This article will discuss a case of how measuring vibration can help detect cavitation issues. It will also show how this can relate to a bigger problem at the plant. Sometimes a proposed change of pump type is too drastic. Instead, detailed analysis and thinking should occur before replacement.
A chemical plant in the southeastern U.S. used six horizontal split case cooling water pumps to provide cooling water to a production facility. These six pumps operated in banks of two units taking suction from three cooling towers and feeding it to three plant processes:
- Process A (cooling tower A, feeding pumps A and B)
- Process B (cooling tower B, feeding pumps C and D)
- Process C (cooling tower C, feeding pumps E and F)
Plant management wanted to evaluate the pumps’ operation—their reliability and energy consumption. A field test was conducted to review and assess these issues. Pump C, driven by a 150-horsepower motor at 1,775 rpm with an 11.4-inch impeller diameter, was selected for the initial study.
As shown in Figure 1, a best efficiency point (BEP) was at 4,800 gallons per minute (gpm) for the 11.4-inch impeller. The flow varied between 2,800 gpm and 5,000 gpm, depending on the process demand. This corresponded to power consumption between approximately 130 horsepower (at 69 percent efficiency) to 140 horsepower (at 83 percent efficiency). The required variation of flow was accomplished by the main process (its piping and valving), while the pump isolation valves always remained fully open and were operated only when the pump was removed for maintenance, or as in this example, as a temporary method of performance reconstruction during the pump field test.
To verify the pump’s performance, the flow was varied by throttling the pump discharge valve—starting at 2,800 gpm, where the pump was operating because of the process demand at that time, to 1,600 gpm. The data was plotted over the original performance (see Figure 2). The pump appeared to closely follow the head-capacity curve (according to the 11.4-inch impeller size) and calculated efficiency, based on the measured flow, head and power (via amps). It tracked closely to the original values, no more than about 2 percent reduced as compared to the original values.
In addition, the flow and amps of all six pumps were recorded at their normal operation during the test, showing that the crystallizer condenser demand (pumps A and B) happened to be significantly greater (nearly 4,000 gpm) compared to pumps C and D and E and F, each of which were slightly below 3,000 gpm (see Figure 3).
Therefore, from a hydraulic performance standpoint, the pump operated as expected, provided satisfactory flow over the required range of operation and remained near its original efficiency with essentially negligible degradation (see Figure 3).
The main indicator of reliability degradation—typically indicating a need for repair or overhaul—is an increased level of vibration. To assess that, the overall level of vibration was measured at two locations—the pump’s inboard bearing housing and the suction/inlet portion of the casing housing.
Vibration data (also tabulated in Figure 2) are plotted as a function of flow in Figure 4 (discharge valve was closed in 5 percent increments). Vibration measurements taken at the bearing housing remained steady at approximately 0.18 to 0.19 inches per second, which was below the warning level (field criteria for the warning level was 0.30 inches per second and 0.50 inches per second). While this level was elevated compared to what would be expected from the new installation, it was low and acceptable for a typical field case example.
The vibration levels at the suction housing were not measured to evaluate their absolute values—which do not have an acceptable industrial criteria—but to help evaluate relative differences between the pump behavior at different flows. The “jumping around” of vibrations became more violent and showed a larger spread between the minimum and maximum values as the pump operated farther away from low flow. In addition, the sound of cavitaton became more pronounced as the pump approached higher flow.
When the valve was fully open, the net positive suction head available (NPSHa) was 21.6 feet, and the required (per OEM curve) was 16 feet. However, the net positive suction head required (NPSHr) was greater (above 20+ feet, exceeding the available) when the pump ran farther out on its curve, marked “operation” in Figure 1. Therefore, cavitation levels varied depending where on the curve it occurred specific to the demand of the process.
The need for action is usually determined by measurable economic impact on the pump life and satisfaction—or lack of—with overall plant performance. If pump performance fails to deliver the required flow for the process, a repair or change to the pump or the system should be considered. In this example, the pump flowed close to its original curve and operated in the expected operating region. The pumps satisfied the plant’s flow requirements.