Retrofit High Service Pumps with VFDs

[djcbgn]

Hello Everyone,

We are installing VFDs on our High Service Pump Station pumps at the WTP. There are currently 3 large (duty pt.:6,000 gpm @ 360' TDH)and 2 small(3,000 gpm @ 360' TDH) constant speed split case centrifugal pumps. The pumps have been running out on the curve causing excessive maintenance and repairs due to cavitation. Therefore VFDs are being installed to run the pumps closer to the BEP, reducing costly repairs and energy costs. VFD will be initially installed on 2 large pumps and 1 small pump. I need to develop a functional description for operating these pumps. This project was developed by another engineer at our water system who has since left and the VFD installation is almost complete so there are no other options to consider for the original problem(like impeller trimming or control valves).


WTP staff prefers to operate the pumps based on elevated storage tank levels versus pressure or flow. I am going along with this for now. With varying demands, typical scenarios will be one small pump running, one large pump running, one large and one small pump running, and possibly two large pumps running. All these scenarios will have pumps running at less than full speed (still need to work this out, though). The tricky part is running a small and large pump together in parallel(could have the large pump on VFD and one small constant speed pump combination). I see almost endless combinations. Any suggestions on speed control using tank levels and how to approach the varying pump combinations.

Thanks in advance.

 

[BigInch]

The tricky part is running the pumps at lower speeds and getting the discharge head you need to get up to the tower tank. There may be a case that if you buy pumps with the capacity to deliver your head at low speed, at high flows they will overpressure your pipeline.

Get some discharge pressure control valves and you'll probably be a lot happier.

I hate Windowz 8!!!!

 

[LittleInch]

By the sound of it you're going to need a decent PLC controller looking at the base control point (tank level) then also considering pressure and flow from each pump and it total to vary the speed output up to a maximum value chosen by you and a minimum value chosen by you. When the tank levels keep falling and the pump is at its new max speed, then start another one. Whether this is auto or human controller is up to you.

When you have two pumps running this is where you need to pre-determine the flow rate at which you switch between small pump, large pump, small + large, large + large and large + large + small. Then the system switches from one mode to the other and controls on level by varying the speed of the pump.

Not too hard, but you need to work out the flows and pressures and then write it all down and a controls engineer then programs the PLC accordingly.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

Don't forget to get new motors specifically rated for use with VFDs.

I hate Windowz 8!!!!

 

[Artisi]

djcbgn; How about posting in one forum - not 2.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[nornina10]

Hi djcbgn

How about following simplified functional description for controls. Your objective was to run the pump at or near BEP

Elevated Tower Level -100 %full (FL)- All pumps off
1.
ETL- (FL - 2 ft)- Start small pump and run at a 80% full speed and a flow corresponding to BEP. If the elevated tower level starts to increase, then small pump will be able to fill the tank and shut off. When to start the small pump( i.e 2 ft below full level or some other value) depends on how often you want to start the small pump.
2.
ETL- (FL-2.5ft)- Increase the speed of small pump to 100%.
3.
ETL- (FL-3 ft)- Start the one large pump while the small pump is running at 100% speed. Run the large pump at a speed so that it delivers a flow comparable to the smaller pump (3000 gpm). If the larger pump is running below 60% full speed (36 HZ)then I would not run smaller pump and a larger pump at the same time.
4.
ETL (FL-3.5 ft)- Stop the smaller pump and ramp the speed of the larger pump in stages to see when you start gaining level in the elevated tower. Maintain the speed of the larger pump at that speed until ETL is 100% full and stop the pump.
5.
ETL (FL-5 ft)- Start the second large pump and run both large pumps at same speed and adjust their speed until you notice ETL starts to increase. Maintain that speed until ETL is 100% full and then stop both pumps.

6. Repeat the steps 1 through 5 for next cycle.