The Great Pump Teardown
There was a pump in that room, and the program said there would be a teardown.
Teardowns aren't uncommon in technical schools, but not at trade association meetings. This one was being held at the Refrigerating Engineers and Technicians Association (RETA) annual convention in soggy Florida. Most technical sessions that take place during association meetings use PowerPoint presentations, handouts, and maybe a tool or small component passed around the classroom. This particular RETA meeting included teardowns of a valve in addition to the pump.
The pump teardown was one of the technical session highlights, and crowded to the point that instructor Steve Hess of Cornell Pumps had to offer an unscheduled second class later in the day. It even bumped up attendance of his pump troubleshooting session, held before the teardown. It also got people's minds off the previous night's Molly Hatchet concert.
HANDS-ONHess pulled open a Cornell open-drive pump, which any technician might need to do in order to provide the small bit of maintenance it needs: lubrication. He started by pointing out that the tech would have already evacuated the ammonia from the system. Then he got down into it.
The first step was to loosen and remove the backplate bolts. "An external line connects to the reservoir," he said. Hess backed the unit out, pulled off the impeller, and pulled off the backplate.
"Use STP to lubricate seal elastomers, the shiny seal faces," Hess told his audience. Avoid getting finger oils onto the stationary element.
He removed the backplate from the second stationary element and turned the pump to the outboard side. "Look for the shiny side of the stationary element, the black elastomer," Hess said. "Make sure the backplate area is clean."
He noted that the line comes in, feeds oil to the pump, and goes into the fitting. When tearing down the unit for maintenance, "You need to avoid cutting the O-ring."
Hess applied a little more STP to the shaft sleeve. "The groove prevents leakage along the seal," he said. "The sleeve prevents wear to the shaft." The O-ring and sleeve then go over the shaft. "Put on the gland - try to avoid dragging it," he said.
The bronze rotating element and elastomer are put on next. "Without STP it sets a lot quicker," he said. However, there should be no refrigeration oil on mating surfaces. "It will take longer for them to wear in," Hess said.
He made sure the pumping ring was facing out, then installed the TeflonÂ® spacer, first compression spring, and secondary compression spring (facing out). The final steps were to attach the impeller and tighten up the springs.
The exercise was missing a little of the ambience of an on-the-job or even a shop teardown; but the lights stayed on and there was no PowerPoint, for which the audience seemed very grateful.
TROUBLESHOOTING SCENARIOSIn his more academic session, Hess explained that there are two different scenarios for pump troubleshooting - the operational scenario and the mechanical scenario.
The operational, hierarchal method troubleshoots for cavitation, recirculation, and vapor entrainment. "Cavitation relates to net positive suction head," he said.
Vapor entrainment can be perceived as cavitation. It can result in:
These considerations may affect whether or not the standby pump is susceptible to vapor lock. "I like to see the valve as close to the vessel as possible."
The pump bypass line, Hess said, needs to be at least 3/4 inch inner diameter. Total capacity equals the bypass plus the system requirement. The system uses a needle or globe valve.
Line velocities for the drop leg are 1/4 foot per second (fps), 3 fps for the pump leg. "You don't want turbulence."
The suction vent line needs to be 1-1/4 inch diameter. An eccentric reducer and isolation valve can help prevent vapor entrainment. The volute vent line facilitates startup, vapor evacuation, and pump draining.
MECHANICAL TROUBLESHOOTINGThe mechanical aspects of troubleshooting, Hess said, comprise:
Loss of prime, he said, may be caused by incorrect volute vent line piping, a closed vent line (not fully evacuated), or incorrect pump startup. Loss of prime may be caused by cavitation, recirculation, or vapor entrainment - "all three issues are main culprits," Hess said.
Motor overloading on new installations may be caused by:
Motor overloading on existing installations may be caused by oil ingestion.
Publication date: 01/09/2006