Exercise 4-2 Centrifugal Pumps EXERCISE OBJECTIVE Familiarize yourself with the basics of liquid pumps, specifically with the basics of centrifugal pumps. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Pumps Basic operation of a liquid pump Types of liquid pumps The centrifugal pump Velocity head. Performance chart. Cavitation. NPSHR and NPSHA. DISCUSSION Pumps A pump creates the flow in most processes using a fluid as the medium. To create flow, the pump converts the mechanical rotational energy supplied by a prime mover into a force that pushes the fluid into the system. Most pumps operate on the same basic principle, they draw fluid by increasing the space inside the pump and they discharge the fluid by decreasing the space inside the pump. Figure 4-18 illustrates this principle using a manual pump as an example. When the handle is pulled out, the space inside the pump increases. This reduces the pressure inside the pump and the fluid is drawn into the pump. When the handle is pushed back in, the space inside the pump decreases. This increases the pressure inside the pump and forces the fluid out of the pump. Check valves prevent the fluid from flowing in the wrong direction. Force Force Check valves Check valves Figure 4-18. Basic pump operation. Festo Didactic 86005-00 83
Ex. 4-2 Centrifugal Pumps Discussion Basic operation of a liquid pump Figure 4-19 shows the basic elements of a liquid pump. The housing contains a rotating mechanism connected to a shaft. A drive, such as an electrical AC or DC motor, turns the shaft to create flow. When the drive is in operation, the suction line brings liquid from the vessel to the pump inlet and the discharge line forces the liquid out of the pump outlet into the system. Atmospheric pressure Inlet Pump Outlet Discharge line To system Suction line Figure 4-19. Operation of a liquid pump. The pump reduces the pressure at the inlet near to an absolute pressure of 0 kpa (0 psia) which causes liquid from the vessel to flow to the inlet. Since this pressure is lower than the atmospheric pressure, the liquid moves through the suction line and into the pump inlet port. The pump then forces the fluid out of the outlet port and discharges it into the system. The displacement is the volume of liquid that one complete revolution of the pump shaft discharges into the system. The greater the pump displacement is, the greater the flow rate is, for any given rotation speed. Types of liquid pumps There are two categories of liquid pumps: positive-displacement and dynamic. Figure 4-20 shows a flow chart that gives the common variations between these two categories. For a positive-displacement pump, the displacement of the pump stays approximately constant when the pump outlet pressure changes. Positivedisplacement pumps are either rotary or reciprocating. Rotary pumps produce a smooth, constant flow, whereas reciprocating pumps produce a pulsating flow. Systems that use a positive-displacement pump can develop very high pressure. They must include a pressure relief valve to bypass the pump output flow directly to the vessel if the system pressure becomes too high. Otherwise, the motor may stall or components may burst if the pumped flow becomes blocked or severely restricted. For a dynamic pump, the displacement of the pump and, therefore, the pump output flow-rate, are not constant. The displacement is greatest at low pump outlet pressure, and it decreases as the pump outlet pressure increases. With this type of pump, a pressure relief valve is not needed to protect the system because the pumped liquid backslips within the pump if the pump outlet pressure becomes too high. However, dynamic pumps should not be allowed to run for prolonged periods with their output flow blocked, because the liquid backslipping within the pump tends to overheat, which may damage the pump seals. Dynamic pumps are either centrifugal, mixed flow, or axial. Most of the dynamic pumps in 84 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Discussion the industry are centrifugal. Centrifugal pumps can generate very high flow rates at moderately high pressure. Positivedisplacement pumps Rotary Reciprocating Piston Lobe Vane Gear Flexible Impeller Peristaltic Screw Piston Ram Diaphragm Radial Axial Internal External Helical Progressive Cavity Archimedean Dynamic pumps Axial Mixed Flow Centrifugal Singlesuction Doublesuction Multistage Submersible Sealed Magdrive Figure 4-20. Types of liquid pumps. The centrifugal pump The most common type of centrifugal pump is the single-suction centrifugal pump. Figure 4-21 shows a typical single-suction centrifugal pump. Such a pump consists of an impeller that rotates inside a stationary casing. The liquid enters at the center (eye) of the impeller, where the impeller vanes collect it. The impeller rotation accelerates the liquid at a high speed and expels it radially into the volute chamber. As the discharged liquid leaves the impeller periphery, the pressure reduces at the impeller eye, which forces new liquid to enter the pump. This results in a constant flow of liquid through the impeller. Festo Didactic 86005-00 85
