Exercise 5-2. Bubblers EXERCISE OBJECTIVE DISCUSSION OUTLINE. Bubblers DISCUSSION. Learn to measure the level in a vessel using a bubbler.

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1 Exercise 5-2 Bubblers EXERCISE OBJECTIVE Learn to measure the level in a vessel using a bubbler. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Bubblers How to measure liquid level using a bubbler. Advantages and limitations. DISCUSSION Bubblers In some cases, it is preferable not to expose the differential-pressure transmitter to the process fluid, either because this fluid is corrosive, radioactive, or because it contains solids. In this situation, a bubbler is an option to consider for level measurement. Bubblers also use hydrostatic pressure to measure the level of liquid in a vessel, although the bubbler method does not involve direct measurement of the hydrostatic pressure at the bottom of the vessel. Instead, it uses a tube (i.e., the bubbler) that extends down into the vessel. A purge gas flows into the tube and the pressure builds up in the tube until bubbles escape from the tip of the bubbler. A notch at the tip of the bubbler usually facilitates the formation of small bubbles. To escape from the bubbler, the pressure of the purge gas in the tube must compensate for the hydrostatic pressure that the water above the notch exerts. When bubbles start to exit the bubbler, the air pressure in the tube almost equals the hydrostatic pressure of the column of liquid above the tip of the bubbler. The pressure indicator on the bubbler setup enables the reading of the pressure inside the tube when bubbles can barely escape the tube. This pressure is proportional to the level of liquid above the tip of the bubbler. Figure 5-10 shows a typical bubbler setup. Needle valve Purge gas source Bubbler Minimum measurable level Figure Typical bubbler setup. Festo Didactic

2 Ex. 5-2 Bubblers Discussion How to measure liquid level using a bubbler To measure the level of liquid in an open vessel using a bubbler, you must first ensure that the tip of the bubbler is at the minimum level you want to measure. The notch at the tip of the tube allows a steady stream of fine bubbles to flow from the bubbler rather than periodic bursts of large bubbles. On industrial installations, a flowmeter such as a rotameter is sometimes added to the setup of Figure 5-10 to monitor the purge gas flow through the bubbler and ensure a steady flow. Once the bubbler is at the desired level, fill the vessel up to the maximum level you want to measure and connect the setup to a source of purge gas. The purge gas is usually an inert gas such as nitrogen (N 2 ) or air if the process does not prohibit it. Adjust the flow of purge gas until bubbles escape from the bubbler very slowly. At this point, the pressure in the bubbler is almost equal to the hydrostatic pressure of the liquid above the tip of the bubbler. Since the level of liquid in the vessel is at its maximum, the air pressure that builds up in the bubbler is also at its maximum. If the level of water decreases, more bubbles escape from the bubbler and the pressure inside the bubbler decreases proportionally to the new level. The pressure inside the bubbler is proportional to the height of liquid above the tip of the bubbler as the following equation shows: (5-10) where is the distance between the tip of the bubbler and the surface of the liquid is the pressure inside the bubbler is the density of the fluid is the acceleration due to gravity is the local atmospheric pressure Advantages and limitations The measurement of the level of liquid with a bubbler is a method that is relatively inexpensive, that allows the installation of the pressure transmitters on the top of the vessels, and that can be used with corrosive liquids on condition that the bubbler purge gas is compatible with the process liquid. However, bubblers are not suitable for process liquids that can solidify. Indeed, solidification would result in an excessive gas pressure within the bubbler, causing the pressure transmitter to read a level higher than the actual liquid level. In tall vessel applications, the bubbling rate may increase markedly when the level decreases from the maximum to minimum level. Thus, the bubbling rate may be so high at low level that the liquid agitation causes inaccurate level measurements. For tall vessel applications, a flow-regulating device such as a differential pressure regulator must be installed in the bubbler line to maintain a constant gas flow through the bubbler over the entire level measurement range. 122 Festo Didactic

3 Ex. 5-2 Bubblers Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Using the level measurement mode of the differential-pressure transmitter. 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 5-5. Table 5-5. Devices required for this exercise. Name Model Identification Differential-pressure transmitter (low-pressure range) LI 1 Solenoid valve S Electrical unit Pneumatic unit Accessories Calibrator ---- Festo Didactic

4 Ex. 5-2 Bubblers Procedure Pneumatic Unit Vent tube Open to atmosphere Pressure reducing valve Bubbler Calibrator (4-20 ma) 24 V from the Electrical Unit Figure P&ID. 124 Festo Didactic

