Centrifugal pumps priming

The precise flow-decay pattern will depend on the type, size, and dimensions of the pump. Flow for a typical centrifug pump will begin to decay at the NPSH point, but some additional fluid transfer will usually occur before a steady backflow of air through the pump begins. At that point, the pump s priming is completely lost. 

Gas content is another important effect. It is well known that centrifugal pumps will not pump high gas content mixtures, as flow breaks down (the pump loses prime ) when the gas/liquid ratio rises beyond 15 per cent. 

Positive displacement pumps are self-priming by their normal operating action and are designed to cope with mnning dry. Centrifugal pumps are not inherently selfpriming, and need to be provided with assistance in the form of ejectors, as in Figure 32.56 or dry vacuum pump systems. In these systems, a dry vacuum pump allows air to be drawn out and where an automatic valve prevents... 

Equation 4.10 shows that when a centrifugal pump runs on air, the pressure developed is very small. In fact, a conventional centrifugal pump can never prime itself when operating on a suction lift. 

Electric motors are frequently used to drive small piston pumps that prime the main centrifugal pumps. 

The only real difference between a sump pump and an ordinary centrifugal pump is that the sump pump is more difficult to prime than the ordinary pump. 

Although centrifugal pumps are the major kinds in use, a great variety of other kinds exist and have limited and sometimes unique applications. Several kinds of positive displacement types are sketched in Figure 7.12. They are essentially self-priming and have a high tolerance for entrained gases but not usually for solids unless they may be crushed. Their characteristics and applications are discussed in the next section. 

PUMP PRIMING. Equation (8.21) shows that the theoretical head developed by a centrifugal pump depends on the impeller speed, the radius of the impeller, and the velocity of the fluid leaving the impeller. If these factors are constant, the developed head is the same for fluids of all densities and is the same for liquids and gases. The increase in pressure, however, is the product of the developed head and the fluid density. If a pump develops, say, a head of 100 ft and is full of water, the increase in pressure is 100 x 62.3/144 = 43 (2.9 atm). If the pump is... 

Thus, to get a centrifugal pump going, it is not enough to start the motor. One must also replace the air in the system y/ith liquid. This is called priming. Numerous schemes for performing this function are available in addition, special self-priming pumps have been patented and manufactured. 

Most pumps in HCl service are nonmetallic. Wetted parts may be PVC, CPVC, PP, PTFE, PVDF, FRP, ceramic, or carbon. Self-priming centrifugal pumps are a standard, but diaphragm-type metering pumps also are widely used. 

The main disadvantages of centrifugal pumps are the limitations of delivery pressure and their inability to prime themselves. The former can be overcome by using twin or multi-stages, usually on the same axis, and driven by one motor. The fitting of a self-primer, described in Chapter 6, will eliminate the latter disadvantage. 

The main disadvantages of centrifugal pumps are the sealing of the impeller shaft and the inability of the pump to be self-priming. The latter is only a disadvantage if the pump cannot be positioned so that its suction is flooded (see Fig. 6.2). 

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