Ex. 4-2 Centrifugal Pumps Discussion Outlet Impeller Shaft Inlet Impeller eye Vane Volute casing Figure 4-21. Typical single-suction centrifugal pump. Velocity head A pump can create a liquid column at its outlet as a result of the kinetic energy imparted to the discharged liquid. The velocity head of a centrifugal pump corresponds to the vertical height of this column of liquid. The velocity head is measured in meters (m) in SI units and in feet (ft) in the US customary units. Manufacturers often use the velocity head instead of pressure to describe the outlet performance of centrifugal pumps. The velocity head does not change if liquids of different specific gravities are used, as Figure 4-22 (a) shows. On the other hand, the maximum pressure the pump can develop at its outlet is dependent on the specific gravity of the liquid. Thus, liquids of differing specific gravities rise to different heights for the same pump outlet pressure, as Figure 4-22 (b) shows. The equation below is used to convert a velocity head into a gauge pressure: (4-12) where is the gauge pressure is the velocity head is the specific gravity of the fluid is a conversion constant, 0.102 m/kpa (2.31 ft/psi) The diameter of the impeller and the speed at which it rotates determines the velocity head a centrifugal pump can develop. The higher the rotation speed, the greater the velocity head. Similarly, the greater the diameter of the impeller, the greater the velocity head. 86 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Discussion Outlet pressure 400 kpa (58 psig) (a) The specific gravity of the fluid has no influence on the velocity head. Outlet pressure 333 kpa (48 psig) Outlet pressure 400 kpa (58 psig) (b) Liquids of differing specific gravities rise to different heights. Outlet pressure 400 kpa (58 psig) Figure 4-22. The influence of the fluid specific gravity on a centrifugal pump. Performance chart In order to design a new process control system or to analyze the operation of an existing one, it is important to know that the head a centrifugal pump develops at its outlet varies with flow rate. Most pump manufacturers publish charts or tables that show the relationship between the velocity head of the pump and a range of flow rates. Figure 4-23 shows a typical performance chart for a centrifugal pump, the chart has three sections: the upper-right corner, the upper part of the chart, and the lower part of the chart. Be aware that some pump performance charts show the curve for different pump speeds, but for a fixed impeller diameter. The upper right-hand corner of the chart indicates the pump size, the pump speed, and the maximum and minimum diameters of the pump impeller. In this example, the chart describes a pump that has an inlet port of 20 cm (8 in), an outlet port of 15 cm (6 in), and a maximum impeller diameter of 43 cm (17 in). The chart is valid only for a pump speed of 1160 revolutions per minute. The maximum and minimum diameters of the pump impeller are 43 cm and 28 cm (17 in and 11 in), respectively. Festo Didactic 86005-00 87
Ex. 4-2 Centrifugal Pumps Discussion The upper part of the chart shows the head-versus-flow curve of the pump for impellers of different sizes rotating at 1160 r/min. The chart shows that the head is maximum when the flow rate is minimum (i.e., zero), that is, when the flow is blocked. The head decreases as the flow rate increases. The chart also shows that the head increases as the impeller diameter increases, for any given flow rate. The choice of the impeller diameter for a particular application depends on the maximum head and flow rate that the application requires. The liquid will not flow in the system unless the pump develops a head higher than the sum of all the pressure losses due to the components downstream. In other words, liquid flow does not occur unless the pump is able to develop enough