when there are two pumps in parallel, there could be two cases:

1. 1) One pump operating and another pump spare
   2) Both pumps operating in parallel
   
   But there is a third type of operation for parallel pumps: lead lag operation.
   There are two things to know about lead–lag operation:
   (i) both pumps are not operating all the time, and
   (ii) it is not the case that the “spare” pumps are put into operation when the first pump fails.
   In lead–lag operation, two (or more) pumps work together as a team to do one specific task.
   There are two types
   
   1st type: At the beginning, only one pump starts to run (the “lead pump”), and when/if it cannot manage the situation, the second pump (the “lag pump”) starts to run automatically to help the first pump.
   
   2nd type: The other type of lead–lag operation is the type where the lead pump is operating continuously and the lag pump comes on to the scene when the lead pump cannot keep up with the flow.
   Example: A lead–lag pump system is normally used for vessels that have an unpredictable level because of unpredictable flow fluctuations, such as in a sump.