Pump System Trouble

[robs68charger]

So a customer has a filtration system of ours that includes a centrifugal pump. The pump is rated for 500GPM at 70 TDH. The system draws from a basin. The suction is a flooded suction, ie the inlet of the pump and inlet piping are below the waterlevel in the basin. The system loses about 8 psi through the filtration, when clean, and the water is discharged back into the basin throw eductors. The eductors are rated at 20 psi. Neglecting the minor losses in the pipe we are looking at 28 psi or about 65 feet of head. There is a pressure gauge on the discharge of the pump and it is reading 30 psi, which is 70 feet of head. The problem is the pump sounds like there is gravel in the impeller. There is no air getting into the suction piping. When I start to close the butterfly valve of the discharge side of the system the pump starts to sound better. When it is half way closed the pump sounds as it should. I am at a complete loss at why the pump is not operating correctly. Does anyone have any suggestions? The total number of eductors allow the appropreaite flowrate, and the system suction does not suggest that the pump is being starved.

 

[JohnGP]

If the pump is that noisy at rated flow, it "sounds" like you have cavitation. You need to check available NPSH against the NPSH required by the pump at that flow. This view is reinforced by the fact that it quietens down when flow is reduced.

Cheers,
John

 

[MikeHalloran]

Put a vacuum gage on the pump suction; that should help you figure out what's going on.



Mike Halloran
Pembroke Pines, FL, USA

 

[Artisi]

Sounds like the pump is cavitating as it is running too far out on its performance curve.
This seems to be confirmed by your statements, 1) sounds like gravel in the impeller 2) closing the discharge valve reduces the noise - throttling the discharge increases the total head across the pump, reduces flow rate and lowers NPSHr so that the pump is no longer cavitating.

You need to establish the flowrate.
How accurate is the pressure reading.
What is the suction head ie, the water level over the pump inlet.

I would suggest that the pump is oversized for the duty, you need to establish what the flow and total head for the system under operating conditions should be and then size the pump for this duty. This might only be an impeller diameter change.

 

[itsmoked]

I'm not clear on the filter placement. It sounds like it's in the suction side. This this true? If not, is there any screen or device that could be getting restricted?

Is the system new? Or has this problem appeared over time?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dabluffrat]

What is the flow rate when the valve is fully open and 1/2 closed? You might be trying to run the pump out beyond the end of the curve with the valve wide open. Does discharge pressure drop off when the valve is fully open?

Did you know that 76.4% of all statistics are made up...

 

[hbw]

The pump is cavitating. This is caused by the pumped liquid vaporizing in the impeller. The NPSH available in the system is less than the NPSH required for the pump. Reducing the flowrate reduces the NPSHR (required) by the pump The gravel sound is caused by vapor bubbles (cavities) collapsing against the impeller. This erodes the surface of the impeller vanes and degrades the pump performance.

You should contact the pump manufacture or obtain a pump curve to determine the NPSHR for the pump versus flowrate. Then calculate NPSHA (available)
NPSHA = static head + absolute suction pressure - vapor pressure - suction losses

You can increase the static head or reduce the suction losses or reduce the flowrate or replace the pump with another having lower NPSHR or a combination of these to fix the problem.

 

[dabluffrat]

hbw, it's not necessarily an NPSH issue. There are many reasons for pump cavitation.

The valve and piping maybe over sized for this application and pump. Even with adequate NPSHa a pump won't run beyond 120% - 150% of the impeller design BEP flow.

Closing the discharge valve increases the system head and creates back pressure for the pump to run against.

Did you know that 76.4% of all statistics are made up...

 

[Artisi]

Seems as though Rob has left in his 68 charger.

 

[robs68charger]

Well I got the piping details and did the calculations and actually had to find the delta P from the strainer via testing and found that the customer does need to improve the piping because his NPSHa was below the NPSHr. I told the customer he had redo the piping, the cheapest, or buy a pump with a NPSH that will work with his piping but in the end will probably have to make adjustments to the piping anyways. Now it is up to him and we will see. Thanks for all the input thus far from everyone.