pressure to push the liquid through the circuit piping and valves. If, for example, the application requires a maximum head of 24 m (80 ft) at a flow rate of 3800 L/min (1000 gal US/min), the pump must have an impeller with a diameter of at least 36 cm (14 in), as Figure 4-23 shows. The flow rate can then be varied by restricting the discharge flow (creating pressure loss) with a valve. Finally, the lower part of the chart shows the break power (BP) curves associated with each of the head-versus-flow curves. The top BP curve corresponds to the top head-versus-flow curve, etc. A BP curve indicates the minimum amount of power the motor of the pump must develop to operate at different points of the head-versus-flow curve. The amount of power is determined from the scale in the lower left-hand corner of the chart. Operation point Pump model: X Size: 20 x 15 x 43 cm (8 x 6 x 17 in) Speed: 1160 r/min Impeller max. diam.: 43 cm (17 in) Impeller min. diam.: 28 cm (11 in) Velocity head, m (ft) Break power, kw (hp) NPSHR, m (ft) Flow rate, L/min (gal/min) Figure 4-23. Typical performance chart for a centrifugal pump. In our example, a 36 cm (14 in) diameter impeller generates a maximum head of 24 m (80 ft) at a flow rate of 3800 L/min (1000 gal US/min). As Figure 4-24 illustrates, the corresponding BP curve shows that a motor of at least 24 kw (32 hp) is required. 88 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Discussion Operation point Pump model: X Size: 20 x 15 x 43 cm (8 x 6 x 17 in) Speed: 1160 r/min Impeller max. diam.: 43 cm (17 in) Impeller min. diam.: 28 cm (11 in) Velocity head, m (ft) NPSHR, m (ft) Break power, kw (hp) Flow rate, L/min (gal/min) Figure 4-24. Determining the amount of power the drive must be capable of developing at operation point. Cavitation Figure 4-25. Liquid and gaseous phase of a liquid at equilibrium in a closed container. Liquids, as well as solids, tend to evaporate. When a liquid evaporates, some of the molecules at its surface go from the liquid state to the gaseous state. In a closed container, the liquid and gaseous phases of the substance come to equilibrium when the number of molecules returning to the liquid equals the number of molecules leaving the liquid by evaporation. The pressure that the saturated vapor exerts on the container is called the vapor pressure,. The vapor pressure of a substance changes with temperature. When the temperature is higher, the molecules have more kinetic energy and escape more easily from the liquid. When the vapor pressure of a substance equals the surrounding pressure, the liquid starts to boil; that is, bubbles form in the substance as it changes from the liquid state to the gaseous state. Even at low temperature, a liquid can vaporize if the surrounding pressure falls under its vapor pressure. The vapor pressure of a liquid plays an important role in the phenomenon of cavitation. When the velocity of a fluid in a pipe increases, the pressure decreases. This is known as the Bernoulli effect. When the velocity of a liquid increases in a pump, the pressure sometimes decreases enough to reach the vapor pressure of the liquid. When this occurs, bubbles may form in the fluid. These bubbles are cavities in the fluid, this is why this phenomenon is called cavitation. When these bubbles go into a region where the pressure is higher, they collapse. If the collapsing occurs far from a solid boundary the implosion of the cavity is symmetrical and does not cause damage to the pump. It produces only less spectacular undesirable effects such as loss in the pump capacity, pressure head, and efficiency. However, if a cavity collapses close to a solid surface, such as the pump impeller, it creates large pressure transients close to the implosion and releases tremendous amounts of energy that can cause damage to the pump, which may result in the eventual destruction of the pump Figure 4-26 Festo Didactic 86005-00 89
