Centrif Pump with Varying Discharge
Centrif Pump with Varying Discharge
(OP)
I've got some multiple-stage centrifigul pumps installed in a vertical configuration. Normal inlet pressure is 100 psig. Normal discharge pressure is 1,300 psig. Flow is up. Fluid is 60F water (SG 1.03). Pump is electrically driven.
Several times a week downstream upsets cause discharge pressure to drop to about 100 psig (rarely less than suction pressure, but that does sometimes happen) for several hours at a time.
The pump location is inaccessible and the closest location where I could install a backpressure valve is about a mile away (and the source of the upset is between the pump discharge and the location where I could put the backpressure valve).
Is there any significant risk of damaging the pump in this scenario?
Several times a week downstream upsets cause discharge pressure to drop to about 100 psig (rarely less than suction pressure, but that does sometimes happen) for several hours at a time.
The pump location is inaccessible and the closest location where I could install a backpressure valve is about a mile away (and the source of the upset is between the pump discharge and the location where I could put the backpressure valve).
Is there any significant risk of damaging the pump in this scenario?
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem





RE: Centrif Pump with Varying Discharge
Regards checman
RE: Centrif Pump with Varying Discharge
RE: Centrif Pump with Varying Discharge
Regards checkman
RE: Centrif Pump with Varying Discharge
RE: Centrif Pump with Varying Discharge
The upset comes from a relatively small amount of natural gas coming with the produced water. If I put 1,000 SCF in the 1.996 ID tubing, then the gas displaces enough liquid to drop the pump discharge pressure from 1,300 psig to about 100 psig.
Downhole gas separators help, but nothing prevents the gas from getting into the pump discharge. Usually the gas is in the form of very small bubbles (that don't tend to collapse and cause cavittion damage due to their size) and/or disolved gases that evolve out of solution as the water moves up the piping (due to temperature changes as the fluid passes through different temperature strata).
When a slug of gas is large enough to fill the pump with gas, you run into heat and friction problems that destroy pumps pretty quickly. I'm trying to determine the damage I can expect if the liquid-full pump sees a rapid decrease in discharge pressure.
Low-pressure trips are not practical because you'd have to put the sensor on the surface and you don't often see surface indications of the problem.
An orifice on the discharge of the pump won't work because the downstream side of the orifice would see such a wide range (i.e., an orifice that gives you 100 psid at normal flow rate would let the pump drop to 200 psig if the column backpressure drops to 100 psig).
The pump is not stonewalling. Every one of them that I've seen has been on its pump curve and able to move across the pump curve as upstream and downstream conditions have changed.
David
RE: Centrif Pump with Varying Discharge
If this is the case, then the pump would stay on its curve, flow volume would stay somewhat stable, forces would stay in balance in the pump, and horsepower use would drop during a swing. Damage that would normally be seen when a pump runs too far from BEP would not be present. (vibration, recirculation, shaft deflection, etc) However, I would expect that the less dense mixture being pumped could cause heat/rubbing damage at bushings and wear rings, due to loss of lubricity and a reduced ability to carry away the heat produced in the pump.
If the above sounds right, and the pumps have been seeing these swings with no apparent drop in performance, it could be that nothing needs done except to trend any damage seen as the pumps are routinely repaired. If in service times prove too short, self lubricating materials might be looked at. A no flow switch to trip the pumps may be a good idea to limit damage when they totally gas up.
SWAGs:
Not sure why/how the discharge pressure (measured at the pump) drops so much. Maybe there is some pumping affect by the column of bubbles (like in an aquarium riser). If it is very short duration drops to below suction pressure, maybe the momentum of the water further up the discharge pulls along the lighter fluid for a time, or the pressure transducer overreacts to the quick swing.
RE: Centrif Pump with Varying Discharge
You have it just about right. The big issue is the discharge column simply gets lighter due to the gas in the vertical pipe. We don't see terribly long pump lives, but they aren't rediculously short either. I was thinking that lubricity and heat removal were the big concerns, but I wanted to see if there was any different input from pump guys (as opposed to downhole guys).
A lot of people have tried pump-off-control schemes with these pumps (based on water rate, full-stream reaction to a heated probe, and current) and some of them work well at telling when the pump is trying to pump gas. The big difficulty is deciding what to do with this information--most of the time if you turn the pump off, solids in the produced liquids will settle onto the top of the pump and it won't be able to develop the necessary starting torque.
These pumps are pretty popular in one particular field and I'm working on a class on deliquifying gas wells and I needed to get some additional input on the risks.
Thanks for your input.
David