PSV to common scrubber header

[tjpengineer]

I am sizing a PSV for fire case on Toluene that will discharge to a common scrubber header. What is the accepted method to determine superimposed backpressure on the PSV from the header? Normal running condition, all reliefs on the header discharging at the same time, some reliefs discharging, etc.?

 

[Chance17]

For the first pass use the normal operating pressure.

It is very unlikely all reliefs will discharge to the header at the same time.
However "some" group venting can be credible especially for simultaneous Fire.
The latter takes some analysis on your part to determine which equipment is involved.

I am aware of one refinery that typically has 2-psi header pressure.
However calculations show sectional Fires may back presure the Flare header by 24-psi.
Remediation work is underway.