Ex. 4-2 Centrifugal Pumps Discussion shows a cavitation bubble collapsing close to a solid surface. Figure 4-27 illustrates how the pressure varies in the pump along the path that the liquid follows. If the pressure drops sufficiently to fall below the vapor pressure of the liquid, cavitation is likely to occur and cause damage to the impeller. Figure 4-28 shows the areas on an impeller that are susceptible to cavitation damage. Cavity Collapsing cavity Liquid jet formation Figure 4-26. Cavitation bubble collapsing. Liquid jet damage Pressure Pump suction Eye Vapor pressure Pump discharge Suction Liquid path Discharge Figure 4-27. Pressure along the liquid path in a pump. Areas subject to cavitation Rotation direction Figure 4-28. Areas of an impeller susceptible to cavitation. 90 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Discussion Figure 4-29. Actual cavitation in a typical centrifugal pump. NPSHR and NPSHA To avoid cavitation, the pressure at the pump inlet must be kept above a minimum level called Net Positive Suction Head Required (NPSHR). The NPSHR is measured in meters (m) in SI units and in feet (ft) in US customary units. The pump manufacturer determines the NPSHR and plots it as a function of the flow rate on the performance chart. Figure 4-31 shows an example. The NPSHR information appears as a single curve, labeled NPSHR, plotted just below the head-versus-flow curves. The NPSHR can be determined from this curve and the scale in the middle right of the chart. For example, with an operation point of 24 m (80 ft) at a flow rate of 3800 L/min (1000 gal US/min), the NPSHR would be about 1.2 m (4 ft). Festo Didactic 86005-00 91
Ex. 4-2 Centrifugal Pumps Discussion Operation point Pump model: X Size: 20 x 15 x 43 cm (8 x 6 x 17 in) Speed: 1160 r/min Impeller max. diam.: 43 cm (17 in) Impeller min. diam.: 28 cm (11 in) Velocity head, m (ft) NPSHR, m (ft) Break power, kw (hp) Atmospheric pressure Vapor pressure Flow rate, L/min (gal/min) Figure 4-31. Determining the NPSHR from the pump performance chart. To determine whether the pressure at the pump inlet is above the NPSHR, one must know the Net Positive Suction Head Available (NPSHA) at that point. A formula that takes into account both the vapor pressure of the liquid and the configuration of the system around the pump inlet is used to estimate the NPSHA. Figure 4-30 shows a centrifugal pump installed below a vessel open to atmosphere; this is the most common type of configuration for centrifugal pumps. Figure 4-30. Vessel of liquid placed above the centrifugal pump. With this configuration, the liquid flows to the pump inlet by gravity. To draw liquid in, the pump does not need to reduce its inlet pressure as low as when it is located above the vessel level, thus reducing the risk of cavitation. With this configuration, the formula used to calculate the NPSHA is: (4-13) where is the Net Positive Suction Head Available (m or ft) is the head equivalent to the absolute pressure on the surface of the liquid in the vessel is the height that the liquid in the vessel is above the pump impeller eye head equivalent to the total loss of absolute pressure in the suction line is the head equivalent to the absolute vapor pressure of the liquid To prevent cavitation, the NPSHA must be kept greater than or equal to the NPSHR plus a 0.6 m (2 ft) safety margin. The higher the height of the vessel or the pressure on the surface of the liquid are, the greater the NPSHA. On the other hand, if the pressure losses in the suction line are high or if the vapor pressure of the liquid is high, the NPSHA will be lower. 92 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Procedure Outline To convert absolute pressure into head, use the following formula: (4-14) where is the head is the absolute pressure is the specific gravity of the fluid is a conversion constant, 0.102 m/kpa (2.31 ft/psi) If the temperature of the process fluid increases, so does its vapor pressure. From Figure 4-32, we can observe that cavitation is more likely to happen with a warmer liquid. Therefore, if the process fluid temperature increases significantly, it might be necessary to modify the pump installation to ensure that the NPSHA always remains greater than the NPSHR. Pump discharge Pressure Pump suction Eye Vapor pressure of a warmer liquid Vapor pressure of a cooler liquid Liquid path Figure 4-32. Higher fluid temperature and cavitation. PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Pressure versus flow PROCEDURE Setup and connections 1. Connect the equipment as the piping and instrumentation diagram in Figure 4-33 shows and use Figure 4-34 to position the equipment correctly on the frame of the training system. Use the basic setup presented in the Familiarization with the Training System manual 1. Table 4-7 lists the equipment you must add to the basic setup to set up your system for this exercise. 1 This exercise does not require the column. Festo Didactic 86005-00 93
