The Discussion of this exercise covers the following points:

Size: px

Start display at page:

Download "The Discussion of this exercise covers the following points:"

Valentine Heath
5 years ago
Views:

Exercise 2 Float Switch EXERCISE OBJECTIVE Learn the working principle of float switches and how to use the float switch, Model 46935.

supplied float switch Commissioning the float switch Installing the float switch DISCUSSION Instrument symbol Float switch Introduction A float switch is a point-level detection device used to

A float switch that stops a pump when a tank is full is an example of a typical float switch application. Figure 9 shows a point-level detection mechanism that uses a simple float switch. Figure 9. Float switch application.

1 Exercise 2 Float Switch EXERCISE OBJECTIVE Learn the working principle of float switches and how to use the float switch, Model DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Introduction Reed-switch-type float switches Industrial applications Advantages and limitations Description of the supplied float switch Commissioning the float switch Installing the float switch DISCUSSION Instrument symbol Float switch Introduction A float switch is a point-level detection device used to actuate another device when the level in a vessel rises above, or drops below, a set point. Like all pointlevel sensors, float switches allow level detection of a single discrete level. A float switch that stops a pump when a tank is full is an example of a typical float switch application. Figure 9 shows a point-level detection mechanism that uses a simple float switch. Figure 9. Float switch application. In this example, the float of a float switch is displaced when its buoyancy makes it move upward when it comes in contact with the liquid. The float can be linked mechanically to an actuator, such as in the example of Figure 9, or via another mechanism, such as a reed switch and magnet mechanism, like the one on the float switch of the Instrumentation and Process Control System. The working principle of float switches using this type of mechanism is described in the section below. Festo Didactic

2 Exercise 2 Float Switch Discussion Reed-switch-type float switches Reed-switch-type float switches have two main components: a reed switch and a float with an encased magnet. Depending on its position along a stem, the float actuates (or not) the reed switch. The reed switches used in float switches usually have a single-pole single-throw (SPST) or a single-pole double-throw (SPDT) contact configuration. The reed switch in the float switch of the Instrumentation and Process Control System is of the SPST type. How a reed switch works The reed switch was invented in 1936 by Walter B. Ellwood from the Bell Telephone Laboratories. As stated in the original patent application, this device eliminates the complicated mechanical and magnetic structures of the usual electromagnetic relays. Figure 10 shows a typical reed switch diagram. This type of switch is named after the thin metal reeds enclosed in its glass capsule. The capsule is filled with an inert gas to prevent the reeds from rusting. The portions of the reeds that come in contact when the switch closes are plated with precious metal to ensure good conductivity. Glass capsule Reed Inert gas Terminal Gap Contact plating Figure 10. Reed switch. The working principle of a reed switch is simple. The reed switch is normally open. The gap between the two reeds isolates them and no current can flow through the switch. If a permanent magnet is placed close to the switch, the magnetic flux generated by the magnet draws the two reeds together. As shown in Figure 11, the plated tip of one of the reeds becomes a magnetic north pole, while the tip of the other reed becomes a magnetic south pole 2. Since the tips of the reeds have opposite magnetic poles, they attract each other and the two reeds snap together to close the electric circuit. When the magnet is removed from the proximity of the switch, the stiffness of the reeds brings them apart and the electrical circuit is open again. 2 Note that each reed also has an opposite pole at its other extremity. Magnetic poles always come in pairs; a magnetic monopole has never been observed. 18 Festo Didactic

The float has a cylindrical shape and the stem passes through a hole along its axis. The float is made of a material of a lesser density than water so that it floats.

3 Exercise 2 Float Switch Discussion Permanent magnet Magnetic flux S N N S N S Contact closed Figure 11. Actuated reed switch. The float and the stem In a float switch, the reed switch described above is located in the stem, a nonmagnetic tube. The stem is waterproof and protects the reed switch from water. The float has a cylindrical shape and the stem passes through a hole along its axis. The float is made of a material of a lesser density than water so that it floats. A permanent magnet is embedded in the float and, when the float moves along the stem, the magnet changes the state of the reed contact. Construction and operation The working principle of the float switch can be summarized as follows. The float is made of a material that is lighter than water and incorporates a permanent magnet. As the float rises and falls with the changes in the water level, it moves the magnet along the stem, thereby opening and closing the reed switch contact, which consists of two thin moveable blades of ferromagnetic metal. The free ends of the reeds are the contact points and form the actual reed contact. Float switches can be normally-open or normally-closed switches. For example, the factory default configuration for the float switch, Model 46935, is with the reed contact acting as a normally-closed contact. With this configuration, the magnet in the float closes the reed contact when the level is below the float. When the level rises above the float, the magnet rises with the float and, because it is too far from the reed switch, its magnetic field is too weak to keep the reed switch contact closed. Thus, the two reeds separate from each other and the circuit opens. Figure 12 shows how a float switch operates in NC configuration. The configuration of most float switches can be reversed to transform them into normally-open switches. This is generally done by inverting the position of the float. Festo Didactic

