Pumps and its operation characteristic and performance
- 2. • WHAT IS PUMP? It converts mechanical energy to hydraulic energy. • NEED OF PUMP. To increase the flow rate To increase the pressure. To move the fluid from lower elevation to higher elevation
- 3. CLASSIFICATION OF PUMP DYNAMIC POSITIVE DISPLACEMENT Dynamic : Continuous transfer of energy to fluid. Positive Displacement : Positive amount of fluid is pressurized in a given time
- 4. Classification of positive displacement pump
- 5. CLASSIFICATION OF DYNAMIC PUMP
- 6. Centrifugal Pumps • A centrifugal pump is a rotating machine in which flow and pressure are generated dynamically. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or casing. The function of the casing is to collect the liquid discharged by the impeller and to convert some of the kinetic (velocity) energy into pressure energy.
- 7. • Centrifugal pumps can be classified in several ways. Pumps may be classified: • in terms of energy conversions (volute type and diffuser vane ring type); • in terms of fluid flow through the pump (radial, mixed flow and axial flow); • in terms of number of stages; • according to the design of the casing; • according to the design of the impeller; and • in terms of their application. Classification of Centrifugal Pump
- 8. Number of stages • A centrifugal pump with a single impeller that can develop a differential pressure of more than 150 psi between the suction and the discharge is difficult and costly to design and construct. • A more economical approach to developing high pressures with a single centrifugal pump is to include multiple impellers on a common shaft within the same pump casing. • Internal channels in the pump casing route the discharge of one impeller to the suction of another impeller.
- 9. Design of the casing. • Centrifugal pumps are also classified in terms of axially and radially split single casings. • The discharge connections are usually located in the lower half thereby enabling the upper half to be lifted and the rotor laid bare for inspection or removal without disconnecting the pump, either from its foundation or from the suction and discharge piping. The disadvantage of this type is the difficulty in maintaining a seal at the joint when at higher pressures. • As the working pressure increases, radially split casings are preferred because of the inherent strength in their construction.
- 10. • Impellers can be open, semi-open, or enclosed. • The open impeller consists only of blades attached to a hub. • The semi-open impeller is constructed with a circular plate (the web) attached to one side of the blades. • The enclosed impeller has circular plates attached to both sides of the blades. Design of Impeller
- 11. • Impellers can be either single suction or double-suction. • A single-suction impeller allows liquid to enter the center of the blades from only one direction. • A double-suction impeller allows liquid to enter the center of the impeller blades from both sides simultaneously. Design of Impeller
- 12. Radial Flow • The impeller discharges fluid at right angles to the shaft axis. In this centrifugal pump in which the pressure is developed wholly by centrifugal force. The radial type pumps are used for the application of high head and low discharge. • In radial flow pumps ratio of impeller outside Dia ( D2) to eye Dia ( D1) is 2 or more and impeller having narrow width.
- 13. Mixed Flow : • The flow direction is partly axial and partly radial. Hence has a result the flow is diagonal. The mixed flow type pumps are used for the application of medium head and high discharg • In mixed flow pumps ratio of impeller outside Dia ( D2) to eye Dia ( D1) less than 1.5and impeller having wider width.
- 14. Axial Flow: • The flow through impeller is parallel to shaft axis low head and very high discharge. The axial flow type pumps are used for the application of medium head and high discharge.
- 15. Suction Phase • Suction Pipeline Design The root cause of many pump problems and failures can be traced to poor upstream, suction-side, pipeline design. Common problems to avoid are: • Insufficient fluid pressure leading to cavitation within the pump. • Narrow pipes and constrictions producing noise, turbulence and friction losses. • Air or vapor entrainment causing noise, friction and loss of performance. • Suspended solids resulting in increased erosion. • Poor installation of pipework and other components.
- 16. Cavitation • If the pressure of the fluid at any point in the pump is lower than its vapor pressure, it will literally boil, forming vapor bubbles within the pump. The formation of bubbles leads to a loss in throughput and increased vibration and noise but the big danger is when the bubbles pass on into a section of the pump at higher pressure. The vapor condenses and the bubbles implode, releasing, locally, huge amounts of energy. This can be very damaging, causing severe erosion of the pump’s components. • To avoid cavitation, you need to match your pump to the fluid, system and application.
