Parallel Pumps

[DerekLJ]

Just i quick question, i would like opinions.

I have 3 IDENTICAL centrifugal pumps running in parallel. The only difference is in the driver; 2 have 30HP electric motors, the other has a 40 HP electric motor.The rated(required) power is 25.3 HP each.

Does anyone see a problem in this configuration during normal flow/start-up/and low flow?

 

[dickon17]

No common problems. However, double check each pump for identical impeller diameters (particularly the 40 hp unit). Also, look at each pump curve to ensure discharge head pressure falls constantly from zero flow to max flow. Pump curves should never rise or be horizontal at any flow position.

 

[BigInch]

As long as you can keep and maintain the performance of all pumps exactly equal, no problems. If the performance of one pump varies from the others, due to impeller ware for example, you may have some flow redistributions.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

As per the same question in your earlier posting,

No problems.

 

[JJPellin]

Between the two posts, most of the potential issues have been covered. However, one more occurs to me. Since the larger motor can provide more torque during start-up, you should check to make sure that the couplings can take the higher torque. We would normally rate a coupling with a 1.5 to 2.0 service factor for the maximum power of the driver. The pump with the larger driver could see higher torque during start-up (especially in a viscous fluid). Be sure that the coupling can take it. Thanks to BigInch for reminding us of the potential for higher start-up torque.

Johnny Pellin

 

[BigInch]

I learned about torque the hard way. After flying small HP airplanes, I moved up to a big one, pushed the throttle in all the way and found myself making a left turn on the runway. Torque effects never left my mind after that. Afterwards I also found out it affected many late Korean War pilots moving up from Hellcats to the F8F Bearcat that had 2,100 HP with a water-methanol injection short-term boost power to 1,790 kW (2,400 HP). Several made unintentional upside down carrier landings instead of the go-around they wanted to do when they went for full power after the wave-off.

http://virtualpipeline.spaces.msn.com

 

[Valvecrazy]

The larger motor may also not develop as much rotor slip as the smaller motors. This would cause the pump with the larger motor to develop a few extra RPM's. Since head increases with the square of the speed, this pump might produce slightly more than the others. If extra pumps are not shut down during low flow conditions, slightly extra head from the pump with the larger motor, might cause a flow problem for the other pumps.

 

[BigInch]

There is always a possible flow stability issue with two or more pumps running if controlled only by the on switches.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

Can't see any problem of one unit running a few RPM above the other 2 units, it just means the other 2 units will move left up the curve a little - and in any case with any multi-unit installations it is unlikely that all units will run at exactly the same speed and that the individual pumps will have exactly performance.

 

[prochem]

DerekLJ,

Do you have enough details for the pumps e.g. curves and driver detail?Is it a new installation?what is the arrangement for emergency shut-down?what is the service?Provide sufficient detail about your system.

I believe you may have VFD drivers for your pumps.

Regards,
prochem