A newsletter on pumps and reliability

Minimum continuous stable flow is defined by API-610¹ as the lowest flow at which a pump can operate without exceeding acceptable vibration limits. This month we are going to discuss flow instability, its causes and some guidelines for avoiding it.

  Dale B. Andrews - Editor

Vibration is caused by cyclic loading. Hydraulic instability is the major source of non-mechanical cyclic loading in a centrifugal pump. The primary causes of hydraulic instability are cavitation, recirculation, and stalling. Vibration from recirculation usually manifests itself in the form of cavitation or hydraulic stalling. Therefore, it will be presented in those contexts.

Cavitation

Pump buyers specify NPSHA conditions at design and rated flow conditions. The pump manufacturer selects a pump with an adequate NPSH margin for the pump to provide cavitation free operation when operating between the design and rated flows (Fig 1). Cavitation may occur when pumps are operated outside of this design/rated envelope (Fig 2).

For example, assume a system where inlet frictional losses are negligible and liquid is near its saturation temperature (boiling point). The NPSHA is approximately equal to height of liquid above the impeller inlet. This is a typical scenario for boiler feed pumps, and many volatile chemical and refinery applications. If the pump is operated in a zone where NPSHR starts to exceed NPSHA, cavitation and instability will occur. For a more detailed discussion on cavitation and its causes follow this link to our Oct. 2004 Newsletter.

Low flow also causes recirculation and turbulence at the impeller exit. If the flow into the eye of the impeller is inadequate to meet the demands of the design, low pressure regions will set up within the vane passage. Figure 3 depicts how fluid, from the high pressure zone at the impeller exit, recirculates back into an adjacent vane passage. Non-clogging impellers with few vanes and large passages are especially susceptible to this. In many cases discharge recirculation will extend to the impeller eye and cause cavitation at the pump inlet. Severe discharge recirculation will even set up cavitation at the impeller outlet. Discharge cavitation is evidenced by damage to the high pressure side of the vanes at the impeller outlet.

Hydraulic Stalls

Impeller inlet vane angles are established to convey a specific volume of liquid through the impeller during each shaft revolution. This is the optimum flow rate, or the best efficiency point flow, of the impeller (Fig 4). As the pump operating flow moves away from the best efficiency point, there is an increasing difference between the angle of attack of the fluid approaching the inlet of the impeller and the inlet blade angle.

Stalling occurs when there is a severe mismatch between the impeller or diffuser vane angle and the angle of attack of the approaching fluid. Figure 5 depicts a reduced flow condition. The angle at which flow approaches the blade generates recirculation within the impeller inlet. In a stall event, the turbulence caused by the mismatch between fluid angle and the vane angle momentarily blocks flow from entering the Impeller passage. Dynamic forces within the impeller then cause the blockage to release. Once flow is reestablished, a new blockage occurs and the cycle repeats, manifesting itself in the form of pressure pulsations, vibration and damage to the pressure side of the vanes at the impeller inlet.

Fig 6 -; (multiply SI values by 51.64 to obtain US specific speed units)

The severity of the vibration that occurs as a result of cavitation or hydraulic stalling is dependent on the severity of the event and the energy levels present. High powered pumps, and large flows, have more energy available for damage than low flow, low power units. There are no simple equations that can be universally applied to determine allowable operating limits. However, fig 6 provides some guidance for allowable operating ranges based on suction specific speed².

¹American Petroleum Institute Standard – 610, Centrifugal Pumps for Petroleum, Heavy Duty Chemical, and Gas Industry Services.

² Process Industry Practices RESP001 Design of Pumping Systems That Use Centrifugal Pumps.

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Lawrence, Massachusetts U.S.A. 01843, Tel:800-434-5248, Fax:978-975-4291