continuous shutdown of a pump [b]

[danielanarine]

Hi all!

Can anyone tell me the long-term effects of continuous shutdown and restarting of a pump? For example there was one night where the pump stalled four times. I suspect that the NPSHA was too low and I know that causes cavitation. I would, however, like to know what are the effects of the actual continuous stalling and restarting.

Thank you!

 

[friartuck]

a. Motor wear. Generally motors are continuously rated unlike compressors that have a design number of starts per hour.

If the motor is oversized, then it will take a bit more hammer...but some manufacturers don't leave much of a margin.

b. Stalling/NPSH problems cause vibration and eventual shaft/bearing wear.

You need to stop the problem . If low inlet water conditions are the problem, fit a low pressure switch or level indicator to deactivate under these conditions.

Friar Tuck of Sherwood

 

[tstead]

Daniel,

Your major concern with repeated starts and stops is with the motor because each start heats up the motor just a little more than usual. Make sure you keep the starts/hour within the motor mfgr's recommendation.

Of course, other issues abound like what friar mentioned. For example, the pump should not "stall" so you need to figure out why it is doing that and correct it. Otherwise, you could damage the pump, just like friar said, but it won't be due to that frequent starts - it'll be due to the degraded condition that causes the pump to "stall" (I'm not sure what you mean by that, can you please elaborate?).

This of course depends on your pumped fluid. Are you pumping clean water, a slurry, a shear-sensitive fluid, or other non-newtonian fluid that can cause problems when the fluid is at rest?

Tim S.

 

[tonyh]

What do you mean by "stall"? If the motor is stopping of its own accord and going out on motor protection (thermal overload), then your problem is unlikely to just be cavitation. You must be taking far more power than you should be, or something is jamming (and that will do damage)
Alternatively, if you are losing flow and then tripping the pump for that reason, it would seem likely that you are vapour locking your pumps. This can cause problems for the seal. Actually stopping and starting (in a controlled manner) a number of times a day will have some effect on pump and motor life but provided that the motor can cool down before restart and the system is reasonably robiust that wouldn't worry me too much.