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I asked a young engineer. "What is the main purpose of system depressuring ?". The answer given to me was a surprise but seem "logical" to some extent.
Young engineer responded "A conventional depressuring system consist of blowdown valve (BDV) and restriction orifice (RO). The purpose of depressuring system is to limit the flow rate to flare system in order to avoid large capacity flare system. It is similar to the RO provided on the level control valve (LCV), to avoid large gas blowby rate in case LCV failed open."What do you think about above statement ?
The young engineer made analogy of BDV/RO with LCV/RO. It is true that the RO upstream (or downstream) of LCV is to limit the flow during gas blowby and to minimise relieving load of downstream pressure relief device (PRD). However, this principle may not be the primary purpose for depressuring system.
My opinion...
The main purpose of depressuring is 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 This has been discussed in "Depressuring within 15 minutes no longer applicable ?".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 RO downstream of BDV in depressuring system primarily is to limit flow so that the temperature will not drop below the allowable lowest temperature limit of material.
For some system (generally low pressure), quick depressuring may not be required as the internal pressure deduced stress may still well below the maximum allowable stress of depressured system. Under this scenario, the RO is to limit the flow in order to minimise disposal and flaring capacity (this pretty inline to what the young engineer explained to me).
Related Topic
- Depressuring within 15 minutes no longer applicable ?
- Bug in ASPENTECH HYSYS 2006 Dynamic Depressuring Fisher Valve model
- Another descrepancy found in API Std 521Jan 2007
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- How to apply valve equation in HYSYS Depressuring ?
- ERRATA - API Std 521, Pressure Relieving and Depressuring Systems
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