Piston Pump Discharge Pressure

[junah]

Dear Members,

I have two piston pumps A and B (2x100% operation) used in Hydraulic power unit for wellhead valves. During functional test, we started pump A and automatically stop the pump when it reached 6000 psi pressure. We put check valve on discharge line and pressure gauge to monitor the pressure. but we also install pressure gauge between check valve and pump discharge (this is for maintenance purpose when Operator wants to test the pump without shutting down the well). My question is, when we stop the pump A, the pressure gauge downstream check valve showed 6000 psig, but the pressure gauge upstream of the check valve suddenly decreased to 0 psig. We did the same test for pump B and somehow both pressure gauges upstream and downstream check valve showed 6000 psig.

Then we tried to HPU unit B (we have 3 units), and it showed the same result. However this time, instead of Pump A, it was upstream check valve on pump B that decreased to 0 psig (so we eliminate possibility of error in tubing connection line).
To add more confusion, HPU unit C both pressure upstream of check valve showed pressure at 6000 psig and none decreased to 0 psig.

We talked to pump Vendor about this matter, and they said because piston pump is not leak free. What does it mean by not leak free ? I check with drawing inside the piston pump, and there suppose to be internal check valve and also outlet check ball. In my understanding, it should keep the pressure. And when it leaks, does it mean the fluid back to suction line and equalized with suction pressure ? FYI, we tried to block the suction line at the same time we stop the pump, and the result still the same, the discharge pressure still decreased to 0 psig. where did the pressure go ?

Please advice. Does piston pump work that way ?

 

[HPost]

Hello,

The piston pumps will have 3 primary connections, aside from any pressure control lines. Those 3 connections are the inlet, the outlet and the casing drain.

All piston pumps have some internal leakage. That means that oil leaks through the clearances between the pistons and the block in which they operate.

The one pump that did not show a pressure drop just happened to stop in a position that trapped the pressure. Given time, that 6000 PSI would have decayed to 0 as well.

The oil leaks back through the pistons and into the casing, leaking out through the casing drain.

What you have seen is perfectly normal.

It's not a very good way to test the pumps though. Just because a pump can generate pressure, doesn't mean that it's in the good shape.

HPost CEng MIMechE

 

[HPost]

It also may be the case that the pressure is trying to "motor the pump". That is the pressure on the pistons is trying to rotate the pump. The pressure would only be enough to make the pump twitch, there will not be enough flow to actually rotate it. It could be that the pump that held the pressure was stiff enough to resist rotation, thus holding the pressure.

Either way, what you are seeing is to be expected.

Cheers

HPost CEng MIMechE

 

[LittleInch]

I agree with the above. For a hydraulic system and for the length you talk about the volume to go from 6000 to 0 is vary small and any minor leak past the pump outlet check valves will result in this. If the pump is a multi cylinder affair then simply the point at which it stops could equalise between cyclinders.

Either way your main check is holding.

You could always replace the check valve which is leaking and see if that fixes it....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[HPost]

This has to be a port plate piston pump. If this was a radial piston pump or a single cylinder reciprocating pump, the outlet check valve or valves would trap the oil.

In a port plate piston pump, the oil can leak back into the pistons or across the gap between the end of the cylinder block and the port plate. As stated, it leaks into the casing and then out through the casing drain.

Do not be tempted to restrict the casing drain, you will blow the shaft seal out.

Cheers

Hpost CEng MIMechE