Centrifugal pumps in series and parallelDocument Transcript
Centrifugal Pumps Pumps are machines that are used to transfer liquid from a location of low elevation to ahigher elevation. There are various types of pumps and they are classified in two major categories:(1) dynamic or kinetic and (2) positive displacement. Dynamic or kinetic are types of pumps in which energy is continuously added to the fluid toincrease its velocity. Centrifugal, jet and turbine pumps falls under this category. Positive displacement pumps, however, are types, in which energy is continuously added byapplication of force to an enclosed volume of fluid and resulting to a direct increase in its pressure.Reciprocating, rotary and diaphragm pumps falls under this category.CENTRIFUGAL PUMPS A centrifugal pump consists of a stationary casing and an impeller connected in rotatingshaft. Liquid enters the center of the rotating impeller and leaves at a high velocity and passes to astationary volute casing which transforms kinetic energy into pressure. The term “centrifugal” camefrom the centrifugal force created as the water move outward from the center of the impellerrotation. Figure 1.1 shows a schematic diagram of a centrifugal pump. Discharge Connecting shaft Impeller center Volute Suction Impeller FIGURE 1.1: A wind turbine converting kinetic energy from the wind to produce electrical powerAdvantages and Disadvantages of Centrifugal Pumps Advantages Simple and compact Easy to maintain Adaptability to motor with high rpm Little vibrations Flow can be controlled from full to non-discharge without shutting the pump
Disadvantages Poor suction power Usually needs priming Cavitation may develop during operation Needs multistage to increase discharge pressure Cannot handle very viscous fluid Check valve is required to avoid back flowPUMP INSTALLATION The following are some of the guidelines and precautions in installing a pump. It is alsoimportant to consult the pump manufacturer for additional recommendations and furtherinformation regarding the pump to be used. Note that the task of the designer is to keep the headlosses at a low value as possible. Figure shows a typical pump installation of a pump. Make sure that the suction line is air tight. Any air leaking in the suction line forms air pockets that reduce the pump capacity. Plan to install the suction line of the pump in a manner that air pockets cannot form inside it. Both the suction and discharge lines should be supported independently to protect the casing from strains that may cause distortion to the pump. All piping lines should be short and straight as possible, with minimum elbows, valves and fittings. The suction line should be at least one or two sizes larger than the suction flange. Use eccentric reducer between the suction pipe and suction flange to have a better fluid flow. The inlet end of the suction line should be at least 3 to 6 feet below the minimum water level of the water source. Keep the suction inlet end away from agitation, because there is a tendency for the air to mix with the liquid and may impair the operation of the pump. Maintain a length of straight piping of at least four to six pipe diameters long between the elbow and suction flange of the pump to even out the flow of the water before it enters the pump. An elbow attached at the suction side should have a large radius. Install a gate valve and a check valve at the discharge line near the pump. The gate valve is used to regulate the flow and the check valve prevents backflow of liquid into the pump. Install a foot valve at the end of the suction line to hold the water in the suction line of the pump. If foot valve is not used, end of the suction end should be belled out to reduce the entrance velocity of the liquid. Install the pump in a secure location and at the same time, available for inspection and maintenance. If possible, place the pump as near the water source to keep suction lift at a minimum. The foundation should be heavy and rigid to reduce vibrations that may cause misalignment between the pump shaft and motor or prime mover.
PUMP IN SERIES AND PARALLEL Pumps can be installed in series or in parallel operation to achieve addition total dynamichead or capacity.Pumps in Series Pumps in series are done by staging two pumps as shown in Figure 1.2. The total dynamichead is increased at a given capacity as shown in the performance curve. TWO PUMPS IN SERIES CENTRIFUGAL PUMP 160 TOTAL DYNAMIC HEAD IN FEET 140 Curve for 2 pumps in 120 series 100 Curve for 1 pump 80 60 40 20 0 40 80 120 160 200 240 GALLONS PER MINUTE FIGURE 1.2: Pumps installed in seriesPumps in Parallel Pumps in parallel are the result of installing two pumps as shown in Figure 1.3. The capacityis doubled while maintaining the total dynamic head. TWO PUMPS IN PARALLEL CENTRIFUGAL PUMP 160 TOTAL DYNAMIC HEAD IN FEET 140 120 Curve for 2 pumps in 100 Parallel 80 Curve for 1 pump 60 40 20 0 40 80 120 160 200 240 GALLONS PER MINUTE FIGURE 1.3: Pumps installed in parallel