4 Pump Alternation With Failure

I'm sure this question has been asked a lot but i cant seem to find a response I understand. Im fairly new to programming so as much help as possible would be great.
I have 4 pumps that are sitting in a wet well. I need to change the sequence of the pumps after every shutdown of the lead pump. 1 of the pumps will be a standby.
The part that i am having trouble with is that the program also needs to adjust the alternation if 1 or 2 of the pumps are not available.
Example being if pump 1 and 3 fail then pump 2 and 4 should alternate between lead and lag till one or all pumps become available again.
I am trying to do this in proficy machine edition.

I appreciate any help!

Write out what you are trying to accomplish using pseudo code first plus your assumptions/constraints.

Ex. -
Pumps must have status' - InService/OutofService , Lead/Lag/Spare.
Pumps(that are InService) must rotate lead/lag status on x event,

etc etc.

Then, when you have the logic detailed out, you can work to convert that pseudo code in machine logic. Easier than starting directly in the machine code.

Create a truth table for each possible combination of pairs that could be in alternating sequence as a duplex control scheme. Then use those truth tables nested into other truth tables to create the triplex and quadraplex schemes, using JSRs to engage them based on which, if any, pump is not available in the rotation.

I did this on a quadraplex pump panel used to pump water off of the emergency backup runway for landing the space shuttle at Edwards Air Force Base, which is near the Mojave desert. The prospect of rain on the runway at the exact moment of an emergency shuttle landing was slim to none, but it didn’t matter, they had 4X 400HP pumps on that pump station to get rid of the runoff just in case... I seriously doubt it was ever used other than in tests. For the test, they used giant bags of water around the sump and detonated charges on them to burst them all at once. Ahhhh the military...

the program that I attached to the following post might be helpful to you ...

http://www.plctalk.net/qanda/showthread.php?p=226769&postcount=61

it should be fairly easy to change the program layout to handle four pumps instead of three pumps ...

hope this helps ... stay safe ... stay well ...

Another thing to do - search for air compressor rotation controls.

This is done frequently in shops with more than one compressor, and 4 or 5 is common.

Rotate start-up based on runtime hours, as demand increases start the next one due, and switch off to take one (or more) out of rotation for service.

Thanks for this one