Pressure Pulsations

Cause and Effect Pressure pulsations are present in both the suction and the discharge of any centrifugal pump. The magnitude and frequencies of the pulsations depend upon the design of the pump, the head produced by the pump, the response of the suction and discharge piping, and the point of operation of the pump on its characteristic curve. The observed frequencies in the discharge may be the running frequency, the vane passing frequency, or multiples of each. In addition, random frequencies with pressure pulsations higher than either the rotating or the vane passing frequencies have been observed. The cause of these random frequency pulsations is sometimes difficult to determine. System resonance, acoustic behavior, eddies from valves and poor upstream piping, and so on, are sometimes involved. However, such random pressure pulsations should not be dismissed as spurious or irrelevant data in any analysis of symptomatic operational problems.

The observed frequencies in the pump suction are much lower than in the discharge. Typical frequencies are in the order of 5 to 25 cycles/s, and they do not appear to bear any direct relation to the rotational speed of the pump or the vane passing frequency.

Diagnosis from Pump Operation In most pumping installations of 435 lb/in2 (3MPa) [that is, 1000 ft (305 m) of head in water] or less of head per stage, there is little outward manifestation of pressure pulsations during normal pumping operation. Other than for specialized applications, such as white water pumps for paper machines (where the discharge pressure pulsations may affect the quality of the paper) or quiet pumps in marine service, there are few external symptoms of internal pressure pulsations. For high-head pumps, however, suction and discharge pressure pulsations may cause instability of pump controls, vibration of suction and discharge piping, and high levels of pump noise.

Diagnosis from Visual Examination of Damage In the case of high-head pumps, any failure of internal pressure-containing members should be investigated with consideration given to the possibility that the failures are fatigue failures from internal pressure pulsations. Examination of the fracture will determine whether the failure is a fatigue failure or not. Fatigue failures may have one or more origins. Characteristic markings, known as striations, are often present on the fracture surface. Metallurgical examination of the fracture surface will also disclose striations on a microscopic scale. These markings represent growth of the crack front under cyclic stress.

If it is a fatigue failure, the cause can usually be traced to high cyclic stress induced in the pressure-containing member from high-frequency pressure pulsations.

Instrumentation Pressure pulsations are usually measured with piezoelectric pressure transducers and recorded as peak-to-peak pressure pulsations over a broad frequency band. Recorded on tape or strip charts, a spectral analysis may be performed for any operating condition.

Corrective Procedures A spectral analysis of the pressure pulsations at the suction and at the discharge of the pump is necessary before a strategy for corrective procedures can be developed. After the spectral analysis is available, problems associated with pressure pulsations can usually be reduced by implementing the procedures shown in Table 1.

TABLE 1 Corrective procedures for various problems

Problem

Corrective procedure

1. Vibration of suction or discharge piping

2. Instability of pump controls

3. Fatigue failure of internal pressure-containing components of the pump from pressure pulsations a. Search for responsive resonant frequencies in the piping or supports. If any part of the system responds to the frequency of the pressure pulsations, alter the system to shift the resonant frequencies.

b. If possible, increase the output of the pump by changing the mode of operation or by installing a bypass from the discharge to the suction of the pump.

c. If the piping responds to the vane passing frequency of the pump, the impellers can be replaced with a unit containing either one fewer or one more vane.

a. If possible, increase the output of the pump by changing the mode of operation or by installing a bypass from the discharge to the suction of the pump.

b. Install acoustical filters to reduce the magnitude of the pressure pulsations.

a. If possible, increase the output of the pump by changing the mode of operation or by installing a bypass from the discharge to the suction of the pump.

b. Redesign the failed components to reduce the induced cyclic stresses to below the endurance limit of the material.

c. If the spectral analysis shows that the maximum pressure pulsations correspond to the vane passing frequency of the impeller, the impeller can be replaced by one having either one fewer or one more vane of the same design.

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