Flow to rotate impeller backwardly

[shahyar]

How can I get the flow needed, from discharge to suction, to rotate pump backwardly?
(I am going to size a RO for pump warm up)

Thanks

 

[MikeHalloran]

Huh? The pump needs warming up? Why?

Getting a (centrifugal) pump to run backwards is pretty easy; just pump stuff through it backwards.

Maybe you need to restate your question using a few more words.







Mike Halloran
Pembroke Pines, FL, USA

 

[lilliput1]

It is not advisable to make the pump rotate backwards because if switched to start, it will draw more than its rated full load amps and trip the fuse or circuit breaker. The check valve would prevent fluid from running in the opposite direction.

 

[MikeHalloran]

I want to be clear on this; I said it was easy; I didn't say it was a good idea.

In addition to the problems it can cause for an electrically driven pump (not a fact in evidence so far), it can damage certain kinds of seals.

I have also associated it with an overheated bearing, but the bearing may have had other problems.

Anyway, centrifugal pumps are typically fitted with check valves and/or anti- reversing ratchets to prevent them from being driven backwards, so I too am really curious to hear why anyone would want to do it.



Mike Halloran
Pembroke Pines, FL, USA

 

[sprintcar]

Running any centrifugal pump backwards by driving the impeller with reverse flow requires special design considerations - like locking the impeller on the shaft. Most are just threaded on and can back off, causing a number of problems with personal injury potential.
Consult your pump vendor before you go backasswards.

"If A equals success, then the formula is: A = X + Y + Z, X is work. Y is play. Z is keep your mouth shut."
-- by Albert Einstein

 

[Zoobie]

sprintcar makes a very good point. I have personally seen the damage caused by reverse flow through a pump that was not meant to handle it (unthreading off the shaft). Fortunately the only thing hurt was the pump....but boy was it a mess inside.

 

[shahyar]

Guys!
I am not gonna back rotate impeller, I like to know what the max flow is for NOT having back rotation.

Thanks for your replies.

 

[MikeHalloran]

Ah.

Pretty close to zero, unless there's friction in the shaft.

Best practice where backflow is a possibility is a backstop ratchet _and_ a check valve.



Mike Halloran
Pembroke Pines, FL, USA

 

[StoneCold]

Shahyar
Why not run the discharge from the online pump through the RO and to the suction of the offline pump. Then is is running forwards and there are no doubts.

StoneCold

 

[JJPellin]

What you are describing is a common practice in our oil refinery. We routinely drill a hole in the flapper of the check valve to allow back flow to keep the pump hot. Since this is normally done in heavy oil pumps, we do not tend to see backwards rotation. As far as sizing the hole (orifice), it is not based on the max flow that can be passed without reverse rotation. It is based on the flow needed to keep the pump hot enough. This tends to be provided by the pump manufacture. It is one of those complicated calculations that tend to produce the same result over and over. In the smaller pumps (200 HP or less) it is generally about 1/4" diameter. In larger pumps (200 HP +) it is sometimes as large as 3/8" diameter. The exact size is not super critical. I would recommend that you drill a 1/4" hole and then verify that the heat up rate is adequate and no backwards rotation occurs. You can always open the hole up if you don't get enough heat.

 

[Artisi]

"Running any centrifugal pump backwards by driving the impeller with reverse flow requires special design considerations - like locking the impeller on the shaft. Most are just threaded on and can back off, causing a number of problems with personal injury potential.
Consult your pump vendor before you go backasswards. "

A pump running backwards form reverse flow will not unscrew the impeller from the shaft as the pump and motor rotate as a unit, if it were a diesel drive unit or similar where the drive unit cannot or is not permitted to rotate in reverse - there is not sufficient torque developed to "unscrew the impeller".
However, running backwards from reverse flow is an entirely different matter than from being driven backwards by starting the pump in the wrong direction with a screw on impeller, this usually ends up in a disaster.

Naresuan University
Phitsanulok
Thailand

 

[vanstoja]

The reverse flow breakaway torque of the impeller shaft is usually the governing factor for reverse flow breakaway of a centrifugal pump. Usually, the shaft bearing breakaway friction controls this and it will be quite low for a shaft supported by rolling element bearings. With a fluid film thrust bearing, breakaway torque can be much higher particularly if the pumpshaft is vertical which adds the rotor weight to the bearing surface friction. To determine centrifugal pump reverse flow breakaway and reverse rotation speeds it usually requires Complete Pump Characteristic (CPC)data which has been reported for only a limited number of pump specific speeds and may necessitate interpolation of the available data for your particular specific speed. Sometimes it is allowable to have a reverse flow-driven reverse rotational speed up to or even greater than the rated foward flow speed but this depends on particular design features such as lekage seal geometry.
I agree generally with the previous discussion of swing check valve seat bypass orifices for keeping warm an idle pump in a multipump flow system. We used a bypass orifice in the check valve body rather than through the disk for this purpose. The CPC data would not be useful for this kind of reverse flow since the CPC data would only apply to a stuck-open check valve where the reverse flowrate could be quite high like 60% of forward rated flow.

 

[lilliput1]

Would insulating the pump work in keeping it warm? Insulation could be flexible removable cover type to allow servicing.

 

[shahyar]

Thanks, specially JJPellin.
Just to let you know, I found a guideline for warm up line sizing. It is in Chemical Engineerig Magazine Nan 1995 Page 89.

 

[EdStainless]

I used to face the issue of unintended reverse flow through centrifugal pumps. We actually had a check in our VFDs to see if there was a back EMF being generated by the motor. If there was you couldn't restart. If hte pump/motor are windmilling slowly then it doesn't hurt to soft start it. Just be careful of the starting torque.

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