Pumps

Pumps

1. 1 Pumps

2. 2 Introduction

3. 3 Introduction A pump is a mechanical device which can transfer rotational energy (mechanical energy) of the machine to the potential or kinetic energy of the liquid. A pump may be defined as machine when driven from some external source (electric motor, turbine or an engine) transfer/lifts liquid or semi-solid fluid from one place to another. 

4. 4 Introduction

5. 5 Classification Dynamic Positive displacement Reciprocating Rotary Peripheral Centrifugal Plunger Piston Vane Screw Gear Special high head Radial Mixed Axial Diaphragm Lobe

6. 6 Pumps Application range

7. 7 Reciprocating pump A reciprocating pump is a positive displacement machine It traps a fixed volume of liquid at near-suction conditions, compresses it to discharge pressure, and pushes it out the discharge nozzle The basic principle involved is that a plunger or piston will displace a quantity of liquid equal to its swept volume. In Figure, plunger A is lowered into the container, displacing liquid which flows into container B Plunger In a reciprocating pump, reciprocating motion is accomplished by a piston or plunger, or diaphragm.

8. 8 Reciprocating pump Figure depicts the suction stroke of a plunger pump. When the plunger moves away from the head end of the cylinder, the discharge check valve is held closed by the higher pressure in the discharge pipe compared to the lower pressure in the liquid cylinder. This lower pressure in the liquid cylinder also causes the suction valve to be opened by the higher pressure in the suction line. Fluid then flows into the cylinder until the plunger reaches the end of its travel.

9. 9 Reciprocating pump Figure depicts the discharge stroke of a plunger pump. As the plunger moves toward the head end, the increasing pressure in the cylinder closes the suction valve. The pressure in the cylinder continues to rise until it exceeds the pressure in the discharge line and the discharge valve opens, releasing the volume of fluid displaced by the plunger.

10. 10 Reciprocating pump

11. 11 Rotary pump Rotary pumps are positive displacement pumps, but unlike reciprocating pumps, have relatively steady, non-pulsating flow. Rotation of the rotor(s) within the casing traps pockets of liquid at suction conditions, elevates the fluid pressure, and then pushes the fluid out the discharge. Can handle debris Used to raise the level of wastewater Abrasive material will damage the seal between screw and the housing Grain augers use the same principle

12. 12 Rotary pump Gear Pump fluid is trapped between gear teeth and the housing Two-lobe Rotary Pump (gear pump with two “teeth” on each gear) same principle as gear pump fewer chambers - more extreme pulsation

13. 13 Rotary Pump Disadvantages precise machining abrasives wear surfaces rapidly pulsating output Uses vacuum pumps air compressors hydraulic fluid pumps food handling

14. 14 Peristaltic Pump Fluid only contacts tubing Tubing ID and roller velocity with respect to the tubing determine flow rate Tubing eventually fails from fatigue and abrasion Fluid may leak past roller at high pressures Viscous fluids may be pumped more slowly

15. 15 Centrifugal Pump Centrifugal pump is a machine consisting of a set of rotating vanes enclosed within housing or casing. Centrifugal pump convert energy of a prime mover (a electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes occur by virtue of two main parts of the pump: Impeller - is the rotating part that converts driver energy into the kinetic energy Volute or diffuser- is the stationary part that converts the kinetic energy into pressure energy.

16. 16 Centrifugal Pump The process liquid enters the suction nozzle and then into eye (center) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force.

17. 17 Centrifugal Pump The faster the impeller the faster the liquid moves . Centrifugal force pushes the liquid outward from the eye and enters the casing . Thus liquid velocity decreases & its pressure increases. The head (pressure in terms of height of liquid) developed is approximately equal to the velocity energy at the periphery of the impeller expressed by the following well-known formula: where: H = Total Head developed in feet v = Velocity at periphery of impleller in ft/s g = Acceleration due to gravity = 32.2 ft/s 2

18. 18 Components of Centrifugal Pump

19. 19 Components of Centrifugal Pump A centrifugal pump has two main components: Stationary Components: Casings: A Casing is provided for housing the impeller & supporting the bearings provided with the shaft. Also casing has a provision for connecting with suction & discharge pipe liens. Casing are three types: Volute Casings Volute with vortex or whirlpool casing Diffuser or turbine casing

20. 20 Components of Centrifugal Pump Volute casings : A volute is a curved funnel increasing in area to the discharge port. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. These casings can convert only a small amount of velocity head into pressure head and a large amount of velocity head is lost in eddies, thus produce comparatively low heads.