- 18. NPSH • To avoid cavitation, the pressure of the fluid must be maintained above its vapor pressure at all points as it passes through the pump. • NPSH-R – this is their minimum recommended fluid inlet pressure, expressed in meters. • A manufacturer’s NPSH-R rating is the minimum recommended inlet head pressure: A pump is already experiencing cavitation at this pressure. Consequently, it is important to build in a safety margin of 0.5 to 1m to take account of this and other factors such as: • The pump’s operating environment – is the temperature constant? • Changes in the weather (changes in temperature and atmospheric pressure). • Any increases in friction losses that may occur occasionally or gradually during the lifetime of the system.
- 20. • Centrifugal pumps, work most efficiently when the fluid is delivered in a surge- free, smooth, laminar flow. Any form of turbulence reduces efficiency and increases wear and tear on the pump’s bearings, seals and other components. • There should be at least 5 pipe diameters’ worth of straight piping connecting to the pump. Never connect an elbow, reducer, valve, or strainer within this final run of pipework. If you connect an elbow directly to the pump flange, the fluid is effectively centrifuged towards the outer curve of the elbow and not directed into the eye of the impeller. This creates stress on the pump’s bearings and seals which often leads to wear and premature failure. • Sometimes, it’s just not possible to make provision for a sufficient settling distance in the pipework before the pump. In these cases, use an inline flow conditioner or straightener. • It’s standard practice to employ suction-side piping one or two sizes bigger than the pump inlet - Never use any piping that is smaller than the pump’s inlet nozzle. Turbulence and Friction
- 21. Turbulence and Friction • Small pipes result in larger friction losses, which means it costs more to run your pumping system. On the other hand, larger diameter pipes are more expensive – so you need to weigh up the increased cost with the likely energy saving resulting from reduced friction losses. • It also makes sense to keep the run of pipework to a minimum by positioning the pump as close as possible to the fluid source. • A reducer is a constriction and requires careful design to avoid both turbulence and the creation of pockets where air or vapor might collect. The best solution is to use an eccentric reducer orientated to eliminate the possibility of air pockets. • As a general rule of thumb, suction pipe velocities should be kept below 2 m/s. At higher velocities, the greater friction causes noise, higher energy costs and increasing erosion, particularly if the fluid contains suspended solids. If your system contains any narrow pipes or other constrictions, bear in mind that the pipe velocity will be a lot higher at these points.
- 23. • Entrained gases cause a loss in pump performance, increase noise, vibration and component wear and tear. • It should be fully submerged. If it’s too close to the surface of the fluid, the suction creates a vortex, drawing air into the liquid and through the pumping system. • The feed pipe should also be clear of any other pipes, agitators or stirrer-paddles – anything that might drive air into the fluid. In shallow tanks or ponds, it may be advisable to use a baffle arrangement to protect the feed pipe from air entrainment. Air or Vapor Entrainment
- 24. • The feed pipe isn’t too close to the bottom of the tank. If it is, the suction may draw up solids or sludge. • Install a filter or strainer. Filters can create a large pressure drop and be responsible for cavitation and friction-loss. The filter screen should have at least three times the free area of the pipe cross-section. • Use a differential pressure gauge across the screen to look out for any increased pressure drop before clogging problems arise. This will also help in the accurate assessment of NPSH-A Suspended Solids
- 25. • Pumps should be securely located, Don’t use one to support the other. All other components must be just as securely located and create no stresses or strains on any other parts of the system. • Ensure that the pipe connecting to the pump’s inlet flange is aligned precisely with it. • If you need to install non-return valves or flow control valves fit them on the discharge side of the pump, and never in suction-side pipework. Installation
- 26. • Problems in suction side pipework often have damaging consequences for the system pump and can be avoided by following these guidelines: • Ensure that conditions do not favor cavitation. This requires careful selection of the pump, its positioning and the head pressure. • Position the feed pipe to minimize entrainment of air/vapor and solids. • Minimize friction and turbulence by choosing appropriate pipes and components • Use pipes with a diameter twice that of the pump’s suction side flange. • Ensure that the pipework is aligned with the pump’s flange and straight for at least 5 pipe diameters. • Use an eccentric reducer orientated to eliminate air pockets. • Keep the pipe velocity below 2m/s. Summary
- 27. Strainer • Filters are normally constructed from a fibrous material, which creates paths for the process stream to flow through of varying, random apertures. The debris is retained at various points in the material and the filter element eventually becomes loaded and has to be replaced. • Strainers typically provides a single barrier to the process stream and has a fixed opening.
