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Cyanogen281

Siemens S7 PLC - Lead and Lag

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Hey guys, I'm having a little difficulty understanding Lead and Lag when it comes to pump applications with the Siemens S7 PLC I am converting a 3 pump system to a 4 pump and I have been dropped right in the middle of it with little to no prior knowledge of Lead and Lag. Any advice or examples would be greatly appreciated.

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A typical lead-lag system will allow someone to select the lead pump manually, or automatically rotate all pumps through the lead. For example, in Auto lead, pump 1 might be the lead pump for 1 day, then pump 2 is the lead pump for 1 day, etc. The lead pump is the one that is always running. The lag pump(s) is the one that will start up if the lead pump can't keep up. The way i have done it in the past is watch for the lead pump to be at max speed for a short amount of time. If it is there for more than a couple of seconds, then start up the lag pump. You then trigger the drop out of the lag pump by watching lead speed. If is drops below a certain value, then it is OK to drop out the lag pump(after a time delay). Depending on your process, you could have more than one lag pump. I always make the lag pump the next one after the lead pump.

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