Hi All
I am having trouble understanding the rationale behind using 10% accumulation for non-fire case and 21% accumulation for fire case for the same PSV.
Let me elaborate by providing example.
I have a vessel with two possible credible scenarios 1) Fire 2) Blocked Outlet.
I will size for both the cases with an accumulation 21% and 10% respectively. Both the cases area requirements are pretty close.
Let's say fire case turned out to be the governing scenario( slightly larger area). So my PSV will be set to open completely at an upstream pressure of 1.21*set pressure. But for blocked outlet case, i have used an accumulation of 10%. At 10% accumulation my PSV will not be 100% open and the area available is less than actual discharge area. That available area of the PSV at 10% accumulation could be lower than my required area i arrived by my calculation. Is this possible. Are we adopting the correct methodology by comparing the area requirement estimated with 10% accumulation against a valve which opens completely at 21% accumulations.
Consider the other case. If blocked outlet is governing, then the PSV should open completely at 1.1*set pressure. But we have used 21% accumulation for fire case. As i stated previously the area calculated are pretty close. So should i change the accumulation to 10% and check the area requirement, as the PSV cannot experience 21% accumulation even under fire case???
Please clarify