Sec 10 Pumps

Understanding Pump Stations Pump Station – a combination of pumps and electrical control equipment prefabricated on a mounting plate (skid), with piping, valves and sensors to transfer fluid (generally water) from one point to another. A. Methods of OperationVariable Speed ElectricConstant Speed ElectricEngine Drive B. Pump StylesTurbineSubmersibleCentrifugal Any combination of “A” and “B” except “engine drive” – “submersible”, can become a pump station. Pump – since this is a common generic word, the following is a description of the most common types of pumps used in irrigation. Any of these pump types can be used to build a pump station. Centrifugal Pumps Centrifugal pumps use centrifugal force to convert mechanical energy into pressure and velocity. http://marineengineeringonline.com/centrifugal-pumps/ The pump is mounted on a shaft through the eye of the impeller. The shaft is driven by a motor. As the shaft turns it spins the curved vanes that make up the impeller. Water enters the pump through the eye of the impeller. Water is then moved to the outside of the impeller and into the volute where most of the volute energy is converted to pressure. “The higher the velocity at the outside rim of the impeller, the greater the energy transfer to the water. The speed of the impeller’s perimeter can be increased by either increasing the speed of rotation or by increasing the diameter of the impeller.” Principles of Irrigation 3rd Edition Irrigation Association March 2015 Pg 200 Centrifugal pumps are used most common where the water comes from surface sources (lakes, dugouts, etc) The intake side of the pump is usually larger than the discharge side. This helps prevent cavitation. The size of pump refers to the diameter of the discharge fitting. A 2” pump would have a 2” discharge fitting. Centrifugal pumps require priming if mounted above water level. Vertical Turbine also known as lineshaft turbine, generally pumps from a vertical well. The main components are bowl assembly and impellers (actual pump), the discharge head (turns the water from a vertical direction to a horizontal direction and holds the motor), the motor, the column (connects the bowl assembly to the discharge head) and the shaft (connects the impellers inside the bowl assembly to the motor). Vertical turbine pumps are best suited for deep well applications or applications where high pressure is required. They are sometimes installed in wet wells in large areas like golf courses where dugouts are used as the water source. http://www.dep.state.fl.us Vertical turbine pumps are Multistage. They consist of a series of bowls vertically mounted on a rotating shaft. The motor is mounted on top of a lineshaft or pumpshaft in the pump column. It is normally above grade. Because of this the pump must sit vertical. Water is pulled up the lineshaft to the bowls. As water leaves the first impeller, diffuser vanes in the bowl of the pump direct the water to the eye of the second impeller. This is repeated for all the bowls present in a particular pump. This process adds head to the water as it passes through each stage. TDH increases with the size and number of bowls in the pump. General movement of water is vertical. At the top of the pump the water is channeled horizontally into the piping system by a device called a discharge head. Size refers to the minimum well casing diameter in which the pump can be installed. Vertical turbine pumps do not require priming because at least the first bowl and impeller are usually submerged below water. Sometimes all the bowls are submerged. Submersible Pumps Submersible pumps can be used for shallow or surface water and some can be mounted horizontally A submersible pump is a vertical turbine pump driven by a submersible motor. The motor is mounted below the bowls and connected to the impeller. Since there is no line shaft in the pump column the discharge piping is not required to be perfectly straight. Submersible pumps can be installed at an angle anywhere from 5-90 degrees The wiring to the motor is on the outside of the pump column. Submersible pumps require special motors that are designed to run underwater and can be small enough to fit into a well casing. Since the motor is below ground there is less noise and less vandalism. Submersible pumps do not require priming because the motor is submersed. ‘The controls for single-phase units are usually a control box and a pressure switch. These will turn the pump on at a set low pressure and turn the pump off at a set high pressure. Many domestic water systems are set to turn the pump on at 30psi and off at 50 psi. With a pressure storage tank in the system, the pump will turn on and off very rapidly every time any water is used. This short cycling of the pump and motor will cause motors to fail very quickly. To prevent short cycling, the pressure tank should allow for not less than one minute of run time on the pump and motor and two minutes recovery time for each cycle.” Principles of Irrigation, Irrigation Association March 2015 pg 207 VFD (Variable Frequency Drive) – A variable frequency drive is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Frequency