4 replies to this topic

[ReddeerC]

Dear Colleagues,

 

A PSV is installed on a process line with a higher pressure rating. The PSV is to protect downstream lower pressure system. When calculating PSV inlet pressure loss for 3% rule, can we ignore the minor loss on the “T” between the PSV inlet pipe and the main process line?

One of my colleague suggests not to consider the pressure loss on the “T” since the PSV is installed on the main process line with a design pressure of 5000 kPag and the main process line doesn’t need PSV protection.

The reason why my colleague made this suggestion is because: If the pressure loss on the “T” is not considered, the PSV inlet pipe non‐recoverable pressure loss can then meet 3% rule.

 

 

Please refer to the attached file.

 

 

Thanks for your inputs and time in advance.

 

 

Best Regards,

Attached Files

•  PSV Inlet Pipe Non-Recoverable Pressure Loss.pdf   82.3KB   43 downloads

[Bobby Strain]

You should follow the API recommended practice, not your colleague's idea.

 

Bobby

[fallah]

ReddeerC,

 

In general, not only you can't ignore considering the pressure loss on the "T" in 3% pressure loss evaluation in PSV inlet line but also you should provide a round entry branch connection to minimize the entrance loss at the "T" as one of the non-recoverable pressure losses. Indeed, to evaluate 3% rule you should add the incremental pressure loss in the process line, i.e. pressure loss at relieving flow conditions minus pressure loss at normal flow conditions, to the pressure loss in normally non-flowing PSV inlet line. Then, if the distance of PSV inlet line branch connection from the point of design pressure change in main process line wouldn't be as minimum as possible, it might the low pressure part of the main process line would be subject to the pressure higher than the relevant design pressure.

[ReddeerC]

Thanks, Fallah and Bobby, for your time and kind inputs!

[colt16]

Why do you break the design pressure after the relief valve though? The relief valve is then protecting the pipe with higher design pressure and the pipe will never see this design pressure because the relief valve will always open.

 

Since the relief valve is set at the lower pressure system design pressure, the break point of design pressure can be downstream of the isolation valve of the control valve but before the relief valve. That would make more sense. 

Edited by tanykiat, 16 December 2014 - 09:58 PM.