I have a vessel runs at 2 psi during normal operation but if the outlet gets plugged, we use 150 psi steam to clear the plug.

Since this vessel is a small vessel (2ft length and 16" dia), we designed it for 180 psi which is equal to the PSV set pressure on steam header. MAWP is 220 psi. For blocked outlet we dun need a psv since its MAWP is greater than set PSV on header.

Do we need a PSV if the vessel is filled with steam with outlet and inlet blocked? This is an insulated vessel so I am not sure if there will be nay thermal expansion. Fire case is not valid here ( vessel not in flammable area)


Thank You

Mahesh Datla

Mahesh,
The system you describe does not need a PSV FOR THE BLOCKED OUTLET CASE as you have already noted (and for the reasons you noted). Common usage of the term "blocked outlet case" is for a situation where there is no outlet flow and the vessel is connected to potential inflows at their design conditions. Given the information you provided, during the blocked outlet scenario your vessel would typically be connected to its "normal" feed piping. This supplies an unspecified material at 2 psi (g??). You have a second blocked outlet scenario wherein you are attempting to clear a blockage using 180 psig maximum steam. Since your vessel's MAWP exceeds the maximum inlet pressure, it appears you do not have a blocked outlet case that would impact your PSV sizing.

Your final paragraph confuses me. Here you are talking about a steam filled vessel with the outlet and INLET blocked. This is NOT a blocked outlet case. Likely, your vessel will cool, the steam will tend to condense, and you may induce a vacuum. You must consider that and provide vacuum relief if appropriate. I don't see a thermal expansion situation here. Still, if you say there is no fire case, and if you cannot identify a relief scenario, you should follow the oft-recommended solution in this forum and include a minimum sized "thermal expansion" PSV at the vessel to satisfy code requirements.
Doug

Mahesh,
I just have a simple question...

You said that :
"...Fire case is not valid here (vessel not in flammable area)..."

Is this the conclusion of Plant Quantitative Risk Assessment or similar safety studies ?



Doug,
Probably what Mahesh meant Blocked-In-Heat-On.