3 replies to this topic

[vlcmstne04]

I have a question about the way API recommends we calculate the required relieving flow for Gas Expansion PSVs.

 

Per API 521  - 4.4.13.2.4.3 we should calculate discharge area with the F prime that accounts for the wall temp, Relieving temp, and  specific heat ratio.

 

This all makes sense in my head, but where they lose me is in calculating the relieving temp, the equation (11) suggests we use the Ideal gas equation and assume Z=1 to get a relieving temperature. 

 

I understand the relieving temperature is changing, however using this calculated temperature and the associated properties will give us a much smaller flow than if we had iterated the compressibility to convergence, and used that new relieving temperature.

 

maybe someone can help me with this concept.

 

Thank you

[Bobby Strain]

You can use whatever method you desire. As long as the results are a greater flow and orifice than the accepted procedure. Also, the relief temperature often greatly exceeds the design value for the vessel.

 

Bobby

[serra]

you can estimate values and calculate PSV area in different ways, see for example this thread

 

https://www.eng-tips....cfm?qid=384407

 

for estimating initial conditions (PSV opening) I solve a V-P flash operation,

this method works for vapor, two-phase etc. cases....

[Technical Bard]

As Bobby has pointed out, a fire engulfed vessel filled with gas with be overheated about the yield strength of the vessel, perhaps before the PSV lifts, and even if the PSV lifts it doesn't reduce the internal pressure.  For this reasons, a depressuring system is preferred for gas filled vessels.