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Terminology for process capacity and redundancy

Terminology for process capacity and redundancy

Terminology for process capacity and redundancy

(OP)
I have a come across terminology used in process design that describes how many units ( pumps, heat exchangers etc.) required, available and how many on standby.
but I cant remember exactly how it works. Say if you need to deliver 500m3 /hr of cooling water and you have 2 X 500m3/hr pumps , one of which is on standby , that is called N+1, but what is it called if you have three 250m3/ hr pumps.
Same thing goes for electrical generating plant. I.e. if plant load is 1.2 MW, you may have 3 X 600kw machines or 4 X 400 etc.
More generally , what are other ways of describing these kind of configurations.

RE: Terminology for process capacity and redundancy

Capacity is usually referred to as N x X%,

so in your case it would be
2 x 100% for the 500m3/hr pumps,
3 x 50% would be what you call the 250m3/hr pumps

then 3 x 50% or 4 x 33% for the generating plant case.

But sparing is still N+1 in all cases.

Which one to go for during design is up to you and different factors will come into play.

Whilst 2 x 100% may seem excessive and need higher cost for the "spare", but the efficiency may be higher and overall 2 sets of something may be much the same cost as three (3 x 50%) etc of something a little smaller.

Of course you then throw in turn down or start up and that could impact which one you choose if you're limited in available power, you might not have enough starting capacity for one big motor and need to install more smaller ones.

Lots of options and each plant will have different issues as to which one is which.

A few companies opt for 2 x 50% and no spare on the basis that overall they save CAPEX and even one unit will probably give you 60-70% in reality whilst you repair / replace the second unit, either with one you keep in the stores or even if you have to fly it in you still save money by not installing hundreds of spare units which sit idle for 95% of the time, but you still need to maintain them or swap them over on a regular basis. Up to you.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Terminology for process capacity and redundancy

In some plants, where there are critical utilities, Operators may opt to go for N+2, the rationale being that they want 1 spare inline machine even when another is taken off line for maintenance while the plant is running. A case for this is plant air compressors.

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