Rotating in reverse direction

[sowhatso]

Assuming that the pump is OFF, and the check valve is leaking and the water is comming baack to the pump causing a reverse rotation. what is the bad effect of this rotation if the pump is OFF, and no one run the pump during the reverse rotation.

Thanks for your input

 

[Artisi]

Generally there is no problem if the pump runs backwards from reverse flow - however, if fitted with mechanical seal/s there could be a problem as some seals are designed to run in one direction only - this you will have to discuss with the seal manufacturer and you must ensure that there is no attempt to start the pump while it is running in the reverse direction.

 

[bingopin]

Depending on the amount of leakage, be concerned about the impeller coming unscrewed, if the pump is so constructed. A serious concern is the potential to overspeed the pump and driver. Seen this a few times and it ain't pretty.

 

[21121956]

Hello everybody:

If the pump is the "lead pump" and starts periodically, when the moment arrives for returning to work, suddenly the abnormal rotation will be inverted (to the normal one) and it is here when, with this repetitive action that the mechanical seal of the pump begins to be damaged.

I have experienced a lot of these problems with submersible pumps and, the remedy has been to solve properly the leakage through the check valve.

Another associated problem with the leakage in the check valve is the risk that the pump starts with a voided discharge pipe and, you will have to add to your problems the consequences that this involves.

 

[MikeHalloran]

The potential seal damage and the potential for the impeller unscrewing are certainly cause for concern.

As long as the motor is guaranteed to remain OFF, those might be manageable risks. But what happens when you need to start the pump to put the fluid back where it was? Is there a means provided to stop the reverse rotation? Will everyone bother, all the time?

Starting a motor that's already running backwards is pretty much guaranteed to cause severe electrical problems.

By way of illustration, I have one data point: "Plugging" a small 1/15HP 115VAC PSC motor will blow a 20A breaker right out of the panel. I don't know if the breaker exploded before launch, or on impact with the floor. It made a surprising amount of noise.



Mike Halloran
Pembroke Pines, FL, USA

 

[JJPellin]

Actually, a screwed impeller should not unscrew itself in this case. If the motor is wired up for reverse rotation, the torque from the motor will unscrew the impeller. But spinning in reverse from backwards flow results in the same torque direction as normal rotation. The concern about starting up the motor while spinning backwards is very real. We recently failed a coupling with more than a 2.0 service factor. I believe this was the cause of the failure.

Johnny Pellin

 

[ScottyUK]

Starting big motors with the machine running backward usually results in either the motor protection relay operating, or the motor going off site for a rewind. We came close to the latter recently on a circ pump: a fair bit of smoke and some quite disturbing noise from the motor, then the protection relay tripped the breaker and ended the episode before things got any worse. That was on a 1750HP machine; not something we can afford to rewind frequently!


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Sometimes I only open my mouth to swap feet...

 

[zeusfaber]

There's a few pumps out there where reverse rotation will rapidly write off the thrust bearing.

A.

 

[GPRnD]

Depending on the specific speed of the pump, the maximum speed in reverse rotation can be significantly higher than the normal running speed (up to 180% in some cases).

This could cause the pump to run at or through a rotor resonant frequency. This in turn could result in damage to the rotating element.

One other consideration that has not been mentioned is that some bearing lubrication systems do not function correctly in reverse. This can be checked with the manufacturer.

 

[jnmaney]

I have a system i want to get some advice on. Large milling machine, hyd system = counterbalance circuit : 2 counterbalance cylinders, pilot operated check valve, pressure reducing valve and a nitrogen charged accumulator. pressure line going out to 1 other area of machine. My experience with this system is when hydraulics are shut off pump pressure immediatly drops to zero and counterbalance gage does not drop, holding system pressure at the check valve. What I have on this machine is pressure is maintained at the pump for a couple seconds after pump is shut off, then a noise like fluid rushing back reversing the pump. Hyd gage drops to zero. C/bal gage also drops to zero, then quickly return's to system pressure. Is a brand new machine, have replaced the pilot operated check valve and the pressure reducing valve. Initially with the original components the c/bal pressure was slow to creep back up to system pressure after shut down, now with new valves = quicker response to system pressure.

 

[vpl]

jnmaney

Please post this as a new question. You will get better responses that way.

Thanks

Patricia Lougheed

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