21. 21 Components of Centrifugal Pump Vortex or whirlpool casing: Like volute casing with a circular vortex or whirlpool chamber between the impeller & the volute. Vortex chamber converts some of the kinetic energy into potential energy with slight loss by friction. More efficient than volute casing or volute pump.

22. 22 Components of Centrifugal Pump Diffuser or turbine casing: In this system the impeller is surrounded by a series of stationary guide vanes or by a diffuser ring with guide vanes which by their divergence furnish gradually expanding passages for the liquid to follow after leaving the impeller. In this process direction of flow is changed and velocity head is converted to pressure head before the liquid enters the volute. Velocity head of the liquid leaving the impeller is completely converted into pressure than in the volute type Under variable conditions of speed and discharge the efficiency of the pump goes down since the diffuser is generally designed for one rate of discharge at a given impeller speed. Costlier than volute pump Pumps having diffuser type casing are commonly known as Turbine pumps.

23. 23 Components of Centrifugal Pump Casing Wear rings- act as the seal between the casing and the impeller to restrict leakage of high pressure liquid back to the pump suction. Suction and Discharge Nozzles .

24. 24 Components of Centrifugal Pump Seal Chamber and Stuffing Box: When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. Both the seal chamber and the stuffing box have the primary function of protecting the pump against leakage from the gap between the pump casing and the shaft. The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control.

25. 25 Components of Centrifugal Pump Glands The gland is a very important part of the seal chamber or the stuffing box. It gives the packings or the mechanical seal the desired fit on the shaft sleeve. It can be easily adjusted in axial direction. Bearing Housing The bearing housing encloses the bearings mounted on the shaft. The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water.

26. 26 Components of Centrifugal Pump Rotating Components Impeller Impeller wearing ring Shaft Shaft sleeve Couplings

27. 27 Impeller It is the main part of pump assembly fitted with a series of backward vanes (or blades). The function of the impeller is to force the liquid into a rotary motion by centrifugal force. On the basis of construction can be classified as: Closed or shrouded impeller : contains two shrouds (or side walls) in which plain or curved vanes are inserted. Semi-open impeller : Vanes are fixed on one shroud only. Open type impeller : Vanes are directly fixed on the web. There is no shroud.

28. 28 Impeller

29. 29 Shaft The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. It must do this job with a deflection less than the minimum clearance between the rotating and stationary parts.

30. 30 Shaft sleeves Pump shafts are usually protected from erosion, corrosion, and wear at the seal chambers, leakage joints, internal bearings, and in the waterways by renewable sleeves. Unless otherwise specified, a shaft sleeve of wear, corrosion, and erosion-resistant material shall be provided to protect the shaft. The sleeve shall be sealed at one end. The shaft sleeve assembly shall extend beyond the outer face of the seal gland plate. (Leakage between the shaft and the sleeve should not be confused with leakage through the mechanical seal). .

31. 31 Coupling Couplings can compensate for axial growth of the shaft and transmit torque to the impeller. Shaft couplings can be broadly classified into two groups: rigid and flexible. Rigid couplings are used in applications where there is absolutely no possibility or room for any misalignment. Flexible shaft couplings are more prone to selection, installation and maintenance errors. Flexible shaft couplings can be divided into two basic groups: elastomeric and non-elastomeric.

32. 32 Axial flow pump An axial flow pump is one in which the fluid enters parallel to the axis of rotation and leaves in the axial tangential plane. Axial flow pumps are normally designed for conditions where low head high flows Axial flow pumps are sometimes called propeller pumps because they operate essentially the same as the propeller of a boat.

33. 33 Axial flow pump

34. 34 Radial flow pump Radial flow pumps operate at higher pressures and lower flow rates than axial and mixed flow pumps. Also called Centrifugal Pump pumps broad range of applicable flows and heads higher heads can be achieved by increasing the diameter or the rotational speed of the impeller Radial flow pumps are those where the fluid enters the impeller in a direction parallel with the axis of rotation and leaves the impeller in the radial tangential plane.

35. 35 Mixed flow pump Mixed flow pumps, as the name suggests, function as a compromise between radial and axial flow pumps fluid experiences both acceleration in radial and the axial direction As a consequence mixed flow pumps operate at higher pressures than axial flow pumps while delivering higher discharges than radial flow pumps

36. 36 Mixed flow pump

37. 37 Starting of centrifugal pump A typical starting sequence for a centrifugal pump is : • Ensure that all valves in auxiliary sealing, cooling, and flushing system piping are open, and that these systems are functioning properly. • Close discharge valve. • Open suction valve. • Vent gas from the pump and associated piping. • Energize the driver. • Open discharge valve slowly so that the flow increases gradually.

