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Depressuring Flow - Quick Manual MethodSunday, January 25, 2009

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Depressuring system is provided in Oil and gas, Gas & LNG plant,etc to evacuate the inventory from process system as fast aspossible so that the reduced internal pressure stresses is keptbelow the rupture stress. This has been discussed in"Depressuring within 15 minutes no longer applicable ?".Nevertheless, quick depressuring may lead to other problem suchas low temperature embrittlement, excessive noise andvibration, etc. Depressure a high pressure would lead to lowtemperature of depressured system and failure due to lowtemperature embrittlement. Higher the depressuring rate, lower thetemperature can be experienced by depressured system. Thus, therestriction orifice (RO) downstream of Blowdown Valve (BDV) in

depressuring system primarily is to limit flow so that the temperature will not drop belowthe allowable lowest temperature limit of material. This has been discussed in "Don'tmisunderstood depressuring".

Although depressuring shall be implemented within the shortest time possible, excessivedepressuring may potentially lead to damage to equipment such as compressor seal, solidbed, etc. Thus, there are two type of depressuring as discussed in "Controlled andNon-controlled Type Depressuring". Nevertheless, it is emphasized again here,depressuring system shall be designed to bring plant to safe level without any tolerance.

Many depressuring systems are designed to depressure the system within 15 minutesfollow recommendation in API 521. Nevertheless, one shall take note that the 15 minutesis sort of arbitrary and may be good for some system and configuration. Thus, in mostrecent API STD 521, the requirement has slightly changed where a depressuring systemshall be designed such that the stress induced by internal pressure of a system is lowerthan stress allowable by the system. This may lead to shorter depressuring time asdiscussed in "Depressuring within 15 minutes no longer applicable ?".

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Depressuring can be conducted using simple depressuringmodule in process simulator such HYSYS, PRO-II, etc. One shallunderstood there are limitation and accuracy issue in abovementioned depressuring modules and shall be used with care.There are other depressuring simulator such as LNGDYN byTECHNIP, BLOWDOWN by Imperial College, etc which arecalculated rigorously and tested with real plant data. It is alwaysadvisable to use these simulator for specific case.

AssumptionIn this post, a manual depressuring method is introduced. Thismethod is first introduced by Grote and are derived base onfollowing assumptions :

i) Critical flow throughout entire depressuring processii) Constant mass flow throughout entire depressuring processiii) System being depressured is maintained as gaseous throughout entire depressuringprocessiv) Constant temperature, molecular weight and compressibility

MethodologyFollowing is the derivation of the manual equation.

How to apply valve equation in HYSYSDepressuring ?High pressure and moderate/low temperature operationare common in oil and gas industry. High pressuresystem with large vessels will lead to large inventoryand it significantly increases hazardous level topersonnel and assets...

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August 2012Concluding RemarkEquation [5] may be used for manual depressuring if a system inventory (initial mass, M0),depressuring time (t), initial (P0) and final pressure (P) are are known. One shall check theassumptions are valid before it is used. This equation may be used as quick method to

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determine the depressuring flowrate for quick estimate, however it is not recommendedduring detailed design.

Related PostAdditional Concerns in Controlled DepressuringDepressuring - Save Some Time in HYSYS - FLARENET IterationFew Recommendation on Manual Blowdown LineA refresh to Process Engineer on few phenomenons in restriction orificeBug in ASPENTECH HYSYS 2006 Dynamic Depressuring Fisher Valve modelHow to apply valve equation in HYSYS Depressuring ?Why Restriction Orifice is some distance from Blowdown valve ?

Labels: Depressurization, HYSYS

posted by Webworm, 4:41 PM

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