As an engineer at a water injection facility I was asked to check out the status indication of one of three river water pumps. The pump was running, but the indication said it had stopped. I took a look at the pumps and noticed that all three were, in fact, running.
I looked at the motor control center, but the pump with the suspect indicator was tripped. Weird! I reset the breaker, but it instantly tripped again. Obviously, something was amiss.
Then I went back to the pump house and had another look, just to make sure I had seen everything correctly. Yes, all three pumps were running. This was extremely odd. A motor-driven pump was seemingly running without electricity.
Then I had an idea. I grabbed a pencil with an eraser on the end -- we still used pencils in those days -- and I touched the eraser lightly to the shaft of one of the "good" pumps. The pencil twitched to the left.
I did the same to the second good pump and got the same result. Then, when I touched the suspect shaft that was giving the incorrect reading, the pencil twitched to the right. The pump was running backwards. No wonder the breaker kicked out when I put power to it.
Each pump has a check valve to the header to keep it from spinning backwards when it's off and the others are on. That was the rub. I had a non-functioning check valve, which allowed the pump to run backwards, which then gave me the incorrect reading.
This entry was submitted by Walter Driedger and edited by Rob Spiegel.
Walter Driedger is a PEng, TÜV FS Eng ID-No. 3243/11. He is currently the chief engineer for process control at WorleyParsons in Canada. WorleyParsons is a provider of professional services to the energy, resource, and complex process industries.
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