Course Objectives. Introduction to Pumps. Introduction to Pumps 3/6/2014

Transcription

1 Introduction to Pumps Robert von Bernuth Course Objectives Throughout this course, students will learn to identify situations where pumps are needed. understand what information is needed to select a pump. gain a basic understanding of how centrifugal pumps work. learn how to use the information provided on typical pump curves. understand pump and system curve interaction. 2 Section 1: When Is a Pump Needed? The need for a pump depends on the system and the source. In irrigation, there are two scenarios where pumps are needed: The water is located below where it is needed. well pond stream The available source does not provide adequate pressure to run the system. 3 1

2 Why Is a Pump Needed? Water isn t where we want it. Water doesn t have enough pressure to do what we want. Sprinkler or drip systems need pressure. Municipal water systems often have enough pressure to run sprinkler or drip systems. Water from ponds, rivers, or wells must be moved and pressurized. 4 Irrigation System Pressure Requirements All irrigation systems require some pressure. high pressure > 60 psi for large sprinklers or rotors medium pressure psi for small and medium rotors psi for sprays and rotors low pressure < 30 psi for microirrigation (drip and microspray) 5 Water Source Potable water It is most likely a pressurized system. Is there adequate pressure to run the type of system desired? Well A pump will be required to both lift the water and pressurize it. Pond or stream A pump will be required. Some lift will probably be required, and the system must be pressurized. 6 2

3 What Is a Pump? A device that moves fluids raises fluids pressurizes fluids 7 What Information Is Needed to Choose a Pump? You need to know the flow and the pressure. The flow is the maximum flow for any zone the sum of the flows of all the emitters or sprinklers running simultaneously. If a pump is needed, the flows of the zones should be nearly the same. The pressure is the operating pressure of the emitters or sprinklers plus friction loss. If the source is pressurized, only the additional pressure needs to be provided by the pump. 8 How Does a Pump Work? Most irrigation pumps are centrifugal pumps. Centrifugal is the term used to describe the outward force produced by a rotating body. An electric motor or combustion engine drives a shaft that is connected to an impeller. The water enters the impeller and is spun, creating a centrifugal force. The centrifugal force creates velocity, which is converted to pressure. 9 3

4 Typical Centrifugal Pump Cross Section Engine or motor spins the impeller. Water enters suction (impeller eye). Water is spun by impeller. By centrifugal force it accelerates (gains velocity) toward the volute. 10 The volute shape converts most of the velocity to pressure. End Suction Centrifugal Pump 11 Pump Curves The performance of a pump is characterized by a pump curve. Typically, the head (feet of water) a pump produces is plotted against the flow rate (gallons per minute). 12 4

5 Typical Pump Curve A typical pump curve decreases in head produced as the flow rate increases. The cause can be thought of as increasing friction loss with flow. Most pumps are designed to operate in a limited range of the full pump curve. They are most efficient in that range. Deviating from the design operating point decreases efficiency. 13 Typical Pump Curve P1 P2 P3 14 Design Operating Range Design operating range 15 5

6 Pump Curves Expanded Most pump curves include a family of curves for different impeller sizes or different speeds. They also include efficiencies. 16 Expanded Pump Curves Other Speeds and Efficiencies E3 E2 E3 < E2 < E1 E1 N3 N2 N1 N3 < N2 < N1 17 Pump Power It takes power to spin the impeller. The power depends on the flow rate and the total head. Where hp = horsepower Q = flow in gallons per minute h=total dynamic head in feet E = efficiency (decimal) 18 6

7 Summary Section 1 Centrifugal pumps develop pressure by spinning water through an impeller and converting the velocity to pressure. Pumps are characterized by a plot of flow [Q] against head [h], and they have a design operating point. Pump curves typically include a family of curves for different speeds or impeller diameters and efficiency curves. We can calculate the horsepower required by a pump. 19 Section 2: Pump Operating Conditions Determine total flow requirement. Determine total dynamic head. 20 Selecting Pumps Head and flow must match the system. Head is total dynamic head. Total flow is determined by plant/crop needs and available capacity. The next few slides show how to determine total dynamic head. Pump requirements must be met. adequate inlet pressure to prevent cavitation adequate power 21 7

