Up Time Magazine - December 2008/January 2009 - (Page 56) vibration upload A Tough Diagnosis The Saga of the Never Ending Problem by Greg Davison Y ou are often told that there is never just one problem with a machine. My very first vibration class taught me that a phase and magnitude vector was a combination of all the vibration from all of the forces acting upon the machine. Likewise, a spectrum also contains all of the frequencies from all of the forces acting upon a machine. So, it is never just imbalance, or just misalignment. It is always some combination of many forcing frequencies. This is precisely why wall charts and cookie cutter solutions do not always work. What follows is a story of multiple problems of mythical proportions. The story begins along the east bank of the Arkansas River at Oklahoma Gas and Electric’s Muskogee Generating Station, about three miles east of Muskogee, Oklahoma. The station consists of three large coal fired units, each producing approximately 505 net megawatts, and one smaller 180 net megawatt gas fired unit. On Muskogee Unit 4, one of the coal fired units, condensate is pumped by two 60% capacity pumps driven by two identical Westinghouse 1000 HP condensate pump motors. The motors are vertical Westinghouse 6808 P30 frame, 4160 Volt, 3 Phase, 60 cycle motors with a 1.15 service factor. In October of 2006, Operations alerted the Reliability Technician of a potential problem with the MK4B Condensate Pump. The technician proceeded immediately to the pump to determine if the vibration had increased. Indeed, the vibration level had doubled since the last monthly route data had been uploaded two weeks earlier. A doubling of the amplitude was an obvious indication that there had been a change in machine condition. Was the change in the pump, or the motor, or both? This is usually the first question asked, quickly followed by, “How long will it last?” One of the easiest ways to answer the first question is to divide and conquer. Uncouple the motor. Run the motor solo and see if the vibration goes away. If the vibration goes away, the problem is most likely in the pump. If the vibration is still higher than normal, the problem is probably in the motor. If Dispatch could allow the unit to come to half load, this fairly common troubleshooting technique could cut the problem in half. As luck would have it, system needs required the unit to remain at full load, if at all possible. So, other methods had to be devised to diagnose the problem. The top of the vertically mounted motor is approximately ten feet above floor level (see Figure 1). The bottom of the pump is almost twenty feet below floor Figure 1 – Vertically mounted motor at OG&E’s Muskogee Generating Station in Muskogee, OK. level. Figure 2 shows a multi-stage pump being lowered into position. Without permanently mounted vibration probes, it was impossible to get direct measurements from the pump. We took several readings on the motor and along the motor base, and we used a fishtail to obtain shaft readings adjacent to the coupling and stuffing box. The highest readings were from the motor outboard. The amplitudes gradually decreased as the probe was moved from the top of the motor to the floor, which might lead one to conclude that the problem was in the motor, since this was where the highest readings were. But, past experience had shown that sometimes the tail wags the dog. In this particular arrangement, a vibration caused by forces in the pump can cause the top of the motor to move back and forth as the whole assembly pivots about the floor, similar to a lever and a fulcrum. Or, could it be just something terribly wrong in the top of the motor? Frequency analysis would help to identify the problem. The spectra contained a peak close to 0.5x running speed that was higher than the running speed peak (see Figure 3). There was also what appeared to be a harmonic at 1.5 x running speed. These ½ harmonics are december/january 2009 56
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