Running 2 small pumps in parallel vs one single pump 1

[planck121]

Hi,
Is it possible to show that running two pumps (smaller) is cheaper than running one single centrifugal pumps. I believe the energy savings are better in running two smaller pumps (esp given process variability in a running plant) since they are able to handle this better than running a larger single pump and then throttling flow at the discharge to meet requirements. However, I would like to know if there is some way to show based on general calculations from pump performance curves that running two smaller pumps is more economical than a single pumps. I do understand reliability is a different story (2 pumps provide better reliability in case part of the flow is useful) compared to a single pump. However if 100% flow is required for a particular process than a single pump would be a better option.

In this case however I am trying to look at it from a cost stand point and I would assume it makes better sense to look at it further from the lifecycle costing stand point of the pump rather than just purchase cost.

Thanks

 

[Artisi]

planck121: starting to get somewhere now, I would say had this info' been given in the 1st or 2nd post the discussion may well have taken a different direction.
One point is still unclear, a flow of 643 M3H using 1.3MW and 322 M3H using 671 kW would indicate that the head in the system is something like 500 metres, is this correct?

I think my first post in reply summed up the situation fairly well and still holds true, but you know better than anyone else the operating conditions and what would / should constitute the best selection based on actual operation and depending on turn-down and frequency, you maybe better running 3 x 33% pumps, this gives a wider range of flow conditions and from a power input consideration may be advantages.

Pump efficiencies seem very low, also mentioned by DubMac, I would have expected 80 to mid, ever high 80's for pumps of this flow.

However, it's your call - best of luck.

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.)

 

[planck121]

Thanks for the input Artisi, Much appreciated.
Yes the head is around 450m (475 for 1 pump and 525 for 2 pump senerio at the max impeller size) however this is not max impeller.

I agree with you on the pump efficiencies, they are certainly low...however stuck in an interseting situation where I am managing an EPC lum sum contract and cost is coming into picture. As we all know when it comes to good engineering and cost. It is usually cost that wins the battle.

Its been good input nonetheless and gives me a broader prespetive after this discussion.

cheers

 

[TenPenny]

Is it possible to show? Yes, it is, if it happens to be true.

Take your load profile, number of hours at various operating points.

Using the performance of the single large pump, calculate the energy used.
Using the perflormnce of the two smaller pumps, determine your operating scheme (1 vs 2 pumps) and calculate the energy used.

 

[psv1990]

This is a very interesting thread. Based on the information provided, the energy savings of going to a larger more efficent pump could exceed $100,000 per year.

 

[Artisi]

Even more if the pumps finally selected are more efficient than now being considered.

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.)