centrifugal pump low discharge pressure implications

[timbones]

First, some background...

I have a closed loop cooling water system in which the pressure losses were over estimated. The circulation pump is a 100 hp centrifugal with a 14.5" impellor. The design flow rate is 1300 gpm with a TDH of 180 feet. When the pumps were started up the TDH was about 90 feet and the flow meter was reading totally off scale and off the pump curve (in excess of 2000 gpm). The plant manager was concerned that the high flow rates would lead to erosion problems in the exchangers and piping so some orifice plates were installed to boost the TDH and knock the flow rates down. The system now runs near the design conditions.

I would like to trim the pump impellor down to 13" from 14.5" to improve the efficiency of the system. If I leave in the current orifice plates or put in slightly larger orifices, I should be fine in terms of getting sufficient flow. If I completely remove the plates, I estimate the TDH will drop to 60 feet and the flow increase to 1600 gpm. This point is somewhat off the pump curve but I am not sure what that means. Will I run into problems with stable operation of the pump at such a low discharge pressure? Are there implications to the life of my pump? Will I draw more current through my motor than if I left the plates in (making it almost pointless to remove them)? Suggestions?

 

[scalleke]

Note that when you trim the impeller you will not follow the affinity laws 100% as there is a large trim of the impeller this will make that there is recirculation at the discharge side near the volute area.

Is there a pressure reducing tollerance at the OD of the impeller. You will need to make sure that you respect the construction tollerances of the pump. Maybe call the manufacturer and see if he can help you with the trim. If he does that you will maintain warranty.

An advantage of orrifice plates is that they create a nice intersection. between the pump curve and the system curve. removing the orrificces might indeed cause more unstable operation.

By trimming the impeller the bearing load will change. Question is whether the bearing can cope with the change. Here again ask the manufacturer.

Best regards.

Scalleke

 

[timbones]

I don't have any concern with actually trimming the impellor. The pump I have can be ordered with impellors between 13" and 16.75" in diameter. My concern is stable operation of my pump, its electric motor and my system at high flows and very low differential head.

tim