Lab Report #1: Basic Hydraulic Technology. Organization and presentation of report /20. Grammar and spelling /20

Transcription

1 Lab Report #1: Basic Hydraulic Technology Organization and presentation of report /20 Grammar and spelling /20 Technical presentation, content, analysis /60 Total /100 Monday, August 20, 2012 ME/ECE 4710 Motion and Control Instructor: Dr. James Kamman Mechanical and Aeronautical Engineering Western Michigan University Kalamazoo, MI 49008

2 Abstract A hydraulic test bench is used to study the relationship between pressure and flow rate of fluid through various hydraulic circuits. Three simple circuits are used in total. The first measures the flow rate of the pump at various system pressures. The second measures the flow rate and pressure difference through a restriction in order to calculate the constant of proportionality, and the third uses LabView software with pressure sensors to export pressure data to text files. These experiments are used to understand the relationship between the volume flow rate and the pressure of the fluid. Other concepts that were learned include building fluid circuits on the test bench, safe start-up and shut-down procedures, and using the LabView interface for recording measurements. The experiments found that the test bench is flow-limited to 2.75 GPM, and the flow over the relief valve must be considered to understand the dynamic behavior of the system. iii

3 Contents Abstract... iii Introduction... 1 Procedure... 2 Part 1:... 2 Part 2:... 3 Part 3:... 4 Results:... 5 Analysis and Conclusions... 7 iv

4 Introduction Hydraulic systems are used to transmit power through fluid to plants such as actuators. The hydraulic systems used for these experiments are made up of a tank, a pumping device, and a circuit including a flow meter, pressure gages, and a variable restriction valve. The pumping device is a fixed displacement pump that has a theoretical maximum flow rate of 3 GPM. The tank has a filtered return line that returns the fluid at atmospheric pressure, and the gages are used to measure the pressure and flow rate of the fluid at various points in the circuit. The experiments are divided into three parts. The first part records the flow rate of the pump at various system pressures resulting in the maximum flow rate of the pump. The second part uses pressure readings at the inlet and outlet of a restriction to find the coefficient of proportionality (k) of the restriction, and the third part uses electronic sensors in conjunction with LabView to export spreadsheets of data to text files. The experiments are designed to help the participant understand the behavior of basic hydraulic components while deriving the maximum pump flow rate, coefficients of proportionality, and the power lost over a restriction. Complete instructions for the laboratory procedure are found on the class website under Lab #1: Basic Hydraulic Technology. 1

5 Procedure Part 1: The first exercise is used to measure the flow rate of the pump. A simple circuit is constructed using a hydraulic test bench. The pump is connected to a parallel configuration of a flow meter and a pressure relief valve with a dead-headed pressure gage as shown in figure 1. The circuit is assembled using hoses fixed with quick connects attached to permanently fixed valves and gages on the test bench. The pressure relief valve is initially set to 250 psi and decreased in random steps until the pressure relief valve is fully open. System pressure and flow rate are recorded for each change in the set pressure of the relief valve. Figure 1: Experimental Setup 1 2

6 Part 2: In part two, the hydraulic system is connected to a variable flow control valve. The variable flow control valve restricts the flow of the system as it is closed; resulting in a higher pressure. The pump is connected to a pressure gage as well as to the inlet of the variable flow control valve. The outlet of the valve is then connected to a flow meter and another pressure gage. This all is finally connected back to the tank through a filter (see Figure 2). The pressure relief valve is initially set to 250 psi. After assembling the system, the system pressure is recorded as the flow control valve is slowly closed from 100-0% in 10% increments. After varying the flow valve, the next step in the experiment is to set the flow valve and vary the relief valve to see at what pressure the flow begins to decrease in the circuit. Begin with the variable flow valve set to 60% open, and record system pressure as the relief valve set pressure is decreased in steps of psi. Figure 2: Experimental Setup 2 3

7 Part 3: For part three, a hydraulic circuit is fixed with sensors that are connected to LabView software as shown in figure 3. Two pressure sensors are attached to the inlet and outlet ports of a variable restriction valve. The valve is in series with a flow meter. Set the pressure relief valve to 250 psi and the variable flow valve to 50% open. Record the inlet and outlet pressures and the system flow rate as the relief valve is opened similar to the procedure for part 2. Figure 3: Experimental Setup 3 4

8 Results: The pressure and flow rate values are recorded by visually inspecting coarse lines on the flow meter and radial tick marks on the pressure gage. The course scale of the flow meter may result in some human error, and the pressure gage does not have tick mark indications below 30 psi resulting in estimated readings for that range. The values are satisfactory for finding relationships between the variables but are not satisfactory for detailed system modeling. Part three initiates the use of electronic pressure sensors that are both more accurate and precise than previous readings of the pressure gages, however the electronic pressure sensors sample at 1000Hz. The recorded measurements display all of the fluid pulsations resulting from the pump and the geometry of the system. The sensors are also susceptible to noise from other electronic devices. Consequently, test data appears as a noisy oscillating function when graphed. By averaging the data over a certain range and combining it, a more filtered pressure can be used for analysis and modeling. The pressures and flow rates recorded from LabView are much more accurate than the data collected visually in earlier exercises. By taking away the human error of visually reading the gages and relying on the accuracy range of the electronic sensors, the values become much more precise. The test data in part three is, therefore, more reliable and can be used for modeling purposes. In part one of the experiment, pressure and flow rate were recorded in table 1. The fluid pressure in the circuit is limited to the set pressure of the relief valve. Table 1: Pressure Vs. Flow as relief valve is opened Pressure (psi) Relief Valve Open Flow Rate (GPM) Part two of the experiment results in four sets of data. The first set is from the configuration with the relief valve set to 250psi and the flow valve closed incrementally. The data was recorded in table 2. The second, third, and fourth data sets have the relief valve set to 250 psi and the flow valve 5

