What I mean is, if pump 1 passes the criteria (lowest runtime, etc.,) move "1" into NEXT_PUMP and it's current runtime (say '800' hrs from cardosocea's post) replaces the 99999999 in RUNTIME_CHK.
Now the pump 2 check will see if it can beat, not 99999999, but 800. If it can, then NEXT_PUMP = 2 and RUNTIME_CHK becomes that lower number (250). No one else will beat 250, so at the end, NEXT_PUMP = 2, which should trigger the start logic for pump 2. It will also reset some bit or timer before executing those 8 rungs of logic again, until the system has had time to verify whether the addition of pump 2 has or has not brought your pressure back in line.
Just as you clear NEXT_PUMP with a zero to verify that something won, you need to seed RUNTIME_CHK with a ridiculously large number, since you are looking for something to be lower. If float, then 10^34; if DINT, then 2147483647, etc. I just used lots of 9's for simplicity.