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[ayan_dg]

If a gas flows through a orifice where the downstream pressure is less than critical pressure , the flow through the orifice is critical flow and depends on the inlet pressure , it is independent on the outlet pressure.
Now for conventional spring loaded relief valve we limit outlet back pressure to 10% of set pressure ( say for blocked discharge case ) , why this limitation is required because at say 15% back pressure or even at 25% back pressure the flow should be critical for most gases and hence independent of outlet pressure.

[fallah]

Now for conventional spring loaded relief valve we limit outlet back pressure to 10% of set pressure ( say for blocked discharge case ) , why this limitation is required because at say 15% back pressure or even at 25% back pressure the flow should be critical for most gases and hence independent of outlet pressure.

 

ayan,

 

Because the maximum accumulation in pressure vessel is 10% and it would offsset the maximum back pressure of 10% in order to the valve being full open. Then if the back pressure in conventional PSV to be higher than 10%, the PSV can't be in full open postion, hence rated capacity, otherwise the accumulation would reach the exceeded back pressure that in turn the code limitation would have been passed...