Relief Valves

in case of a pressure relief valve if both the valves are in line and both pop up at the same time,then is it going to affect the upstream vessel??

Relief valves are supposed to be designed so that one valve is enough to vent the vessel under typical fire conditions. Two valves on line would vent faster, to be sure, but that is not the issue.

if both the valves are venting then wont it affect the upstream vessel?wont it cause sudden pressure drop?

kkunal...

This is an excellent question for those people in your organization that have access to the system PIDs and the relief valve calculations that were done.

In most complex chemical plants, you must evaluate an entire system at once.

In the US we have what is called a HAZOP, where various system upset scenarios are evaluated.

In this type of document you would find the answer to your questions.

You do have PIDs and back-up documentation.....right ?

we do have PIDs but we dont have access to calculations documentations..i have found this trend of keeping the stand by PSV isolated throughout my refinery..but havent got any suitable explanation to it..is it the same way everywhere??

How sudden the pressure drop would be depends on the capacity of the relief valve(s) in relation to the volume of the vessel. Two valves would result in a faster pressure drop than one valve. They might then reseat themselves sooner, but the total amount of process fluid released would be about the same.

As to your post 6, what was not suitable about the prior explanations?