3.2 Pump Characteristics - TU Delft OCW

ial outflowo Mixed flow impellor with axial inflow radial, axialand tangential outflowSuctionshieldPropellorpumpo Axial flow impellor with axial inflow and axial tangential outflowThe different types are illustrated in figure 3.11.Figure 3.12 gives a five stage centrifugal pump. Theblades of this pump are closed.Figure 3.13 gives a two stage ground water abstraction pump. The motor is under the stages and alsosub merged. This is a common way of powering thesekind of pumps. Long shafts are difficult to balanceand sensitive to failure and are not preferred to a direct connection between pump and motor.PressureshieldSealingWeir inletAxleWeirWeir outletFig. 3.12 - Five stage centrifugal pumpCentrifugal impellors can be open or closed. The picture of the five stage pumps shows closed blades. Infigure 3.14 a picture of an open blade in the form af apropellor is shown.HouseFig. 3.10 - Principle impellor pump4Figure 3.15 gives an example of a cut open pump withan open blade.

Pumps and pumping stationsThe axial propellor pumpsare best applied for large volume flows and low heads(less than 1 ato or 10 meter). The application formixed flow pumps is in therange of 3 to 30 meters headand is characterised with afairly constant efficiencywith varying heads.Centrifugal pumps are usedwhen high heads are required (above 20 meters).With serial or multi stagepumping heads up to 1000meter can be reached.Fig. 3.14 - Propellor3.3.4 Specific speedSpecific speedNs n Q3H4Fig. 3.15 - Glass pumpcentrifugalMixedflowaxialEfficiency in %To classify geometricallysimilar pumps, the numerical quantity ‘specific speed’has been adopted. Specificspeed is the speed requiredfor delivery of unit flowagainst unit head; it will varyin accordance with the system of units usedPressure in mN is the pump impellerspeed in rpm; Q is outputat maximum efficiency (m3/s); H is delivery head atmaximum efficiency (m) ascan be read from the efficiency curve. Specificspeeds are an indication ofwhat type of impeller pumpcan be used for a specificFig. 3.13 - Ground water application.pump withFollowing the definition oftwo stagesspecific speed a pump witha low specific speed willgenerally give a low volume flow with a high pressure.A radial flow pump will be most suited. A high specificvolume flow in m3/sspec. speedspeed in r/minFig. 3.16 - Relation between specific and actual speed,5

CT5550 - Water Transportspeed will result in a relative large volume flow at arelative low pressure, resulting in a preference for anaxial pump.Figure 3.16 gives a relation between specific speed,actual speed, pressure, volume flow and efficiency.3.3.5 Working principle impellor pumpsThe rotation of the axle accelerates the fluid addingkinetic energy to the fluid. Within the house or diffuserof the pump this kinetic energy is partly transformedto static pressure, increasing the head of the fluid.At the suction side of the impellor the fluid accelerates as well and pressure at that point will be lowered, following the laws of Bernoulli. Energy is conserved and at the suction side of the pump the pressure of the fluid will be partly transformed to kineticenergy. This requirement is expressed in the Net Positive Suction Head (NPSH) of a pump. At high volumeflows, the NPSH will increase, because the kineticenergy at the suction side will be higher due to thehigher fluid velocity.The form of the blades on the impellor affects the characteristics of the pump. Different shapes of bladesare given in figure 3.17. Every blade has its own set ofpump curves.and the head realised by a single pump (figure 3.3).On the other hand the network served can be characterised with the pipe characteristic. Every set orsystem of pipes can be virtually represented by asingle pipe with equivalent hydraulic features. In theend a complete network can hydraulically be represented by one pipe, called the network characteristic.The hydraulic losses in this characteristic pipe are

pumps. The impeller pumps can be further catego-rised based on the type of impellor in Radial flow pumps, mixed flow pumps and axial flow pumps. All types of pumps will be described in this chapter. 3.2 Pump characteristics The hydraulic properties of a pump can be described by some characteristics: o Q-