Recip Pump - Falling Discharge Pressure with Increasing Suction Pressure

[DanTh3Man]

Hello,

I'm troubleshooting a pump system and am hoping the community here can provide some insight into what kind of issue I might be dealing with.

First some background:
[ul]
[li]System consists of two parallel centrifugal pumps feeding two parallel recip pumps[/li]
[li]Centrifugal pumps are on VFDs but have no other means of control[/li]
[li]Recip pumps have recycles. One recycle is blocked in because the valve is washed, suspect the other is washed as well but not as bad[/li]
[li]Fluid being pumped is water[/li]
[li]Pump discharge PSVs are not passing[/li]
[li]Discharge pressure at the centrifugal pumps has remained constant (i.e. is not falling)[/li]
[li]There is a few kilometers of pipe between the centrifugal pumps and the recip pumps[/li]
[/ul]

About 10 days ago we noticed that this system was moving a lot less fluid than would be expected. After some troubleshooting we found that the recip pump recycle valves were passing. The worst offender was blocked in but the other remains open. When this was done, and since then, there has been a consistent (but small) differential between the discharge pressures of the parallel pumps (with the blocked in recycle line having the higher pressure).

Since this was done the flow through the system has been slowly trending down. The recip pump discharge pressures have also been falling while the recip pump suction pressure is increasing.

The simple explanation seems to be that the condition of the remaining recycle valve is worsening but in this case wouldn't the differential in discharge pressures increase?

The only other source of the problem that I can see is a pump valve/fluid end issue. This makes more sense to me because it could cause the discharge pressure of the pump to fall which would in turn lower the system pressure and decrease the discharge pressure of the other pump. But then again maybe the same logic regarding the discharge differential pressure applies here as to the recycle valves...

Anyone able to share some insight?

 

[TugboatEng]

Falling discharge pressures and rising suction pressures? I'd have a look at the piston and cylinder. The same mechanism that "washed" the recycle valves may have "washed" the pump liners.

 

[btrueblood]

Had the same thought as Tug, cylinder/plunger sealing may be compromised.

 

[DanTh3Man]

Thanks for the input, that was my thought as well. Always nice to know other have similar thoughts before telling them to shut 'er down.

 

[LittleInch]

Are the two pumps discharge combined again or separate?



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

 

[DanTh3Man]

They discharge into a common header. Each pump has a check valve before the flows re-combine.

 

[quadtracker]

I would check condition of both suction and discharge valves in the fluid end of the reciprocating pumps. Also replace leaking recycle valve. Get your pump efficiency back to normal. Replace packing in the stuffing box if necessary.

 

[georgeverghese]

Agreed, sounds like leaking suction or discharge valves in the recip pumps, or scratched cylinder liners or broken piston rings.