Need a little help diagnosing an issue with a new pump I installed at the lake to replace an old worn out one. I replaced an old 1/2 HP jet pump + tank combo with a newer 3/4hp + tank combo and for some reason the unit keeps tripping the built in overheat sensor and shutting off.

I've drawn up a diagram of what the install looks like.



The old unit ran fine and the main reason we replaced it was because the fittings were flat worn out or rusted so bad (from age) that we couldn't sufficiently repair the constant leaks.

I would have assumed that a higher horse pump would be a pretty simple install, but no dice.

The main thing that concerns me is the head differentials between the connections. We're on a hill and never had any issues, however with the lake being ridiculously low (it's usually about 5'-6' above the foot valve), I'm thinking the pump is having to work too hard to create enough lift and pressure to send up the hill. Thus tripping the heat sensor on the motor. The motor will cool and then kick on again.

Additionally, the pressure gauge will only get up to 10-15psi, when it should be hitting 30psi minimum. I didn't think to check the pressure of the tank while I was up there, but I would assume it would come already pressurized from the factory. Maybe folly on my part.

At any rate, hopefully it's a simple fix and I'm not screwed by having to wait for the lake level to rise again, whenever that'll be.

TIA!

NPSHr on the pump?

You installed a larger pump. I'm assuming you re-sized your pump cable & other electrical components accordingly for the new demand load?

Ragoo said:

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NPSHr on the pump?

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This.

Just looking at it, you are using a monster amount of the pump capacity on the suction side, with only .25 psi (+/-) of head on the valve, odds are you are getting some cavitation at the impellers because you have 9-1/2 feet of lift before you even get to the pump.

Between that and the comment about upsizing your power cables, thermal overload is in play.

*not an engineer, just a contractor that deals with pumps all the time

This is what I was able to find for the pump.



Without doing any calcs, I would think the low lake level on the suction side is what is causing the motor to cavitate and go into thermal protection. My options on that front are to somehow try to lower the RPMs of the motor or add some length of pvc extension out farther into the lake to try and get more head on the foot valve.

As for the power cable, it's sized properly and on a dedicated 20A breaker.

civilized05 said:

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This is what I was able to find for the pump.

Capacity chart (friction loss not included):
SUCTION 0' 5' 10' 15' 20' 25'
PSI
0 985 955 798 690 591 502
20 945 917 765 652 567 582
30 925 897 749 638 555 472
40 655 635 531 452 393 334
50 370 359 300 255 222 189
USGPH

Without doing any calcs, I would think the low lake level on the suction side is what is causing the motor to cavitate and go into thermal protection. My options on that front are to somehow try to lower the RPMs of the motor or add some length of pvc extension out farther into the lake to try and get more head on the foot valve.

As for the power cable, it's sized properly and on a dedicated 20A breaker.

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extending the foot valve won't do any good. You are normally lifting water 4.5 feet where you are now lifting 9.5 feet. That extra 5 feet is too much for the pump.

You need a pump that can lift at -10 feet as a worst case.

Or you need to greatly reduce the pressure losses down stream of the pump.

This pump is supposed to be able to lift at -25', at least per the specs.

Burcam Model
http://burcam.com/en/products/water_in_pumps_and_systems/jet_pumps/shallow_well_stainless_steel/sw_stainless_steel_jet_pump-3_4hp_115_230v
http://burcam.com/files/Web/Infos/Anglais/506518SS.pdf

civilized05 said:

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This pump is supposed to be able to lift at -25', at least per the specs.

Burcam Model
http://burcam.com/en/products/water_in_pumps_and_systems/jet_pumps/shallow_well_stainless_steel/sw_stainless_steel_jet_pump-3_4hp_115_230v
http://burcam.com/files/Web/Infos/Anglais/506518SS.pdf

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the -25 feet depends on the volume to be pumped and the pressure to overcome downstream.

Does it sound like cavitation when the pump is running?

or get a submersible to eliminate the reduction in capacity on the suction side

schmellba99 said:

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or get a submersible to eliminate the reduction in capacity on the suction side

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a booster to feed the main charge pump.

That would work, though there are complications that can come in with boosters that are not properly sized.

In the OP's scenario, if the lake levels are going to be low for a while, he might want to add a submersible and tank for the main pump to suck out of versus the lake, or size a submersible booster to provide a more flooded suction on the main pump and reduce the loss on the suction side.

Pumps generally are not efficient when you have a lot of suction, which is why in almost all of the designs I see and install the engineers go for a flooded suction as much as possible, and when that's not practical they typically go with as short a suction run as possible and up the HP (vertical turbines are a good example in my world).

NPSH is the biggest factor, and I honestly don't fully understand it. My dad was a card carrying guru when it came to pumps and rotational equipment and he tried to explain it to me once. Pretty sure he got mad when i didn't pick up on every aspect and he gave up.

The submersible booster is what I had in mind. The submersible plus storage tank would require more infrastructure and probably be an eyesore.

Some of the submersibles we installed last year are clogged up with Zebra Mussels on Lake Austin.

You can also enlarge your suction line. You might need to depending on how much the pump flow increased.