8 Relationship Between psi and feet of head 1 foot of head = 1 foot of elevation 1 foot of elevation = psi Example: 200 feet of head = (0.433) (200) = 86.6 psi 22 Total Dynamic Head Total dynamic head [TDH] is the total pressure head that must be delivered by a pump expressed in feet of head of water. TDH depends on the physical situation and system requirements. 23 Components of TDH Suction pipe friction loss Suction lift Suction entrance losses Discharge pipe friction losses Discharge lift System operating pressure Fitting losses 24 8

9 Total Dynamic Head 6. System pressure requirement 4. Discharge pipe friction loss 1. Suction pipe friction loss 5. Discharge lift 2. Suction lift 7. Miscellaneous fittings losses 3. Suction entrance 25 Total Dynamic Head 6. System pressure requirement 4. Discharge pipe friction loss 1. Suction pipe friction loss 5. Discharge lift 2. Suction lift 7. Miscellaneous fittings losses 3. Suction entrance 26 Total Dynamic Head 6. System pressure requirement 4. Discharge pipe friction loss 1. Suction pipe friction loss 5. Discharge lift 2. Suction lift 7. Miscellaneous fittings losses 3. Suction entrance 27 9

10 Total Dynamic Head 6. System pressure requirement 4. Discharge pipe friction loss 5. Discharge lift 1. Suction pipe friction loss 2. Suction lift 7. Miscellaneous fittings losses 3. Suction entrance 28 Total Dynamic Head 6. System pressure requirement 4. Discharge pipe friction loss 1. Suction pipe friction loss 5. Discharge lift 2. Suction lift 7. Miscellaneous fittings losses 3. Suction entrance 29 Total Dynamic Head 6. System pressure requirement 4. Discharge pipe friction loss 1. Suction pipe friction loss 5. Discharge lift 2. Suction lift 7. Miscellaneous fittings losses 3. Suction entrance 30 10

11 Total Dynamic Head 6. System pressure requirement 4. Discharge pipe friction loss 1. Suction pipe friction loss 5. Discharge lift 2. Suction lift 3. Suction entrance 7. Miscellaneous fittings losses 31 Now You Try It Determine the TDH of a pump with the following conditions: System operating pressure is 50 psi. Suction friction loss and entrance losses together total 2 feet. Suction lift is 5 feet. Discharge lift is 20 feet. Miscellaneous fitting losses total 2 psi. Discharge friction loss is 10 psi. 32 Total Dynamic Head Example 1. Suction friction (4-in. steel pipe, 20 ft long) = 2 ft 2. Suction lift = 5 ft 3. Suction entrance loss (included in 1. above) = 0 ft 4. Discharge friction (4-in. steel pipe, 1,000 ft long) = 23 ft 5. Discharge lift = 20 ft 6. Pressure required for system operation = 116 ft 7. Miscellaneous fitting losses = 5 ft TDH = 171 ft 33 11

12 Summary Section 2 Two parameters determine the pump operating point: flow which is determined by plant/crop needs and available capacity TDH made up of seven components The largest are system pressure and discharge lift. Suction losses and suction lift are very important in net positive suction head. 34 Section 3: Special Considerations Consideration must be given to net positive suction head [NPSH] to prevent cavitation. Net positive suction head available [NPSHa] must be determined. NPSHa depends on the air pressure (altitude) of the installation and temperature of the water. As long as NPSHa exceeds net positive suction head required, cavitation should not occur. 35 Cavitation Cavitation is a phenomenon that occurs when the pressure inside the pump isn t high enough and water momentarily converts to vapor and then reconverts back to water. This sudden change can cause severe damage to the pump. It causes a sound similar to small claps of thunder or rocks inside the pump. To prevent cavitation, adequate inlet pressure is required. This is called net positive suction head required [NPSHr]

