The need for Standby pumps when VSD is avialable

[EnOm]

Hi
I was wondering whether standby pumps are required when the lead pump is coupled with a Variable Speed Drive (Sometimes also called Variable Frequency Drive).
Say I have a design flow rate of 12.5 m³/hr at 30 m Head (55 GPM at 98.43 ft).
There is a pump with a Variable Speeed Drive that can provide that flow at the required head along with a reasonable efficiency.
Would there be a benefit from splitting that flow over 3 pumps (33% each) with Approx 4.17 m³/hr at 30m Head, also each coupled with VSD?
Or since I have VSD would it be better to go with a (one duty and one alternate) arrangement? As in the pumps take turns in serving the demand.

In Summary, which is the overall better of the two arrangements:

[ul]
[li]3 pumps 4.17 m³/hr at 30m (18.4 GPM at 98.43 ft) each c/w VSD (1nos duty and 2nos standby)[/li]
[li]2 pumps 12.5 m³/hr at 30 m Head (55 GPM at 98.43 ft) c/w VSD (1nos duty and 1nos alternate)[/li]
[/ul]

The application is a residential booster pump set for a 4 storey apartment building which is required to maintain constant pressure in the network.

Best Regards

 

[BigInch]

If you use 3 pumps, each with 1/3 of the required system flow capacity, Q_t , it may be possible to do away with the VFDs entirely, but it depends on your exact system curve and the head required at 1/3 Q_t, that pump will probably be able to pump within 1/3* Q_t-25% to Q_t+10%. The attached spreadsheet calculates flow ranges at reasonable efficiencies for 3 pumps. You'll be able to cover 25 to 100% of your system flow at reasonable efficiencies using only a control valve. Granted, there's some holes in the range at 40% and 70%, but that just means that the efficiency might be just a little bit lower out than you might want idealy, but probably not too far out to make it any more expensive than using a VFD.

With 2 pumps there will be more holes in the range, and lesser reliability.

There's a lot of reliability, standby and capacity games you can play. You might go for 3 pumps, 6 m3/h each, 2 running, one on standby, if reliability is critical. That'll give you 12 pretty easily all the time.

If you can do with partial flow when one pump is down for maintenance, 3 pumps, 4 m3/h ea, 1 on standby will give you reasonably good reliability for an 8 m3/h flow, but a bit less for 12 m3/h.

Independent events are seldomly independent.

 

[Artisi]

Best advice I can give you is to go to

http://www.grundfos.com

they have a large array of pumps and systems that are designed for the application you have.

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.)

 

[stanier]

Also Lowera pumps from ITT

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---B.B. King

http://waterhammer.hopout.com.au/