Pumps cavitation, reasons

It is positively my experience that the most common reason for pumps cavitation is partial plugging of draw nozzles. This problem is illustrated in Fig. 25.5. This is the side draw-off from a fractionator. Slowly opening the pump s discharge control valve increases flow up to a point. Beyond this point, the pump s discharge pressure and discharge flow become erratically low. It is obvious, then, that the pump is cavitating. 

It is positively my experience that the most common reason for pumps cavitation is partial plugging of draw nozzles. This problem is illustrated in Fig. 36.5. This is the side draw-off from a fractionator. 

Be aware that in some cases, you ll have to live with cavitation. Many pumps suffer cavitation for reasons of inadequate design, hor example, when operating only one pump in a parallel system, this pump tends to go into cavitation. Pumps that perform more than one dut through a valve manifold tend to suffer cavitation. Pumps that fill and drain tanks from the bottom tend to suffer cavitation. The last pump drawing on a suction header tends to cavitate. And of course vacuum pumps and pumps in a high suction lift arc candidates for cavitation. 

The major operating problem of piston pumps is dissolved air and the formation of bubbles in the eluent. Bubbles in the pump heads cause pulsation of volume flow and pressure pulsation. Bubble formation and cavitation problems are promoted at the inlet check valve because the minimum pressure in the system is reached here. For this reason the eluent must be suitably degassed. This can be done online by a membrane degasser, by pearling helium offline through the eluent or by the use of an ultrasonic bath. 

Pumps often cavitate in spite of a designer s best efforts to meet NPSH requirements of pumps. This behavior is not well understood, and could occur for a number of reasons, including ... 

This means that a pressure gauge placed on the suction of a pump would show a relatively large loss in pressure while the pump was being lined out. Once the flow has been increased to its steady-state value, the suction pressure would come back up. Hence, if the operator opens the discharge valve too quickly, he can cause an excessive loss in suction pressure and the pump may cavitate during start-up. This is the reason many pumps experience seal failure due to cavitation when they are first put on-line. 

Pumps are designed to handle gas-free liquids. If a centrifugal pump s suction supply contains any appreciable quantity of gas, the pump wiU cavitate. In the e imple of cavitation due to entrainment, the liquid is reasonably stable, unlike with the change of phase described in the preceding section. Nevertheless, the entrained gas has a negative effect on pump performance. While this form of cavitation does not seriously affect the pump s internal components, it severely restricts its output and efficiency. 

The primary causes of cavitation due to entrained gas include two-phase suction supply, inadequate available net positive suction head (NPSHa), and leakage in the suction-supply system. In some applications, the incoming liquid may contain moderate to high concentrations of air or gas. This may result from aeration or mixing of the liquid prior to reaching the pump or inadequate liquid levels in the supply reservoir. Regardless of the reason, the pump is forced to handle two-phase flow, which was not intended in its design. 

Figure 10.4a shows the cylinder casing of a diesel engine which was water cooled. Vibrations caused cavitation resulting in pitting which penetrated the casing. The lower Fig. 10.4b shows the blades of the water pump in the diesel which had also corroded for the same reasons. 

From a performance viewpoint, cavitation in cryogenic fluids, such as liquid oxygen, has produced very erratic operation of propulsion systems. Tests of complex flow systems have indicated once cavitation bubbles have been formed there is very little reduction or recondensation of them before they reach the combustion chamber, A comparison of the physical characteristics of the cryogenic fluids with water indicates some of the reasons for the reduced recondensation of cavitation bubbles in these liquids. Recent performance tests of valves and pumps adapted for use with liquid oxygen indicate a large amount of research and development work must be conducted before the performance of this equipment with the cryogenic fluids is comparable to that of similar equipment with water. 

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