CENTRIFUGAL PUMPS e-notes: Dr.N.Balasubramanya Professor, Department of Civil Engineering M.S.Ramaiah Institute of Technology, Bangalore

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2 CENTRIFUGAL PUMPS e-note: Dr.N.Balaubramanya Profeor, Department of Civil Engineering M.S.Ramaiah Intitute of Technology, Bangalore-5654 A pump i a hydraulic machine which convert mechanical energy into hydraulic energy or preure energy. A centrifugal pump i alo known a a Rotodynamic pump or dynamic preure pump. It work on the principle of centrifugal force. In thi type of pump the liquid i ubjected to whirling motion by the rotating impeller which i made of a number of backward curved vane. The liquid enter thi impeller at it center or the eye and get dicharged into the caing encloing the outer edge of the impeller. The rie in the preure head at any point/outlet of the impeller i Proportional to the quare of the tangential velocity of the u liquid at that point (i.e, α ). Hence at the outlet of the impeller where the radiu i g more the rie In preure head will be more and the liquid will be dicharged at the outlet with a high preure head. Due to thi high preure head, the liquid can be lifted to a higher level. Generally centrifugal pump are made of the radial flow type only. But there are alo axial flow or propeller pump which are particularly adopted for low head. Advantage of centrifugal pump:-. It initial cot i low. Efficiency i high. 3. Dicharge i uniform and continuou 4. Intallation and maintenance i eay. 5. It can run at high peed,without the rik of eparation of flow Claification of centrifugal pump Centrifugal pump may be claified Into the following type.according to caing deign a) Volute pump b) diffuer or turbine pump. According to number of impeller a) Single tage pump b) multitage or multi impeller pump 3. According to number of entrance to the impeller: a) Single uction pump (FOR FIGURES DOWNLOAD PRESENTATION) b) Double uction pump 4. According to dipoition of haft a) Vertical haft pump b) Horizontal haft pump 5. According to liquid handled a) Semi open impeller b) Open impeller pump

3 6.According to pecific peed a) Low pecific peed or radial flow impeller pump b) Shrouded impeller c) Medium pecific peed or mixed flow impeller pump c) High pecific peed or axial flow type or propeller pump. 7. According to head (H) Low head if H<5m Medium head if 5<H<4m High head if H>4m In the cae of a volute pump a piral caing i provided around the impeller. The water which leave the vane i directed to flow in the volute chamber circumferentially. The area of the volute chamber gradually increae in the direction flow. Thereby the velocity reduce and hence the preure increae. A the water reache the delivery pipe a coniderable part of kinetic energy i converted into preure energy. However, the eddie are not completely avoided, therefore ome lo of energy take place due to the continually increaing quantity of water through the volute chamber. In the cae of a diffuer pump the guide wheel containing a erie of guide vane or diffuer i the additional component. The diffuer blade which provide gradually enlarging paage urround the impeller periphery. They erve to augment the proce of preure built up that i normally achieved in the volute caing. Diffuer pump are alo called turbine pump in view of their reemblance to a reaction turbine. Multitage pump and vertical haft deep-well pump fall under thi category. Centrifugal pump can normally develop preure upto kpa (m). If higher preure are required there are three option.a) Increae of impeller diameter. b)increae of Rpm. c)ue of two or more impeller in erie. The pump look clumy in option (a). The impeller material i heavily treed in option (b) The third choice i the bet and i generally adopted, the impeller which are uually of the ame ize are mounted on the ame haft. The unit i called a multitage pump. It dicharge the ame quantity of fluid a a ingle tage pump but the head developed i high. There are centrifugal pump upto 54 tage. However, generally not more than tage are required. In the cae of the double uction impeller, two impeller are et back to back. The two uction eye together reduce the intake. The two uction eye together reduce the intake velocity reduce the rik of cavitation. Mixed flow type double uction axial flow pump beide are capable of developing higher head. For convenience of operation and maintenance, horizontal haft etting are the preferred etup for centrifugal pump. The exception are deep-well turbine pump and axial flow pump, thee have vertical haft. Retricted pace condition uually require a vertical haft etting. Centrifugal impeller uually have vane fitted between the hroude or plate. The crown plate ha the uction eye and the bae plate i mounted on a leeve which i keyed to the haft. An impeller without the crown plate i called the non-clog or emiopen impeller. In an open impeller both crown plate and the bae plate are abent. Only clear liquid, can be afely pumped by a hrouded impeller pump. The emi-open impeller i ueful for pumping liquid containing upended olid, uch a ewage, molae or paper pulp. The open-vane impeller pump i employed for dredging operation in harbour and river. Shrouded and emi open impeller may be made of cat

