ArticlePump Vibrations. Part 1: Monitoring
by Kevin R. Guy
This is the first of two papers by the author on the topic of pump vibration. The first paper discusses the most common causes of pump vibration: flow-related issues. The need to operate a pump at or near its best efficiency point (BEP) is emphasized and why it is important. Descriptions of the opposite flow problems of low-flow and cavitation are given. The need to gather periodic data on pressure and flow along with vibration is stressed. Suggested machine locations for pump vibration monitoring are given.
“A qualitative proactive maintenance program for pumps depends on a quality vibration monitoring and analysis program. Such a program should evolve over an interval from 18 to 24 months and include a database (objectives, time, discipline). It is also necessary to develop an equipment information file and an equipment data sheet.
Data on operating speed and bearing type (i.e., rolling element or Babbitt) are readily available. Information about specific bearing type and style (i.e., angular contact, tilting pad, pressure dam) can be useful in determining where periodic data should be collected. Operational information is necessary to ensure that the pump is operating at its best efficiency point (BEP). It is important to try to collect data at the same operating conditions.
BEP information can be found on the pump curve supplied by the manufacturer. Figure 1, a typical pump curve, shows that the pump discharge flow is 2,250 GPM at 291 feet of discharge head. The conversion formula from head (in feet) to pressure is
discharge pressure = (discharge head/2.31)
The 291 feet of discharge head converts to a discharge pressure of 126 psi; data on this pump should thus be taken at 126 psi discharge pressure, which provides a discharge flow of 2,250 GPM.
Pump vibration can be caused by cavitation and low flow. Cavitation occurs when the pump provides more flow at a lower discharge pressure than it is designed for at BEP. Low flow causes the pump to operate at a higher pressure, resulting in internal recirculation.
Cavitation is characterized by low discharge pressure and broad-band noise in the spectrum (Figure 2). Broad-band noise can excite bearing natural frequencies that cause high readings when data are collected as spike energy, HFD, HFB, or VHFB. Operational parameters should be reviewed to determine that a high reading is not a false reading of a bearing problem. Low flow is characterized by higher than normal discharge pressures and axial vibration (low-flow shuffling).
Low flow shuttling can cause severe damage to rolling element bearings and thrusting that can break the pump shaft. Figure 3 is a vector diagram from a fixed speed pump. When a pump is operating at BEP, the flow out of the impeller is directed from the impeller blades straight out the discharge volute. When pressure increases, flow decreases and forces water against the housing of the pump (Figure 4). The flow causes the shaft to bow, resulting in higher-than-normal axial vibration. If the situation is not corrected, the shaft may break, typically at the impeller eye.”
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