How to deterimne the required capacity for safety relief valve sizing 3

[liao8093]

Recently we want add a new pressure reduceing valve in the existing steam piping, the upstream piping size at pressure reduceing valve inlet is 4" and downstream pipe size at pressure reduceing valve outlet is 6" respectively, The piping is designed as per code ASME/ANSI B31.1. For safety consideration, we need install a new safety relief valve in the downstream piping of pressure reduceing valve outlet. Anybody knows how to determine the required overpressure relieved steam capacity (flow rate) for the downstream piping overpressure protection and then we can provide this capacity to potential vendor to purchase this new safety relief valve, the related piping process data as follows:
1.Fluid: superheatd steam.
1.Maximum Process flow for the piping and pressure reducing valve design: 24903 kg/hr.
2.Maximum Pesssure / Temperature at upstream piping of pressure reduceing valve inlet : 21 Kg/cm2.g / 260 C
4. Pesssure at downstream piping of pressure reduceing valve outlet: 7.6 Kg/cm2.g


Thanks

 

[Montemayor]

liao8093:

Go to thread798-113006 and note carefully the importance of identifying the worse case relief scenario for your application. You probably won't like the answer: you have to do extensive and accurate process analysis on the application in order to fully identify ALL the applicable and credible pressure relief scenarios, and subsequently identify the worse (highest capacity) one.

These aren't my rules or much less, my preferences. This is the only acceptable and correct method for an engineer to correctly assert to the operators and the public that the application is a safe one to both humans and equipment. If you haven't any experience in participating in and running Hazops, PSSRs, MOCs, and as-building P&IDs you have very little business in trying to size PSVs -- that's my honest opinion and I'll defend it against any other engineer who believes otherwise. PSVs are applied for the specific purpose of protecting human life and welfare as well as the equipment they are installed on. I place a very high priority on the first item - human welfare - and I have never seen or heard of another method that can better serve to ensure that the correct and safe PSV is fully identified and justified.

That's why I continue to maintain and expound that "sizing" the correct capacity that a PSV should have is a job for an experienced and capable engineer who knows the process in question as an expert, not as a good learner or as an eager beginner. The consequences of not doing a 100% accurate job are far too serious to assign this task to an un-experienced individual. Plant management (& their legal counsel) fully know the downside of having a wrong PSV installed and having it fail in operation with resulting human casualities and damages. That's why I recommend you read the above-mentioned thread and heed the advice given.

Art Montemayor
Spring, TX

 

[athomas236]

I would design the safety valve capacity based on following conditions:

1. Maximum upstream pressure

2. Maximum downstream pressure, that is set presure at which safety valve with lowest set pressure is set to lift

3. Pressure reducing valve fully open, that is maximum valve Cv

4. The flow calculated from the above should then be increased by 10%

athomas236