4 iron Or cat teel. Open vane impeller are uually made of forged teel. If the liquid pumped are corroive, bra, bronze or gun metal are the bet material for making the impeller. A radial flow impeller ha mall pecific peed (3 to ) & i uitable for dicharging relatively mall quantitie of flow againt high head. The direction of flow at exit of the impeller i radial. The mixed flow type of impeller ha a high pecific peed (5 to 5), ha large inlet diameter D and impeller width B to handle relatively large dicharge againt medium head. The axial flow type or propeller impeller have the highet peed range (5 to,). They are capable of pumping large dicharge againt mall head. The pecific peed of radial pump will be <N<8, Axial pump <N<45, Mixed flow pump 8<N<6. Component of a centrifugal pump The main component of a centrifugal pump are: i) Impeller ii) Caing iii) Suction pipe iv) Foot valve with trainer, v) Delivery pipe vi) Delivery valve. Impeller i the rotating component of the pump. It i made up of a erie of curved vane. The impeller i mounted on the haft connecting an electric motor. Caing i an air tight chamber urrounding the impeller. The hape of the caing i deigned in uch a way that the kinetic energy of the impeller i gradually changed to potential energy. Thi i achieved by gradually increaing the area of cro ection in the direction of flow. Suction pipe It i the pipe connecting the pump to the ump, from where the liquid ha to be lifted up. Foot valve with trainer the foot valve i a non-return valve which permit the flow of the liquid from the ump toward the pump. In other word the foot valve open only in the upward direction. The trainer i a meh urrounding the valve, it prevent the entry of debri and ilt into the pump. Delivery pipe i a pipe connected to the pump to the overhead tank. Delivery valve i a valve which can regulate the flow of liquid from the pump. Priming of a centrifugal pump Priming i the proce of filling the uction pipe, caing of the pump and the delivery pipe upto the delivery valve with the liquid to be pumped. If priming i not done the pump cannot deliver the liquid due to the fact that the head generated by the Impeller will be in term of meter of air which will be very mall (becaue pecific weight of air i very much maller than that of water). Priming of a centrifugal pump can be done by any one of the following method: i) Priming with uction/vacuum pump. ii) Priming with a jet pump. iii) Priming with eparator. iv) Automatic or elf priming. Head on a centrifugal pump: Suction head (h): it i the vertical ditance between the liquid level in the ump and the centre line of the pump. It i expreed a meter. Delivery head (hd): It i the vertical ditance between the centre line of the pump and the liquid level in the overhead tank or the upply point. It i expreed in meter. 3

