CENTRIFUGAL PUMPS

What is centrifugal pump?It is the type most widely used in the chemical industry for transferring liquids of all types- raw materials, materials in manufacture, and finished products—as well as for general services of water supply, boiler feed, condenser circulation, condensate return, etc.

These pumps are available through a vast range of sizes, in capacities from 0.5 m3/h to 2 × 104 m3/h (2 gal/min to 105 gal/min), and for discharge heads (pressures) from a few meters to approximately 48 Mpa (7000 lbf/in2). The size and type best suited to a particular application can be determined only by an engineering study of the problem.

A centrifugal pump is of kinetic energy type - it imparts energy to a liquid by means of centrifugal force produced by a rotating impeller. A positive displacement pump imparts energy by mechanical displacement. Piston, diaphragm, plunger, screw, vane, and gear pumps are some examples.

Advantages of Centrifugal PumpSimplicityLow first costuniform (non-pulsating) flowsmall floor space low maintenance expense quiet operationadaptability for use with a motor or a turbine drive

Working Principle

Works on the principle of centrifugal force. This is the force that pushes the liquid away from the centre(in tangential direction).Converting Prime Mover energy into Mechanical energy through shaft .Converting Mechanical energy into fluid energy through impeller.Converting kinetic Energy into pressure energy through the volute casing.

Major Parts

• Shaft: It transmit the torque/Power. and supporting the impeller and other rotating parts. Shaft is protected from erosion ,corrosion and wear at the seal chamber through renewable sleeve.

• Impeller: An Impeller has vanes that pushes the liquid through the impeller. Transmit energy into the fluid (hydraulic energy).

• Volute/Casing: Impeller are fitted inside the casing. Volute casing is a curved shaped ,increasing in cross sectional area. Volute reduces the velocity of the liquid and increases the pressure.

Major Parts

Volute Casing

Impeller/ Blades

Shaft

Types Of Impeller

OPEN SEMI OPEN CLOSED

Types Of ImpellerSingle suction   - pump with single suction impeller (impeller has suction cavity on one side only); simple design but impeller is subjected to higher axial thrust imbalance due to flow coming in on one side of impeller only.

Double suction - pump with double suction impeller (impeller has suction cavities on both sides); has lower NPSHR than single suction impeller. Pump is considered hydraulically balanced but is susceptible to uneven flow on both sides of impeller if suction piping is not done properly.

Types of VoluteSingle volute   - pump volute has single lip which is very easy to cast. Is usually used in small low capacity pumps where a double volute design is impractical due to relatively small size of volute passageway which make obtaining good quality commercial casting difficult. Pumps with single volute design have higher radial loads.

Double volute - pump volute has dual lips located 180 degrees apart resulting in balanced radial loads; most centrifugal pumps are of double volute design.

Action of a Centrifugal Pump

Power from an outside source is applied to shaft A, rotating the impeller B within the stationary casing C. The blades of the impeller in revolving produce a reduction in pressure at the entrance or eye of the impeller. This causes liquid to flow into the impeller from the suction pipe D. This liquid is forced outward along the blades at increasing tangential velocity. The velocity head it has acquired when it leaves the blade tips is changed to pressure head as the liquid passes into the volute chamber and thence out the discharge E.

System CurvesIn addition to the pump design, the operational performance of a pump depends upon factors such as the downstream load characteristics, pipe friction, and valve performance.

Typically, head and flow follow the following relationship:

(Q2)^2 = h2

(Q1)^2 h1

where the subscript 1 refers to the design condition and 2 to the actual conditions. The above equation indicates that head will change as a square of the water flow rate.

System CurvesFigure 10-42 shows the schematic of a pump, moving a fluid fromtank A to tank B, both of which are at the same level. The only forcethat the pump has to overcome in this case is the pipe friction, variationof which with fluid flow rate is also shown in the figure.

System CurvesOn the other for the use shown in Fig. 10-43, the pump in addition to pipefriction should overcome head due to difference in elevation betweentanks A and B. In this case, elevation head is constant, whereas the headrequired to overcome friction depends on the flow rate.

System CurvesFigure 10-44 shows the pump performance requirement of a valve opening and closing.

Pump SelectionOne of the parameters that is extremely useful in selecting a pump for a particular application is specific speed Ns.Specific speed of a pump can be evaluated based on its design speed,flow, and head:Ns =NQ^1/2 H^3/4Where N=rpmQ= flowrate in gpmH= head in ft.lbf/lbm

Specific speed is a parameter that defines the speed at whichimpellers of geometrically similar design have to be run to dischargeone gallon per minute against a one-foot head.

In general, pumps with a low specific speed have a low capacity and high specific speed, high capacity. Specific speeds of different types of pumps are shown inTable 10-14 for comparison.

Another parameter that helps in evaluating the pump suction limitations,such as cavitation, is suction-specific speed.S = NQ^1/2 (NPSH)^3/4

Typically, for single-suction pumps, suction-specific speed above 11,000 is considered excellent. Below 7000 is poor and 7000–9000 is of an average design. Similarly, for double-suction pumps, suction specific speed above 14,000 is considered excellent, below 7000 is poor, and 9000–11,000 is average.

Figure 10-45 shows the schematic of specific-speed variation for different types of pumps. The figure clearly indicates that, as the specific speed increases, the ratio of the impeller outer diameter D1 to inlet or eye diameter D2 decreases, tending to become unity for pumps of axial-flow type.

Typically, axial flow pumps are of high flow and low head type and have a high specific speed. On the other hand, purely radial pumps are of high head and low flow rate capability and have a low specific speed. Obviously, a pump with a moderate flow and head has an average specific speed.