The operating conditions of all the depropanizers were very similar and we could not identify any deviation ofconsequence regarding operating conditions. From analyzing his-

torical data the following two interesting observations were made:• Originally, the columns had a hot vapor bypass control for

overhead pressure and later the control system was changed to aflooded control system. The relief-valve problems started subse-quent to the control system change.

• Most of the relieving incidents were reported to occur in thenight. Due to variable pressure controls, the columns are operatedat lower pressures during the night (180 to 200 psig) against amaximum of 250 psig during the day.

The fact that the relieving incidents occurred during the night, when operating pressures are lower, indicated that the problem was not linked to any rapid fluctuations in operating pressures.Hence, it was decided to look at any difference in installation

between these columns. The piping drawings were reviewed andfield measurements were made to determine the relative eleva-tion of the relief valves with respect to the bottom flange of theair-fin coolers. The relief valves that used to chatter had a relativeelevation of +20 in. and +46 in. above the outlet flange of the aircoolers. The relief valve on the system with the water cooler hada relative elevation of +87 in. The system that did not have anychattering problem had a relative elevation of only +1 in. Fig. 4 isa depiction of relative height.

Although the control systems were very similar, the pressure inthe column with the water cooler never had drastic changes in theoverhead pressure due to the reasonably steady water temperature. When the water temperature varied, the variation in pressure was

slow and the control system did not make drastic changes. The varia-tions in air temperatures were more drastic and the pressures reachedthe maximum-allowed pressure of 250 psig in a span of a few hours. Also, in the night, the pressure was reduced up to 180 psig.

During the day, when the ambient temperature is high, thecontrol system will tend to push the liquid level in the condenserdown. In the cases where the relief valves are placed above thebottom of the condenser, this will lead to the draining of liquidand developing vapor space in the inlet pipe. In the night, whilethe ambient temperature reduces the vapor-inlet line might becollapsing and the higher liquid level in the condensers will forcethe liquid to rush into the transient low-pressure zone, causingchattering of the relief valves.

Solution. The preferred solution was to reduce the elevation ofthe relief valves. However, this could not be done since the flareheader was located at a marginally lower elevation with respect tothe relief valves. Reducing the relief-valve elevation would haveled to pockets in the relief valve outlet line. It was verified thatthe relief valve on the column had enough capacity to cater toboth the column and the reflux-drum relieving cases. Hence, it was decided to remove the relief valve on the reflux drum and toisolate it close to the vessel. This way, the possibility of formingvapor space that may lead to pressure transients was eliminated. All the block valves between the reflux drum and the relief valves were locked open to provide a clear relieving path. This modifica-

tion was implemented only in the depropanizer that use to havefrequent failure. HP

LITERATURE CITED

1 Kirsner, W., “Steam condensation-induced water hammer,” HPAC , January1998.

PIC

PIC

PIC

PIC





Set pr280 psig

Set pr280 psig

Air cooler

DepropanizerReflux drum

Reflux drum

Reflux drum

Reflux pump

Boosterpump

PCV

Rundown

Air cooler

DepropanizerReflux pump

Boosterpump

PCV

Air cooler

DepropanizerReflux pump

Boosterpump

PCV

Set pr280 psig

Water cooler

Depropanizer

Reflux drum

Reflux pump

Boosterpump

PCV

Depropanizer overhead system.FIG. 3

George Mani is a process engineering specialist with BP

Canada Energy Co. Currently, his responsibility is giving process

engineering support to multiple natural gas processing and NGL

facilities in operating and implementing projects. Mr. Mani has 30years of wide-ranging experience in the oil and gas idustry.

Relativeheight

Reflux drum

Aircooler

Relative location of relief valve.FIG. 4

Avoid Condensation-Induced Transient Pressure Waves

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