Pump - stand by pump pressurization
Pump - stand by pump pressurization
(OP)
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





RE: Pump - stand by pump pressurization
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Pump - stand by pump pressurization
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.
RE: Pump - stand by pump pressurization
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
RE: Pump - stand by pump pressurization
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Pump - stand by pump pressurization
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
RE: Pump - stand by pump pressurization
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Pump - stand by pump pressurization
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?
RE: Pump - stand by pump pressurization
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso