Centrifugal pumps head losses

It is important to recognize that a centrifugal pump will operate only along its performance curve [10, 11]. External conditions will adjust themselves, or must be adjusted in order to obtain stable operation. Each pump operates within a system, and the conditions can be anticipated if each component part is properly examined. The system consists of the friction losses of the suction and the discharge piping plus the total static head from suction to final discharge point. Figure 3-51 represents a typical system head curve superimposed on the characteristic curve for a 10 by 8-inch pump with a 12-inch diameter impeller. 

Centrifugal pumps, 181 Discharge systems, 187 Example calculation, 186 Flow friction losses, 185. 186 Friction losses, pipe, see Chapter 2 Friction, 188 Pressure head, 184—186 Static head, 184-186 Suction head, 184, 185 Suction lift, 184, 185 Suction systems, 186 Hvdroclones, 265—267 Application system, 267 Ignition, flammable mixtures, 493 Impellers, centrifugal, reducing diameter, 203 Impellers,... 

The head against which the liquid is to be pumped. This will be determined by the difference in pressure, the vertical height of the downstream and upstream reservoirs and by the frictional losses which occur in the delivery line. The suitability of a centrifugal pump and the number of stages required will largely be determined by this factor. 

A centrifugal pump is to be used to extract water from a condenser in which the vacuum is 640 mm of mercury. At the rated discharge the net positive suction head must be at least 3 m above the cavitation vapour pressure of 710 mm mercury vacuum. If losses in the suction pipe account for a head of 1.5 m. what must be the least height of the liquid level in the condenser above the pump inlet ... 

The static pressure difference will be independent of the fluid flow-rate. The dynamic loss will increase as the flow-rate is increased. It will be roughly proportional to the flow-rate squared, see equation 5.3. The system curve, or operating line, is a plot of the total pressure head versus the liquid flow-rate. The operating point of a centrifugal pump can be found by plotting the system curve on the pump s characteristic curve, see Example 5.3. 

Lack of available NPSH may also be caused by high frictional loss in the suction piping. If this is the case, a small reduction in flow will not noticeably increase the pressure at the suction of the pump. A properly designed suction line to a centrifugal pump should have a frictional head loss of only a few feet of liquid. However, having a large-diameter suction line, and a relatively small draw-off nozzle, usually will lead to excessive loss of available NPSH. 

Plot the effect of pump wear. When a pump wears, there is a loss in capacity and efficiency. The amount of loss depends, however, on the shape of the system-head curve. For a centrifugal pump (Fig. 6.30), the capacity loss is greater for a given amount of wear if the system-head curve is flat, as compared with a steep system-head curve. 

ACTUAL PERFORMANCE OF CENTRIFUGAL PUMP. The developed head of an actual pump is considerably less than that calculated from the ideal pump relation of Eq. (8.21). Also, the efficiency is less than unity and the fluid horsepower is greater than the ideal horsepower. Head losses and power losses will be diseussed separately. 

When a centrifugal pump is taking suction from a tank or other reservoir, the pressure at the suction of the pump is the sum of the absolute pressure at the surface of the liquid in the tank, plus the pressure due to the elevation difference between the surface of liquid in the tank, and the pump suction less the head losses due to friction in the suction Une from the lank to the pump. 

SeUgrem et al. (2000) conducted tests on sand as well as sand-clay mixtures pumped by centrifuged pumps. The phosphate clays had a diameter d Q between 1 p-m and 50 p,m. The sands were much coarser with of 0.64 mm (0.025 in), 1.27 mm (0.05 in), and 2.2 mm (0.09 in). The presence of clay and other particles finer than 75 p,m and a concentration smaller than 20% had a beneficial effect by reducing the head loss and efficiency derating factor. The data recorded by SeUgrem et al. (2000) should not be applied to a higher concentration of clays because the viscosity effect introduces a new component to the equation. 

Later in this chapter, we formally set out the tools used for sizing head losses. The elements used for selecting pumps are introduced in Chapter 5. Different types of pumps are presented (centrifugal pumps, axial pumps, positive displacement pumps), and the reasoning that guides the choice of a pump, based on the levels of flow rate and head to be delivered, is discussed. 

Centrifugal pumps exhibit slippage. They impart momentum to the fluid, which is converted to velocity head. At no-flow conditions, rota tion of an impeller of a given diameter at constant speed produces the maximum head which the pump is capable of delivering. As flow increases, the head falls by an amount equivalent to frictional losses within the pump itself. It should be noted that the pressure which a centrifugal pump is capable of delivering varies with the density of the fluid, since pressure equals head times density. 

Small pumps are usually driven by constant-speed electrical motors. Flow may be controlled by throttling a valve in the discharge line. The suction should never be throttled, because a centrifugal pump requires a positive suction head to operate. Low suction pressure causes cavitation and loss of flow. 

The net suction lift possible with a pump amounts to the barometric pressure less the following items (1) true vapor pressure of liquid at the pumping temperature, (2) pressure caused by weight and spring of suction valve, and (3) entry friction losses. Should this computation come out to be negative it means that a net positive suction head is necessary, that the pump can produce no lift. Centrifugal pumps should always be provided with a flooded suction (positive head), because there is no convenient way to prime or fill them with liquid and they cannot operate without liquid in the case. 

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