13 Net Positive Suction Head Required Every pump has an NPSHr, and NPSHr curves are plotted on pump curves. A pump must have NPSHr to operate properly. If NPSHa is greater than the NPSHr, then there is not a problem. NPSHa depends on pump conditions, location altitude, and water temperature. 37 Net Positive Suction Head Available Equation NPSHa = H a H s H f H vp H a = atmospheric pressure at elevation of pump {ft} H s = static lift between the water surface and center line of pump {ft} H f = suction line friction losses {ft} H vp = vapor pressure of liquid being pumped {ft} Vapor pressure depends on the temperature of water. This is the pressure at which water boils for the given temperature. At sea level it is 14.7 psi or 34 feet. 38 Atmospheric Pressure Altitude {ft} Water {ft} {psi} Sea level , , , , , , , ,

14 Suction Lift Suction lift Friction losses 40 Vapor Pressure of Water Temperature Vapor pressure { F} { C} {ft} Under normal conditions, vapor pressure isn t a factor unless water temperature is more than 80 F psi = 34 ft 41 Computing NPSHa Given: Centrifugal pump is located at 5,000 feet elevation. Water temperature is 50 F. Pump is located 10 feet above lake surface. Suction friction losses are 7 feet. Find: NPSHa 42 14

15 NPSHa = H a H s H f H vp Centrifugal pump located in Denver, CO. Pump located 10 feet above lake surface. Suction friction losses are 7 feet. Water temperature is 50 F. 43 NPSHa = H a H s H f H vp 44 NPSHa = H a H s H f H vp 45 15

16 NPSHa = H a H s H f H vp = = 10.9 ft 46 More Information on NPSH For more information on NPSH, the following are two excellent YouTube online videos: This video explains how lowering the pressure leads to boiling. v=oryyp4f8ltu&feature=related This video explains how low NPSHa leads to cavitation. 47 Section 4: Understanding Pump Curves Information on the pump curves Using the information to select a pump 48 16

17 Selecting a Pump Selecting a pump means matching its performance to the needs of the system. The needs of the system are the TDH and the target flow rate. Pick a pump that produces the target flow rate at the desired head at or near its peak efficiency. Make sure that the set up has NPSHa greater than NPSHr. Check the required power. 49 Typical Published Pump Curve 1. TDH {ft} 2. Capacity {gpm} 3. Impeller diameter 4. NPSHr 5. Brake horsepower 6. Efficiency 7. Target operating point 8. Impeller speed curves ABC Pumps 17

18 Ref. point 2 ABC Pumps 52 Ref. point 3 ABC Pumps 53 Ref. point 4 ABC Pumps 54 18

19 Ref. point 5 ABC Pumps 55 Ref. point 6 ABC Pumps 56 Ref. point 9 ABC Pumps 57 19

20 Pump Capacity Charts Some pump manufacturers publish pump capacity charts to aid in selecting a pump. The chart provides guidelines as to which model of pump to choose, and selection of the specific pump results from reviewing the specific pump curves. Many manufacturers now use computer programs to select the correct model of pump. 58 Example of Pump Capacity Chart From Armstrong Pumps 59 Example of Pump Capacity Chart ce.engr.ccny.cuny.edu/courses/ce365/pump_selection.pdf 60 20

21 Summary Section 4 In this section we learned what information is provided in typical pump charts and how to use them to select a pump. The following information is generally included in a pump chart: TDH flow impeller diameters impeller speeds NPSHr efficiency power suction size discharge side size 61 Section 5: Pump and System Operating Point Where the system curve and pump curve intersect Common point Changes with change in flow 62 Operating Point It is rare that a pump is selected that exactly matches the flow and head conditions of the system. As a result, the operating point (where system and pump operate) will be slightly different from the target flow and head. If the flow requirements change, the delivered pressure (head) of the pump will change

22 System Curves System curves will change as flow requirements change. For example, if the number of sprinklers operating changes, the flow and head will change. The following slide depicts this concept. 64 Operating Points 65 Summary Section 5 In this section we learned about how the pump and system curves interact as flow requirements in the system are changed. We also learned that with typical pump curves, opening a valve to an identical second lateral does not result in a doubling of the flow rate, and it does result in a decrease in system pressure

23 Introductory Pumps Summary 1. What is a pump? 2. Parts of a pump 3. Pump curves 4. Pump efficiencies 5. Impeller diameters 6. Pump power 7. Pump operating conditions 8. Total dynamic head 9. Net positive suction head 10. Cavitation 11. Information on pump curves 12. Pump and system curves 67 23