There's no rule of thumb.
IMO, the number of standby units should be selected considering two primary factors,
1) Criticality of the demand.
2) The flowrates operating ranges you need to maintain.
Of those, the most important is criticality of demand. If it makes no difference if your system is down for the time that it takes to repair a pump, there may be no justification for having more than one pump, but many systems have some flowrate that must be supplied at all times. For example, if this is a life support system, the overall reliability required might be 0.995 If the pumps you install have an individual reliability of 0.85, The question you need to answer is, how many will be needed to reach the overall reliability requirement of 0.995
1 pump gives 0.85, and is not sufficient.
2 pumps give 1-(1-0.85)^2 = 0.9775 and is still not sufficient.
3 pumps give 1-(1-0.85)^3 = 0.997, THAT is > 0.995, so you would need 3 full-size pumps to meet that reliability criteria. If your reliability criteria specified that 1/2 design flow was sufficient to sustain life, then you could use 3 x 50% capacity pumps.
This can also be evaluated in terms of costs alone when the cost of losing flow is known. If you always need to produce at least 0.90 percent of revenue, you'll need 2 pumps, selecting both pumps with a capacity of at least 90% of design flowrate.
As for factor 2 of letting flowrate operating ranges make that decision, partial flows could be meet by using a VFD or a control valve, so if those options provide an economic solution, you may not need to use multiple pumps, running 2 x 50% pumps with a control valve when you need 75% system flow, or turning one pump off when you only need 50% flow. Since those options, and perhaps others as well, such as adding tanks, etc.) can be considered to permit operation at partial flowrates, I prefer not to dwell on partial flow criteria to decide on the number of pumps and revert to the criticality level of the system to make the number of pumps determination. You could evaluate the running costs at those parital flowrates multiplied by the time you spend running at those partial flows to see if such options might have cost advantages over providing additional pumps. Usually they do and additional pumps would not be required for partial flowrate operation, but ... sometimes they don't, if you run a lot of time at very low partial flowrates.
I think all life-support equipment on the International Space Station have at least 3 units in parallel, if not 3 entirely duplicated circuits.
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"The problem isn't finding the solution, its trying to get to the real question." BigInch
