ASME Inlet PSV Pressure Drop-3% Minimum Question 1

[RJB32482]

Here's a question for the ASME Boiler and Pressure Vessel Code that states the inlet pressure drop in piping to a PSV is not to be greater than 3%. Here's the situation: we have a relief that would open when our jacket is overpressurized. Five pipes go into a common header then vertically goes up into the inlet of the PSV. The jacket is a double-pipe system. Can the pressure drop calculation occur at the top of the header to the PSV or should it occur at the bottom of the header to the PSV. The relief valve is set at 165 psi and the MAWP for the jacket is 180 psi. Does that account for any issues with the calculation?

Thanks

 

[codeeng]

By definition the MAWP is always taken as the top and you should allow for any other loads below such as liquid head, delta P, etc.

 

[RJB32482]

So I would not have to calculate pressure drop from the bottom of the reactor jacket?

Thanks

 

[codeeng]

Correct, but you should verify any unusually high pressure drop due to pressure relief does not over stress the bottom. Basically, you can't over stress any part more than 3% during pressure relief.

 

[InspEngr]

As I recall, the pressure drop is to be calculated at the PRV inlet. In other words, it does not matter how long the inlet piping is or what the geometry looks like, you have to calculate what the pressure drop will be when the PRV opens. This means you must add up the cumulative pressure drops from the vessel all the way to the PRV inlet and that pressure drop cannot exceed 3%. One of the reasons for this is to prevent PRV "chatter".

It does not have to do with overpressure of the equipment being protected. ASME allows for 10% built-up pressure for single relief valves and 16% for multiple relief valves. The exception is for the fire-relieving scenario in which case a built-up overpressure of 21% is allowed.

For instance, if a PRV is set for 100psi, the inlet pressure due to pressure drop cannot exceed 3psi. So, when the PRV pops at 100, there will be an immediate pressure drop (due to the static fluid becoming dynamic - so to speak), but that pressure drop cannot result in less than 97psi at the PRV inlet.

In the same scenario, if the overpressure event was, say, due to a blocked outlet, the vessel pressure is allowed to build up to as high as 110psi for a single RV or 116psi if multiple RV's are present.

Regard,

-InspEngr

 

[robsalv]

INSPENGR is on the right tram.

More simply put, if you have a vessel at 100psi and protect it with a PRV set to 100psi, then the pressure drop in the inlet piping between the vessel and the PRV cannot be more than 3% or in this example 3psi.

The 3% limitation is to reduce the risk of chatter - a greater pressure drop could make the SV "think" the process pressure had fallen below the blowdown pressure and therefore the SV would try to close, only to have the process pressure instantly rocket back up to set pressure and open the PRV again. This cycle repeats and is known as chatter.

If I read into your words RJB, I think you were wondering whether you could set the SV to the pressure dropped figure, in my example above that would be 97psi. There'd be no point doing that, because by default a PRV inlet line is a dead leg, so when the vessel got to 97psi, the PRV would lift. The inlet line would then experience a pressure drop applicable to the flow rate at 97psi and then we're back to square one again!

Hope this helps.

Rob

 

[robsalv]

...er, that shouldn't have been "more simply put", but rather, "reiterating the point". Sorry Inspengr!