Pump - stand by pump pressurization

[hemvig]

I have a HAZOP question

Pump A and B will be installed in parallel with one pump acting as standby spare.
Both pumps have tight shut-off orbit type valves on the suction and discharge.
If the suction side orbit valve on the spare pump is accidentally closed then I expect the discharge pressure from the on line pump to pressurize the standby pump all the way back to the pump suction isolation valve. We have a check valve on the discharge.
Also do we consider check valve in the HAZOP.

Question 1: Is the above sceanario real, if so is there any solution.(These are Barrel type pumps with suction pressure of 1200 psig and discharge of 2656 psig)The pumps are rated for 2 MAWP suction side MAWP: 2656 psi and discharge side MAWP 3000 psi)
Flow: 500 gpm
temperature 100F

 

[BigInch]

It is possible to backspin pumps, or leakback through the pump and pressure the suction side, which is why you should have suction and discharge blocks closed on standby pumps when not in service.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[pennpiper]

The poster does not say what kind of facility these pumps are in. He (or she) also does not say if the pumps are on "Run/Rest" cycle with Automatic start.

So I would say there are two cases'
A). Not Auto-Start - keep the Suction and discharge valves closed on the stand-by pump and only open them when the pump is "manually started at the pump (not remote). Check Valves may be considered optional in this case by some. I would prefer Check Valved in the Discharge lines of all Centrifugal Pumps.

B). Auto-Start (as in common in most Refineries) - include a good quality Check Valve in the Discharge of both pumps. Keep the Suction and Discharge Valves open on both pumps in case there is a failure of one pump and the other pump kicks on due to the Auto-Start.

 

[JJPellin]

In a HAZOP analysis, you should not take any credit for the check valve. I work in an oil refinery. It is most common in our facility to leave both the suction and discharge block valves fully open on all standby pumps. Even if they are not on auto-start, we normally want them hot, flooded, pressurized and ready to run.

But, for your specific situation, you should never have the discharge valve open and the suction valve closed. Either both are open or both are closed. And you should always open suction first and close suction last.

If an operator accidentally closes the suction valve, you are still within the MAWP of the suction side, so no additional layers of protection are needed.


Johnny Pellin

 

[BigInch]

Would not a closed suction valve send the pump to shutoff head and tend to pressure the suction of the other to shutoff head too? That's what I took the question to mean. Not taking credit for any discharge checks that might be installed, the only alternative seems to be closed blocks.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[JJPellin]

I have participated in a number of detailed HAZOP analyses. In that method, you consider possible hazardous situations that could arise from things like: High Flow, Low Flow, High Pressure, Low Pressure, etc. If you find a hazardous situation, you consider the consequences and likelihood. Based on that, you determine how many layers of protection you need to protect against that outcome. In that analysis, a check valve cannot be considered for credit as a layer of protection. This is because check valves frequently stick open. This failure cannot be predicted or prevented. And, even if the check valve does close, there is a very good chance it will leak over time.

This does not mean that check valves do not serve a useful purpose. It does not mean that they are not needed. We install check valves on the discharge of all centrifugal pumps (expect molten sulfur) and most rotary positive displacement pumps.

I took the question to ask if you could consider a check valve as a tight shut-off, leak tight valve that would prevent the discharge pressure of the running pump from pressuring up the standby pump if he suction valve to the standby pump were closed. If that is the question, the answer would be no.

I am assuming that each pump has its own suction block valve and those are the valves the OP was asking about. If the suction block valve was accidentally closed on the running pump, I would expect that pump to fail. But I would not expect any effect on the standby pump.

If I had a pump that was not rated for the discharge pressure of the other pump running in parallel, I would not consider the check valve as protection. If the likelihood and consequences of an operator accidentally closing the suction block valve were high, I would insist on another protection such as a PSV inside the block valves. If the overpressure potential was less than the hydro-test pressure of the pump, but still above its pressure rating, I might consider a lesser protection such as a high pressure alarm. Since the block valve can only be closed if an operator is present, I might be able to take credit for operator intervention. But, if the check valve were to leak slowly over many hours, the operator could be long gone before the overpressure event occurred. All of this would be taken into account in the standard HAZOP method we use.


Johnny Pellin

 

[BigInch]

I did not consider the check either. And, rereading the OP I see now that the suction side is equally rated for discharge pressure, so leak back through the check and continuing through to suction will not overpressure the standby pump's suction.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[bimr]

If you are not taking credit for a discharge check, it would seem that you need an automatic valve to open with the pump start.

If you have a pump sitting there with the suction and discharge valves open on a manifold in parallel with another pump; what is there to prevent backflow through the non operating pump?

 

[BigInch]

No. Taking "credit" for check valves as a safety feature, or considering that they positively block flow during maintenance is usually not permitted, however you can still depend on it to stop backflow under normal day to day operating circumstances.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso