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.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
Actually, the pump being driven by flow was my initial guess. Of course, I have a bit of experience with controls for pumping systems.
I had an interesting diagnostics challenge a while back that involved a large pump driven by a large motor. It was in a hydraulic power unit that we had loaned a customer while we were repairing their failed power unit.
The complaint was simple: the pump will not start. Because the customer was an hours drive away, I first asked a few questions, then asked them to have their electricial check the 3 100 amp fuses in the 480 volt power feed circuit. He reported that one 100 amp fuse had failed, I suggested that he replace it and try starting the system again, He replaced the fuse and the replacement fuse failed as soon as they tried to start the motor. So now I had to head over to the customers site and find the problem. My approach was first to do a complete inspection, since that sort of short circuit shaould provide some evidence of itself. But all of the visible wiring looked good. Next, I did a resistance check at the motor terminals of the starter-overload assembly, which showed an open circuit in one phase. That was a good clue. I opened the connection box on the side of the motor and found that, because it had been assembled with the splice pressing against the cover of the box, the one connection had slowly cold-flow, penetrated the tape wrapping and contacted the box cover, short circuiting the phase to ground and evaporating part of the connection. The repair was simple, which was to cut off the damaged end, install a split-bolt splice instead of the lug and bolt splice, and tape the new connection. Then I was careful to position the wires in the splice box so that they did not press against the cover, and replaced the cover. After installing a new 100 Amp fuse, the systm started and ran correctly.
Note that I did switch off the service to the system before I started working on it.
I agree, Chuck. That's what makes these Sherlock so much fun. And it's always the super simple answer that wins -- like the magnet that isn’t magnetized or the resistor what was marked incorrectly.
Yes, good observation, Tim. Jennifer is right about how these Sherlock postings show some excellent deductive logic. It's not named "Sherlock" for nothing. What I find most impressive is that many of the solutions arise when the Sherlock involved refuses to make any assumptions.
Good job double checking something that your eyes and ears were telling you. Sometimes it is good to take a step back and ask if what you are seeing is the whole situation.
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