The Discussion of this exercise covers the following points:

Transcription

1 Exercise 5-3 Wet Reference Leg EXERCISE OBJECTIVE Learn to measure the level in a vessel using a wet reference leg. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Measuring level in a closed tank Dry reference leg Measurement errors. Wet reference leg Measurement errors. DISCUSSION Measuring level in a closed tank In a closed tank, it is possible to measure the level using a differential-pressure transmitter. However, it requires a special installation for the transmitter impulse lines that takes into account the pressure inside the tank. In a closed tank, the pressure at the bottom of the tank is the sum of the static pressure, the hydrostatic pressure due to the weight of the gas above the liquid, and the hydrostatic pressure of the fluid as Figure 5-13 illustrates. Gas (density = g) Liquid (density = l) Figure Pressure at the bottom of a closed tank. Festo Didactic

2 Ex. 5-3 Wet Reference Leg Discussion Using the definition of static pressure and hydrostatic pressure from Unit 4, the total pressure at the bottom of a closed tank is: (5-11) where is the total pressure at the bottom of the tank is the density of the gas above the liquid is the height of the column of gas is the density of the liquid in the tank is the height of the column of liquid In most cases, the density of the gas is much less than the density of the process liquid, thus the pressure due to the weight of the gas can be neglected and Equation (5-11) becomes: (5-12) There are two types of installations to measure a level in a closed tank using a differential-pressure transmitter. Figure 5-14 illustrates both of them. Figure 5-14 (a) illustrates a dry reference leg installation and Figure 5-14 (b) illustrates a wet reference leg installation. a) Dry reference leg b) Wet reference leg Figure Measuring level in a closed tank. Both the wet reference leg and the dry reference leg installation use the same principle. The reference leg allows the low-pressure side of the differentialpressure transmitter to receive the pressure of the gas above the process fluid. The difference between a dry reference leg and a wet reference leg lies in the filling of the reference leg. In a dry reference leg installation, the process gas fills the reference leg while in a wet reference leg installation, the process fluid or an inert liquid fills the reference leg. A wet reference leg installation is required if the vapors are corrosive, if they can clog the pipe or the transmitter, or if the vapors can condense in the reference leg. Dry reference leg In a dry reference leg installation, the differential-pressure transmitter compares the pressure in the reference leg (P R ) to the pressure at the high-pressure tap of 132 Festo Didactic

3 Ex. 5-3 Wet Reference Leg Discussion the tank (P T ) and returns the pressure differential. This pressure differential corresponds to the hydrostatic pressure created by process fluid above the highpressure tap. From this pressure differential, one can easily deduce the level from Equation (3-7). Modern differential-pressure transmitters enable the conversion of the pressure differential to a level, if the density of the process fluid is known. Figure 5-15 shows a typical dry reference leg installation. Shut-off valve Reference leg Shut-off valve Minimum measurable level Three-valve manifold Shut-off valve Separator Drain valve Figure Dry reference leg installation. Measurement errors When using a dry reference leg installation, level-measurement errors usually come from changes in the temperature of the liquid in the tank or from an accumulation of liquid in the reference leg. If the temperature of the process fluid increases, its density decreases and the fluid takes more space in the tank. Thus, the actual level of liquid in the tank is higher than the level that the transmitter indicates. To prevent such an error, a transmitter with a temperature compensation mechanism can be used. Liquid can find its way to the reference leg either if the tank is filled with liquid above the tap connecting the reference leg to the tank or if the gas at the top of the tank condenses into the reference leg. In both cases, the liquid in the reference leg exerts an additional pressure on the low-pressure side of the transmitter and the transmitter displays a level lower than the actual level. Festo Didactic

4 Ex. 5-3 Wet Reference Leg Procedure Outline Wet reference leg In a wet reference leg installation, the differential-pressure transmitter still compares the pressure in the reference leg to the pressure at the high-pressure tap of the tank but, in this case, a liquid fills the reference leg. A condensing chamber or pot is sometimes present at the top of the reference leg to facilitate the condensation of the vapors. The pipe connecting the condensing chamber to the tank is sloped toward the tank to allow the condensate to return to the tank. The process fluid is usually used to fill the reference leg. If the process fluid is corrosive and might damage the differential-pressure transmitter, an inert liquid is used instead. Figure 5-16 shows a typical wet reference leg installation. Shut-off valve Condensing chamber Reference leg Shut-off valve Drain valve Drain valve Minimum measurable level Three-valve manifold Drain valves Figure Wet reference leg installation. Measurement errors Similar to a dry leg installation, measurement errors in a wet leg installation usually come from an uncompensated difference of temperature between the fluid in the reference leg and the fluid in the tank. PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections PROCEDURE Setup and connections 1. Connect the equipment as the piping and instrumentation diagram in Figure 5-11 shows and use Figure 5-12 to position the equipment correctly on the frame of the training system. To set up your system for this exercise, start with the basic setup presented in the Familiarization with the Training System manual and add the equipment listed in Table Festo Didactic

5 Ex. 5-3 Wet Reference Leg Procedure Table 5-8. Devices required for this exercise. Name Model Identification Differential-pressure transmitter (low-pressure range) LI 1 Solenoid valve S Digital pressure gauge B PI 1 Electrical unit Pneumatic unit Accessories Calibrator ---- Vent tube Wet leg Calibrator (4-20 ma) 24 V from the Electrical Unit Figure P&ID. Festo Didactic

