Parallel pumps, quantity and capacity

[MisterDonut]

Consider a generic parallel pump system in which flow can vary from 0 to 100% of design flow rate. Is there a "rule of thumb" on how many pumps is ideal? Which of the following arrangements has the lowest cost over the long term, conisdering first costs and energy efficiency?
- 2 @ 100% (2 pumps, each capable of 100% design flow rate)
- 3 @ 50%
- 4 @ 33%
- 5 @ 25%

 

[nickelkid]

Is this for Centrifugal or Positive Displacement type pump?

 

[MisterDonut]

Centrifugal

 

[nickelkid]

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

MisterDonut,

You may find the above link of interest.

 

[BigInch]

Looking at parallel pump capacities when planning standby needs is comparing apples to oranges.

The number of pumps with the percentages you indicate say you are considering standby capacity, which is evaluated based on the improved reliability provided by n, n+1, n+2 spares X spare capacity provided by n+1, n+1, n+2, etc.

Standby capacity is decided by the amount of money lost or other penalties imposed upon failure to meet contract deliveries during pump failure VS cost of pump configuration. Note that penalties could be non-monetary, such as when a pump to a life support system fails, but for evaluation purposes, we will only consider penalties in terms of monetary cost.

In most cases, energy used over the lifetime is at the very least an order of magnitude greater than capital cost, so you must design all pumps for their proportion of 100% at equal BEP efficencies. You do not mention if BEP efficiency varies significantly, so I will assume the same efficiency is achievable at all proportional BEPs in any configuration. Since you are always trying to pump 100% in all cases, and the same efficiency for all pumps is possible, the energy cost of pumping is equal for all configurations and the energy cost variable drops out of consideration.

Secondly, contract penalties are also likely to exceed the capital cost. Contract or other penalties will be incurred upon failure of a certain number of pump units such that pumping capacity drops below 100% of design flow. As such it is the reliability of the pump configuration in their ability to deliver at least 100% of design flow that you must evaluate against the cost of providing n, n+1, n+2, n+3 pumps.

In my experience, it is seldom economic, nor necessary to provide more than 3 x 50%, but if you have an extremely critical need to deliver 100% for 99.995% of the time, it is possible that you could need more.



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

From a purely hydraulic perspective, the number of pumps operating closest to their best efficiency (BEP) for the maximum amount of time over the life of the project
would be the best way to go particulary when the pumps have to run a between 0 and 100%.
As pointed out by BigInch, energy input will usually outstrip capital costs in a very short time period.

From a point of reliability - 3 x 50% is a good choice but in your case this must be weighed against the hydraulic consideration.

You need to make a full hydraulic / cost analysis for all combinations of flows / number of pumps / pump efficiencies / capital cost / running costs / maintenance costs.

 

[BigInch]

Artisi, I think only reliability and penalty costs are significant; the others usually show small differences, except when space or weight are very costly, as on an offshore platform, of if you do need to operate a lot at 30-60% of the design flow and you can shut a lot of the smaller units off. THAT could be significant. If that is not the case, than net pumped volume would be equal, net power equal, maintance is 2 big ones, vs 3 or n+1 slightly smaller ones; almost the same. Nothing ever adds up enough to favor one over the other; you're left with incremental reliability vs penalty for underdeliveries.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

BigInch, reliability today with good quality and correctly selected equipment properly serviced and maintained tend to be extremely reliable and failure due to mechanical reasons rare, that's why I consider 3 x 50% to be ideal on the off-chance that you for some reason that one unit go down.
As for penalty costs, this is only a factor if penalties apply or need to be considered, however,I guess that lost production can be considered a penalty (in some cases).

The OP asked for lowest cost over the long term
" ....considering first costs and energy efficiency?"

Therefore I still consider the least number of pumps operating near to BEP the most reliable cost effective alternative.

Have a great new year

 

[BigInch]

Art, that's exactly what I was saying too. I just think its not so hard to prove.

And yes TYVM. A most happy new year to you ...and all.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

I've done some further work on this question over the last couple of days. Provided you use hydraulically similar pump curves (same relative head and efficiency to relative BEP flows for each respective BEP flow per pump in all configurations), power consumption can be higher for multiple units, if operation about one primary flowrate is the objective. Dividing up a weighted average "target" system flowrate amongst multiple units results in smaller BEPs for each unit, and the pump head increases with smaller flows as it approaches dead head power, higher heads and power can be required, however a gain in efficiency is also possible when considering many units can more closely track system flows closer to individual BEP flows and gain efficiency. One thing I didn't consider before. Closly tracking multiple target flowrates can result in substantial savings when very wide flowrate ranges and significant operating times at extreme ranges are necessary and you are debating between 1 and 2 units, or 3 units, but that close tracking incremental savings quickly decrease when the total units added exceed 3.

Net result is, if wide flowrate ranges are needed, the decision could be based on energy savings, but if one flowrate is the primary objective, nuimber of units should be based on criticality of maintaining flow in case of trouble with one or more units.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

BigInch, I had similar thoughts but haven't got round to posting anything as MisterDonut seems to have been gobbled up in the system. Knowing the full operating conditions it is likely that a number of variously sized pumps could be the best combination, ie, 1 x 50% 1 x 30% 1 x 20% which when reading your last comments seems to be the what you are saying, " Provided you use hydraulically similar pump curves (same relative head and efficiency to relative BEP flows for each respective BEP flow per pump in all configurations),"
This of course doesn't give 100% flow if 1 pump should go down but then everything in life is a compromise.

 

[BigInch]

If you have 3 x 50%, you're OK.

I actually had equal units in mind as I don't like to make configs out of different capacities because it tends to complicate things more, but no reason why you can't.

The way I analyzed it was to assume a typical curve shape relating each H-P-Eff point on a curve to its corresponding BEP H, P and Eff. Then you can easily substitute different BEP flowrates and see the effect of using 1 X 100% Q, 2 x 50% Q, 3 x 33% Q units, etc. on a generic system curve of an assumed quadradic equation that has an intersection on the pump curve at 100% Q and how all unit discharge heads have to be adjusted to meet the system curve by use of control valves to allow you to make up your total req'd H at any system flow. If all units are the same, relative power reduces to y% Q x Hsystem x %Q/%BEPQ x number of units running x efficiency at that %Q/%BEPQ. So, for equal units flowing at N x unit BEP, power is equal among all configurations. Where they deviate from BEP=100%Q
/Npumps, to make up intermediate flows, they lose some eff, so with more units the variation away from each individual BEPQ isn't so great, which results in tracking the total flowrate range at slightly higher efficiencies with each additional units. Over the life of the system, it can make a difference in power consumption, especially between 1 and 2 units, and again with 3, but over that, the relative gain in net total eff is much less as you increase number of units. All in all, 3 equal 33%Q units looks like a generally good optimal config that can run a range of flowrates from 25%Q to 115%Q. Possibly 3 x 40%Q etc., if the high flowrate range is greater than 115%Q.

Now to see what adding VFD would do. Probably nothing. I think it will work out such that the more units you have the less benefit VFD would have, as with more units they would already be able to track max possible efficiency without a VFD at most all reasonable flowrates.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/