Mislabeled submersible pump?

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brantmacga

brantmacga

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I recently installed a VFD for a 20HP submersible pump used for irrigation.

The customer added a backup 20HP pump last week, so we installed a double-throw switch ahead of the VFD to connect both.

The new pump is causing an overload shutdown on the drive.

Existing 20HP pump pulls 55A. The new pump pulls 66A. The box clearly says it’s a 20HP pump, and per the manufacture chart should pull 54.3A.

Could the FLA vary that much, or is it likely mislabeled?

Per the same chart the 25HP has an FLA of 65A. Just by the numbers I think they installed a 25HP.


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brantmacga said:
I recently installed a VFD for a 20HP submersible pump used for irrigation.

The customer added a backup 20HP pump last week, so we installed a double-throw switch ahead of the VFD to connect both.

The new pump is causing an overload shutdown on the drive.

Existing 20HP pump pulls 55A. The new pump pulls 66A. The box clearly says it’s a 20HP pump, and per the manufacture chart should pull 54.3A.

Could the FLA vary that much, or is it likely mislabeled?

Per the same chart the 25HP has an FLA of 65A. Just by the numbers I think they installed a 25HP.


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Just a thought are the motors physically the same size?
 
Do you have same flow rate, pressure and amount of lift for both pumps? Vary any of those and the motor shaft sees a different load. Difference in pump speed or impeller design can change flow or pressure when everything else is the same.

ADD:

In fact a restriction in one output line that goes unnoticed will lessen flow and will have lesser load on that pump with everything else being the same.
 
ptonsparky said:
Submersibles often run into SFA. Is the pump running open discharge? Worst case scenario esp if it happens be a bit further down the hole.
Click to expand...

It is ... they haven’t plumbed it into existing system yet. But so was the existing motor when I installed the Vfd.


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kwired said:
Do you have same flow rate, pressure and amount of lift for both pumps? Vary any of those and the motor shaft sees a different load. Difference in pump speed or impeller design can change flow or pressure when everything else is the same.

ADD:

In fact a restriction in one output line that goes unnoticed will lessen flow and will have lesser load on that pump with everything else being the same.
Click to expand...

+1 on this. try partially closing a discharge valve on the pump that's going into overload and see if the amps drop. There's one or two smaller pumps that engineering design got wrong at my plant and we ended up just throttling the discharge valve. Not ideal but a cheap and easy solution.
 
Saturn_Europa said:
+1 on this. try partially closing a discharge valve on the pump that's going into overload and see if the amps drop. There's one or two smaller pumps that engineering design got wrong at my plant and we ended up just throttling the discharge valve. Not ideal but a cheap and easy solution.
Click to expand...

I’ll try that. Thanks


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drcampbell said:
The depth of the water, not the depth of the pump, determines the pressure difference across the pump.
(along with other factors that are presumably constant)
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By that I assume you mean the amount of vertical rise one must pump the water? One can put the submersible deeper into the well but it gets loaded pretty much the same as the rise above the water level in the well is what makes a difference in pumping load.

Now if there are two different wells and one has different "draw down" characteristics that can have some impact in this aspect of the installation.
 
So are these two pumps discharging into a common header/pipe? If so, are there check valves on each one? My thinking is that if they are flowing into a common pipe and they didn’t add check valves, that second pump is now discharging not only into the pipe, but it’s flow is now going back into the source through the other pump, so it’s more like open channel flow from the pump’s standpoint.

When they had only one pump they would not have needed a check valve. I can see a scenario where they added the second pump later and maybe thought of putting a check valve on it, but they didn’t pull pump one and add one there. So the back flow only happens to pump 2. I’ve seen that before, more than once.

Even if they did add check valves to both, the one on the first pump might be stuck open.
 
Jraef said:
So are these two pumps discharging into a common header/pipe? If so, are there check valves on each one? My thinking is that if they are flowing into a common pipe and they didn’t add check valves, that second pump is now discharging not only into the pipe, but it’s flow is now going back into the source through the other pump, so it’s more like open channel flow from the pump’s standpoint.

When they had only one pump they would not have needed a check valve. I can see a scenario where they added the second pump later and maybe thought of putting a check valve on it, but they didn’t pull pump one and add one there. So the back flow only happens to pump 2. I’ve seen that before, more than once.

Even if they did add check valves to both, the one on the first pump might be stuck open.
Click to expand...

The scenario of no check valve would have to be an OLD installation. Maybe older than my first born. Stuck open would be a very good possibility.
 
ptonsparky said:
The scenario of no check valve would have to be an OLD installation. Maybe older than my first born. Stuck open would be a very good possibility.
Click to expand...
That might depend on how system is set up, what the rules are and who is there to enforce them.

I still see a lot of pasture pumps (to fill livestock water tanks) that only pump on demand through an open ended pipe that dumps into the tank. No check valve on those pumps as they want water to drain back down so the pipe doesn't freeze in cold weather when not in use. Those may or may not be legal - IDK, but there are still many like that around. The fact it has an air gap and can't siphon tank water back down the well possibly makes it legal though.

Field irrigation they also don't want the pipe to freeze when not in use so no check valve at the pump but is a check valve as about the first thing above ground, this prevents water in the rest of the system from flowing back into the well. Such water could have had fertilizer or pesticides injected into it and they don't want that going back down the well. The rest of above ground piping can still be drained to prevent freezing but can not drain back down the well.

I'm sure you are well aware of most of this, just presenting it for others not so familiar with this kind of system.
 
brantmacga said:
I recently installed a VFD for a 20HP submersible pump used for irrigation.

The customer added a backup 20HP pump last week, so we installed a double-throw switch ahead of the VFD to connect both.

The new pump is causing an overload shutdown on the drive.

Existing 20HP pump pulls 55A. The new pump pulls 66A. The box clearly says it’s a 20HP pump, and per the manufacture chart should pull 54.3A.

Could the FLA vary that much, or is it likely mislabeled?

Per the same chart the 25HP has an FLA of 65A. Just by the numbers I think they installed a 25HP.


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Click to expand...
Keep in mind he never said it was a well pump or underground. Having two pumps down hole is very unusual, so I took this to mean it was a submersible pond/canal pump. With an air gap discharge into an irrigation ditch, you don’t need a check valve for one pump (at least not around here). But as soon as you add the 2nd pump going into a common outlet pipe, you need them on both.
 
Jraef said:
Keep in mind he never said it was a well pump or underground. Having two pumps down hole is very unusual, so I took this to mean it was a submersible pond/canal pump. With an air gap discharge into an irrigation ditch, you don’t need a check valve for one pump (at least not around here). But as soon as you add the 2nd pump going into a common outlet pipe, you need them on both.
Click to expand...
I immediately think of of our aquifer when hearing submersible. Open discharge on a submersible that is pumping against virtually no head pressure would cause grief.
 
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