6 Ex. 5-3 Wet Reference Leg Procedure Figure Setup. 2. Connect the control valve to the pneumatic unit. 3. Connect the pneumatic unit to a dry-air source with an output pressure of at least 700 kpa (100 psi). 4. Wire the emergency push-button so that you can cut power in case of an emergency. 5. Do not power up the instrumentation workstation before your instructor has validated your setup. 6. Connect the solenoid valve so that a voltage of 24 V dc actuates the solenoid when you turn the power on. 136 Festo Didactic

7 Ex. 5-3 Wet Reference Leg Procedure 7. Install the differential-pressure transmitter and configure it to read the pressure differential between the wet leg and the bottom of the column. 8. Configure the pressure gauge to read the pressure at the top of the column. 9. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements for the proper completion of this exercise. This checklist is not exhaustive, be sure to follow the instructions in the Familiarization with the Training System manual as well. f All unused male adapters on the column are capped and the flange is properly tightened. The solenoid valve under the column is wired so that the valve opens when the system is turned on. The hand valves are in the positions shown in the P&ID. The control valve is fully open. The pneumatic connections are correct. The vent tube is properly installed and HV3 is open. The wet leg is properly installed and its hand valve is open. The low-pressure range differential-pressure transmitter is used. The pressure-reducing valve of the pressure unit is closed. 10. Ask your instructor to check and approve your setup. 11. Power up the electrical unit. 12. Use the calibrator to send a 4 ma signal to the current to pressure converter of the control valve. 13. Test your system for leaks. Use the drive to make the pump run at low speed in order to produce a small flow rate. Gradually increase the flow rate, up to 50% of the maximum flow rate the pumping unit can deliver. Repair all leaks. 14. Close HV5 and let the level of water in the column rise to about above 25 cm (10 in). Use the calibrator to close the control valve and stop the water flow to the column. 15. Bleed the differential-pressure transmitter. Festo Didactic

8 Ex. 5-3 Wet Reference Leg Procedure 16. Empty the column and adjust the zero of the differential-pressure transmitter to read a pressure differential of 0 kpa (0 psi). 17. To measure the level inside the pressurized column using the wet leg, you must fill the wet leg with water. To do so, close the hand valve of the wet leg, fill the column with at least 80 cm (32 in) of water, open the wet leg valve to allow water to fill the wet leg, and quickly close the valve of the wet leg to capture the water in the wet leg. Make sure the wet leg is completely full. If not, try again. Refer to the Familiarization with the Training System manual for details on the installation and utilization of the wet leg. 18. Once the wet leg is completely filled with water, adjust the zero of the differential-pressure transmitter to read a pressure differential of 0 kpa (0 psi) when the column is empty. 19. Close HV3 and HV5 to pressurize the column. 20. Open the control valve, and let the level of water in the column rise. Since the column is pressurized, the level should stabilize around 65 cm (26 in). Keep an eye on the pressure gauge connected to the top of the column. Make sure the pressure inside the column does not rise above 240 kpa (35 psi). 21. When the level is stable, use the calibrator to close the control valve and keep the column pressurized. 22. Record below the level at which the water has stabilized in the column and the pressure at the top of the column. Level: Pressure inside the column: 23. What phenomenon do you observe on the wall of the column? 24. Use HV4 to adjust the level inside the column to 60 cm (24 in). The column is pressurized, be sure to open HV4 slowly when adjusting the level inside the column. 25. On the differential-pressure transmitter, read the pressure between the wet leg and the bottom of the column and record it in Table Festo Didactic

9 Ex. 5-3 Wet Reference Leg Conclusion 26. On the pressure gauge, read the pressure inside the column and record it in Table 5-9. Table 5-9. Pressure measurement using a wet leg installation. Level cm (in) P kpa (psi) Pressure inside the column kpa (psi) Level calculated using P cm (in) 60 (24) 27. Use HV4 to decrease the water level in the column by steps of 5 cm (or 2 in) until the column is empty. In Table 5-9, record the pressure reading of the differential-pressure transmitter and the pressure reading of the pressure gauge. For each reading, wait for the pressure to stabilize before recording it. 28. Using the pressure differential recorded in Table 5-9, calculate the theoretical level of water in the column. 29. Use the data in Table 5-9 to plot a graph of the pressure differential as a function of the level (as read on the column rule). 30. Open HV3 to depressurize the column and open HV4 to empty the column. 31. Use the main switch to cut the power to the Instrumentation and Process Control Training System. CONCLUSION In this exercise, you learned to measure the level inside a pressurized vessel using a wet leg installation Festo Didactic

10 Ex. 5-3 Wet Reference Leg Review Questions REVIEW QUESTIONS 1. Which type of installation is required to measure the level inside a pressurized vessel using a differential-pressure transmitter? 2. When can you neglect the pressure due to the weight of the gas above the liquid when measuring the level in a pressurized vessel using a differentialpressure transmitter? 3. In which condition is a wet leg installation required to measure the level inside a pressurized vessel? 4. What is the difference between a wet leg and a dry leg installation? 5. What can cause an error in the level measured with a wet reference leg installation? 140 Festo Didactic