Ex. 4-2 Centrifugal Pumps Procedure Table 4-7. Devices required for this exercise. Name Model Identification Digital Pressure Gauge 46761-B PI 1 Electrical Unit 46970 Pneumatic Unit 46971 Accessories 46993 Calibrator ---- Figure 4-33. P&ID. 94 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Procedure BACK VIEW Figure 4-34. Setup. 2. Wire the emergency push-button so that you can cut power in case of an emergency. 3. Do not power up the instrumentation workstation before your instructor has validated your setup. 4. Configure the pressure gauge so that it gives readings in the desired units. 5. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements for the proper completion of this exercise. This checklist is not exhaustive, be sure to follow the instruction of the Familiarization with the Training System manual as well. Festo Didactic 86005-00 95
Ex. 4-2 Centrifugal Pumps Procedure f The hand valves are in the positions shown in the P&ID. The pressure gauge is installed properly. 6. Test your system for leaks. Repair all leaks. 7. Fill the pipes completely with water and bleed the pressure gauge. 8. Set the zero on the pressure gauge. Pressure versus flow This exercise is not designed for a setup with two teams working at the same time on the trainer. When two teams use the same pump, the flow is divided between the two teams. 9. Use the drive to make the pump run at its maximum speed to produce the maximum flow rate the pumping unit can deliver. 10. Close HV2 to stop the flow. Do not let the pump run with the flow blocked for a long time. This may damage the pump. 11. Read the pressure delivered by the pump on the pressure gauge. 12. Record this value in Table 4-8. 96 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Procedure 13. Adjust the opening of HV2 to read a flow rate of 6 L/min (1.5 gal/min) on the rotameter. Read the pressure on the pressure gauge and record it in Table 4-8. Table 4-8. Pressure at the pump outlet as a function of the flow rate. Flow rate L/min (gal/min) 0 (0.0) 6 (1.5) Pressure kpa (psi) 14. Use the ball valve HV2 to increase the flow rate by steps of 6 L/min (or 1.5 gal/min) until you reach 60 L/min (16 gal/min). For each flow rate, record the pressure reading in Table 4-8. 15. When the drive output frequency is maximum (60 Hz), the pump rotates at 3450 revolutions per minute (r/min). Change the drive output frequency so that the pump rotates at 3000 r/min. a Consider the pump rotation speed to be directly proportional to the drive output frequency. 16. Again, measure the pressure at the outlet of the pump as a function of the flow rate. Use HV2 to increase the flow rate by increments of 6 L/min (1.5 gal/min). Record the pressure drop for each flow rate in Table 4-9. Take measurements until you cannot increase the flow rate further. 17. Repeat this operation for a pump rotation-speed of 2500 r/min and 2000 r/min and fill the empty columns in Table 4-9. Since the pump rotation speed is smaller than 3450 r/min, you may not be able to obtain the higher flow rates listed in the table below. Festo Didactic 86005-00 97
Ex. 4-2 Centrifugal Pumps Conclusion Table 4-9. Pressure at the pump outlet as a function of the flow rate. Flow rate L/min (gal/min) Pressure at 3000 r/min kpa (psi) Pressure at 2500 r/min kpa (psi) Pressure at 2000 r/min kpa (psi) 0 (0.0) 6 (1.5) 18. Use the data in Table 4-8 to plot a graph of the pressure developed at the pump outlet as a function of the flow rate. 19. On the same graph, plot the data of Table 4-9. CONCLUSION In this exercise, you have measured the pressure at the outlet of the pump for various flows and for various pump rotation speeds. REVIEW QUESTIONS 1. What is the basic working principle of most pumps? 2. What is the difference between positive-displacement pumps and dynamic pumps? 3. To which of the liquid pumps categories do the centrifugal pumps belong? 98 Festo Didactic 86005-00
Ex. 4-2 Centrifugal Pumps Review Questions 4. What is the head of a pump? 5. Is cavitation more likely to occur in a pump if the pumped liquid is hot? Why? 6. Equation (4-13) gives the NPSHA for an installation where the liquid flows to the centrifugal pump inlet by gravity. Find the equivalent equation for the installation below where the pump is located above the vessel. Figure 4-35. Festo Didactic 86005-00 99
Ex. 4-2 Centrifugal Pumps Review Questions 7. Is the pump in the figure below able to force water out of the open vessel under standard temperature and pressure conditions? Explain using the formula found in question 6. 10.3 meters (33.9 feet) 15 meters (50 feet) Figure 4-36. 100 Festo Didactic 86005-00