5 Static head (H): It i the vertical difference between the liquid level In the overhead tank and the ump, when the pump i not working. It i expreed a meter. Therefore, HS= (h+ hd) Friction head (hf): It i the um of the head lo due to the friction in the uction and delivery pipe. The friction lo in both the pipe i calculated uing the Darcy equation, hf=(flv/gd). Total head (H): It i the um of the tatic head H, friction head (hf) and the velocity head in the delivery pipe (Vd /g). Where, Vd=velocity in the delivery pipe. Vd H () m h hd hf g Manometric head(hm): It i the total head developed by the pump. Thi head i lightly le than the head generated by the impeller due to ome loe in the pump V Vd Hm H g g Working of a centrifugal pump: A centrifugal pump work on the principal that when a certain ma of fluid i rotated by an external ource, it i thrown away from the central axi of rotation and a centrifugal head i impreed which enable it to rie to a higher level. Working operation of a centrifugal pump i explained in the following tep. ) Cloe the delivery valve and prime the pump. ) Start the motor connected to the pump haft, thi caue an increae in the impeller preure. 3) Open the delivery valve gradually, o that the liquid tart flowing into the deliver pipe. 4) A partial vacuum i created at the eye of the centrifugal action, the liquid ruhed from the ump to the pump due to preure difference at the two end fo the uction pipe. 5) A the impeller continue to run, move & more liquid i made available to the pump at it eye. Therefore impeller increae the energy of the liquid and deliver it to the reervoir. 6) While topping the pump, the delivery valve hould be cloed firt, otherwie there may be back flow from the reervoir. It may be noted that a uniform velocity of flow i maintained in the delivery pipe. Thi i due to the pecial deign of the caing. A the flow proceed from the tongue of the caing to the delivery pipe, the area of the caing increae. There i a correponding change in the quantity of the liquid from the impeller. Thu a uniform flow occur in the delivery pipe. Operation difficultie in centrifugal pump a) Pump fail to pump the fluid. 4

6 Caue ) Improper priming due to leakage of foot valve or incomplete filling. Remedial Meaure Repair or replace the foot valve, prime completely. ) Head more than deign head Reduce the head or change the pump 3) Clogging of impeller, uction pipe or trainer Clean the upected part 4) Suction lift may be exceive Reduce the height of pump above the ump 5) Speed more than deign peed Connect another prime mover of higher peed 6) Direction of rotation of impeller i wrong Change the direction. B) Pump doe not give the required capacity a) Leakage of air through the uction pipe or through the gland packing Stop the leakage b) Damage to ome part of the pump by wear & tear Replace the damaged part c) Clogging of impeller paage Clean the impeller C) Pump ha poor efficiency a) Higher than deign peed Reduce the peed 5

7 b) Low head & higher dicharge Reduce the dicharge c) Impeller touching, the caing or improper alignment of haft Carryout the neceary repair. D) Pump top working a) Air entry into uction pipe Stop the pump, plug the leakage, reprime and tart b) Suction lift i high Reduce the uction lift. Efficiencie of centrifugal pump Manometric efficiency (η): it i the ratio of the manometric head to the head actually Hm ghm generated by the impeller n mano Vw u/g Vw u Mechanical efficiency(η mech): It i the ratio of the impeller power to the power of the impeller power motor or the prime mover. n mech motor power Overall efficiency(ηo): It i the ratio of the work done by the pump in lifting water againt gravity and friction in the pipe to the energy upplied by the motor. work done againt gravity friction no power of the prime mover or motor Velocity Triangle of a Centrifugal Pump Figure how the inlet and outlet velocity triangle for a centrifugal pump.it may be noted that the inlet velocity triangle i radial,(velocity of whirl i zero at inlet or VW = Depending on the geometry of the blade at outlet it can be: Forward:if the blade angle<9,radial if Φ=9, c) Backward if 9 Work done by the impeller of a centrifugal pump: Figure how the velocity triangle at the inlet and outlet tip of a vane fixed to the impeller. Let N=peed of the impeller in RPM D= Diameter of the impeller at inlet 6

8 D=Diameter of the impeller at outlet U = Tangential velocity of the impeller at inlet πd N/6 U = tangential velocity of the impeller at outlet πd N/6 V =abolute velocity of the liquid at inlet V = abolute velocity of the liquid at outlet. Vf & Vf are the velocitie of flow at inlet and outlet. Vr & Vr Relative velocitie at inlet and outlet Vw whirl velocity at outlet angle made by V with repect to the motion of the vane blade angle at inlet = blade angle at outlet For a erie of curved vane the force exerted can be determined uing the impule momentum equation Work=force x ditance. imilarly the work done/ec/unit weight of the liquid triking the vane= (Vw u Vw u ) g But for a centrifugal pump Vω Work done/ec/unit weight Vw u g Q And the work done/ec Vw u (4) g Where Q=volume of liquid flowing per econd = Area x velocity of flow Q πdbvf (5) In eq (5), B i the width of the impeller at the outlet. Deign factor of centrifugal pump: a) Rim diameter D πdn Rim velocity or impeller velocity u K u ghm 6 Rim diameter D 6 gku Hm πn = 85 K N Hm u Where N= peed in RPM Hm= manometric head, m Ku U/ ghm peed ratio Value of varie from.95 to.8 depending on the pecific peed. b) Pipeline diameter: The diameter of ection and delivery pipe are deigned to give velocitie not exceeding.5 to 3 m/ on ection and delivery ide. c) Dicharge (Q): the dicharge or capacity of a centrifugal pump i given by Where k =factor which account the reduction in flow area due to To thickne of impeller vane, D =Rim diameter,b =Rim width, 7

