Exercise 2-3 Centrifugal Pumps EXERCISE OBJECTIVE In this exercise, you will become familiar with the operation of a centrifugal pump and read its performance chart. You will also observe the effect that increasing flow rate or rotation speed has on the maximum pressure (head) that a centrifugal pump can develop. 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 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 2-26 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 valve (open) Check valve (closed) Figure 2-26. Basic pump operation. Festo Didactic 87996-00 63
Basic operation of a liquid pump Figure 2-27 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 2-27. 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, the greater the flow rate, for any given rotation speed. Types of liquid pumps There are two categories of liquid pumps: positive-displacement and dynamic. Figure 2-28 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 64 Festo Didactic 87996-00
pumps are either centrifugal, mixed flow, or axial. Most of the dynamic pumps in 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 2-28. Types of liquid pumps. The centrifugal pump The most common type of centrifugal pump is the single-suction centrifugal pump. Figure 2-29 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 87996-00 65
Outlet Impeller Shaft Inlet Impeller eye Vane Volute casing Figure 2-29. 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 2-30a 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 2-30b shows. The equation below is used to convert a velocity head into a gauge pressure: (2-10) 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. 66 Festo Didactic 87996-00
Outlet pressure 400 kpa (58 psig) Outlet pressure 333 kpa (48 psig) (a) The specific gravity of the fluid has no influence on the velocity head. Outlet pressure 400 kpa (58 psig) Outlet pressure 400 kpa (58 psig) (b) Liquids of differing specific gravities rise to different heights. Figure 2-30. The influence of the fluid specific gravity on a centrifugal pump. System curve 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. As seen in the previous exercises, the pressure losses in a system also varies with flow rate. If we neglect the variation with flow rate of the Darcy friction factor (major losses, see Equation (2-7)) and the loss coefficient (minor losses, see Equation (2-8)), then we can say that the total pressure loss varies with the square of the flow rate: (2-11) where depends on the pipe's diameter and length, friction factors and loss coefficients Festo Didactic 87996-00 67
The centrifugal pump in a system must provide just enough pressure to counterbalance the total pressure loss and a possible increase in elevation of the fluid. This is summarized in the following equation, also known as the system curve: (2-12) where is the sum of the total pressure loss and the increase in elevation of the fluid is the increase in elevation of the fluid is the pressure head to be delivered by the pump Most pump manufacturers publish charts or tables that show the relationship between the velocity head of the pump and a range of flow rates. A typical peformance chart for a centrifugal pump will be presented later on. If we combine the system curve and the velocity head of the pump on the same graph, as shown in Figure 2-31, we obtain the operating point of the pump. Head (h) Operating head Operating point Pressure A Pump curve (kpa) Pressure B System curve (kpa) Operating flow rate Output flow rate (Q) Figure 2-31. System curve and operating point. If a valve is present in the system and is further closed or opened, then the system curve becomes more or less abrupt (the factor changes in Equation (2-12)). New system curves give new operating points, as shown in Figure 2-32. In the procedure of this exercise, this is how you will obtain the performance chart of the centrifugal pump: by changing the system curve through different valve openings. 68 Festo Didactic 87996-00
Head (h) New operating point (valve more closed) New operating point (valve more opened) Pressure A Pump curve (kpa) Pressure B System curve (kpa) System curve (valve Pressure C more closed) (kpa) System curve (valve Pressure D more opened) (kpa) Output flow rate (Q) Figure 2-32. Variation in operating point with a change in the system curve. Performance chart Figure 2-33 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 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. 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 does 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/min), the pump must have an impeller with a diameter of at least 36 cm (14 in), as Figure 2-33 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. Festo Didactic 87996-00 69
Operating 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 2-33. 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/min). As Figure 2-34 illustrates, the corresponding BP curve shows that a motor of at least 24 kw (32 hp) is required. Operating 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 2-34. Amount of power required by the drive at operating point. 70 Festo Didactic 87996-00
Cavitation Figure 2-35. 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 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, or cavities, may form in the fluid. This is 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 2-36 shows a cavitation bubble collapsing close to a solid surface. Figure 2-37 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 2-38 shows the areas on an impeller that are susceptible to cavitation damage. Cavity Collapsing cavity Liquid jet formation Liquid jet damage Figure 2-36. Cavitation bubble collapsing. Festo Didactic 87996-00 71
