To decide if a spare pump is required you can ask yourself, "If this pump were to fail, how long would it take to repair/replace and at what loss?"

If this pump is critical then you definitely need a spare.

Another question that I pose along the lines of spare pumps is how often should the spare be run?

Perhaps my previous email was not clear enough but I would like to know if there is any standard policy of how operating pumps that have spare pump.

How often would you switch from the main to the spare?
How often do you test/start the spare pump?
For how long do you run the spare pump?
Do you run to failure the main pump?
If a failure occurs to the main pump, do you run the spare pump only for the time required to repair the main pump? Or is your spare pump becoming the main pump?

Any kind of information will be appreciated.

The last refinery I was in used to switch between the main and spare about every 2 weeks.  If both were the same driver type (eg. turbine), then the new 'lead' pump was run for 2 weeks before being switched over.  If the drivers were different and for example, because of energy prices, the motor was normally used, the turbine would be started up and run for a couple of days before switching back to the motor.

There is no standard API or anything on the spare pump philosphy. But normal practice differs based on the importance of the pump. Such as if the pump is very critical, then you will test start the spare pump every morning shift i.e once a day.  if it is not that critical then you do it at least twise a week.

If both main and spare pump have same driver then you will swichover the pump after the maintance of the main pump i.e you will run alternatively each pump.

But if driver is different for both pumps then the decision will be based on the driver choice.

All this is true only if the spare pump is of the same capacity as the main pump.
Feel free to ask if you have further Q's.