9 Vf=Contant velocity of flow through the impeller. Generally k= i conidered. PROBLEMS.A centrifugal pump running at 8 Rpm i working againt a total head of. m. the external diameter of the impeller i 48mm and outlet width 6mm. If the valve angle at outlet i 4 and manometric efficiency i 7% determine a)abolute velocity of water leaving b) Flow velocity at outlet The valve. c) Angle made by the abolute velocity at outlet with the direction of motion at outlet. d) Rate of flow through the pump. D N x.48x8 Soln: velocity of valve at outlet u.m 6 6 ghm manometric efficiency n Vw u, 9.8x. mano.7 Vw x, Vw 4.8m. Vf From the outlet velocity triangle tan u Vw Vf tan 4 x(.4.8) 5.5m Abolute velocity of water leaving the valve V i given by V Vf Vw m Angle made by the abolute velocity at outlet with the direction of motion i given by Vf 5.5 tan Vw 4.8 Rate of flow through the pump Q D B Vf x.48x x.457m 3/. A centrifugal pump impeller having external and internal diameter 48mm and 4mm repectively i running at Rpm. The rate of flow through the pump i.576 m3/ and velocity of flow i contant and equal to.4m/. the diameter of the ection and delivery pipe are 8mm and mm repectively and ection and delivery head are 6.m(ab) and 3.m(ab) of water repectively. If the power required to drive the pump i 3.3KW and the outlet vane angle i 45 determine. a) inlet vane angle b) Overall efficiency c) manometric efficiency of the pump Soln: tangential velocity or impeller velocity at inlet D N x.4x u.56m 6 6 Vf.4 From the inlet velocity triangle tan. 9 u.56.8 ( inlet vane angle) 8

10 Overall efficiency n rqhm 9.8x.5xHm n.387hm () P 3.3 p V p V but, Hm Z Z r g r g 4Q 4x.567 where, V V 5.m d dd x. / 4Q 4x.567 where, V V.3m d x.8 / let Z Z i.e pump inlet and outlet are at ame level. p h 6.m( p ) h 3.m( ) r r ab d Hm m x9.8 x9.8 3 n, overall efficiency of pump =.387x5.3=.597=59.7% Velocity of the impeller at outlet D N x.48x u 5.3m 6 6 From the outlet velocity triangle Vf.4 tan, tan 45 u Vw 5.3 Vw, Vw.73m ghm 9.8x5.3 Manometric efficiency n mano.43 43% Vw u.73x It i required to deliver.48m3/ of water to a height of 4m through a 5mm diameter and m long pipe by a centrifugal pump. If the overall Efficiency of the pump i 75% and co efficient of friction f=. for the pipe line. Find the power required to drive the pump. 4Q Soln: velocity of water pipe V Vd V m d 4x.48.7 / x.5 rqhm 9.8x.48x7. 37 Overall efficiency n.75, P 7. KW P P 4. The impeller of a centrifugal pump i of 3mm diameter and 5mm width at the periphery and ha blade whoe tip angle incline backward 6 from the radiu. The pump deliverie 7m3/min of water and the impeller rotate at Rpm. Auming that the pump i deign to admit radically. calculate a)speed and direction of water a it leave the impeller, b)torque exerted by the impeller on water c) Shaft power required 9