NPSHa = Atmospheric pressure + delta elevation - fluid vapor pressure - suction line friction losses

Vapor pressure is pretty low for ambient water. Delta elevation is negative since it's a lift pump. If you have a small suction line, it could easily eat up all your NPsHa. A larger foot valve might also help as it will have lower pressure drop.

Extending your suction line isn't going to help. Your lift is from the water surface to the pump centerline. No matter how deep your foot valve is, the surface to centerline dimension is fixed. In fact going deeper might hurt you since it will add suction friction losses.

Last thing to think about is hard pipe has less loss than flex hose, so consider swapoing to PVC if you're currently using hose.

Check out the drawing I provided. It's already 1.25" PVC.

Would reducing the horsepower or lowering the rpms help?

The motor is going to pull whatever amps it needs to run at design speed and supply the torque needed on the working end. I'm not electrically savvy enough to tell you how to do what you want outside of getting a new motor. It may not realistically be possible with the motor you have either depending on it's design.

I concur that it sounds like your suction is starved. From your sketch and the info in it, you should have around 20' of suction head and be getting around 9 gpm of flow.

Are there a lot of elbows in the suction line or is it fairly straight? Every elbow adds significant lenght to the suction line and will reduce the numbers I listed above.

Is this new suction piping or did you reuse the old pump's suction? Are you sure it's free and clear of sediment and debris?

What happens when you throttle the discharge valve, how does the pump behavior change? If the pump is cavitating which I agree it sounds like, then it should sound like it's pumping rocks. Throttling the discharge will back it up on the curve and allow it to pump without cavitating by reducing NPSHr.

Is there pressure on the discharge system or is it empty / depressured? Pumps can also cavitate if they run off the right end of the curve (too much flow / very low discharge pressure) as the NPSHr rises with flow.

Since you upsized the pump, I think you may still need a larger suction line. For suction piping, short runs likely need to be one line sze larger than the suction nozzle. Long runs may have to be increased more than one line size. Also, there should be a straight run of piping the same diameter as the pump suction directly upstream of he pump.

It may also be possible that your suction line or foot valve are full of trash. This is especially possible if it sits idle for long periods. A bigger pump might have also pulled trash into the suction that the smaller pump couldn't previously.

It does look like a starved suction issue, and that would explain the low discharge pressure, but I'm struggling to see how that would cause the motor to overheat, unless maybe the motor relies on flow for cooling or it's gaining heat from the pump. Manual says the motor is fan cooled, and is rated for continuous duty.

Can you tell if the motor or the pump seems to be generating more heat?

Are you sure it's shutting down on overheating? Also cycles off/on based on tank pressure and motor load.

Your suction piping is causing another 3-5 feet of friction losses (assuming you don't have more than 2-3 90s in there) on top of the 9.5 feet of head difference from the water surface to the pump suction. Shortening that horizontal run or increasing the pipe diameter to 1.5" or 2" would reduce that. But you still seem to be inside the manufacturer's 25 foot suction claim.

It looks like your flow rating is closer to 7-9 gpm based on your suction and discharge pressures.


Have you confirmed you have adequate power supply (voltage, wire/cable gage)? Inadequate voltage at the motor can cause high motor amps which would cause high heat.

Are you running 120V or 240V? Is that toggled correctly on the motor?

I'm wondering if your pressure switch is set correctly. Here are the directions from the manual. Did you make any adjustments on it? If that is not set correctly for your application, it could cause it to start/stop too frequently or stop before it builds adequate discharge pressure.

Quote:

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Pressure switch setting (start/stop 20/40 or 30/50) has been made in factory. An adjustment may be done to give other operating pressures.
Adjustment or modification of start/stop setting of pressure switch have to be done carefully.
Turn adjustment nut half turn at a time.
Turn nut 1 clockwise to raise start and stop pressure setting. Never turn nut 2. This will change the 20 PSI range between start and stop pressure and may damage your tank'sbladder or modify the efficiency of your water system. Check system operation after each adjustment.

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Your pressure tank pre-charge setting shouldn't matter too much unless it's higher than your pressure switch start pressure, but then your pump would never want to start running.

Rethought the problem while I took a shower and I think I got the answer now. You don't have enough precharge pressure in your tank, so you have a high flow, high amp, high motor temp, low differential pressure issue.


Set your pressure switch to kick on at 30 psi and off at 50 psi, and charge your tank to 28 psi.

The switch settings should be at 30/50 psi from the factory. Haven't messed with those.

The one variable that I didn't check was to see if the tank was precharged to 28psi from the factory. I'll do that next time I'm up there.

If you have a way to measure amps that would help troubleshoot a lot. Or if not you could disconnect the discharge line and let it flow to the ground, ideally with a valve on the end where you could control backpressure. This could help you determine if it's a low suction pressure issue or high flow issue.

Low discharge restriction/backpressure->high flow->high amps->high motor heat
Fix: increase tank pre-charge pressure

Low discharge restriction/backpressure->high flow->high suction pressure required->insufficient suction pressure->suction cavitation and vaporization->low discharge pressure
Fix: increase tank pre-charge pressure

Long/skinny suction pipe and low water level below pump suction or suction plugging->low suction pressure->suction cavitation/vaporization->low discharge pressure->low flow (and low amps)->pump heat buildup
Fix: shorten suction length, or clear plugging, or increase suction pipe diameter, or remove extra elbows in suction pipe