PSV for derating piping

[ixchawla]

Codes like API-520,521 says about PSV location and inlet line loss for vessels. What if the PSV is provided for derating the piping, when no vessel is there. How the following is taken care in design:
1. If PSV is located far-off from the derating point, is it possible to give the PSV set pressure as pressure at derating point plus/minus line loss upto the PSV location ( corresponding to relieving conditions).
2. Does the 3% rule apply for such cases?

Thanks in advance.

 

[NiravK]

While designing safety systems we need to think about the system behaviour along with as codes & stds. No matter whether it is provided on vessel or pipe. The reason is you design PSV with a specifid 'Set Pressure'.

There should not be too much of difference between the pressure at the point where you want protection, and the pressure just at the inlet of the PSV. If the difference is large, either PSV will not open at actual over pressure or it may 'chatter'.

You may set the PSV at the pressure equal to 'derating point' minus 'line loss'. This is the reason why API calls for max 3% of line losses. I would definitely consider the same for piping system.

regards,
nirav

 

[pack12]

Use the 3% rule for gas pipeline. These require "pop action" safety valves. For a liquid pipeline using linear spring safety valves, you simply have to perform a system analysis to insure the pipeline is not over pressured.