Pressure Pump suction Eye Vapor pressure Pump discharge Suction Liquid path Discharge Figure 2-37. Pressure along the liquid path in a pump. Areas subject to cavitation Rotation direction Figure 2-38. Areas of an impeller susceptible to cavitation. Figure 2-39. Actual cavitation in a typical centrifugal pump. 72 Festo Didactic 87996-00
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 2-40 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 operating point of 24 m (80 ft) at a flow rate of 3800 L/min (1000 gal/min), the NPSHR would be about 1.2 m (4 ft). Operating 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 2-40. 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 2-41 shows a centrifugal pump installed below a vessel open to atmosphere; this is the most common type of configuration for centrifugal pumps. Atmospheric pressure (h a) Vapor pressure (h vpa) Figure 2-41. Vessel of liquid placed above the centrifugal pump. Festo Didactic 87996-00 73
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: (2-13) 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 of the liquid in the vessel 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, 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 is lower. To convert absolute pressure into head, use the following formula: (2-14) 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 2-42, 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 2-42. Higher fluid temperature and cavitation. 74 Festo Didactic 87996-00
Ex. 2-3 Centrifugal Pumps Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Measuring the pressure-versus-flow curve of the pump for different rotation speeds End of the exercise PROCEDURE Set up and connections 1. Set up the system shown in Figure 2-43. Make sure the rotameter and pressure gauge are mounted vertically on the expanding work surface. Figure 2-43. Measuring pump outlet pressure versus flow rate. 2. Make sure the reservoir of the pumping unit is filled with about 12 liters (3.2 gallons) of water. Make sure the baffle plate is properly installed at the bottom of the reservoir. 3. On the pumping unit, adjust pump valves HV1 to HV3 as follows: Open HV1 completely. Close HV2 completely. Set HV3 for directing the full reservoir flow to the pump inlet. 4. Turn on the pumping unit. Festo Didactic 87996-00 75
Ex. 2-3 Centrifugal Pumps Procedure Measuring the pressure-versus-flow curve of the pump for different rotation speeds a We will consider that the pump rotation speed is directly proportional to the drive frequency (Model 6510-1) or to the analog input (Model 6510-0 and 6510-1), as shown in Figure 2-44, Figure 2-45, and Figure 2-46. Maximum frequency : 49.5 Hz 50.0 40.0 Drive frequency (Hz) 30.0 20.0 10.0 0.0 0 25 50 75 100 Pump rotation speed (%) Figure 2-44. Pump speed versus frequency (Model 6510-1 only). 5.0 4.0 Analog input (V) 3.0 2.0 1.0 0.0 0 25 50 75 100 Pump rotation speed (%) Figure 2-45. Pump speed versus 0-5 V input. 76 Festo Didactic 87996-00
Ex. 2-3 Centrifugal Pumps Procedure 20.0 15.0 Analog input (ma) 10.0 5.0 0.0 0 25 50 75 100 Pump rotation speed (%) Figure 2-46. Pump speed versus 4-20 ma input. 5. Make the pump rotate at maximum speed. 6. Close valve HV1 completely in order to read a null flow rate [0 L/min (0 gal/min)] on the rotameter. 7. In Table 2-2, record the pressure displayed by the pressure gauge at 100% pump rotation speed for a flow rate of 0 L/min (0 gal/min). 8. Adjust valve HV1 to increase the flow rate by steps of 2 L/min (0.5 gal/min) until you reach 12 L/min (3.0 gal/min) on the rotameter. After each new flow setting, record the new pressure reading in Table 2-2. Table 2-2. Pressure at the pump outlet as a function of the flow rate. Flow rate L/min (gal/min) Pressure at 100% speed kpa (psi) Pressure at 90% speed kpa (psi) Pressure at 80% speed kpa (psi) Pressure at 70% speed kpa (psi) Pressure at 60% speed kpa (psi) 0 (0.0) 2 (0.5) 4 (1.0) 6 (1.5) 8 (2.0) 10 (2.5) 12 (3.0) Festo Didactic 87996-00 77
Ex. 2-3 Centrifugal Pumps Conclusion 9. Make the pump rotate at 90% of the maximum speed. 10. Repeat procedure step 8, recording your measurements in the Pressure at 90% speed column of Table 2-2. 11. Repeat steps 0 and 10 for pump rotation speeds of 80%, 70%, and 60%. a Because of the decreased pump capacity at lower rotation speeds, you may not be able to obtain the higher flow rates listed in Table 2-2. 12. Stop the pump and turn off the pumping unit. 13. Using Table 2-2, plot the pressure-versus-flow curves of the pump for rotation speeds of 100%, 90%, 80%, 70%, and 60%. 14. From the curves you obtained, how does the pump outlet pressure vary with flow rate? 15. What happens to the pump outlet pressure when the rotation speed is increased for any given flow rate? 16. From the curves you obtained in step 13, what is the approximate rotation speed required to develop a head of 70 kpa, gauge (10.2 psig) at a flow rate of 6 L/min (1.5 gal/min)? End of the exercise 17. Disconnect the circuit. Return the components and hoses to their storage location. 18. Wipe off any water from the floor and the training system. CONCLUSION In this exercise, you measured the pressure at the outlet of the pump for various flows and pump rotation speeds. 78 Festo Didactic 87996-00
Ex. 2-3 Centrifugal Pumps Review Questions 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 category of liquid pumps do centrifugal pumps belong? 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? Festo Didactic 87996-00 79
Ex. 2-3 Centrifugal Pumps Review Questions 6. Equation (2-13) 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. 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 identified in question 6. 80 Festo Didactic 87996-00