- 28. Specification Requirements of Strainer The following information is fundamental to ensure the correct specification of strainer equipment for its intended purpose. • Filtration Size • Strainer Unit Sizing • Solids Content • Maximum Allowable Pressure Drop • Maximum Design Pressure/Temperature Range • Materials
- 29. Filtration Size • The common purpose of a strainer is to remove debris from a process stream. Strainer elements are available with a range of filtration sizes from as large as 10mm perforations down to 25 micron (0.025mm) fine woven wire mesh. • If the filtration size is too large then escaping debris could seriously affect the performance of pump. A filtration size that is too small could result in increased maintenance to clean the strainer and/or increased pressure drop. • Correct selection of the filtration size is very important and should relate to the maximum particle size that pumps is capable of handling.
- 30. Mesh Size Micron Rating Gap Size mm Gap Size inch 20 Mesh 740 microns 0.74 mm 0.030 in 30 Mesh 500 microns 0.50 mm 0.020 in 40 Mesh 400 microns 0.40 mm 0.015 in 60 Mesh 250 microns 0.25 mm 0.010 in 80 Mesh 200 microns 0.20 mm 0.008 in 100 Mesh 150 microns 0.15 mm 0.006 in 120 Mesh 125 microns 0.125 mm 0.005 in 150 Mesh 100 microns 0.10 mm 0.0039 in 200 Mesh 76 microns 0.076 mm 0.0030 in 300 Mesh 50 microns 0.05 mm 0.0020 in Strainer Mesh Comparison Guide
- 31. Strainer Unit Sizing • The strainer size should be determined from the potential dirt volume that the unit expects to retain and the maximum allowable pressure drop for the strainer. An undersized strainer will result in increased maintenance required to continually clean the elements. In the worse instances it can create pressure drop problems in a process stream.
- 32. Solids Content • The potential solids content of a stream is crucial, when considering the sizing of the strainer body. The element should be sized to adequately cater for the potential solids loading expected from the process stream. • Where the solids loading is expected to be particularly high, duplex strainers, Automatic backwash strainers should be used to make the process of element cleaning quicker and easier.
- 33. Maximum Allowable Pressure Drop • Downstream equipment such as pumps that require protection by strainers are normally also dependent upon a minimum head pressure in order to function correctly. • It is therefore important that the maximum allowable pressure drop for the strainer is determined to ensure that the design of the strainer will not present problems to downstream process equipment.
- 34. Maximum Design Pressure/Temperature Range • It is important to specify the design pressure and temperature range. • It is important for each strainer that the specific design conditions are provided.
- 35. • Strainer materials should reflect the materials specified for the pipeline. • This information is provided in the relevant piping class which normally also provides details on the relevant Material Data Sheet (MDS). It is standard practice for the MDS requirements to cover pressure retaining parts such as the strainer body and cover, but not items such as the strainer element. • The element material should be of similar, or better, material standard to the body. Materials
- 36. Pumping Phase Parts of centrifugal Pump • Shaft • Shaft sleeve • Impeller • Casing • Impeller lock nut • Bearings (Radial and Thrust) • Bearing housing • Backup plate • Bearing pedestal • Stuffing box • Sealing arrangements (Mechanical seal or gland packing) • Wear ring (Casing and Impeller)
- 37. Specific speed • Specific speed ( Ns ) of the centrifugal pump identifies the approximate acceptable ratio of the impeller eye diameter (D1) to the impeller outside diameter (D2) in designing a good impeller. • Specific Speed (Ns): 500 to 5000; > 1.5 – radial flow pump Ns: 5000 to 10000; < 1.5 – mixed flow pump Ns: 10000 to 15000; = 1 – axial flow pump
- 38. • The performance of the centrifugal pump can be changed by the change in impeller diameter or its rotational speed. • The affinity laws state how such changes influence the pump’s performance. These laws are summarized in the following points. • • The flow rate or capacity is directly proportional to the pump speed: double the speed / double the flow. Q ∝ n • • The pump head is directly proportional to the square of the pump speed: double the speed/multiply the pressure by four. Hp ∝ n2 • • The power required by the pump motor is directly proportional to the cube of the pump speed: double the speed/multiply the power by eight. P ∝ n3 • The Affinity laws give approximate results. This discrepancy is due to hydraulic efficiency changes The affinity laws,
- 39. Priming in Pumps • Centrifugal pumps are capable of pumping incompressible fluids only. • Centrifugal Pump can not suck the liquid, but it pushes the liquid from suction to discharge. Due to pressure difference created by the liquid pushed to the discharge with an additional push on liquid from the atmospheric pressure in the storage tank connected to pump suction piping, more liquid enter in the suction side of pump provided suction line is completely filled with liquid (primed). • Its sort of that pushes the liquid out and pulling effect is not so prominent. During the start up of the pump if any air pocket is present at the suction side, then pump will push the air. As a result air present in the suction side will try to expand and it will block the liquid from entering into the centrifugal pump.
