Pump System Trouble
Pump System Trouble
(OP)
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.





RE: Pump System Trouble
Cheers,
John
RE: Pump System Trouble
Mike Halloran
Pembroke Pines, FL, USA
RE: Pump System Trouble
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.
RE: Pump System Trouble
Is the system new? Or has this problem appeared over time?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Pump System Trouble
Did you know that 76.4% of all statistics are made up...
RE: Pump System Trouble
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.
RE: Pump System Trouble
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...
RE: Pump System Trouble
RE: Pump System Trouble