PEMP RMD M.S. Ramaiah School of Advanced Studies

Transcription

1 Hydraulic Pumps Session delivered by: Prof. Q.H. Nagpurwala 1

2 Session Objectives This session is intended to discuss and understand the following: Classification of hydraulic pumps Construction and working principle of centrifugal pumps Loss mechanism in centrifugal pumps Performance characteristics of centrifugal pumpsp 2

3 Classification of Pumps Kinetic Pumps Positive Displacement Centrifugal Peripheral Special Reciprocating Blow case Radial flow Mixed flow Axial flow Single-stage Multistage Jet (ejector boosted) Gas lift Hydraulic ram Electromagnetic Piston plunger Diaphragm Single rotor Single suction Double suction Single suction Self priming Non self priming Double acting Single acting Double acting Vane, Piston Flexible, screw Self priming Non self priming Single-stage Multistage Single-stage Multistage Simplex Duplex Simplex Duplex Multiplex Simplex Multiplex Open Semi open Closed impeller Open Closed impeller Steam Power Fluid operated Mechanical operated Rotary Piston, screw Multiple rotor Gear, Lobe Circumferential 3

4 Centrifugal Pumps Centrifugal means directing or moving away from the axis. Centrifugal pumps use an impeller and volute to create the partial vacuum and discharge pressure necessary to move water through the casing. Radial flow and mixed flow pumps are commonly referred to as centrifugal pumps. The rotating element of a centrifugal pump is called the impeller. The open impeller consists of a hub to which vanes are attached, while closed impeller has plates or shrouds on each side of the vanes. The open impeller is less efficient compared to closed one but suited to handling liquids containing solids. Radial pumps are provided with a spiral casing often referred as a volute casing which guides the flow from the impeller to the discharge pipe. The ever increasing flow cross section around the casing tends to maintain a constant velocity with in the casing. Some pumps have diffuser vanes instead of a volute casing. Some pumps are of the double suction type. Higher the pressure drop or head, lower will be the flow rate. 4

5 Types of Pumps Pumps can be broadly classified into: Kinetic Axial Pump Centrifugal Pump Mixed Flow Pump Positive Displacement Reciprocating Piston or Plunger Gear Pump Screw Pump Lobe Pump 5

6 Basic Difference in Pump Types Kinetic Pumps Head generators Positive Displacement Pumps Flow generators 6

7 Kinetic Pumps (Turbomachines) Axial Shaft Shaft Shaft Rdil Radial Mixed flow In axial flow pumps, the entry and exit is parallel to the axis of the impeller. In radial flow pumps, the exit is perpendicular to the flow at the inlet. In mixed flow pumps, the flow at the exit of the impeller is at an angle to the axisof the impeller. 7

8 Positive Displacement Pumps Reciprocating piston or plunger External gear pump Sliding vane Double screw pump Three lobe pump Double circumferential piston 8

9 Comparison of Pumps Positive Displacement Pump Kinetic Pump (Turbomachine) Creates thermodynamic and mechanical action between near static fluid and a relatively slow moving Creates thermodynamic and dynamic action between a flowing fluid and a rotating element and involves energy surface and involves a volume change transfer with pressure and momentum or displacement of fluid. changes. Commonly involves a reciprocating In principle, it involves a steady flow motion and unsteady flow of fluid, of fluid and pure rotary motion of the though it is not impossible for the mechanical elements. It may also machine to have a purely rotary motion involve unsteady flow for short periods with near steady flow. of time especially while starting / Since the fluid containment is positive, stopping or during load changes. stopping a positive displacement However, in most instances the machine during its operation traps a machine is designed for steady flow certain ti amount of fflid fluid and maintains iti it conditions. indefinitely in a state different from As there is no positive containment of that of the surroundings, provided heat the fluid in a kinetic pump, stopping of transfer and leakage are absent. the machine will let the fluid state change rapidly and become the same as that of the surroundings. 9

10 Comparison of Pumps Positive i Displacement Pump Kinetic Pump (Turbomachine) Employs rather low speeds and is relatively complex in mechanical design. Usually heavy per unit output. t Employs valves which are open only part of the time as in other reciprocating machines. Heavy foundations are usually needed because of the reciprocating masses and consequent vibration problems. Mechanical features are rather complex in these machines Positive containment and near static energy transfer process result in higher efficiency. Low component life because of opening and closing of valves during continuous operation. Employs high rotational speeds. Simple in design principle and light in weight ihtper unit power output. t Foundations may be quite light since vibrations problems are not severe. Do not employ valves that open and close in steady state operations. Dynamic action in kinetic pumps results in lower efficiencies. During steady state operation, the valves are open all the time and volumetric efficiency differs negligibly from 100%. Since valves are always open and fluid velocities are high, a kinetic pump has a high fluid handling capacity per unit weight of machine. 10

