Transcript Document
Experiment 3-1 Pumps Design Types Basic Operation Performance Experiment 3-1 Objectives: 1. Calculate the theoretical pump flow rate based on shaft speed, pump displacement, and volumetric efficiency. 2. Describe the effect of pressure on pump flow rate. 3. Measure the volumetric efficiency of a pump. 4. Describe the effect of oil temperature on volumetric efficiency. 5. Explain basic pump operating principles. Pump Operating Principles The maximum amount of flow that runs through a hydraulic system is determined by the size and speed of the pump. In a pneumatic system, maximum possible flow is determined by the size of the compressor. All pumps or compressors operate by development of a pressure differential across their ports. When the internal rotating group begins to move, a vacuum is generated at the inlet. Atmospheric pressure causes fluid to rush into the inlet because fluids, like electricity, always seek the path of least resistance. When in the pump or compressor, the fluid is forced to leave via the outlet and into the system. Pump Types There are three basic designs of pumps which are gear, vane, and piston. All pumps operate by the same principles. A vacuum develops at the inlet and pressure at the outlet causing fluid to be forced into the system. Another way to explain this action is by saying that the pump generates a pressure differential across it’s ports or that it has an increasing and decreasing volume. However it is stated, all pumps develop flow and in the same way. Determining Pump Flow There are two primary factors that control pump and compressor flow, which are, their physical size and the speed at which they are ran. The following formula illustrates the relationship between speed, size, and flow rate. This formula is used to calculate theoretical flow. Flow Rate = Displacement (cubic inches/revolution) x Shaft Speed ( revolutions/ minute) divided by 231 Volumetric Efficiency Volumetric efficiency has to do with the ability of any given pump to maintain the same rate of displacement while under pressure. Most hydraulic pumps have efficiencies that are in the range of 80 to 90%. While we may think of a positive displacement pump as producing the same amount of flow with each revolution, they in fact loose some oil flow to internal leakage past moving parts. Overall Efficiency Overall efficiency takes into account a pump’s volumetric efficiency as well as it’s mechanical efficiency and combines them. This is determined by dividing the hydraulic horsepower delivered to the system by the input horsepower of the prime mover. Pressure Rating and Power When the term “pressure rating” is used in reference to a pump, it can sometimes be confused with the function of the pump. The pressure rating of a pump refers to the pressure of the environment in which the pump is to operate and not to mean that the pumps produce pressure when in fact, they do not. Pump life is in part determined by the pressure of its operating environment and ratings are designed to give the pump the greatest possible amount of service life. The amount of power required by a given system will depend largely on the pressure and flow requirements. Both internal leakage and friction have to be taken into account when calculating the power needed because they affect the pump’s overall efficiency. Calculating Power Calculating the amount of power needed in a system will ultimately lead to calculating horsepower of the power unit. Many factors are considered but the primary ones are the work load and the speed at which we need to perform the work. There are two formulas which can assist in this process; theoretical horsepower, and, actual horsepower. Theoretical horsepower does not take into account volumetric or mechanical deficiencies of a pump and therefore is for “ballpark” figures. Theoretical Pump Horsepower: Flow Rate X Pressure 1714 Actual Pump Horsepower: Flow Rate X Pressure 1714 X Overall Efficiency Review 1. What action causes oil to flow from the reservoir to the pump’s inlet? 2. Name the three types of hydraulic pumps commonly used. 3.What is the main advantage of a gear type pump? 4. Which type of pump would you use if you needed a high efficiency pump? 5. Calculate theoretical pump flow given the following information: Displacement = 3.0 cubic inches Speed = 1725 Pressure = 1000 psi Pump Type = piston 6. Using information found in the volumetric efficiency chart of this presentation and in question 5, calculate the actual flow rate of the pump. Review 7. Explain how pressure can affect the flow rate of a pump. 8. Using the data from the experiment, plot the flow rate vs. pressure data on the graph below. Label your graph. Review 9. Based on the curve, or lack there of, what happens to volumetric efficiency as discharge pressure increases? 10. Calculate the power needed to run a unit providing 7.5 gpm at 1500 psi. The pump’s volumetric and mechanical efficiencies are 90 and 95% respectively.