Discharge PSV piping 2

[jcme7212]

Dear coleagues.

In accordance to API Recomended Practice 521, the criteria of dimensioning the discharge piping of a gas safety valve is do not exceed the permissible backpressure, however this code doesn't state clearly the restriction about the Mach Number, Is it possible to have a choked flow at the discharge piping without affect the performance of the safety valve?. Is it feasible to have supersonic flow i.e, Mach > 1.?
I know a common criteria for main header should no exceed 0.8 Mach, but what about the lateral segments of discharge valves piping.
If someone can help me i would appreciate a lot.
thanks in advance.

 

[Montemayor]

jcme7212:

The possibility of achieving "choked flow" is a very real and common one when dealing with gas exhaust or venting. It is your responsibility to check your flow characteristics to make sure that the mandated maximum backpressure is not exceeded. API Recomended Practice 521 cares about you ensuring that and expects that you will do a proper design to achieve it. Checking for choked flow is part of any gas flow system design.

Don't worry about supersonic flow and even about choked flow in designing your relief valve discharge headers. Follow the mandated backpressure requirements and be sure that you have not achieved choked flow at those conditions and you won't have any problems. I presume you're doing your calculations using your worst case scenario flow conditions. The basic design to follow is simple: do not exceed the maximum backpressure and built-up backpressure limits and stay out of choked flow. You'll find both comply with each other nicely. API does not go into your conventional design of the discharge vent headers with respect to sizing because this is basic fluid flow design that you should know backwards & forwards.

Hope this addresses your questions and concerns.

Art Montemayor
Spring, TX

 

[EGT01]

jcme7212,

To add to Montemayor's post, you might be interested in reading the following thread
thread798-43158

Also as a new user, you may also want to see FAQ731-376 for tips on how to make the best use of Eng-Tips.