Chemical & Process Technology

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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 as possible so that the reduced internal pressure stresses is kept below the rupture stress. This has been discussed in "Depressuring within 15 minutes no longer applicable ?". Nevertheless, quick depressuring may lead to other problem such as low temperature embrittlement, excessive noise and vibration, etc. Depressure a high pressure would lead to low temperature of depressured system and failure due to low temperature embrittlement. Higher the depressuring rate, lower the temperature can be experienced by depressured system. Thus, the restriction orifice (RO) downstream of Blowdown Valve (BDV) in depressuring system primarily is to limit flow so that the temperature will not drop below the allowable lowest temperature limit of material. This has been discussed in "Don't misunderstood depressuring".

Although depressuring shall be implemented within the shortest time possible, excessive depressuring may potentially lead to damage to equipment such as compressor seal, solid bed, etc. Thus, there are two type of depressuring as discussed in "Controlled and Non-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 minutes follow recommendation in API 521. Nevertheless, one shall take note that the 15 minutes is sort of arbitrary and may be good for some system and configuration. Thus, in most recent API STD 521, the requirement has slightly changed where a depressuring system shall be designed such that the stress induced by internal pressure of a system is lower than stress allowable by the system. This may lead to shorter depressuring time as discussed in "Depressuring within 15 minutes no longer applicable ?".


Depressuring can be conducted using simple depressuring module in process simulator such HYSYS, PRO-II, etc. One shall understood there are limitation and accuracy issue in above mentioned depressuring modules and shall be used with care. There are other depressuring simulator such as LNGDYN by TECHNIP, BLOWDOWN by Imperial College, etc which are calculated rigorously and tested with real plant data. It is always advisable to use these simulator for specific case.

Assumption
In this post, a manual depressuring method is introduced. This method is first introduced by Grote and are derived base on following assumptions :

i) Critical flow throughout entire depressuring process
ii) Constant mass flow throughout entire depressuring process
iii) System being depressured is maintained as gaseous throughout entire depressuring process
iv) Constant temperature, molecular weight and compressibility

Methodology
Following is the derivation of the manual equation.