38. 38

39. 39 Pumps performance

40. 40 Pumps Selection Material Compatibility Solids Flow Head NPSH a Self priming requirment

41. 41 Pumps performance Effects of Changing Liquid Specific Gravity Specific gravity (S.G.) has the following effects on pump performance, assuming constant rpm and impeller diameter: 1. Flow rate (quantity) is unchanged by S.G. (although the flow reading on a differential-pressure flow meter varies.) 2. Pressure varies directly with S.G. (Although pressure varies, head is constant.) 3. Horsepower varies directly with S.G. These relationships are important when converting a pump to another service or if significant changes to fluid gravity are anticipated. For example, converting from a light hydrocarbon service to water service may significantly overload an existing driver.

42. 42 Matching pumps to system characteristics Good piping system design Match system characteristics to pump curve Trimming pump impellers To reduce flow To match partload requirments Pump control Two-speed pumping & motors Variable speed pumping Source distribution pumping

43. 43 Matching pumps to system characteristics Modulation of pump-piping systems Throttle volume flow by using a valve Change flow resistance – new system curve Also known as “riding on the curve” Turn water pumps on or off in sequence Sudden increase/drop in flow rate and head Vary the pump speed System operating point move along the system curve Requires the lowest pump power input

44. 44 Matching pumps to system characteristics Plant loop (at constant flow) (production loop) To protect evaporator from freezing, a fairly constant-volume water flow is required Building loop (at variable flow) For saving energy at partload A differential pressure transmitter is often installed at the farthest end from the pump Primary-secondary loop A short common pipe connects the 2 loops

45. 45 Matching pumps to system characteristics Series and Parallel Operation Often pumps are installed in series or in parallel with other pumps. In parallel, the capacities at any given head are added; in series, the heads at any given capacity are added. Figures show series and parallel pumps curves, a system curve, and the effect of operating one, two or three pumps in a system. In both figures, the operating points for both pumps "A" and "B" are the same only when one pump is operating. For 2 or 3 pumps operating, the points are not the same because of the pump curve shapes. Hence, due consideration should be given to the pump curve shape when selecting pumps for series or parallel operation.

46. 46 Matching pumps to system characteristics

47. 47 Matching pumps to system characteristics

48. 48

49. 49

50. 50 Common problems Low Performance Cavitation Seal Leakage Bearings Failure Vibration Noise

51. 51 Cavitation The formation of vapor bubbles in the impeller suction eye due to fluid flashing or boiling, with subsequent collapse of the bubbles as the pressure rises, is called cavitation . Cavitation may cause vibration, pitting damage, and impaired performance. Cavitation may or may not be serious depending on the pump, HP/stage, impeller design, and the fluid being pumped. In small pumps with low differential head per stage, the energy of collapsing bubbles is much less than in larger, high-head-per-stage pumps. Cavitation is more severe in a single-boiling point fluid (like water) than with a mixture (like petroleum stocks) that have a broad boiling range .

52. 52 NPSH NPSHA : is technically defined as the total suction pressure (in psia ) at the suction nozzle less the true vapor pressure of the liquid (in psia ) at the pumping temperature. For centrifugal pumps, NPSHA is always expressed in feet of the liquid pumped. For reciprocating pumps it includes the acceleration head. NPSHA depends on the system characteristics, liquid properties and operating conditions. NPSHR : The minimum total suction absolute head, at the suction nozzle minus the liquid vapor absolute pressure head, at flowing temperature, required to avoid cavitation. For positive displacement pumps it includes internal acceleration head and losses caused by suction valves and effect of springs. It does not include system acceleration head. NPSHR depends on the pump characteristics and speed, liquid properties and flow rate and is determined by vendor testing, usually with water.

53. 53 NPSH Calculation of NPSHA NPSHA can be calculated as follows: NPSHA = H + S - F – V p where: NPSHA = feet of head of the pumped liquid, at the pump impeller-eye elevation and suction flange face. H = minimum absolute pressure on the surface of liquid pumped, in feet of the liquid. S = static head, or vertical distance between the surface of the liquid and the center of the impeller, in feet. S is negative (-) when the pump is above liquid surface, and positive (+) when the pump is below. F = friction losses, in the suction pipe and fittings, in feet of the liquid. V P = True vapor pressure of the liquid,