Rputvin
Mechanical
- Oct 31, 2017
- 168
I've got an interesting set of circumstances that is confounding all of the engineers at my company and I'm looking for someone that may have some idea of how to troubleshoot this.
Less than a month old, we ran off at our facility, but don't record flow values, only pressure and current on the motors. Closed loop, running a stamped stainless close-coupled centrifugal pump at ~73 GPM and ~25psi differential pressure. The pump curve for this pressure should be giving us ~120gpm of flow (selected slightly oversized), and we're sitting short of the customer's required flow rate of 80 GPM. The system pressure is almost dead-on the calculated drop for the system.
We're moving city water between 75 and 100°F. Using standard pressure gauges at multiple points in the system, before and after the pump. Using IFM Efector flow meters with proper installation runs, one after the pump discharge before branching, and one on each branch before collecting to the return header, all flow meters downstream equal the discharge meter before the header.
We're on the curve through about 40 GPM, then we start getting deviations and we're sitting 40% or so off the curve. The pump manufacturer built us another pump and ran it off through the entire curve to make sure their stamping and impellers hadn't drifted since the curve was established - their pump matched the curve values, albeit with much more ideal and controlled conditions than what I'm dealing with.
Our first step was to change the impeller to the largest available trim, which improved the numbers slightly to where I'm at now, but still significantly lower than expected flow.
The system is pressurized with a diaphragm expansion tank, we started at 12psi and have increased it to 20psi. There's an air separator installed, which is operating as best as we can tell, it does not have a strainer in it. The make-up line is city water connection regulated to 17 psi. The rest of the system is the customer's piping that I'm short on details on, but should follow a standard flow/pressure drop relationship and not figure into the conundrum.
The pump will make deadhead (shutoff/no flow) pressure. As we open the throttle valve (butterfly valve used for isolation mainly) we see a 1-2psi drop in suction pressure. The motor is drawing near nameplate amperage, suggesting we're to the far right on the curve. Required NPSH is all of 2.5psi
Any ideas as to what might cause a pump to... not pump? We're thinking suction restriction is a possible culprit, but there's nothing there to impede flow. Would a small amount of air in the system wreak this kind of havoc?
Any help is appreciated!
Less than a month old, we ran off at our facility, but don't record flow values, only pressure and current on the motors. Closed loop, running a stamped stainless close-coupled centrifugal pump at ~73 GPM and ~25psi differential pressure. The pump curve for this pressure should be giving us ~120gpm of flow (selected slightly oversized), and we're sitting short of the customer's required flow rate of 80 GPM. The system pressure is almost dead-on the calculated drop for the system.
We're moving city water between 75 and 100°F. Using standard pressure gauges at multiple points in the system, before and after the pump. Using IFM Efector flow meters with proper installation runs, one after the pump discharge before branching, and one on each branch before collecting to the return header, all flow meters downstream equal the discharge meter before the header.
We're on the curve through about 40 GPM, then we start getting deviations and we're sitting 40% or so off the curve. The pump manufacturer built us another pump and ran it off through the entire curve to make sure their stamping and impellers hadn't drifted since the curve was established - their pump matched the curve values, albeit with much more ideal and controlled conditions than what I'm dealing with.
Our first step was to change the impeller to the largest available trim, which improved the numbers slightly to where I'm at now, but still significantly lower than expected flow.
The system is pressurized with a diaphragm expansion tank, we started at 12psi and have increased it to 20psi. There's an air separator installed, which is operating as best as we can tell, it does not have a strainer in it. The make-up line is city water connection regulated to 17 psi. The rest of the system is the customer's piping that I'm short on details on, but should follow a standard flow/pressure drop relationship and not figure into the conundrum.
The pump will make deadhead (shutoff/no flow) pressure. As we open the throttle valve (butterfly valve used for isolation mainly) we see a 1-2psi drop in suction pressure. The motor is drawing near nameplate amperage, suggesting we're to the far right on the curve. Required NPSH is all of 2.5psi
Any ideas as to what might cause a pump to... not pump? We're thinking suction restriction is a possible culprit, but there's nothing there to impede flow. Would a small amount of air in the system wreak this kind of havoc?
Any help is appreciated!