4 Exercise 2 Float Switch Discussion Stem Contact open Contact closed Float Lower stop Figure 12. Float switch (NC configuration). Industrial applications Float switches are mainly used to perform on/off control and to start sequential operations in batch processes. They are also used to trigger alarms when high or low liquid levels are detected in vessels. High levels can indicate unusual change in normal load demand, due to clogged pipes or valves accidentally closing, while low levels can indicate pump or liquid supply failure. Advantages and limitations Float switches are relatively inexpensive, reliable, and require minimum maintenance. However, they are limited in their use as on/off control devices or alarm switches. They do not provide continuous measurement of the liquid level. Moreover, some types of float switches are not recommended for processes where dirty liquids are handled. For example, liquids that contain particles of ferrous metal can cause malfunction of magnetically-operated float switches. Description of the supplied float switch The float switch, Model 46935, designed for the Instrumentation and Process Control Training System is shown in Figure 13. A connection box at the top of the float switch gives access to the switch contacts. 20 Festo Didactic

5 Exercise 2 Float Switch Discussion Figure 13. Float switch, Model Table 6. Components of the float switch. Component Contact connectors Non-magnetic tube Mounting screw Stem Float Fault panel Description Connectors for the contact points of the reed switch. Tube holding the float switch in place in the column. The tube can slide in the mounting screw. This allows the float switch to be positioned at different heights. Secures the float switch in the installation hole of the column flange. The stem contains the reed switch and allows the float to slide. The float has an embedded magnet that actuates (or not) the reed switch, depending on its position along the stem. The float can be reversed to switch between a NC and a NO float switch. Contains one switch used to simulate a fault with the apparatus. Commissioning the float switch The float of the float switch, Model 46935, can be reversed to transform the NC float switch into a NO float switch and vice versa. Before installing the float switch at the top of the column, you must make sure that the switch configuration is correct for your needs. The float switch has a NC factory default configuration. Float switches with this configuration are frequently used to cut power to pumps in case of high level alarms. However, for the exercise that follows, the switch is used differently and the float switch must be configured as a NO switch. To check if your float switch is configured as a NC or NO switch, follow the steps below: a. Hold the float switch vertically so that the tip with the float is pointing downward. b. Connect a multimeter (set to the continuity measurement mode) to the jacks on the float switch connection box. c. If the float switch is configured as a NC switch, it should conduct current when pointing downward. If not, it is configured as a NO switch. Festo Didactic

Exercise 2 Float Switch Discussion If your switch is not configured as a NO switch, you must reverse the float to be able to use it to complete the following exercise.

d. Put the lower stop back in place. (a) (b) (c) (d) Figure 14. Changing the float switch configuration (from NC to NO).

6 Exercise 2 Float Switch Discussion If your switch is not configured as a NO switch, you must reverse the float to be able to use it to complete the following exercise. Follow the steps below to reverse the float switch contact configuration. a. Remove the lower stop (i.e., the retaining ring). b. Remove the float from the stem and flip it 180 degrees. c. Put the float back on the stem. d. Put the lower stop back in place. (a) (b) (c) (d) Figure 14. Changing the float switch configuration (from NC to NO). Installing the float switch The float switch is not designed to operate in a pressurized column. To avoid damage to the equipment, be sure the column is open to atmosphere when using the float switch. The float switch is designed to be installed at the top of the column, on the flange. Use Figure 15 and the instructions below to install the float switch on your system. a. First, secure the flange at the top of the column. b. Once the flange is secured, remove the brass hexagonal-head plug using a wrench. 22 Festo Didactic

7 Exercise 2 Float Switch Procedure Outline c. Insert the float switch in the installation hole and screw the mounting screw in place. You can slide the float switch up and down to adjust the height of the float. a) b) c) Figure 15. Installing the float switch. PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup and connections Testing the float switch PROCEDURE Setup and connections 1. Connect the equipment according to the piping and instrumentation diagram (P&ID) shown in Figure 16 and use Figure 17 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 Instrumentation and Process Control Training System manual and add the equipment listed in Table 7. Before installing the float switch on the flange, be sure it is configured as a normally-open switch. Refer to the Commissioning the float switch section for details. Table 7. Material to add to the basic setup for this exercise. Name Model Identification Float switch LSH 1 Electrical unit Pneumatic unit Accessories Festo Didactic

8 Exercise 2 Float Switch Procedure Figure 16. P&ID. 24 Festo Didactic

Exercise 2 Float Switch Procedure Air from the pneumatic unit (140 kpa (20 psi)) Figure 17. Setup. 2. In this exercise, the float switch is used to open the solenoid valve to provide additional drainage to the column if the level of water is too high.

