Throttle Pump Outlet? 5

[mjpetrag]

I currently have a tank water pump that is cavitating. I can throttle the flow by closing the butterfly valve on the discharge, and this stops the cavitation. But I was wondering if there was a more suitable permanent solution (not looking to install an orifice plate since there are 4 pumps in a row that run at different rates). I was thinking of changing the impellar size or its housing.

 

[dcasto]

you certainly need to look at your pump curves and NPSH. Most pumps have a low NPSH as you back up their curve. Slowing the pumps down also lowers the NPSH. Maybe your suction line is too small cause a loss of suction head.

 

[TheBlacksmith]

I'm not sure what 4 pumps in a row means, but if they share a common inlet manifold maybe the pumps are robbing each other. Is this pump further from the tank (others before it)? Does it act up when the others are turned off? Just some thoughts.

 

[mjpetrag]

There are 4 identical pumps that share the same inlet and discharge manifold. Only one is currently running and that pump is cavitating. I am sort of lost right now as to changing the impeller size since I have no pump curve data (there's a chance it may show up still).

 

[BigInch]

Sounds like the low flow scenario is too low for one pump. That pump probably needs to cut-off while the suction supply builds up.

Throttling is acceptable, if you don't have to throttle too much, as long as the throttled flow is still adequate for downstream needs and you can keep above 50% of the pump's BEP flowrate. If the throttled flow where cavitation stops is not adequate, there is little you can do about it except increase suction pressure of that low flow. That would be much better than trimming the impeller. You need a pump with less flow and trimming is more suited to lowering discharge head. Depending on the % flow you need, trimming can be very inefficient. The best way to get less flow w/o throttling is by reducing speed. Any chance of that?

http://virtualpipeline.spaces.msn.com

 

[Valvecrazy]

I have never liked pumps having a common suction. Many times I have had the suction pressure get low enough to draw air from the mechanical seals of the pumps that are not running. It may look like cavitation but, it is actually a suction leak. Valve off the suction to the other pumps and see if the problem goes away. For this reason I always recommend a separate suction line for each pump.

 

[mjpetrag]

The pump runs only at one speed. I was thinking that maybe there could be a blockage in the eye of the impeller, since there is no strainer in this line.

 

[Valvecrazy]

Throttling the output of the pump or slowing the speed of the pump will decrease the NPSHR. Something in the eye of the impeller should have the same effect. The suction is sized large enough for 4 pumps. I don't think this is a NPSH problem. I will still bet you are sucking air in from the mechanical seals of the other pumps. This happens more often than you think.

 

[BigInch]

If he's cavitating as he says, I'd bet its cavitation, not robbing air, ey? I'm convinced its a low supply problem on a system that was built very very big for the present operating scenario. I think the fact that there are four identicle pumps indicates that a very very wide range of flows was to be expected, (its seldom that you must use more than 3) and this time the supply just got too low for even one pump.

Sounds like a tank is just the thing for this installation, then you could stop that pump, use tank flow to feed a continuous process while you wait for supply pressure to build, or otherwise, I hate to say it, try putting one pump on a VSD (or two if you think this condition will last long enough that you'll have to rotate duties) and keep running continuously on low speed.

http://virtualpipeline.spaces.msn.com

 

[Valvecrazy]

If throttling solves the problem, it could be that the head on the discharge line is lower than the pump was designed for. Working to far to the right of the curve can certainly cause cavitation.

 

[JJPellin]

What an incredible range of replies to what appears to be a simple problem. I will offer yet another (although very similar to the last comment by Valvecrazy). If the system was designed for four pumps, I suspect that the suction is adequate. I also suspect that the discharge is adequate for a much higher flow that the single pump is delivering. I would tend to think that this lone pump is simply running against a system that was designed for much more flow (multiple pumps running, not just one). So, with just a single pump, that pump has very little system resistance and is running off the end of the curve. I don't know why you dislike an orifice plate. You could pinch on your valve, determine the DP across the valve that corresponds to smooth pump operation and then size an orifice plate that will produce that same DP. Slowing down the pump could also be a good options, but difficult if it is a motor. Trimming the impeller could work, but if you need a variety of flow rates, it will be difficult to put that material back if you need it later. You can always open up the orifice if you need more flow in the future or if your system resistance increases.

Johnny Pellin

 

[BigInch]

Right! Business is obviously way too slow today.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

I would suggest a study of the hydraulics of the "system" to determine what is going on rather than trying to apply shotgun therapy. That means looking at the single and multiple pump operations in relation to the inlet conditions as well as the discharge conditions including all system losses.

 

[mjpetrag]

Thanks for all the help. I actually just found the pump curve data and it was cavitating since it was running off the curve.

 

[BigInch]

Throttling is acceptable, if you don't have to throttle too much, as long as the throttled flow is still adequate for downstream needs and you can keep above 50% of the pump's BEP flowrate. If the throttled flow where cavitation stops is not adequate, there is little you can do about it except increase suction pressure of that flow. That would be much better than trimming the impeller. You need a pump with less flow and trimming is more suited to lowering discharge head. Depending on the % flow you need, trimming can be very inefficient. The best way to get less flow w/o throttling is by reducing speed.

http://virtualpipeline.spaces.msn.com

 

[itdepends]

What is the pump supplying, what are the flow/pressure requirements of the downstream user(s)?

Without this information it makes it difficult to recommend solutions- although since you've now discovered your on the right hand side of the curve for the pump it's relatively easy to pick out the solution(s) from those offered above.

 

[khardy]

You might look at adding an hydraulic control valve to the discharge to maintain a fixed pressure and keep the pump running on its curve.

http://www.controlvalves.com.

 

[rmw]

Good on JCPellin. You nailed it.

Now, my question is; why not just start a second pump which will half the flow and push the first pump back on its curve and hopefully far enough back so that both pumps will run in a non cavitating mode?

rmw

 

[Valvecrazy]

Hey, that was my second guess but, I think that is what I said as well. I also agree that a control valve would solve the problem. The one I would use is at

http://www.cyclestopvalves.com.

These valves hold a constant outlet pressure but, can also be equipped with pressure sustain pilot that will also maintain a certain preset amount of inlet pressure. This will keep the pump from being able to work to far to the right of the curve, which will eliminate the cavitation.

 

[JJPellin]

RMW,
Starting a second pump might work. But it depends on the system curve. If the downstream system is pretty much wide open with a very flat system curve, starting a second pump might just result in two pumps running with cavitation. If there is no flow control valve downstream, you cannot assume that you will end up with two pumps running at 50% of the original flow rate. But, you will be using up a lot more horsepower and might still have a problem. I would stick with flow control, back-pressure control or simple throttling.


Johnny Pellin