11 d) Lift of the pump. Take mechanical=95% and hydraulic efficiency=75% Soln: tangential velocity of the impeller at the outlet D N x.3x u 5.7m 6 6 From continuity equation Q DBVf, Vf.833 6m x.3x.5 / Vf From the outlet velocity triangle tan u Vw Vf 6 Vw u 5.7.4m tan tan 6 Abolute velocity of water at the outlet tip of the impeller V Vf Vw 6. 4 V 3.63 m ( ) magnitude Vf 6 tan tan 6.5 Vw.4 Torque exerted by the impeller on water rq ( ) x T VwR x(.4x ) g 9.8.5KN m Shaft power (P) impeller or rotor power NT xx KW 6 6 But, mechanical efficiency impeller power n mech i. e,.95 haft power P P 57. 3KW Lift of the pump Impeller power=r(q+q)h Where r=p wt of water=9.8 KN/m3 H =ideal head=(theoretical head-hyd loe) Q=leakage of water m3/ Neglecting leakage q we have 54.45=9.8x.833xH Or h =9.59m We know, hydraulic efficiency Actual head or lift n h ideal head Actual hft n xideal head H ).7x m of water h ( i 5. The following data relate to a centrifugal pump. Diameter of the impeller at inlet & outlet =8mm and 36mm repectively. width of impeller at inlet and outlet=44mm & 7mm repectively. rate of flow through the pump=7.8lp. Speed of the impeller = 5 Rpm. Vane angle at outlet=45 water enter the impeller radially at inlet neglecting loe through the impeller. Find the preure rie in the impeller.

12 Q.78 Soln: velocity of flow at inlet Vf D B x.8x.44 Q.78 Velocity of flow at outlet Vf.m DB x.36x.7 D N x.36x5 Tangential velocity of impeller at outlet u 8.7m 6 6 Vf u Vf coec g x coec 45 x9.8 Preure rie in the impeller i given by the equation 6. A centrifugal pump deliver water at the rate of 8 lpm,to a height of m, Through a.m, dia, 8m. long pipe. Find the power required to drive the pump, if the overall efficiency i 65%, and Darcy friction factor=.. Soln. Dicharge Q=8 lpm=.3 cumec. Delivery head hd =m Dia of delivery pipe dd=.m Length of delivery pipe l d 8m Overall efficiency n.65 f. Vd Total head H h hd hf hfd g So thi prob h h (detail are not given) H h d h fd Vd g f 8x.x8x.3 4x.3 x H=3.65m 5 9.8xπ x. πx. 9.8x Output of the pump= QH 9.8x.3x kw Output of the pump But overall efficieny power require to drive the pump Power required to drive the pump = 9.6/.65=4.8kw 7. A centrifugal pump i required to deliver 8 ltr of water per econd againt a head of 6m, when running at 8rpm. If the blade of the impeller are radial at inlet and velocity of flow i contant and equal to m/ec, find the proportion of the pump. Aume overall efficiency a 8% and ratio of breadth to diameter at outlet a. Soln: the inlet and outlet velocity triangle will be a hown From continuity equation Q DBVf.8 x.xdx

13 D.67m( diameter of the impeller at outlet) B.x.67.67m 6. 7cm (Width of the impeller at outlet). ghm 9.8x6 n mano Vw u.8 Vwu D N x.67x8 Vwu 96. ( i) but u 8.m 6 6 From eq (i) Vw x or Vw 6.99m Vf From the outlet velocity triangle tan. 947 u Vw Vf 5.4 (Blade angle at outlet) tan Vw The following data refer to a centrifugal pump tatic head = 4m, uction height 5m, dia of uction and delivery pipe =.m, lo of head in uction pipe = m, lo of head in delivery pipe = 8m, impeller dia at outlet =.4m, impeller breadth at outlet 5mm.blade occupy % of the outlet area, peed rpm. Exit angle of blade = 5 with the tangent, Manometric efficiency = 8%, overall efficiency = 7%. Find the power required to drive the pump and what preure will be indicated by the gauge mounted on the uction and delivery ide. Soln: Outlet vane angle Delivery head hd H h m Head on the pump H=4++8=5m Vf From the outlet velocity triangle tan u Vw D N x.4x where, u 5.3m 6 6 ghm 9.8x5 Alo from the equation n mano Vw u Vw 4.4m 5.3x.8 Vf ( u Vw ) tan ( ) tan 3 Vf.4m dicharg e, Q KDBVf.9xx.4x.5x. 4.9m 3/ Power given to the liquid P=rQH 9.8x.9x KW P 5.85 Power required to drive the pump 8.36kw.7 Preure gauge reading on the uction ide = h+hf 5 7m of water Preure gauge reading on the delivery ide = hd h fd m 9. Following data were obtained from a centrifugal pump in a laboratory. Preure gauge reading on the uction ide 5cm of mercury, preure gauge reading on the delivery ide 7kN/m. quantity of water raied by the pump =7.5kN/min. vertical height difference