11 Centrifugal Pumps Closed or shrouded impeller Semi open impeller 11

12 Single Stage Double Suction Pump 12

13 Axial Flow Pumps An axial flow pump essentially consists of a propeller in a tube. The propeller can be driven directly by a sealed motor in the tube or by a right-angle drive shaft that pierces the tube. The main advantage of a axial flow pump is that it can easily be adjusted to run at peak efficiency at low-flow/high-pressure and high-flow/low-pressure by changing the pitch on the propeller. Application of axial flow pump Evaporators and crystallizers Waste-water handling Sludge transfer Flood control Flume recirculation Irrigation Regeneration Heat recovery High-volume mixing. 13

14 Pumping Station 14

15 Casing of Centrifugal Pump (a) (b) (c) (d) (a) Schematic of flow through volute (b) Double volute casing (c) Double suction horizontally split volute casing (d) High pressure volute casing with diffuser 15

16 Centrifugal Pump Installation Typical centrifugal pump installation Typical axial flow pump installation 16

17 Centrifugal Pump Stages A single stage pump has only one impeller while a multi stage has two or more impellers arranged in such a way that the discharge from one impeller enters the eye of the next impeller Deep well pumps are turbine pumps (have vanes in casing) and are multistage pumps Pump installation will have foot valve and check valve Some application of multistage pumps Boiler feed pump Condensate extraction pump Deep well submersible pump Apartment blocks Deep well multistage pump 17

18 Centrifugal Pump Application Standard Centrifugal Pumps used in clear water applications have low initial cost and compact design limited solid handling capability can deliver flows up to 8-30 liter/min and heads in the range of meter High Pressure Centrifugal Pumps used where high discharge pressures and flow rates are required can deliver flows in the range of 6-8 liter/min and produce heads in excess of 70 meter Trash Centrifugal Pumps can handle large amounts of debris and mostly used by contractors and the rental industry can produce flows up to liter/min and heads up to 45 meter 18

19 Merits and Demerits Advantages Wide range of capacities that range from a liter/min to 3000 liter/min. Heads of 1 meter to 200 meter are generally available Simple construction (small amounts of suspended matter in the water will not jam the pump) Low to moderate initial cost for a given size Moderate to high efficiency at optimal operation Little space required for a given capacity Disadvantages Efficiency is limited to a narrow range of discharge flows and heads Low capacity that is greatly dependent on discharge pressure Potential for impeller to be damaged by abrasive matter in water 19

20 Pump Specification Booster pump Condensate extraction pump Suction pressure 5.59 bar 0.09 bar abs Vl Volume flow rate 250 m 3 /h 160m 3 h Discharge pressure 9.16 bar 2.6 bar abs Power input 36.3kW 46.5 kw Speed 6000 rpm 4800 rpm NPSH 22 m -NA- Efficiency 81% -NA- Fluid handled Boiler feed pump Condensate Specific gravity of fluid NA- Temperature of ffluid 426 K 315 K * The values are examples 20

21 Specific Speed of Centrifugal Pumps Variation in specific speed with type of pump N s n Q e ( British Units) 3 4 h = n: rpm, h: ft, Q: gpm BEP ω Q ω :rad/s h:m Q:m3 /s e ( N s ) ( SI Uit Units ) SI 3 ( gh) 4 = ω : rad/s, h: m, Q: m 3 /s BEP 21

22 Classification and Selection of Pumps Optimum geometry as a function of BEP specific speed (for single stage rotors) Ref: Pump Handbook by Krassic et al 22

23 Classification and Selection of Pumps Approximate upper limit of pressure and capacity by pump class Ref: Pump Handbook by Krassic et al 23

24 Pump Impeller The pump impeller receives the liquid to be pumped and imparts velocity and pressure to it by drawing power from a prime mover. The speed and diameter of the impeller determines the head or pressure that the pump can generate. The rotational speed and height of the impeller blades determines the flow that t the pump can accommodate. A classic pump impeller receives liquid at its internal diameter (ID). By centrifugal force and blade design, the liquid is moved through the blades from ID to OD of the impeller, where it discharges the liquid into the volute channel. 24

25 Pump Impeller Types Centrifugal pump impeller types are: 1. Forward swept impeller 2. Rdil Radial exit itimpeller 3. Backswept impeller H-Q curves for pump 1. Forward swept impeller is used for low flow rate and head. 2. Backswept impeller is used for high flow rate and head. 3. Radial exit impeller is used for medium head and flow rate. 25

26 Impeller Types and Velocity Triangle 26

27 Pump Velocity Triangles Velocity triangle Vs flow and head more flow (V a2 x A) less head (V w2 x U 2 ) V w2 > V 1 w2 more flow (V 1 a2 x A) less head (V 1 w2 x U 2 ) V <V w2 1 w2 27