is related to the motors speed in RPM’s – the faster the frequency the faster the RPM’s If an application does not require an electric motor to run at full speed, the VFD can be used to ramp down the frequency and voltage to meet the requirements of the motor’s load. “For an irrigation pump this means that if the load is small the motor will ramp up slowly to meet that need. This helps prevent water hammer. https://www.vfds.com/blog/what-is-a-vfd Cavitation in pumps – Cavitation is caused by the formation of bubbles in the water. It develops in areas of low pressure around the impeller of the pump. The implosion of the bubbles triggers shock waves that cause damage or pitting to the impeller of the pump. Sizing Pumps To read a pump curve and size a pump two pieces of information are required – TDH and flow rate. Total Dynamic Head (TDH) “In fluid dynamics, Total Dynamic Head (TDH) is the total equivalent height that a fluid is to be pumped, taking into account friction losses in the pipe. TDH = Static Lift + Friction Loss + sprinkler operating pressure.” https://en.wikipedia.org/wiki/Total_dynamic_head Total Dynamic Head is calculated to Determine the size of pump to install If a pump exists it can be used to determine if there is enough pressure left in the system to run the last head. Total Dynamic Head is comprised of: 1. Friction head – total pressure loss through friction in pipe, valves, fittings etc 2. Elevation head – pressure loss due to elevation change 3. Sprinkler operating pressure – the pressure required to run the sprinkler *4. Suction head – the amount of negative pressure required to draw water upwards to the pump. Includes elevation and friction losses from the intake. ( not usually used for residential installs or where pressure at POC is established) TDH example: A residential irrigation system has a pressure of 55psi at the POC. There is 200 ft of 1” PE pipe running to the worst case head at a flow of 12 gpm. The sprinkler head is an 1800 Rainbird MPR designed to run at 30 psi and is 10 feet above the point of connection. There is 3 psi lost at the valve. What is the TDH of this system? Elevation loss 10 feet Friction loss 3.82 psi/100ft x 200 ft = 7.64 psi 7.64 x 2.31 =17.65 feet Fitting losses 3.00 psi x 2.31 = 6.93 feet Sprinkler pressure 30.00 psi x 2.31 = 69.30 feet Total Dynamic Head of the System 103.88 feet of head Total pressure required = 103.88 ft of head x .433 psi/ft = 44.98 psi Try this: There is 600 ft of 1” Class 160 PVC pipe feeding the system. The flow is 10 GPM. What is the friction loss for this pipe? The POC is 15 feet below the highest sprinkler.What is the static pressure? The sprinklers are operating at 35 psi. What is the TDH? Pump Performance Curves A typical pump performance curve will indicate size of impeller, brake horsepower, TDH capacity, flow in GPM, pump efficiency and NPSHR(Net Positive Suction Head Required) 1. Total Dynamic Head 2. Flow in GPM 3. Impeller size 4. Brake Horsepower 5. NPSH required 6. Efficiency Cheat Sheets: Unwritten Pump Rules ;Larry Bachus; Bachus Company Inc. ; Feb 6, 2013, http://empoweringpumps.com/cheat-sheets-unwritten-pump-rules/ What is the largest diameter impeller this pump comes in? __________________ What is the shut off head of the 10” impeller? What is the greatest efficiency this pump can have? ____________________ If you need the pump to lift 70 ft and push 400 GPM of water what size of impeller would you request? What would be the horsepower requirement? Impeller size ________________ Horsepower ________________________ If the system has a maximum flow rate of 450 gpm and lift needed is 90 feet. What hp and impeller size would you need? What would be the NPSHR? Impeller size __________ HP _____________ NPSHR _____________ What is the maximum GPM and TDH of a 10 hp motor and a 8” diameter impeller? Flow capacity ________________ TDH ____________________ At what flow do you get the greatest efficiency with a 9” diameter impeller? _____________________________________________ Extra Capacity It’s not always possible to find one pump that can deliver the performance you require To increase pressure or TDH pumps can be put in series. This is similar to the bowls of a vertical turbine. The total head produced by PUMPS IN SERIES is equal to the sum of all the pumps. PUMPS IN SERIES INCREASE PRESSURE Pump 1 Pump 2 TOTAL TDH 100 ft 100 ft 200 ft Flow 100 gpm 100 gpm 100 gpm PUMPS IN SERIES INCREASE PRESSURE Pump 1 Pump 2 TOTAL TDH 100 ft 100 ft 200 ft Flow 100 gpm 100 gpm 100 gpm Olds College 2017 To increase flow pumps can be put in parallel. The total flow is the sum of the flow of all pumps. PUMPS IN PARALLEL INCREASE FLOWPump 1 Pump 2TDH 100 ft TDH 100 ftFlow 100 gpm Flow 100 gpm TOTAL TDH 100 ft Flow 200 gpmOlds College 2017 PUMPS IN PARALLEL INCREASE FLOW Pump 1 Pump 2 TDH 100 ft TDH 100 ft Flow 100 gpm Flow 100 gpm TOTAL TDH 100 ft Flow 200 gpm Olds College 2017 Booster Pumps Booster pumps are used to increase the pressure, and achieve higher flow rates. Booster pumps work in conjunction with other pumps. They are designed to ‘boost’ the performance of an existing system.