Hot dog, now we are getting down to it!
Now, if one pump fails and you need to pull it out of the rotation ???
As Alaric has pointed out before, if you run them equal times with equal loads, then they will all fail at about the same time. So now instead of one main pump worn out and needing replacing (while you keep running the other two), you got to shut down and replace all three!
Actually, equal time may not cause them to fail at the exact same time, but in the same fiscal year. The company accountants may not want to wreck the maintenance budget by allocating money for 3 pumps the same year, so....
They don't teach the financial part in PLC school, but just how to run the motors evenly.
In the real world of high company finance:
1st Year, you would buy Pumps 1 and 2 and run evenly, taking a tax break for capital depreciation for 2 pumps.
2nd Year, buy Pump 3 and run Pumps 1 and 3 evenly, taking a tax break for capital depreciation for 3 pumps.
3rd Year, run Pumps 1 and 2 evenly, taking a tax break for capital depreciation for 3 pumps.
4th Year, run all 3 Pumps evenly, taking a tax break for capital depreciation for 3 pumps.
....
Run until Pump 1 fails (most wear), replace it and continue even rotation.
...
Run until Pump 2 fails (next most wear), replace it and continue even rotation
....
Run until Pump 3 fails, replace it, and continue even rotation.
...
Repeat cycle.
Hopefully, the pumps will fail in different years, allowing the accounts time to recover from the shock. But you got Murphy's Law to contend with, so Pump 3 might actually fail first!
In years with sympathic tax laws, you may get Accelerated Depreciation, which makes buying new equipment effectively cost less. So instead of contiuning to run an old pump, it may be cheaper overall to buy a new one, if you can claim enough Depreciation the first year to pay for it. In years of increasing business taxes (like 2011), you may have to skip the replacement altogether. Our screwed-up convoluted tax code is one reason businesses are not expanding.
Now write a program for the above!