Dark,

I would strongly advise that you get something very firm from the client regarding "spare pump is specific and identified". Typically, when you have three pumps where one is a common spare for the other two, identifying the "spare" pump means that it has been sized to handle whichever service has the higher flow requirements and is usually located in the middle.

But, I highly doubt that the client's intent is to just let that pump sit idle forever until one of the other two fails. Clients are going to want to provide more even wear by running the "spare" pump regularly. Plus, it's not going to be good for reliability to just leave the "spare" pump sitting idle for so long, just as you would not want to leave a car sitting for years and then expect it to just crank up and take off down the road.