Doubt - VSF or two pumps?

[EngMecBra]

Hi,

Here I have a pump service that shall operate in two different flows: 150 m3/h or 600 m3/h.
I'd like to know which option (probably) is cheaper: only one pump with VSF or two pumps (one pump for 150 m3/h and another pump for 600 m3/h).

Thanks.

Rodrigo Alves.

 

[EngMecBra]

Sorry... Consider VSD (Variable Speed Driver), not VSF.

 

[GHartmann]

You also need to look at your system resistance line and see what the required head is for each flow.

If you have a system where the required head is mostly static head and final discharge pressure (greatly above suction pressure) then you probably can't achieve the head you need at the lower speeds.

See this thread about VSD:

http://www.eng-tips.com/viewthread.cfm?qid=328365

The presentation linked in one of the posts, gives a good overview of when to use VFD.

 

[BigInch]

One 600 pump is not going to work so well at 25% of max capacity without some recirculation.
With a VSD running at 25% speed, you should not need more than 0.25^2 = 6% of your head at the 600 mch flowrate. In other words, a VSD running at less than 50% of rated pump speed is not a good idea at all.


Acutally you must look very closely at how much time you will spend operating at each flowrate to determine the best answer to this problem. If you will be only 10% time at 150 cmh, I'd buy one pump, no VFD and install a control valve. If you will be 50% time at each flowrate or more, I'd say 2 pumps, 1 X 150 and 1 x 450 and run in parallel when necessary. See what I mean?


"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[GHartmann]

Be careful if you select two pumps with different capacities in evaluating their head curves.

If a good portion of your head is friction loss, then the pump flows are not necessarily additive.

The higher flow increases the head and makes both pumps run up their curves.

http://www.mcnallyinstitute.com/15-html/15-01.htm

Good luck!

 

[bimr]

My understanding is that a VFD will only allow you to slow the pump to approximately 60% capacity. You will need 2 pumps.