Using the pump in rotation will increase its reliability from redundancy and also increases the pumps lifespan. There are two ways of running lead-lag system, first being that the leading pump runs until the process demands increases during which the lag pump (second pump) is started. The other way to adopt the lead-lag system would be to rotate the leading duty between the pumps. That is, if Pump A has been leading for 3 months while Pump B is the lagging pump, Pump B is setup to run as a leading pump for the next 3 months while Pump A would be in lagging mode.

Either of these methodlogy have both advantages and disadvantages, which is not part of the article. Maybe I will cover that in the next article.

The snippet below highlights how to write a simple pump control lead-lag logic in RSLogix5 for PLC-5.

Xybernetics Rockwell RSLogix 5 - Simplified Lead Lag Logic Sample