Centrifugal Pump "Surging"
Centrifugal Pump "Surging"
(OP)
I have a centrifugla pump that seems to "surge" at times. Let me explain.
This pump is circulation pump on a scrubber that is under a slight vacuum. This scrubber has instances every couple of hours where there is a slight surge in vacuum in the scrubber. During this time, the circulation flow drops from about 175 gpm to 140 gpm.
I have examined the NPSHR calcs (~ 2 ft) and I have at least 10 ft NPSHA. It has been suggested that the pump is cavitating because the liquid is at or near its boiling point, however I have a circulation tank that maintains a 50% level to feed this pump.
The pump that we are using is way oversized (BEP ~ 900 gpm on full speed) and about 600 gpm at the reduced speed. WE are currently runnning about 250 gpm. Everyone recognizes that the pump needs to be replaced, but can the oversize be causing the phenomenon that I described prior? Can it be that we are so far back on the flat part of the curve that a very small variation in static surface pressure (< 1 psig) can cause large variations in discharge flows (>40gpm)? Is there a critical point on a centrifugal pump curve that should not be exceeded in this regard such that flows become highly unpredictable or drop off dramatically?
This pump is circulation pump on a scrubber that is under a slight vacuum. This scrubber has instances every couple of hours where there is a slight surge in vacuum in the scrubber. During this time, the circulation flow drops from about 175 gpm to 140 gpm.
I have examined the NPSHR calcs (~ 2 ft) and I have at least 10 ft NPSHA. It has been suggested that the pump is cavitating because the liquid is at or near its boiling point, however I have a circulation tank that maintains a 50% level to feed this pump.
The pump that we are using is way oversized (BEP ~ 900 gpm on full speed) and about 600 gpm at the reduced speed. WE are currently runnning about 250 gpm. Everyone recognizes that the pump needs to be replaced, but can the oversize be causing the phenomenon that I described prior? Can it be that we are so far back on the flat part of the curve that a very small variation in static surface pressure (< 1 psig) can cause large variations in discharge flows (>40gpm)? Is there a critical point on a centrifugal pump curve that should not be exceeded in this regard such that flows become highly unpredictable or drop off dramatically?





RE: Centrifugal Pump "Surging"
PUMPDESIGNER
RE: Centrifugal Pump "Surging"
RE: Centrifugal Pump "Surging"
If the pumps is circulating a liquid with dissolved gases the NPSHA as calculated for the liquid alone may not be sufficient, and the evolved gas inside the pump may interfere with normal operation.
RE: Centrifugal Pump "Surging"
RE: Centrifugal Pump "Surging"
One other issue that I have concern about is the tank turnover. The tank's utilized capacity is about 1000 gallons while the pump circulates 250 gpm (turns the tank over in about 4 minutes). I question whether this alone may be causing air entrainment, but the problem I described is not a continual problem - it only happpens when there is the vacuum surge in the scrubber above it. We are considering a vortex breaker in the pump suction line, but, because of the aformentioned reason, I do not think that this will cure the problem.
RE: Centrifugal Pump "Surging"
Regards checman
RE: Centrifugal Pump "Surging"
RE: Centrifugal Pump "Surging"
The volume of gas released increases as the pressure drops.
It may just be greater than the gas volume that the pump can normally handle creating a dip in flow. Any procedure that increases the NPSHA may be of help.
See thread798-129328 .
RE: Centrifugal Pump "Surging"
Naresuan University
Phitsanulok
Thailand
RE: Centrifugal Pump "Surging"
hbw,
the pipe reducers are flat on top. I am not sure what you mean about the pump suction being "self-venting into the pump"???
RE: Centrifugal Pump "Surging"
Captured vapor in the suction line acts like a spring and cause large variablity in suction pressure. Simply put, trapped vapor in the suction piping causes unreliable performance of the pump.
A flat performance curve can do the same thing. Vapor pockets are eliminated by reconfiguring the suction piping. Flat curve can be made steeper by installing a discharge orifice, larger impeller and motor.
RE: Centrifugal Pump "Surging"
I see 2 possibilities
1. although you say the curve is flat round that flow rate, it is possible that the actual performance may droop in this region. many manufacturers just show the curve in this region as flat even though the curve may droop.
2. the reduction of vac. from 1" to 10" may be sufficient to expand any entrained air in the system to a level beyond what the pump can handle - as pointed out by ----
Quote "25362 (Chemical) 12 Nov 05 0:31
The volume of gas released increases as the pressure drops.
It may just be greater than the gas volume that the pump can normally handle creating a dip in flow. "
Trust this helps.
Naresuan University
Phitsanulok
Thailand
RE: Centrifugal Pump "Surging"
I am trying to visualize the entrained air possibility. I guess air could become entrained in the circulated liquid in the mass transfer interface in the packing or from the turbulence caused by a rapid turnover of the supply tank (<5 minutes).
If this is the case, and I can do nothing about the vacuum surge, am I doomed to just "live" with this? Would using a more open impeller help "pass the gas"?
RE: Centrifugal Pump "Surging"
If the recycle flow change occurs only, when there is a pressure surge (drop) in the scrubber, the phenomen should be triggering the flow reduction.
Having the pump operating on a flow substantially below the BEP and on reduced rotational speed, it could be that the pump is operating below the static stability limit. A pressure drop of the system could trigger irratic pump behaviour.
Another reason could be, having liquid near boiling point, that bubbles vapor from boiling liquid or bubbles released gas from the scrubbed gas (not necessarily air) are released due to the pressure drop and enter the pump suction line. Please note, that due to near boiling liquid, you have maximally the static head only available as NPSHA. Additionally to the reaction heat in the scrubber, the pump will put heat into the liquid. If you prepare a small schematic of the recycle line, with pressures, temperatures, heat input etc added, you could verify whether the pressure drop triggers such bubble formation.
If bubble formation occurs, you could try the obvious: prevent pressure surge, but also investigate less heat input, higher static head or a calming section in the tank.
RE: Centrifugal Pump "Surging"
Naresuan University
Phitsanulok
Thailand
RE: Centrifugal Pump "Surging"
The problem of the surging has not gone away. Essentially the sam symptoms occur... When the sudden process vacuum surge happens, upon returning the process to "normal", the pump flow drops off for a few minutes and then rebounds. We tried shutting the pump off and then starting it right back. The flow comes back up, but over the course of a 30 seconds to a minute. It seems like this is way to long for a centrifugal pump. We tried shutting the pump off and then back on before a process upset and the flow came back within 5 - 10 seconds.
I am guessing that the long return is due to air working its way through the pump, however, the flow rebound is steady as well as the discharge pressure of the pump. I would have thought that air would be more disruptive.
At this point, I am not sure what to try next. I still have a few weeks until the next planned outage.
Any ideas?