9 set to 60% open, 50% open, and 40% open respectively. The data results from gradually opening the relief valve in 20 psi increments and is recorded in tables 3-5. Table 2: Pressure relief valve set to 250psi, flow valve closed incrementally Valve % Open Pin (psi) Pout (psi) Flow (GPM) Table 3: Flow valve at 60% open, relief valve opened incrementally Pin Pout Flow Table 4: Flow valve at 50% open, relief valve opened incrementally Pin Pout Flow Table 5: Flow valve at 40% open, relief valve opened incrementally Pin Pout Flow Table 6 shows the data collected from part three. For part three, the flow valve is set to 50% open. The relief valve is set to 250 psi and closed in 20 psi increments, and LabView is run after each adjustment. LabView records the voltage output of the sensors and writes that information with respect to time in a text file. The table shows the difference between the values visually recorded from the gage and the values electronically recorded in LabView. The values from LabView are averaged over a steady state pressure period for each change in the setting of the relief valve. 6

10 Table 6: Flow Valve 50% relief opened, LabView analysis Pin gage Pin LabVIEW Pout Flow Analysis and Conclusions The fluid pressure in the circuit is limited to the set pressure of the relief valve. As the relief valve pressure increases, less fluid will flow over the relief valve and more fluid will flow through the circuit. From the data collected in part one, fluid power at the flow meter can be calculated using the following equation. The results are shown in table 7. It is clear that as the relief valve of the system is opened, the fluid power of the system decreases due to the alternate path the fluid is allowed to take. HP Power in HP P Pressure in psi Q Volumetric Flow Rate in GPM 1714 Conversion factor for HP Table 7: Calculated fluid power (part 1) Pressure (psi) Flow Rate (GPM) Fluid Power (HP) In part two, the flow rate decreases and the change in pressure across the variable flow valve increases as the flow valve is closed. The power loss through the restriction at each setting can be calculated using the equation at the end of this paragraph. The results are shown in table 8. As the flow is restricted, power increases with increasing pressure differences until the inlet pressure approaches the set pressure of the relief valve. At this point, flow is reduced across the flow valve as fluid begins to 7

11 flow over the pressure relief valve. This shows that the relief valve doesn t open fully when the crack pressure is reached. It opens gradually based on the spring constant inside the valve. HP Q Pin Pout Power Loss Flow Rate Inlet Pressure of the flow valve Outlet pressure of the flow valve Table 8: Horse power with decreasing valve position Flow Valve Position (% open) HP In steps (e) and (f) of part 2 (see lab instructions) experiments were done with constant flow valve position and varying relief pressure. An important value that can be calculated from the data collected is the constant relating the root of the pressure difference to the flow rate as shown in the equation below. The values are stored in tables Q k Pin Pout Flow Rate Constant of Proportionality Inlet Pressure of the flow valve Outlet pressure of the flow valve Table 9: Flow valve 60 % open Q ΔP k Table 10: Flow valve 50 % open Q ΔP k Table 11: Flow valve 40 % open Q ΔP k

12 Q (GPM) From the data in tables 9-11, it is clear that k increases slightly with decreasing set pressure of the relief valve. This trend is due to the fact that the difference in pressure across the flow valve is quickly decreasing; while the flow rate is decreasing at a much slower rate which results in a slight increase in the k value. Based on the trends in the data, k for these valve positions can also be found using a best fit line. The data is plotted in excel and a least squares best fit curve is used to find the k value for each flow valve position. The k value is shown on the graph as the slope of the line. Table 12 compares the average computed k values with the best fit k values. Best fit curve approximation of k y = x y = x y = x y = x Sqrt(Pin-Pout) Part 3 k Valve 60% open k Valve 50% open k Valve 40% open k Linear (Part 3 k) Linear (Valve 60% open k) Linear (Valve 50% open k) Linear (Valve 40% open k) Figure 4: Best fit approximation of k Table 12: % error of average and best fit k values LabView 60% open 50% open 40% open Average k best fit k % error (%)

13 In part three, the system is connected to LabView data acquisition. The sampling rate for the pressure sensors is 1000 Hz. This allows the sensors to read all of the fluid pulsations resulting from the pump and the geometry of the system. The sensors are also susceptible to noise from other electronic devices. The resulting test data appears as a noisy oscillating function when graphed. By averaging the data over a certain range and combining it, a more filtered pressure can be found. The pressures and flow rates recorded from LabView are much more accurate than the data collected visually in earlier exercises. By taking away the human error of visually reading the gages and relying on the accuracy range of the electronic sensors, the values become much more precise. The test data is, therefore, much more reliable and can be used for modeling purposes. The flow rates were also recorded visually throughout this process and used to calculate the flow over the relief valve using the following equation. The values are stored in table 13. Volumetric flow rate over the relief valve Maximum volumetric flow rate = 2.75 GPM Volumetric flow rate through the circuit Table 13: Flow rate of the relief valve (GPM) (GPM)