5 Ex. 5-2 Bubblers Procedure Tee fitting Air from the pneumatic unit Figure Setup. 2. Connect the control valve to the pneumatic unit. 3. Connect the bubbler to the kpa (0-100 psi) air outlet of the pneumatic unit. 4. Connect the pneumatic unit to a dry-air source with an output pressure of at least 700 kpa (100 psi). 5. Wire the emergency push-button so that you can cut power in case of an emergency. Festo Didactic

6 Ex. 5-2 Bubblers Procedure 6. Do not power up the instrumentation workstation before your instructor has validated your setup. 7. Connect the solenoid valve so that a voltage of 24 V dc actuates the solenoid when you turn the power on. 8. Install the differential-pressure transmitter and configure it to read the pressure inside the bubbler. 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. 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. 126 Festo Didactic

7 Ex. 5-2 Bubblers Procedure 15. Bleed the differential-pressure transmitter so that the impulse line and the transmitter are free of water. 16. Empty the column and adjust the zero of the differential-pressure transmitter to read a pressure of 0 kpa (0 psi) inside the bubbler. 17. Close HV5, open the control valve, and let the level of water in the column rise above 75 cm (30 in). 18. Close the control valve to stop the water flow and use the hand valve HV5 to adjust the level in the column to 75 cm (30 in) of water. 19. On the pneumatic panel, slowly open the pressure-reducing valve until air bubbles exit from the tip of the bubbler at a very slow rate. 20. On the differential-pressure transmitter, read the pressure inside the bubbler and record it in Table 5-6. Table 5-6. Pressure in the bubbler as a function of the level of water in the column. Level (rule) cm (in) 75 (30) PBubbler kpa (psi) Level calculated using PBubbler cm (in) 21. Use HV5 to decrease the water level in the column by steps of 5 cm (or 2 in) until the column is empty. Record the pressure reading of the differentialpressure transmitter in Table 5-6. For each reading, wait for the pressure to stabilize before recording it. Festo Didactic

8 Ex. 5-2 Bubblers Procedure 22. Using the pressure inside the bubbler recorded in Table 5-6, calculate the theoretical level of water in the column. 23. Use the data in Table 5-6 to plot a graph of the curve of the level (as read on the column rule) as a function of the pressure inside the bubbler. 24. On the same graph, plot the curve of the calculated level as a function of the pressure inside the bubbler. 25. Is there an offset between the two curves? If yes, explain why. Using the level measurement mode of the differential-pressure transmitter. 26. Use the calibrator and HV5 to increase the water level in the column until it is at the same height as the hole in the bubbler. Adjust the level of water so that you can read the rule precisely to obtain the height of the hole in the bubbler. The hole should be located at about 5 cm (2 in) from the bottom of the tank. Record the position of the hole in the bubbler below. 27. Configure the differential-pressure transmitter for level measurement. Refer to the Familiarization with the Training System manual for details. Configure the differential-pressure transmitter so that the upper calibration point is 75 cm (30 in) of water and the empty calibration point is the position of the hole in the bubbler (recorded above). If your differential-pressure transmitter is properly configured, it should read a level of 75 cm (30 in) when the transmitter output is 100% and a level of about 5 cm (or 2 in) when the transmitter output is 0%. 28. Fill the column with water up to 75 cm (30 in). 128 Festo Didactic

9 Ex. 5-2 Bubblers Conclusion 29. Use HV5 to decrease the water level in the column by steps of 5 cm (2 in) until you reach a level of 0 cm (0 in). Record the level reading of the differential-pressure transmitter in Table 5-7. Table 5-7. Level reading of the differential-pressure transmitter. Level (rule) cm (in) 75 (30) Level (differential-pressure transmitter) cm (in) 30. If the levels recorded in Table 5-7 differ from the actual levels by more than ±0.5 cm (±0.2 in), check the zero and the configuration of the differential-pressure transmitter, then take new measurements. 31. Open HV4 to empty the column. 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 vessel using a bubbler. REVIEW QUESTIONS 1. In which situation is a bubbler useful for measuring the level of liquid in a tank? Festo Didactic

10 Ex. 5-2 Bubblers Review Questions 2. What is the pressure inside a bubbler when bubbles barely escape from it? 3. Using a bubbler, can you measure the level of water below the hole or notch of the bubbler? 4. The level decreases in a tank using a bubbler. Does the rate at which the bubbles escape from the bubbler increase or decrease? Explain. 5. Name one limitation of the bubbler method for measuring the level of liquid in a tank. 130 Festo Didactic