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E-Zine November 2015
Click here to read “Pump Control with a PLC (Part 1)” Here are some design considerations for you to keep in mind when writing the pump sequencer for your PLC. Let’s say you have five pumps. Four will be in the sequence; one will be standby. A good way to equalize wear on these pumps is to have the sequence alternate. First, you should set it up so that the first pump to come on is the first pump to go off (FIFO) as demand drops. So, the first sequence is 1-2-3-4 on and then 1-2-3-4 off. But adding in alternation, the next sequence should be 2-3-4-5 on and then 2-3-4-5 off. The next would be 3-4-5-1 on and then 3-4-5-1 off, and so on. Sometimes, you may want to base your alternation scheme on pump run time, instead of FIFO, or in addition to FIFO. This complicates the programming because now you also have to take into account which pumps have been running the longest. You should also establish a hysteresis, so that your pumps are not bumping on and off. Pumps go on at low level and go off at high level. Your logic should keep them off until low level is reached again. If you have an analog input (flow or level) to the controller, you can begin to deselect pumps as you approach high level, and begin to call pumps on as you approach low level. You need to be able to sense the “pump running” condition. There are lots of ways to do this, and they all take time. Flow, pressure, current, shaft rotation all have been used to sense pump run. Remember to program in the necessary time lag to have the pump come on line and up to speed before calling “pump failure.”
Click here to read “Pump Control with a PLC (Part 3)” From Flow Control (April 2002) ISSN 1538-5280 |
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