PSV Inlet Piping 3% rule

[Glenfiddich]

From API 520 Part 2 Section 2, it talks about the 3% limitation on inlep PSV piping. What if you have a situation where you have liquid going to the PSV and the liquid head in the PSV inlet line exceeds the 3% of the set pressure? Would this be considered a violation of the 3% rule? The frictional pressure drop could be less than 3% but the liquid head could be more.

 

[pleckner]

You are only required to consider nonrecoverable losses and fluid static head isn't one of them. Bottom line, liquid head is NOT considered in the analysis.

 

[Glenfiddich]

shouldn't the liquid head be taken into consideration for a blocked liquid outlet case?

 

[pleckner]

@Glenfiddich,

Your question actually petains to the location and set pressure of a PSV rather than application of the 3% Rule, which is considered during relief.

The answer to your question is that the liquid head must be taken into account when determining the location/set pressure of the relief valve. If you suspect that a liquid block-in scenario is credible, then the liquid head must be considered when determining the PSV set pressure. If the PSV set pressure is at MAWP then the pressure in the vessel will be MAWP plus the liquid head, which is clearly not allowed. The solution is to locate the PSV on the bottom piping with an appropriate set pressure or decrease the set pressure of the PSV if installed at the vessel top.