Centrifugal pumps friction loss

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 well point system consists of several individual well points spaced at 0.6 m to 1.8 m intervals along a specified alignment. A well point is a well screen (length 0.5 to 1.0 m) with a conical steel drive point at bottom. Individual well points are attached to a riser pipe (diameter 2.5 to 7.5 cm) and connected to a header pipe (diameter 15 to 20 cm). At the midpoint, the header pipe is connected to a centrifugal pump. As yield at different well points may vary, a valve at the top of each riser pipe is used to control the drawdown so that the screen bottom is exposed. The pump provides 6 to 7.5 m of suction, but friction losses reduce the effective suction to 4.5 to 5.4 m. 

For onshore facilities, water may be supplied from local public water mains, storage tanks, lakes and rivers. In these cases a conventional horizontal pump is used. The preferred design for onshore fire water pumps is a horizontal centrifugal type with a relatively flat performance curve (i.e., pressure versus quantity). The discharge pressure is determined by the minimum residual pressure required at the most remote location of the facility flowing its highest practical demand with allowances added for piping friction losses. 

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. 

The preceding equations apply for the ideal case in which there are no frictional, leakage, or recirculation losses. In any real pump, however, these losses do occur, and their magnitudes can be determined only by actual tests. As a result, characteristic curves are usually supplied by pump manufacturers to indicate the performance of any particular centrifugal pump. Figure 14-36 shows a typical set of characteristic curves for a centrifugal pump. 

There are no valves in the gear pump to cause friction losses as in the reciprocating pump. The high impeller velocities, with resultant friction losses, are not required as in the centrifugal pump. Therefore, the gear pump is well suited for handling viscous fluids such as fuel and lubricating oils. 

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. 

For a forced vortex, the angular speed is constant and the liquid revolves as a solid body. Disregarding friction losses, Stepanoff (1993) claims that no power would be needed to maintain the vortex. The pressure distribution of this ideal solid body rotation is a parabolic function of the radius. When the forced vortex is superimposed on a radial outflow, the motion takes the form of a spiral. This is the type of flow encountered in a centrifugal pump. Particles at the periphery are said to carry the total amount of energy applied to the liquid. 

Because of the large flow velocities which are required (10-100 m/s) hydraulic friction losses as well as losses due to internal leakage through the rotor clearances are high. The total efficiency of centrifugal pumps depends on the specific speed rtq (Figure 9.8) [11, 12]. 

The centrifugal pump is the most common motive foree for transportation of liquids. But this type of pump is not a souree of constant pressure instead, pressure varies with flow in the manner described by the curve of F ig. 2.11. Frictional losses within the pump cause this variation, much as internal resistance in a battery makes terminal voltage fall as current drain is increased. The equation of the curve is readily derived, with Cr representing the flow coefficient of the internal resistance. If Po is the no flow pressure, the drop within the pump is... 

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. 

Both closed and open impellers have been widely used in centrifugal pumps handling slurries. Open impellers have some advantages when compared with closed ones. For instance, low manufacturing cost and low disc friction loss are considerable advantages of these impellers. Furthermore, they are suited for pumping suspensions. This types of impeller have only one shroud and are open on one side where a tip clearance takes place between open side of the impeller and the stationary casing as is seen in Fig. 1. 

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. 

---