Pumps/pumping cavitation

Noisy Pump. Cavitation Aspirated Air Excessive Suction Lift Not enough NPSHa Bent Shaft Bound Rotor Worn Bearings... 

Bottom draw nozzle too small. Pump cavitation problem. Raising tower 10 feet did not help. Flooded the bottom of the tower. Design error in original plant. 

Problem Three chemical plant recovery train towers were limited to half of design rates by bottoms pump cavitation and high tower pressure drop. 

Feedwater tank water temperatures on package boiler systems not fitted with deaerator heaters should be as high as practically possible, while consistent with the avoidance of FW pump cavitation. 

Feedwater supply duties include checking and recording the demand for FW makeup and maintaining correct FW temperatures to prevent risks of pump cavitation and boiler thermal shock. Periodically checking deaerator performance and inspecting the FW pumps and lines for any signs of fouling or corrosion is also required. 

Formation of vapor bubbles in rapidly flowing or turbulent water causing risk of pumping failure and erosion and/or corrosion. Due to an increase in velocity at the pump head resulting in a localized pressure reduction and the subsequent collapse of the vapor into voids or cavities. Where FW temperatures are high (over perhaps 195-205 °F) the pump velocity can reduce FW vapor pressure below that corresponding to the temperature of the liquid and cavitation can occur accompanied by some noise. Warning of severe pump cavitation is often indicated by a heavy noise. 

The pressure at the inlet to a pump must be high enough to prevent cavitation occurring in the pump. Cavitation occurs when bubbles of vapour, or gas, form in the pump casing. Vapour bubbles will form if the pressure falls below the vapour pressure of the liquid. 

In the above discussion it is assumed that the available NPSH in the system is adequate to support the flow rate of liquid into the suction side of the pump. If the available NPSH is less than that required by the pump, cavitation occurs and the normal curves do not apply. In cavitation, some of the liquid vaporizes as it flows into the pump. As the vapour bubbles are carried into higher pressure regions of the pump they collapse, resulting in noise and vibration. High speed pumps are more prone to cavitation than low speed pumps. 

To avoid pump cavitation on start-up, the experienced operator opens the pump discharge valve slowly. Slowly opening the discharge valve results in reduced acceleration of the liquid in the suction line and a slower rate of the conversion of suction pressure to velocity. 

Let s now assume that we wish to pump 300 GPM, not 250 GPM. If we open the flow-control valve shown in Fig. 25.1, the flow will momentarily increase. But, within a few seconds, the flow will become erratically low as the pump cavitates. The problem is that, according to Fig. 

Answer—yes But why Well, the liquid is cooled by 5°F after it leaves the drum. The cooled liquid is not in equilibrium with the vapor in the drum. It has been subcooled by 5°F. This means that the bubble-point liquid has been cooled, without altering its composition. The vapor pressure of the liquid has been reduced. As can be seen in Fig. 25.3, subcooling this particular liquid by 5°F reduces its vapor pressure by about 2 psi. As the specific gravity of the liquid is 0.58, this is equivalent to an increase in the NPSH by 8 ft. Once again, our objective is to increase the flow from 250 to 300 GPM. Figure 25.2 tells us that the required NPSH increases from 20 to 26 ft. However, when we subcool the liquid by 5°F, the available NPSH increases from 20 to 28 ft. As the available NPSH now exceeds the required NPSH by 2 ft, the flow can be increased without risk of pump cavitation. 

Regardless of their efforts, the pump cavitated so badly that it could not be put in service. Some pumps are like that. The design engineer just never allowed any extra liquid elevation for the starting NPSH. But what could I do ... 

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. 

This matches the required NPSH, at a flow of 110 GPM, so the pump cavitates. But it still seems as if I am missing at least half of the 46 ft of liquid head to the pump. Where is it ... 

Bottom liquid outlets. Sufficient residence time must be provided in the bottom of the column to separate any entrained gas from the leaving liquid. Gas in the bottom outlet may also result from vortexing or from forthing caused by liquid dropping from the bottom tray (a waterfall pool effect). Vortex breakers are commonly used, and liquid-drop height is often restricted. Inadequate gas separation may lead to bottom pump cavitation or vapor choking the outlet line. 

If a pump cavitates, one might try to eliminate or minimize it by removing restrictions at the pump suction. If the pump cavitates at high flows, a second pump should be started at a lower flow. If cavitation occurs at low flows, one might turn off the pump. An extreme option is to inject a compressible gas into the impeller. This reduces pump efficiency and capacity, but it can eliminate cavitation. 

T-l Less 1. Tank runs dry Pump cavitates Damage to pump LIA-1, Can reagent react/... 

P-1 Less 25. V-2 closed Pump cavitates Damage to pump FICA-1 See Event 1. 

Mobile-phase degassing is an important step in the LC/MS experiment and can be accomplished via on-line membrane or vacuum devices, sonication, helium sparging or as part of the mobile-phase filtration step. Degassing will eliminate pump cavitation, ensure reproducible retention times and minimize possible sputtering from the ion source. 

At high temperatures, RTFs break down into low-boiling compounds. Vapor of these compounds needs to be vented from time to time to prevent pump cavitation, reduced fluid flow rate, lower fluid flash point, and reduction in safety because of continuously increasing pressure. 

Dissolved air is not readily drawn out of solution. It becomes a problem when temperatures rise rapidly or pressures drop. Petroleum oils contain as much as 12% dissolved air. When a system starts up or when it overheats, this air changes from a dissolved phase into small bubbles. If the bubbles are very small in diameter, they remain suspended in the liquid phase of the oil, particularly in high viscosity oils. This can cause air entrainment, which is characterized as a small amount of air in the form of extremely small bubbles dispersed throughout the bulk of the oil. Air entrainment is treated differently than foam and is typically a separate problem. Some of the potential effects of air entrainment include pump cavitation, spongy and erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation, component wear due to reduced lubricant viscosity, equipment shutdown when low oil pressure switches trip, microdieseling... 

Abnormal II (All) The pH value exhibits high amplitude, high frequency oscillations (region 4 in Figure 7.12), which could be the outcome of a sensor failure or other equipment malfunction, such as pump cavitation. 

If a piunp is provided, do the pump and the system match Is the pump cavitating ... 

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