9 Exercise 2 Float Switch Procedure Air from the pneumatic unit (140 kpa (20 psi)) Figure 17. Setup. 2. In this exercise, the float switch is used to open the solenoid valve to provide additional drainage to the column if the level of water is too high. Water exits the column only under the effect of gravity. Therefore, the column drains slowly relative to the input flow. To avoid an accidental overflow, you must limit the input flow by reducing the speed of the pump or by restricting the flow using the control valve. 3. Connect the control valve to the pneumatic unit. Details about the installation and operation of the control valve are available in the Familiarization with the Instrumentation and Process Control Training System manual. Festo Didactic

10 Exercise 2 Float Switch Procedure 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 emergency. The Familiarization with the Instrumentation and Process Control Training System manual covers the security issues related to the use of electricity with the system as well as the wiring of the emergency pushbutton. 6. Do not power up the instrumentation workstation yet. Do not turn the electrical panel on before your instructor has validated your setup that is, not before step Wire the float switch and the solenoid valve so that a voltage of 24 V dc actuates the solenoid valve when the contact of the float switch closes. 8. 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 to the proper completion of this exercise. This checklist is not exhaustive, so be sure to follow the instructions in the Familiarization with the Instrumentation and Process Control Training System manual as well. f All unused male adapters on the column are capped and the flange is properly tightened. The ball valves are in the positions shown in the P&ID. The three-way valve at the suction of the pump (HV1) is set so that the flow is directed toward the pump inlet. The control valve is fully open. The pneumatic connections are correct. The solenoid valve is wired so that the valve opens when the contact of the float switch closes. The vent tube is correctly installed. 9. Ask your instructor to check and approve your setup. 10. Make sure it is safe to energize the system for you and for the team working on the other side of the system, if any. When ready, turn on the main power. Testing the float switch 11. Press the S1 button to power all the devices not already active on the station (i.e., the drive, the pneumatic devices, etc.). 26 Festo Didactic

11 Exercise 2 Float Switch Conclusion 12. Test your system for leaks. Use the drive to make the pump run at low speed to produce a low flow rate. Gradually increase the flow rate, up to 50% of the maximum flow rate that the pumping unit can deliver (i.e., set the drive speed to 30 Hz). Repair any leaks. 13. Stop the drive and let the column drain. 14. Start the pump and set the flow rate at about 36 L/min (9.5 gal/min). This flow rate should be obtained by setting the pump to 65% of the maximum flow rate that the pumping unit can deliver (i.e., set the drive speed to 40 Hz, approximately). Do not set the drive speed higher to avoid overflow. You can also use the control valve to adjust the flow rate instead of adjusting the drive speed. 15. Watch the level rise in the column up to the float switch. 16. What happens when the level reaches the float switch? 17. Is this a good way to control the level in a vessel? Why? 18. Stop the pump, open HV4, and turn off the power to the system. CONCLUSION You should now be able to install and use the float switch, Model You should also be able to change the float switch configuration so that it operates as a NO switch instead of a NC switch. REVIEW QUESTIONS 1. What is the contact configuration of the reed switch in the float switch, Model 46935? Festo Didactic

12 Exercise 2 Float Switch Review Questions 2. Name one advantage of using a reed switch. 3. Explain how a reed switch works. 4. Why is the reed-switch glass capsule filled with an inert gas? 5. Name one advantage and one limitation of float switches. 28 Festo Didactic

Exercise 4-1. Flowmeters EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION. Rotameters. How do rotameter tubes work?

Exercise 4-1. Flowmeters EXERCISE OBJECTIVE DISCUSSION OUTLINE DISCUSSION. Rotameters. How do rotameter tubes work? Exercise 4-1 Flowmeters EXERCISE OBJECTIVE Learn the basics of differential pressure flowmeters via the use of a Venturi tube and learn how to safely connect (and disconnect) a differential pressure flowmeter