PSV Backpressure Static or Total ?

[Flarenuf]

Hi all
I am trying to settle a slight difference of opinion here. The backpressure on a flowing PSV is made up of the superimposed BP ie:that caused by other valves flowing into the header system, plus the builtup backpressure caused by the valve itself flowing.
If you use Flarenet to model the relief header you can see the STATIC backpressure on the PSV at say 1.2 barg and you can also choose to view the TOTAL pressure which includes the velocity head of the gas and see T say 1.8 barg.

Now the $1M question, which do you put on the PSV datasheet for the vendor the STATIC value or the TOTAL value.

I maintain its the STATIC pressure as it acts in all directions whereas the velocity head contribution is in the direction of flow and hence does not affect the PSV capacity.

Thoughts please?

 

[Latexman]

Static

Good luck,
Latexman

 

[JAlton]

Static. However, built up backpressure is not simply the result of the PRV flowing. Built up backpressure is a result of discharge piping that is undersized (i.e. too small), too long or too many bends or some combination of these issues. Built up backpressure may restrict capacity regardless of any steps taken during PRV Selection or Testing.

JAC

 

[4Pipes]

I hope you have a decent pair of fire proof pants. Questioning Flarenet back pressures like this is verging on heresy. I would not admit to agreeing with you.

 

[Latexman]

He's not questioning the results, just what format to use on a spec, static pressure or total pressure.

Good luck,
Latexman

 

[Flarenuf]

as one nice man has found
ASME VIII Div 1 1986 (section M-8C) states
"The static pressure developed at the discharge flange of a conventional spring loaded safety valve shall not exceed 10% of set pressure"

 

[skearse]

I would include both. The superimposed pressure would be used to adjust your CDTP, if needed; while the builtup backpressure would be used to check against the 10% rule (for the conventional valve).

 

[4Pipes]

Flarenuf,
I can state from first hand experience in more than one organisation, that some Process designers will not always accept that ASME definition - and take it very personnally. Some will only use total because it is also on the printout. Hence my first comment.

 

[prq123]

The Crosby relief valve Engineering Handbook defines the backpressure as "the STATIC pressure existing at the outlet of a pressure relief device due to pressure in the discharge system".