Mechanical Technology June 2015

⎪ Pump systems, pipes, valves and seals ⎪

While waiting for a flight from Joburg’s OR Tambo Airport, Pump Guy, Larry Bachus receives an email that asks “When is a pump problem not a pump problem?”, along with the answer, “When it is a pipe problem!” This prompts him to relate the experience of a friend, Don Densmore, about an urgent pump problem on a thermal oil application at a solar power plant in California. When is a pump problem not a pump problem?

The Pump Guy

“ I have heard, time after time, that someone was in desperate trouble because a pump wasn’t working properly. That’s right! The darn pump just doesn’t work as it is supposed to. My response is always the same: I go there and find out why the pump isn’t working as designed. Sometimes it is true that the pump didn’t perform as designed, but rarely is it be- cause of a faulty pump,” says Densmore. Following a request from The Pump Guy, Larry Bachus, Densmore visited a town called Daggett, California (just outside Barstow), where the engineer- ing department of a large cogen solar company had asked him to help with a problematic pump. “I called the company and spoke by conference call with Phil Jones and a battery of engineers. They were convinced that something had happened to a critical pump at the solar plant. They needed someone to determine what should be done. We agreed on a fee and two days later, I drove to the job site in Daggett, California,” he relates. “The site engineers first escorted me to the conference room. It was apparent they were anxious to get this pump into production as soon as possible. They were willing to do whatever was neces- sary to fix the pump. Then, we went to see the pump.” The pump had been in service for 25 years, pumping thermal (very hot) oil. The oil temperature varies between 93 and 232 °C, depending on the time of day and the amount of sunshine. The pumps would then push the thermal oil to a generation facility to produce electricity. The pump was a Byron Jackson, heavy-duty single-stage pump with 500 mm suction and discharge ports, both located vertically at the top of the pump. It was one of three pumps that operated in parallel, when required, to meet the flow requirements. The three pumps and the piping were coated with thick insulation to handle thermal oil that was being circulated from the solar panels. But the problematic pump was

in the vicinity of the suction gate valve, which was totally open. “We removed the thick pipe insula- tion before the suction gate. There was a hidden ‘Y-strainer’ at the entrance to the suction gate valve. No one knew it was there. The strainer didn’t appear on the original engineering drawings. Its existence was completely masked by thick pipe insulation that was many years old. After 25-years, the strainer basket was completely full of sludge and oil clots, which were backing up into the suction pipe before the strainer,” Densmore explains. The clots and sludge would settle to the bottom of the pipe before the strainer when the pump was not running. Then on starting the pump, the sludge would lodge into the dirty strainer basket. This was the reason the pressure continued declining as the pump speed increased on start-up. “At first, I recommended they remove the strainer from the pipe. But there was no time to build a spacer pipe or spool to replace the strainer in the suction pipe scheme. So, I recommended they leave the strainer in the pipe, but without the basket. Besides, there were known strainers in other parts of the pipe system to capture and remove clots and sludge.” The client started the pump. The suc- tion pressure was adequate with respect to its margin above the NPSHr. “As I drove home, the pump was running like new. The vibrations, cavitation and stress had disappeared,” he reveals. “In all my years of solving problems with pump systems, I find that almost 70% of the pump problems are not really pump problems but system problems. The pipe system being worked with must, therefore, be understood and, to do so, one should always have a gauge on the suction side as well as on the discharge side of the pump,” advises Densmore. Larry Bachus’ next seminar trip to South Africa is planned for late October this year. Contact Phindi or Tiny (phindi@2kg.co.za, tiny@2kg.co.za) at 2KG Training to register. q

piped in a different configuration than the others. “I noticed the inlet piping was installed in a manner that could not be considered ‘good engineering’. However, it had been operating like that for 25-years. With that in mind, I asked for the pump to be started to allow me to gather information from the instrumentation to determine what was going on. “As the speed was ramped up from the VFD, I could see the pressure on the pump inlet was not as high as it was designed to be. This was a problem. And the inlet pressure began dropping steadily as the speed increased. This indicated an obstruction. I could also hear the pump was definitely suffering cavitation and not pumping as it should. We let the pump run for a couple of minutes and then shut it down. The local managers wanted to remove the pump and get it repaired,” Densmore says. The pump was fed from a manifold located about 30 m away and positioned about 3.0 m below the pump base. The pipe leading to the pump had numerous elbows with inclines and declines leading to a 500 mm gate valve and the pump suction nozzle. There was no discernible logic to the suction pipe arrangement. “We cut a small hole in the insula- tion to reach the pipe wall and installed a pressure gauge at the exit pipe on the upstream manifold. We installed another gauge at the gate valve just before the pump suction nozzle. We started the pump again, and noticed a large dif- ference between the two gauges,” he continues. The elevation differential across the pipe system could explain 10 to 15 kPa of pressure drop between the gauges. The numerous 90° suction elbows accounted for another 15 to 20 kPa pressure drop. But there was nothing to explain the 240 kPa pressure drop across the suction pipe system. “We installed some more strategic pressure gauges in the suction pipe system to track the pressure drop. We determined the major pressure drop was

Mechanical Technology — June 2015

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