- 40. • In Centrifugal Pump the head developed (in meters of liquid that is pumped) depends on the velocities determined by diameter of the impeller and the impeller speed (rpm.). As the pressure developed is related to the head by the equation head = pr / sp. weight, the pressure available will be proportional to the specific weight of the liquid. • This means that the pressure (or pressure difference) created with air will be only around 1/800 times that with water (density of water = 1000 kg/ m3 and dry air at S.T.P has a density of 1.2 kg/m3 ). Therefore, if the pump is not primed, the suction pressure created will not be sufficient to lift water. Priming in Pumps
- 41. • Priming of a pump can be achieved by either layout consideration, or by means of some external arrangements that ensures priming or by use of Self Priming Pumps. Few of the external arrangements that ensures priming of a pump are detailed out below. • Manually • With Vacuum Pump • With Jet Pump • With Separator • Installing Foot Valve Methods of Priming
- 42. Importance of running clearances • All centrifugal pumps must have clearance between the stationary pump casing and the rotating element. • There are number of close tolerance clearances which, if altered, will change one or more of a pump's characteristics.
- 43. • Consider a single stage overhung pump with a semi-open impeller, which has back pump-out vanes
- 44. Front clearance • The front clearance between the impeller and the casing is set to achieve an acceptable pump efficiency. • If this clearance is allowed to grow, either through corrosion or erosion, the pump will lose efficiency. More power will be required to achieve the same flow rate from the pump. This is caused by increased recirculation from high pressure zones to lower pressure zones across the front of the casing. • While this recirculation may not be great, the flow disturbance also contributes to the loss of efficiency.
- 45. Back clearance • The back clearance must also be controlled to allow the pump-out vanes to do their job, which is to reduce the pressure at the back of the impeller • If this clearance is allowed to grow outside of tolerance, an increase in axial thrust can lead to early thrust bearing failure.
- 46. • Consider a single stage overhung pump with a fully enclosed impeller with shrouds back and front of the vanes, which has wear rings:
- 47. Clearance of the wear rings • The clearance of the wear rings must be set within tolerance to achieve normal pump efficiency. Any increase in the clearance of the back or front wear rings will lead to reduced pump efficiency due to increased recirculation from the impeller discharge, through the wear rings, back to the suction area of the impeller. • To allow the back wear rings in a fully enclosed impeller to be functional, the impeller must have balance holes between adjacent vanes, drilled in the impeller back shroud close to the hub diameter. • These balance holes relieve the pressure created through wear ring leakage by allowing the leaked flow to return to the impeller suction.
- 48. • The balancing holes connect the space around the hub to the suction side of the impeller. The balancing holes have a total cross-sectional area that is considerably greater than the cross-sectional area of the annular space between the wearing ring and the hub. The result is suction pressure on both sides of the impeller hub, which maintains a hydraulic balance of axial thrust.
- 49. • The other purpose of back wear rings is to create a thrust balance across the impeller. • Normally the back wear rings will have a diameter only marginally greater than the front wear rings, which compensates for the presence of the shaft at the back of the impeller.
- 50. • Common concerns for a single stage overhung pump of either semi- open or fully enclosed impeller design:
- 51. • As a fully enclosed impeller wear ring clearance increases, the net positive suction head required (NPSHR) will increase. The degree of increase in NPSHR required will vary, dependent upon the specific pump design. Where the wear ring diameter is large in relation to the impeller outer diameter (vane tip diameter) the effect on NPSHR will be greater for a given numerical clearance increase. • Broadly, doubling of wear ring clearance may be considered to increase NPSHR for a specific flow by 50%.
- 52. • The clearance increases between back pump out vanes and the pump cover (semi-open impeller), or the back wear ring clearance increases (fully enclosed impeller), the pressure in the seal cavity will increase. • The degree of increase will depend on the specific flushing arrangement of the mechanical seal. This will result in a greater closing force on the seal faces, greater heat generation at the seal faces, with the potential for more rapid seal wear. Where packing is installed, leakage of fluid may increase and more frequent packing adjustment or replacement may result.
- 53. • Impeller vane tip to volute tip clearance:
- 54. • Gap "B" is the distance between the impeller blade tips and the casing volute tip . As this clearance is reduced, (which happens as impeller diameter is increased), the hydraulically induced shock pulses, created as the vane tips pass the volute tip, will also increase. This means that the overall vibration amplitude, and in particular the vibration peak associated with the vane (or blade) passing frequency, will increase as gap "B” decreases.