28 Pump Velocity Triangles 28

29 Fluid Mechanics of Centrifugal Pumps Rotating impeller imparts energy to the fluid Shape of the blades and the flow pattern in the impeller determines how much energy is transferred Priniciple of conservation of angular momentum is used to find out the theoretical head rise H th Torque T = Q( ρv t2 r 2 - ρv t1 r 1 ) PowerP =Tω = ρqh th g 29

30 Terminology of Centrifugal Pumps Suction Head (h s ): It is the vertical height of the center line of the pump above the water surface in the sump. This height is the suction lift and is denoted as h s Delivery Head (h d ): It is the vertical height between the center line of the pump and water surface in the tank to which water is delivered. It is denoted as h d Static head (H s ): Static head is the vertical distance between the liquid level in the sump and the delivery tank. It is denoted by H s. Therefore static head, H s = h s +h d Manometric Head (H m ) or effective head: It is the total head or lift that must be produced by the pump to satisfy external requirement. It includes all the losses. 30

31 Pump System Installation 31

32 Head Developed by a Pump s s d d Z V P Z V P 2 2 H d H s h = M.S. Ramaiah School of Advanced Studies = s s s d d d Z g Z g 2 2 γ γ

33 Head Developed by a Pump Total dynamic head For a horizontal pump the total dynamic head is defined as For a vertical pump with pumping element submerged, the total dynamic head is defined as 33

34 Head Developed by a Pump 34

35 Effect of Specific Gravity of Fluid These three pumps are developing the same discharge pressure. In this case they develop different theads inversely proportional lto the specific gravity of fthe fluids. 35

36 Performance Characteristics The efficiency of a pump varies considerably depending upon the conditions under which it must operate. When selecting a pump for a given situation, it is important to have information regarding the performance of various pumps, from which the selection can be made. H-Q relation for ideal pump H-Q relation for actual pump 36

37 Performance Characteristics Typical performance curves for a Effect of losses on the performance of centrifugal pump at constant speed a centrifugal pump with backward curved impeller vanes 37

38 Pump Performance, Q Q0 Q1 Performance characteristics of a centrifugal pump at different speeds Operating point of a centrifugal pump System and pump characteristics Ref: DOE Fundamentals Handbook (Thermodynamics, Heat Transfer, and Fluid Flow), Volume 3 of 3, U.S. Department of Energy, Washington Rep No. FSC

39 Centrifugal Pumps in Parallel, Q Pump characteristics for two identical pumps in parallel Since the inlet and the outlet of each pump shown in the figure are at identical points in the system, each pump must produce the same pump head. The total flow rate in the system, however, is the sum of the individual flow rates for each pump. 39

40 Centrifugal Pumps in Series, Q Pump characteristics for two identical pumps in series, Q Operating points for two centrifugal pumps in series Using two pumps in series does not actually double the resistance to flow in the system. The two pumps provide adequate pump head for the new system and also maintain a slightly higher volumetric flow rate. 40

41 Various Pump Efficiencies Manometric efficiency (η m ) is the ratio of manometric head developed by the pump to the head imparted by the impeller (Euler head) η man = H m H m + losses Volumetric efficiency (η vol ) is the ratio of volume of liquid delivered to the actual volume of liquid entering the impeller through suction pipe. Due to leakages, all the liquid sucked into impeller does not pass through the delivery pipe. Qd ηvol = Q L is the amount of water leakage Q + Q d Q l Mechanical efficiency (η mech ) is the ratio of power delivered by the impeller to power supplied at the rotor shaft by the prime mover. η output input power power FluidPower ShaftPower mech = = = γqh Tω hd hydraulic energy output t Overall efficiency, η overall = = ηman ηmech η shaft power input 41 vol

42 Net Positive Suction Head Net Positive Suction Head (NPSH) of a pump is the difference between the suction pressure and the saturation pressure of the fluid being pumped. NPSH is used to measure how close a fluid is to saturated conditions. NPSH = P suction P saturation By maintaining the NPSH available at a level greater than the NPSH required by the pump manufacturer, cavitation can be avoided. 42

43 Suction Specific Speed The way that a pump receives liquid into the impeller, determines the available combination of discharge flow and head that the pump can generate. Essentially, it determines the operating range of the pump. The operating range is quantified or rated by the term 'Suction Specific Speed, Nss'. The Nss is calculated using three parameters: speed, flow rate, and NPSHr. These numbers are obtained from pump's performance curves. where, N = speed of the pump / motor in rpm Q = flow rate at BEP NPSHr = net positive suction head required by the pump at BEP 43

