RobsVette
Mechanical
- Apr 15, 2009
- 94
Hi guys, I am looking for some pump specific advice on a condenser water system that we recently installed at a hospital here in NYC.
The system consists of 2 Marley NC8411 cooling towers to cool the condensers of 2 chillers up on the roof of a hospital. The entire system is relegated to the roof, so the condenser water pumps are directly under the cooling tower. There is about 8 to 9' maximum from the water level surface in the cooling tower to the centerline of the pump impellar.
The pump is an 1800 RPM pump sized at 70' and 3000 gpm (condenser water service obviousl.) The NPSHr is 16'. The pump is driven by an ABB ACH550 VFD and has a 75HP motor. When operating at full speed, the pressure gauge on the suction flange of the pump reads about 2.5 psi. We have installed a large 18" suction header to help keep the suction of the pumps flooded and reduce losses to prevent cavitation. The header is on the same elevation as the pump suction so there arent any elbows or bends that could cause unusual flow profiles at the eye of the impellar either.
The situation I am encountering is that we seem to be consistly low in flow and I am wondering if that has anythign to do with the low suction header causing cavitation. The pump doesn't make any noises that would lead me to believe it is cavitating. It is a little louder and occassionaly does make a tick here and there, but nothing like its pumping rocks or anything bad.
I have installed pressure gauges in various parts of the system and so far the drops all seem in line. I lose a couple PSI lifting the check valve (horizontally installed) then very minimal amounts through other devices in the system (strainer, balancing valves, etc...)
I had put an ultrasonic flow meter on it, to confirm and at 60 HZ I had only about 1500gpm of a total 3000gpm.
Now the question becomes why. The only thing I dont have is the motor current at 60hz from the VFD as I dont exactly remembmer the number.
The question is as follows;
Is it possible the low suction head situation could be contributing to the lack of flow? I have always thought that cavitiating pumps tend to cause loss of flow, so now I am a littel worried, especially not being an expert in this part of the field.
My feeling is that the NPSH should be more than adequate, as I have about 9' of vertical height, plus minimal losses from the large suction piping, plus atmospheric pressure contributing as well.
The strange thing about all this is, I am running the pump through the system and the suction and discharge pressures are about 2.5 psi suction and 33.5 psi discharge which lines up to just about 70'. So I have the correct pressure but I'm missing some flow.
I will confirm the flows tomorrow with an ultrasonic meter and verify the motor power draw, but I have a strange feeling this is going to get back to the pump being 1800rpm where perhaps a 1200 rpm pump would have been better served.
Any insight from you guys would be appreciated. If any further information is needed, I can obtain it fairly easily, so feel free to ask for more information if I have missed anything.
The system consists of 2 Marley NC8411 cooling towers to cool the condensers of 2 chillers up on the roof of a hospital. The entire system is relegated to the roof, so the condenser water pumps are directly under the cooling tower. There is about 8 to 9' maximum from the water level surface in the cooling tower to the centerline of the pump impellar.
The pump is an 1800 RPM pump sized at 70' and 3000 gpm (condenser water service obviousl.) The NPSHr is 16'. The pump is driven by an ABB ACH550 VFD and has a 75HP motor. When operating at full speed, the pressure gauge on the suction flange of the pump reads about 2.5 psi. We have installed a large 18" suction header to help keep the suction of the pumps flooded and reduce losses to prevent cavitation. The header is on the same elevation as the pump suction so there arent any elbows or bends that could cause unusual flow profiles at the eye of the impellar either.
The situation I am encountering is that we seem to be consistly low in flow and I am wondering if that has anythign to do with the low suction header causing cavitation. The pump doesn't make any noises that would lead me to believe it is cavitating. It is a little louder and occassionaly does make a tick here and there, but nothing like its pumping rocks or anything bad.
I have installed pressure gauges in various parts of the system and so far the drops all seem in line. I lose a couple PSI lifting the check valve (horizontally installed) then very minimal amounts through other devices in the system (strainer, balancing valves, etc...)
I had put an ultrasonic flow meter on it, to confirm and at 60 HZ I had only about 1500gpm of a total 3000gpm.
Now the question becomes why. The only thing I dont have is the motor current at 60hz from the VFD as I dont exactly remembmer the number.
The question is as follows;
Is it possible the low suction head situation could be contributing to the lack of flow? I have always thought that cavitiating pumps tend to cause loss of flow, so now I am a littel worried, especially not being an expert in this part of the field.
My feeling is that the NPSH should be more than adequate, as I have about 9' of vertical height, plus minimal losses from the large suction piping, plus atmospheric pressure contributing as well.
The strange thing about all this is, I am running the pump through the system and the suction and discharge pressures are about 2.5 psi suction and 33.5 psi discharge which lines up to just about 70'. So I have the correct pressure but I'm missing some flow.
I will confirm the flows tomorrow with an ultrasonic meter and verify the motor power draw, but I have a strange feeling this is going to get back to the pump being 1800rpm where perhaps a 1200 rpm pump would have been better served.
Any insight from you guys would be appreciated. If any further information is needed, I can obtain it fairly easily, so feel free to ask for more information if I have missed anything.
