Backpressure on PSV
Backpressure on PSV
(OP)
I have a PSV with a set pressure of 90 psig with the case being blocked outlet. I want to pipe the relief header back into the suction side of the pump. Would the backpressure on the valve be the net positive suction head on the pump? Any induced backpressure on the valve from it opening? Thanks.





RE: Backpressure on PSV
No, the BP on the PSV discharge nozzle is not the NPSH.
Any induced backpressure on the valve from it opening?
Probably.
Backpressure needs to be calculated as the "pressure at a point" with the PSV closed and using the "worst case" of your normal operating limits and parameters that may affect backpressure. The "point" is the PSV discharge nozzle.
Superimposed backpressure is bacpressure that comes from other sources, like a pad gas in a discharge header going to a flare or when a different PSV discharges into a header. You probably don't have any in your particular case.
Built-up backpressure needs to be calculated as the "pressure at a point" with the PSV open and using the "worst case" of your normal operating limits and parameters that may affect backpressure.
Good luck,
Latexman
RE: Backpressure on PSV
The relief valve is set at 90 psig. The discharge header is a 1" Sch 40 SS line with 15 ft of straight pipe and 1 90 degree elbow (10 ft vertical drop). The discharge header discharges into the suction side of the pump. The relief max flow is 9.8 GPM. Viscosity is 2300 cps and S.G. is 0.98.
How would I calculate the backpressure on the valve? Do I need to know anything about the inlet PSV header? (Let's say its 10 ft of 1" pipe Sch 40 pipe all straight vertical rise) or the suction pressure of the pump?
Thanks
RE: Backpressure on PSV
As a general comment, I find your description of the relief valve inlet and outlet piping as a "header" to be a little confusing. A header is typically a pipe to which several other pipes might be connected but I don't get that impression from your example. From your example, it sounds like you may have a positive displacement pump with a relief valve on the pump discharge with the relief valve discharging back to the pump suction, is that your system?
Latexman is correct about having to account for the superimposed and builtup backpressures but if the system is a positive displacement pump with a relief valve discharging to the pump suction then superimposed backpressure can be a significant concern.
When talking about systems like the one I've described for a PD pump, it is more typical to think of the relief valve setting in terms of "psi" or "psid" where "d" indicates differential rather than psig (gauge). To be on the safe side, be sure and check what the relief valve cold differential test pressure is to see if there is a difference from what you find for the set pressure.
If for the moment, we neglect the builtup backpressure, then if the pump suction pressure is 10 psig and the relief valve is set at 90 psi then the relief valve will not open until the relief valve inlet pressure is 100 psig. It then becomes important to know the maximum pressure for the pump suction to make sure the valve is set properly to protect equipment and piping downstream of the pump. What's the source for the pump suction? Does the source vessel have a relief valve and what is its set pressure? Unless you have good reason not to use it, you may want to consider the pump suction pressure equal to the source vessel relief pressure plus any static head effects.
With the relief valve setting properly accounting for the superimposed backpressure, you only need to check that the builtup backpressure (psi) is less than or equal to the amount of overpressure (psi) used.
If this doesn't help, come back with more details.
RE: Backpressure on PSV
Good luck,
Latexman
RE: Backpressure on PSV
RE: Backpressure on PSV
The vessel is atmospheric, so the pressure is 0 psig. The liquid head is around 10 feet. So for simplicity if its water, the head pressure is 4.33 psig from the vessel. At the worst case flow rate of 9.8 GPM, the pressure drop in the suction line to the tee where the PSV discharge is located is 1.2 psig. So in Latexman's example, P1 is 3.13 psig. Also, the pressure drop in the discharge line of the PSV at 9.8 GPM is 0.5 psig. So this means the backpressure is 3.63 psig, correct?
Or don't you take in account the frictional losses in the suction side from the vessel to where the discharge line enters? Or am I correct how I approached it?
Thanks again.
RE: Backpressure on PSV
No. With a closed discharge scenario, there will be no flow from the vessel to the tee where the discharge enters, therefore there is no frictional losses. If the highest liquid level in the vessel is 10 feet above the centerline of the suction line where the diccharge tees in, P1 = 4.33 psig. P2 = 4.83 psig. PSV inlet = 94.83 psig.
Good luck,
Latexman
RE: Backpressure on PSV
Good luck,
Latexman
RE: Backpressure on PSV
RE: Backpressure on PSV
Backpressure = 0 psig
Superimposed backpressure = 0 psig
Built-up backpressure = dP in the line
Also in this case, the set pressure would NOT be adjusted so it pops at the right pressure, but you do have to select a PSV that meets the built-up backpressure.
Good luck,
Latexman
RE: Backpressure on PSV
RE: Backpressure on PSV
Good luck,
Latexman