Med/High Visc - PD pumps PSV's Inlet outlet DP's too high per typ. API forms. - always likely? 1

[jamesbanda]

I'm revaliding a PHA for a med/high (100-500cp) plant.

We are going through the cases and taking credit for the positive displacement pumps spill back valves for preventing down stream vessel damage in a lock in case.

The problem I encounter is that almost all of them on the high viscosity cases exceed (by a margin) the inlet and outlet DP cases .

Inlet Dp's range from 12%upto 78% , with outlet DP's a similar range of 21 to 78%. Unfortunately the plant is very old (ish) 1980's vintage, I don't believe the fluid conditions have changed so is there a different standard to spill back pumps I'm not aware off.

"back in the day" did vendors/ engineers not calculate this stuff... I doubt it ".. I know different codes apply. but I'd like to get some experienced engineers background.

 

[met11]

What is the required orifice area vs. the actual area? The PD pumps I'm used to have small flows where even the smallest orifice in an ASME valve is grossly oversized. Also what are the inlet/outlet pipe sizes relative to the PSV connections?

 

[don1980]

Today's engineering practices are indeed different from those in the 80's. Although API 520 Pt II first published inlet pressure loss guidance in the 60's, it wasn't until 1986 that the 3% rule was adopted by ASME Sec VIII (in APP M). The first liquid trim PSV wasn't manufactured until the mid 80's, the date that ASME Sec VIII started certification for liquid PRVs. This change required liquid PRVs to reach full capacity at 10% overpressure, thus the need for a different trim design. Prior to that (prior to liquid trim PRVs) these valves would commonly need ~25% overpressure in order to be fully open.

You describe this as a "spill back valve" so I don't know exactly what type valve you have. It could be (1) a modern liquid trim PRV, (2) an old style PRV, prior to advent of liquid trim, or (3) a fully modulating liquid service valve like a Fulflo. If the answer is #3 then I don't think you need to be concerned.

When you have a high viscosity fluid, it's often impossible to get the inlet losses to 3% or less. Sensitivity to inlet pressure loss started in the mid 90's with the implementation of OSHA PSM. The irony is that today we recognize that the 3% rule is inadequate for predicting chatter.