Selecting the type of PSV is always one of the critical tasks for design engineers. One of the critical parameters for selection of PSV is back pressure as it will impact relieving capacity of a PSV.
Back Pressure is a term used for the pressure which is existing at PSV outlets. It is further classified as:
1) Superimposed Back Pressure
2) Built-up Backpressure
Here, the superimposed back pressure is the pressure existing at PSV outlet when PSV is in closed position. It is the resultant pressure present at the PSV outlet because of some other sources in the relief network. It can be constant or variable. Built-up backpressure is the pressure generated because of fluid flowing when the PSV is in open position.
Following actions to be performed in case of inadequate backpressure:
1. Increase the line size.
2. Ensure that the line size in the downstream network is higher than the PSV outlet flange size.
3. Reduce the number of elbows and tee in the downstream line.
4. Check flow rate considered for sizing. It should be required in case of liquid relief and Modulating operated pilot PSV.
5. Check for the possibility of flashing in the downstream line.
6. Change the type of PSV.