- 55. To Avoid Hydraulic Pulse • Trimming the impeller vanes and shrouds to achieve the required gap "B". • Trimming the vanes only to achieve gap "B", while leaving the shrouds at their original diameter. This will maintain gap "A" • Trimming the vanes at an angle to achieve an average gap "B", • An alternative to trimming the impeller vanes is to remove a small portion of the volute tip by machining or filing to achieve an increase in gap "B". The volute tip should preferably be rounded (bull nosed) to further soften the hydraulic shock pulse. An increase in gap "B" of no more than 8% is recommended with this approach. This approach will result in only a very marginal change in hydraulic performance.
- 56. Effect on Critical speed • In multistage pumps the running clearances at the wear rings and at any internal bushings affect the critical speed of the rotating element. • The calculated dry running critical speed will be different to the wet (or liquid filled) running speed. This may not prove to be a problem in the majority of applications, but can become important where variable speed drive is employed. Where the range of operating speeds was initially well outside the fluid damped critical speeds of the rotating element, any increase in running clearances will usually tend to lower the critical speed. Where the change in critical speed is significant as running clearances increase, encroachment of critical speed to within the pump's operating speed envelope may occur.
- 57. Discharge Phase The discharge nozzle, which is shaped like a cone so that the high-velocity flow from the impeller is gradually reduced. This cone-shaped discharge nozzle is also called a diffuser. During the reduction in velocity in the diffuser, energy in the flow is converted to pressure energy. An optimum angle of seven to 10 degrees is used to most efficiently convert velocity energy to pressure energy.
- 58. Reasons for closing the discharge valve • The starting current can be reduced by closing the discharge valve. The current will be very high when any motor starts. When the pump is started with an open discharge valve, the discharge head will have greater resistance to the pump. Therefore, the motor must give the pump a larger starting torque, which means that the motor needs to draw more current.
- 59. Pump Characteristic Curves • The pump characteristic curves can be defined as ‘the graphical representation of a particular pump’s behavior and performance under different operating conditions’. • Total head, Efficiency, Power, Net positive suction head required (NPSHR) to discharge or pump capacity (Q) are utilized to describe the operating properties of a pump. This set of four curves is known as the pump characteristic curves or pump performance curves. • Classification of Pump Characteristic Curves (i) Main characteristic curves, (ii) Operating characteristic curves, (iii) Constant efficiency curves, (iv) Constant head and constant discharge curves.
- 60. Main Characteristic Curves • In some circumstances, it is necessary to know the performance of a pump at different speeds, which can be best seen from the main characteristic curves of a pump. • In order to obtain the main characteristic curves of a pump, it is operated at different speeds. For each speed, the pump discharge (Q) is varied by means of a delivery valve and for the different values of Q, the corresponding values of Head (Hm), shaft power (SP) and overall efficiency (ho) are measured or calculated. Thereafter, Hm vs Q; SP vs Q, and ho vs Q curves for different speeds are plotted which represent the main characteristics of a pump.
- 61. Operating Characteristic Curves • During operation of a pump, the pump must run constantly with the speed of the prime mover; this constant speed is usually the design speed. The set of main characteristics curves which corresponds to the design speed is mostly used in pump operation, and hence such curves are known as the operating characteristics curves. • From these characteristic curves, it is possible to determine whether the pump will handle the necessary quantity of liquid against the desired head and what will happen if the head is increased or decreased. In addition, these characteristic curves illustrate what size motor will be required to operate the pump at the required conditions and whether or not the motor will be overloaded under any other operating conditions.
- 62. Head versus Discharge Curve • For radial flow impellers, head decreases only slightly and then drops rapidly as discharge (Q) increases from zero • Slope changes along H-Q curves for the mixed and axial flow impellers are not as dramatic as those for radial flow impellers.
- 63. Efficiency versus Discharge Curve . If a different number of stages are needed for a particular situation, efficiencies must be adjusted upward or downward depending on the number of stages.
- 64. NPSHR versus Discharge Curve • A typical radial flow pump, NPSHR gradually increases as the pump discharge increases.
- 65. Constant Efficiency Curves • the Muschel curves facilitate to see directly the range of pump operation for a given efficiency. These curves further serve as a suitable basis for the comparison of pumps, especially from a commercial viewpoint.
- 66. Constant Head and Constant Discharge Curves • The constant head curves and the constant discharge curves are also useful for determining the performance of a variable speed pump having constantly varying speed
- 67. “Power comes not from knowledge kept but from Knowledge shared”