44 Pump Eficiency vs Specific Speed Compilation by Balje (1981) of maximum efficiencies for various kinds of pumps as a function of design specific speed, N D. Since efficiency is also a function of Reynolds number, the data has been corrected to a Reynolds number, UD/ν, of

45 Affinity Laws Small speed reduction (e.g. ½) = large power reduction (e.g. 1/8) 45

46 Affinity Laws 46

47 Flow through Pump Impeller Secondary flows in the impeller passage Separated flow at the volute lip Ref: Hydrodynamics of Pumps by C.E. Brennen 47

48 Losses in Centrifugal Pumps Hydraulic losses Skin friction and diffuser loss - increase with the square of the flow rate and are dominant at high flow rates Volute losses - depends on the impeller exit velocity - increases with decreasing flow rate Incidence losses - increases on both sides of the design flow rate -inlet blade angle should be adjusted to minimise these losses Disk Friction and recirculation loss - affect only the efficiency, not the head Mechanical losses Disc friction between the fluid and the rotor Mechanical friction on the main bearing glands 48

49 Pump Efficiency Manometric efficiency (η m ) is the ratio of manometric head developed by the pump to the head imparted by the impeller (Euler head) η m = H m H m + losses Volumetric efficiency (η vol ) is the ratio of amount delivered pipe to the actual amount of water entering the impeller through suction pipe. Due to leakages, all the water sucked into impeller does not pass through the delivery pipe. pp η vol = Qd Q + Q d Q l Q L is the amount of water leakage 49

50 Pump Efficiency Mechanical efficiency (η mech ) is the ratio of power delivered by the impeller to power supplied at the rotor by shaft connected to the motor. η output power input power FluidPower ShaftPower mechanical = = = γqh Tω Overall efficiency (η overall ) is the ratio of hydraulic energy output by the pump to the shaft power input to the pump η = η η η overall manometric mechanical volumetric 50

51 Pump Efficiency Curves 51

52 Performance Characteristics The efficiency of a pump varies considerably depending upon the conditions under which it must operate When selecting a pump p for a given situation, it is important for the pump p selector to have information regarding the performance of various pumps among the selection is to be made Ideal H vs Q characteristics 52

53 Performance Characteristics These are non-dimensional i curves for the various types of pumps,which h show the head, brake horsepower, and efficiency plotted as a percent of their values at the design or best efficiency point of the pump. Characteristic curve for axial flow pump For a typical axial flow pump, the head and brake horsepower both increase drastically near shutoff as shown in the Figure. Normal or rated capacity corresponds to the point of optimum efficiency or BEP-best efficient point. The shutoff head is that which is developed when there is no flow. in case of the axial pump the shut off head may be as much as 3 times the normal head. 53

54 Performance Characteristics Characteristic curve for radial flow pump The above Figure shows that the head curve for a radial flow pump is relatively flat and that the head decreases gradually as the flow increases. Note that the brake horsepower increases gradually over the flow range with the maximum normally at the point of maximum flow. 54

55 Performance Characteristics Characteristic curve for mixed flow pump Mixed flow centrifugal pumps have considerably different characteristics as shown in the above Fig. The head curve for a mixed flow pump is steeper than for a radial flow pump. The shut-off head is usually 150% to 200% of the design head, The brake horsepower remains fairly constant over the flow range. 55

56 Cavitation The process where the local absolute pressure of the fluid, somewhere in the fluid, drops below the vapour pressure of the fluid and vapour bubbles are formed. Bubbles move downstream and as they reach a region of higher pressure, collapse and create a sudden high pressure pulse that can damage the material of the flow walls. Impeller inlet region is more prone to cavitation since the lowest pressure appears there. Prediction of the exact location and the extent of damage is quite difficult Cavitation is most likely to occur near the point of discharge of radial flow and mixed flow impellers where the velocities are highest. It may also occur on the suction side of the impeller, where the pressure are the lowest. In the case of an axial flow pump, the blade tip is most vulnerable to cavitation. 56

57 Methods of Avoiding Cavitation Cavitation can be avoided if the NPSH available is larger than the NPSH required. The NPSH available is the total energy per unit weight, or head at the suction flange of the pump minus the vapour pressure head of the fluid. The NPSH required is the minimum suction head in order to function properly and avoid cavitation. NPSH required is usually taken as that value of NPSH available, where 3% drop of the total head occurs. 57

58 Session Summary Classification of pumps based on specific speed has been discussed. Working principle of a centrifugal pump has been described briefly. Characteristics of centrifugal pumps with radial, backswept and forward swept vanes are highlighted. Cavitation phenomenon in pumps has been discussed briefly. 58

59 References John Tuzson, Centrifugal Pump Design, John Wiley & Sons Inc, New York pressure cavitation-extract.pdf

60 Thank you 60