a) 5 pumps = 100%, with 1 backup pump at 20%, what is the n value?
b) 2 pumps and 2 backup (all equal size)= n+1 correct?
c) 2 pumps and 1 backup (all equal size)
Assuming that all the pumps are equal capacity, parallel piped on a common header and individually able to be run in place of any other pump...
1. N+1
2. 2N (more common than N+N)
3. N+1
N refers to the number of pumps (or equipment in general) required to meet the design flow (load etc).
N+1 indicates this many pumps and one spare. It could be one pump or a hundred, with one spare. Obviously with 100 duty pumps you might want a few more spares, with N+2, 3, 4 etc. resulting.
2N is a much more substantial deal and refers to a spare for every pump, but is often realized with separate parallel plant so that pipework and accessories and even major plant are replicated. Two pumps, one duty and one spare are usually termed N+1 for this reason.
You can also group redundant equipment - with say one spare for each of four groups of equipment (makes sense in a fan or crac unit context) being 4(N+1).
Mike:
Typical (and recommended) design would be to keep the backup units also running. If more than 1 backups (for n+3 or N+4) etc, one can have a system that will "cycle" the units such that at least one or more backups are operating all the time. You do not want to find out that the back up units did not come on line because of a failure of some other component like a valve or the unit itself.
Again the devil is in the details, of functions and design objectives.
power your pumps, chillers, cooling towers, boilers, etc, each from a separate electrical panel.
So if something goes wrong with the panel, you don't lose your entire plant because a fault in your eletrical panel will render your multiple N to nothing.
