SmilerFDL
Chemical
- Jul 10, 2014
- 5
Hi all,
Yes, I know that the whole API always states that gauge set pressure should be used to check if built-up BP is within allowed limits. Still this question arises frequently and is answered differently. Most engineers I ask intuitively vote for CDTP as reference, me too. I searched the forum and couldn't find an exhaustive answer beyond "leave it to the vendor". Assume the following (extreme) example:
Vessel MAWP: 10 barg
PSV set pressure: 10 barg
Relief pressure: 11 barg
superimposed BP constant: 8 barg
CDTP: 2 bar
Built-up BP: 1 bar
From API point of view the valve will be derated acc. to backpressure ratio and everything is fine. I also have no doubt that a properly selected valve will relieve sufficient massflow at differential pressure from 11 to 9 barg and accumulation is not exceeded. I'm concerned that the valve is operating stable from the point of opening, until the vessel pressure reaches relief pressure. The inlet pressure just managed it to open the valve against a meager spring force of 2 bar and instantly an additional force of up to 1 bar is added to close, but the valve remains open?
The example is in the subcritical flow regime and the lift/flow/backpressure relation is interdependent, which may soften the increase in built-up BP. Please post your comments and which type of valve you would select.
Thanks, Smiler
Yes, I know that the whole API always states that gauge set pressure should be used to check if built-up BP is within allowed limits. Still this question arises frequently and is answered differently. Most engineers I ask intuitively vote for CDTP as reference, me too. I searched the forum and couldn't find an exhaustive answer beyond "leave it to the vendor". Assume the following (extreme) example:
Vessel MAWP: 10 barg
PSV set pressure: 10 barg
Relief pressure: 11 barg
superimposed BP constant: 8 barg
CDTP: 2 bar
Built-up BP: 1 bar
From API point of view the valve will be derated acc. to backpressure ratio and everything is fine. I also have no doubt that a properly selected valve will relieve sufficient massflow at differential pressure from 11 to 9 barg and accumulation is not exceeded. I'm concerned that the valve is operating stable from the point of opening, until the vessel pressure reaches relief pressure. The inlet pressure just managed it to open the valve against a meager spring force of 2 bar and instantly an additional force of up to 1 bar is added to close, but the valve remains open?
The example is in the subcritical flow regime and the lift/flow/backpressure relation is interdependent, which may soften the increase in built-up BP. Please post your comments and which type of valve you would select.
Thanks, Smiler
