Recently I came with this confusion. For the PSV datasheet Vendor Data Specifies three cells as below:
1. Under the Back Pressure : i) Constant
ii) Variable
iii) Total
As I understand
i) Constant Pressure: It means constant header pressure or superimposed back pressure. However my lead engineer says constant means positive pressure generated by the nitrogen purging. Not the superimposed back pressure. So according to my lead engineer constant should be nearly zero but according to me it should be around 1.5 barg as our maximum header pressure.
ii) Variable: As I understand it is built-up backpressure i.e. backpressure generated as a result of PSV opening. However my lead engineer says Variable pressure is total of self built-up backpressure + Backpressure generated by others.
I would appreciate if you can share your thought in order to clarify what exactly is the definition of i) Constant Back Pressure and (ii)Variable Back Pressure
a constant backpressure in a header system is the pressure that exists due to the positive bP maintained by a purge gas ( as you rightly say negligible) plus any bP built up by a constant flowing gas in the system. So for example if a plant was starting up and venting continuously from a separator during this phase then there would be a constant bP throughout the header system.
Any PSV or vent valve would see this bP at the exit flange .
Under normal operational conditions the constant bP in a system ( where hopefully all PSV's and vent valves are closed ) will be very small ) purge gas and any minor leaks into the system only )
a flowing backpressure on a valve is generated by the gas from that valve increasing the backpressure as it flows from exit flange to tip.
some folk make a mistake of only calculating the dP in the lateral/subheader from the valve exit flange to the tie in point of the next header downstream and then adding that to the constant backpressure. This then omits the dP of the flowing gas from tie in point to tip
the total bP at an exit flange is therefore the pressure calculated by the total system gas flow from tip back to tie in point plus the dP contribution of relieving flow from tie in point back to valve flange
Thanks a lot for your reply. Based on your explanation I paraphrase the terms as follows:
1)Constant Back Pressure: It is the positive backpressure generated by the purging gas or other continuously flow in the normal condition.
2) Built-Up Back Pressure: The backpressure generated as the result of PSV itself relieving
Kindly correct if my understanding is wrong.
However I am still confused about the superimposed back pressure. As I understand it is the backpressure generated as a result of other PSV reliefs which are connected to header. When specifying the datasheet then it should be the worst case scenario i.e. all the PSVs releasing at the same time. Is it the part of the variable backpressure?
There are two types of superimposed back pressure,constant and variable.
Constant BP= consultant superimposed BP
Variable superimposed back pressure can be any variable BP at the discharge flange that is not induced by the PSV relieving.
You do not have to assume all the PSVs are relieving at the same time for two reasons:
1. It's not realistic to assume worst case=all psv relieving. Usually we check for the single common worst cause.ie site wide power failure
2, usually we do not assume two PSVs which have no common relief causes will relieving at the same time.
For example such a PSV will only relieving during fire and there is no other vessel in the same fire zone, PSV's variable BP will be 0.