Pumps cavitation problems

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. 

Excessive amounts of foam in a vessel can result in a host of operating and production problems, including reduced product throughput, level control problems, excessive liquid entrainment out the vapor outlet piping, excessive vapor out the liquid underflow piping, and underflow pump cavitation problems. If foaming occurs in a compressor knockout drum it can easily flood a mesh pad or vane (chevron) separator. When this occurs, the foam carryover from the drum can severely damage the gas compressor downstream. 

As the mechanical integrity of the pump system changes, the amplitude of vibration levels change. In some cases, in order to identify the source of vibration, pump speed may have to be varied, as these problems are frequency- or resonance-dependent. Pump impeller imbalance and cavitation are related to this category. Table 10-11 classifies different types of pump-related problems, their possible causes and corrective actions. 

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

Avoid overheating FW tanks and the generation of steam in the tank. If the FW temperature is too high and there is inadequate net positive static head (NPSH) on the FW pump suction, the pump will not operate and cavitation problems may occur. 

COMMENTS The Carnot vapor cycle as illustrated by Example 2.1 is not practical. Difficulties arise in the isentropic processes of the cycle. One difficulty is that the isentropic turbine will have to handle steam of low quality. The impingement of liquid droplets on the turbine blade causes erosion and wear. Another difficulty is the isentropic compression of a liquid-vapor mixture. The two-phase mixture of the steam causes serious cavitation problems during the compression process. Also, since the specific volume of the saturated mixture is high, the pump power required is also very high. Thus, the Carnot vapor cycle is not a realistic model for vapor power cycles. 

Water enters the pump at state 1 as a low-pressure saturated liquid to avoid the cavitation problem and exits at state 2 as a high-pressure compressed liquid. The heat supplied in the boiler raises the water from the compressed liquid at state 2 to saturated liquid to saturated vapor and to a much higher temperature superheated vapor at state 3. The superheated vapor at state 3 enters the turbine where it expands to state 4. The superheating moves the isentropic expansion process to the right on the T-s diagram as shown in Fig. 2.5, thus preventing a high moisture content of the steam as it exits the turbine at state 4 as a saturated mixture. The exhaust steam from the turbine enters the condenser at state 4 and is condensed at constant pressure to state 1 as saturated liquid. 

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. 

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. 

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. 

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... 

In Table 5.10, the deviations Low and No are merged since they often lead to essentially the same discussion. However, they should be used separately where appropriate. For example, Low Level in a tank may lead to little more than production problems, whereas No Level in that tank could create major hazards such as pump cavitation and air ingress into the tank. 

Checklists also provide a means for the operator to communicate with his supervision. For example, he might note that there was a difficulty with one of the steps in the module, although it was not of such severity to prevent the action sequence from being continued. For example, the operator may note that the pump cavitates while he is carrying out the following instruction Open the bypass around FCV-121. Therefore, he could write on to the check list that he had noticed a problem. 

Depending on its severity, cavitation in pumps can result in loss of performance, severe erosion, vibration and noise. All these effects may be minimised by attention to design and operation, and by prudent use of erosion-resistant materials. Pumps vary considerably in design and function, and it is convenient to use the centrifugal pump to illustrate cavitation problems because of its common usage in fluid systems. 

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. 

A true vortex breaker is normally inserted just ahead of the vessel s liquid exit nozzle as shown in Fig. 13.1.1. This is a very important feature in the geometry at hand since the angular momentum of the incoming gas-liquid mixture will produce bulk rotation of the liquid pool. If a vortex is allowed to form, some of the incoming gas may exit out the underflow and create pump cavitation or other problems downstream. The vortex will also act as a type of fluidic choke and restrict the flow rate out the bottom liquid exit nozzle. 

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