PSV of reciprocating pump

[MohdYaseen2010]

Dears

I have chemical injection (diaphragm) pump which has PSV built in where the PSV discharge to the pump suction line.
As the pump discharge is higher pressure than the suction line, I think the suction line will rupture , so I think its wrong setup & the PSV outlet line should discharge into the source tank (ATM)?
The problem that the PSV is built in the Pump, so I don't know how is this can be achieved?
its embarrassing thing?

Mohd Yaseen

 

[RVAmeche]

PD pumps typically come with an internal PSV configured like you mentioned. This is purely to protect the pump from destroying itself. All manufacturer's I've seen have something in their IOM saying to not rely on this device to protect your system.

You should install an external PSV on the piping system. Ideally this relief discharge would go back to the storage tank the pump is pulling from. The issue with going to the suction line is it creates a feedback loop and heats up the fluid going through the pump, psv, and back again.

Are the suction and discharge line different materials/schedules/pressure classes? If so, you may need to review that. Note that there's significant pressure drop across the PSV because it's essentially an orifice.

 

[JJPellin]

Consider the scenarios when that PSV might relieve. The pump has to be running to build pressure. If it is blocked in on the discharge, the PSV could relieve back to suction. But, the fluid passing through the PSV came from the suction of the pump. The fluid is being removed from the suction at the same rate it is being returned. There is no additional source of fluid to build higher pressure at the suction. If the pump was blocked in at suction and discharge, the action of the pump will heat the fluid up. The expansion of the hotter fluid will build pressure since it has no where to go. But, this will not be immediate and as soon as the first tiny leak develops, the pressure would be relieved. The suction would not rupture.

Further, a diaphragm pump can only build as much pressure as the gas or fluid used to drive the diaphragm. If it is air, you are limited to the system air pressure. If it is oil, you are limited by the system that provides the oil which probably also has its own PSV to prevent over-pressure. We have hundreds of these pump in our plant and I have never seen a failure like you describe. I don't believe it is a credible risk.

It is always better to route the outlet of the PSV back to a suction vessel if possible. The concern is not rupture from high pressure but damage from high temperature in a situation where you lose outgoing flow and the pump keeps running with all flow passing back to suction through the relief valve.

Johnny Pellin

 

[MohdYaseen2010]

Dears thanks for your responses

the fluid is chemical & the intended pump is chemical injection pump.
I am wondering that where the discharge class is 300# discharge & the suction is 150#, in case of discharge blockage how the suction low rating can withstand the built up pressure

Mohd Yaseen

 

[RVAmeche]

Is the chemical injection pump an AODD pump limited by the motive air/oil pressure like JJPellin mentioned? Or is it a different type?

What is the set pressure of the internal relief valve and what temperature is the process. Assuming flanged piping, look up the pressure/temperature rating for the appropriate flanges. Note if the discharge into the suction line is kept, the relief temperature will be higher than normal process due to the short circuit path mentioned previously.

 

[LittleInch]

JJ had given you a great explanation.

The worst case is when there is no flow. At that point there is no more fluid entering the pump. Therefore the pressure cannot build up on the inlet side.

This happens in thousands of pumps worldwide with no consequences. Find something else to worry about like global warming.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MohdYaseen2010]

Dear RVAmeche;

it's not AODD, its driven by electric motor.

Mohd Yaseen

 

[hydtools]

The outlet pressure on the discharge side of the prv is not at the pressure causing the prv to open. There is a pressure drop across the prv and its outlet pressure is that of the line into which it is discharging, not the high pressure causing the prv to open. There is a delta p across the prv.

Ted

 

[JJPellin]

Good point hydtools.

If the pump is motor driven, the diaphragm is probably actuated by oil. Ther is an internal relief valve for the hydraulic oil. That will also limit the pressure.

Johnny Pellin

 

[MohdYaseen2010]

Dear hydtools;

Thanks for the explanation, I know that the pressure at the discharge is less than the valve set pressure & it normally calculated backward from the pressure at the end to the valve outlet.
My concern that this line is very short (discharge from valve outlet to pump suction) cause its internal configuration, so with 22 bar (set pressure) in my case with rating 150# (withstand about 17bar) that means the pressure drop need to be equal or more than 5bar.

At conclusion, I really enjoyed with this discussion as I have learned a lot from all participants.

Mohd Yaseen

 

[hydtools]

All the pressure drop occurs across, through the relief valve.

Ted

 

[MohdYaseen2010]

dear hydtools;

thanks for this correction, do you know how I can estimate the pressure drop across the valve, is it by Manufacturer, if by manufacturer, that mean I must give him my requirements on the discharge system(cause normally we are identify the set pressure only on the valve?)

Mohd Yaseen

 

[hydtools]

Learn more about a PSV/PRV
See the attachment

Ted

 

[MohdYaseen2010]

dear hydtools; many thanks

Mohd Yaseen