14 between the gauge =5mm. Total input to the pump = 6.5kw.Find the efficiency of the pump. Soln: Suction head h=.5x3.5=.4m of water Delivery head hd=7/9.8=7.34m of water. Head on the pump = (h+hd+x+vd/g) Since the dia of the delivery pipe i not given, velocity in the delivery pipe i ignored. H ) 9. 88m Dicharge from the pump x.7m ec Output of the pumpqh 9.8x.7x kw Efficiency of the pump =.48/6.5=.38.=38.%. The internal and external diameter of the impeller of a centrifugal pump are 4cm and 8cm repectively. The pump i running at rpm. The vane angle at inlet and outlet are and 3 repectively. Water enter the impeller radially and velocity of flow i contant. Determine the workdone by the impeller per kn of water. D N x.4x o ln : u 5.3m 6 6 D N x.8x u 5.6m 6 6 Vf From the inlet velocity triangle tan u Vf Vf 5.3tan 9.5m From the outlet velocity triangle tanφ=vf/(u-vw)=9.5/(5.6-vw) or, Vw=34.4m/ Work done/ec=/g(vw u) =34.4*5.6/9.8=76.3kn-m//kn. The impeller of a centrifugal pump run at 9 Rpm and ha vave inclined at to the direction of motion at exit. If the manometric head i m and manometric efficiency i 75% Vane angle at inlet. Take the velocity of flow a.5m/, throughout and the diameter of the impeller at exit a twice that at inlet. a) Diameter of the impeller at exit. Soln: From the definition of manometric efficiency mano ghm / Vwu 9.8x Vwu 6.6 ( i).75 Vf.5 From the outlet velocity triangle ( u Vw ). 44 Vw ( u.44) tan 6 tan 6 D N Subtituting the value Vw, ( u.44) u m but u 6 3

15 6x6.9 D 3.59m x9 u 6.9 further u 8.45m Vf.5 From the inlet velocity triangle tan. 959 u ( Inlet Vane Angle). A centrifugal pump deliver 5lp againt a head of m. When the impeller rotate at 5rpm. If the mano metric efficiency i 75% and the lo of head in the pump i.33v, where V i the abolute velocity at exit. The diameter of the impeller a) The blade angle at exit Take the width of the impeller at exit a.4d where D i the diameter of the impeller D N xdx5 Q.5.99 Soln: u 78. 5D Vf 6 6 DB xdx.4d D ghm From the definition of manometric efficiency n mano Vw u Vw u Hm 6.7 g n.75 mano But, manometric head=(workdone the impeller loe in the pump).33v 6.7 V 4.5m From eq (i) and (ii) Vwx78.5D or Vw 9.8 D From the outlet velocity triangle Vf Vw V.99 D D Solving by trial and error D=.4 m u 78.5D 78.5x.4 9m Vf m D.4 / Vw m D.4 / Vf 3.4 tan. 654 u Vw ( outlet vane angle) 3. A centrifugal pump lift water againt a tatic head of 4m. The ection and delivery pipe are each 5cm in diameter. The head lo in the ection and delivery pipe are repectively.m and 7.5m. The impeller i 4cm in diameter and.5cm wide at the month. It revolve at Rpm and the vane angle at exit i 3. if the manometric efficiency i 8%. Calculate the dicharge. Soln: 4 x.5 xv Q V = velocity in the ection pipe 4

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