Centrifugal Pump "Surging"

[dogbertcountry2]

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?

 

[PUMPDESIGNER]

Does the pump have any flow instabilities in the head/flow characteristic (curve)?

PUMPDESIGNER

 

[dogbertcountry2]

No. It is a standard Durco centrifugal pump - especially at the point on the curve that we are operating, it is very flat.

 

[25362]

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.

 

[jet1749]

Just general observations, but if you have a pump with a flat curve, a small variation in pressure (head) can cause a significant change in flowrate. This is something I have observed many times whilst carrying out dynamic pump tests. A suction side under vacuum conditions with a liquid at elevated temperature, close to its vapour point looks suspicious, even if you have 50% tank level, its pressure at the suction nozzle that counts. Is there anywhere in the system that vapour can accumulate due suction side conditions and piping peculiarities that could cause random surging as it eventually finds its way to the pump?

 

[dogbertcountry2]

I do not know of any vapor accumulation points. The pump feed tank is elevated above the pump such that there is 10 feet of static suction head available. I have calcualted the NPSHA to be >20 ft with a NPSHR <4 ft.

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.

 

[checman]

Dogbertcounty2: The vortex breaker might just do the trick. Your tank retention time is low but acceptable for some on hot water service. It is possible that your pump has a fairly high specific suction speed with that low of an NPSHR. Pumps with high specific suction speeds have a tendency to pre-rotate the incoming fluid if they are run back on the curve. The pre-rotation vortex can centrifuge out the air that becomes entrained in your system. The changing suction condition (scrubber surge) can expand and dislodge the air pocket. Part of the air pocket could get stuck at the impeller eye. The air pocket in the pump impeller eye will reduce pump capacity. The air pockets normally remains at the impeller eye until a major change happens in the system or you shut the pump down. But I have seen pumps on systems that will pass the air with a surge of there own. You can have this scenario when ever there is the possibility of developing an air pocket on the suction side of the pump. So if this is what is happening it might be because of a vortex in the suction line or it could be because you have an eccentric reducer in upside-down or some other point to trap air in your system. There is not enough information to pin point for sure what is causing the problem but the aforementioned is a possibility. If you can shut the pump off briefly when the flow drops off and it does come back when you restart the pump air is most likely the problem. Also 20’ of NPSHA must be under ideal conditions. If the water is near boiling all you really have is vertical head minus losses plus velocity head.

Regards checman

 

[hbw]

Most likely cause is vapor in the suction line. Make sure that suction line is completely self-venting into the pump suction. Pipe reducers should be flat on top.

 

[25362]

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 .

 

[Artisi]

You say there is a surge in vacuum every couple of hours, does the pressure reduce (higher vacuumn) or does it increase (lower vacuum), and what is the magnitude of difference?


Naresuan University
Phitsanulok
Thailand

 

[dogbertcountry2]

The "normal" operating pressure is about 3 inches of water column (vacuuum) above the liquid level in the feed tank. The surge is a pressure of about 10 inches of water column (vacuum) above the liquid level in the feed tank.

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"???

 

[hbw]

"Self-venting into the pump" means that the suction piping has no high point pockets that can accumulate and hold vapor. It means that vapor in the suction line will migrate upward to the pump inlet and not be captured in a pocket or branch of the suction line.

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.

 

[Artisi]

You have some conflicting info in your posts, first up you say 175 gpm and in the same msg you say 250gpm.

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

 

[dogbertcountry2]

Sorry for the confusion. The pump circulates a total of 250 gpm but it splits to 3 different locations - 175 gpm to the packing, 75 gpm to a quench spray, and the balance as a transfer out of the system (feed to a process).

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"?

 

[Jalink]

Dogbertcountry2,

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.

 

[Artisi]

A more open impeller may assist if you actually have an entrained air / gas problem. However, a more open impeller will change the performance of the pump which you need to look at.


Naresuan University
Phitsanulok
Thailand

 

[dogbertcountry2]

Update..... We changed the pump to a more appropriate size pump. I asked for an open impeller, but somehow that got forgotten on the spec. We will be installing during the next outage.

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?