4 replies to this topic

[Rookie]

I have a liquid service PSV that discharges to a discharge line of a pump, which goes to an atmospheric tank. This pump discharge line goes about 3/4 way deep inside the tank, and the tank is about 74% liquid full. We can assume that the line submerges all the way inside the tank and the tank is 100% full (worst case). How can I calculate the back pressure of the PSV if I don't know what pressure the pump is running at. I do know the maximum flow that the pump can put out. Is my back pressure atmospheric since the tank that it discharges to is atmospheric, no matter what pressure the pump is running at?

Thanks in advance.

[JoeWong]

Rookie,

First you may needs to advise if the pump is centrifugal or positive displacement type ? Flow vary with pressure for centrifugal pump whist constant (almost) flow for positive displacement type.

If your PSV is located at the pump discharge to avoid the discharge piping to its design pressure, you may always can consider pump operate upto 110% of PSV setpressure (normally the design pressure).

I would think backpressure to the PSV will be the sum of static head due to submerged pipe plus built-up pressure due to relieve assuming No pressure drop on the tank vent pipe.

regards,

JoeWong

[Rookie]

Thanks for the prompt response Mr. Wong. It's a double PD pump (2 pumps running on 1 common driver shaft). PSV is on discharge of 1 pump and goes (discharge) to the discharge of the other pump, which goes back to the tank. The vent is mounted directly on the tank. The PSV is protecting both the pipe and the downstream equipments. So, do I have to determine if the vent is big enough to handle the max flow of the pump first?

[JoeWong]

Rookie,

Theoritically yes but the pressure drop on vent pipe (i suppose your vent pipe is just a short pipe above the tank top) may not be significant with my limited experiences.

You may try to estimate the pressure drop through the vent pipe to prove my statement.

regards,

JoeWong

[pleckner]

Fist we need to know which back pressure you are looking for? Is it the superimposed back pressure (the back pressure the PSV sees before it opens) or is it the total back pressure, which is the superimposed back pressure + the built up back pressure (the pressure caused by the flow of the relieving fluid)?

If it is the total back pressure you want, we can approach it this way:

First get the pressure at the end of the pump discharge pipe, the one sitting in the tank. You do this by adding the vapor space pressure to the liquid static height in the tank from the pipe to the liquid surface (100% full will do just nicely).

A PD pump is a constant volume pump and will deliver the flow you want at any pressure it needs to get the fluid to where you want it. So, knowing your controlling or sizing relieving rate, add this to the maximum flow of the PD pump that you can ever set it for. Now, perform a straight hydraulic calculation using the total flow, working backwards from the end of the pipe (known pressure) to the pump discharge nozzle. Using this pressure as a starting point, and working backwards, perform another straight hydraulic calculation using only the PSV relieving flow and get the pressure at the PSV outlet flange.

If you are only interested in the superimposed back pressure, the calculation is done the same way outlined above but without the PSV relieving rate added to your maximum pumping rate with one exception. You don't have a relief even yet so the superimposed back pressure will be the pressure at the pump outlet flange + any static head against the PSV (if any).

Now, in the beginning I started out by saying get the pressure at the pump outlet pipe by taking the vapor space pressure and adding it to the liquid static height. I used the term "vapor space" rather than "atmospheric pressure" on purpose. You must determine what the pressure will be in the vapor space at the time of relief. You do this by taking the total flow calculated above and calculate the equivalent air, i.e. if you are flowing 100 gpm, convert this to cfm. Now, you do another calculation from the inside of the tank, into the outlet nozzle, through the vent pipe to atmosphere. This will give you the vapor space pressure at the time of relief. Let's hope the vent pipe is big enough!!