Chapter 7 Discrete Control Using Wireless Field Devices

Recycle Tank Level Control Using Point Measurement  A by-product that can be used as a feedstock within a manufacturing process can be created as the result of that process or another manufacturing process  To account for any imbalance when the recycle supply and is less that the process feed requirements, the recycle tank makeup stream maybe automatically regulated  A WirelessHART vibrating fork liquid level transmitter may be used to detect low level and a WirelessHART on-off valve used to regulate the makeup flow.

Control Module – Discrete Point Measurement  When the discrete level control setpoint, SP_D, is set to Auto (Figure 8-2), then on detection of low level the makeup valve is opened for a period of time determined by the ON_TIME parameter and then turned off.  Makeup is only needed to prevent the level from dropping below the low level sensed by the level switch.  This ensures that sufficient room is maintained in the recycle tank to accommodate surges in tank level when the recycle flow exceeds the feed flow.

Recycle Tank Level Control Using Continuous Measurement  A wireless continuous level measurement provides greater flexibility in structuring the control and provides a direct indication of level. For example, a wireless guided wave radar level transmitter can be used to obtain a continuous measurement of level.

Control Module – Continuous Level Measurement  In Automatic mode the makeup valve opens if the tank level drops below the low level target and closes when the level reaches the setpoint.  In manual mode operators can use the OUT_D parameter to open or close the valve. The example module illustrates the implementation of this level control based on a continuous measurement of level.

Storage Tank Temperature Control  There is often a requirement that storage tanks for plant feedstock or, intermediate or final product be maintained at a temperature that is required for pumping or processing.  Temperature control can be automated using a wireless temperature transmitter and a wireless on-off valve to regulate the steam used to heat the tank using a wireless temperature transmitter and wireless on-off valve.

TIC206 Module for Tank Temperature Control  If the mode of the temperature control is set to Auto, the discrete output to the wireless on-off steam valve is opened when the tank temperature falls below the setpoint value.  When the temperature reaches setpoint, the steam valve is turned off and remains off until the temperature drops by more than the one deviation limit.  When the temperature drops below setpoint by more than the DEVIATION value below setpoint the steam valve is opened again.  When the mode of TC206 is changed to Manual the operator may manually open and close the steam valve using the OUT_D parameter.

Gas-cleaning Tower  A gas-cleaning tower with a feedwater inlet/outlet system plays a key role in the production of titanium dioxide in a European chemical plant.  To automate flushing water and sand from the tower, an on-off valve with a 4320 wireless valve positioner mounted to an on-off valve was installed at the bottom of the tower. A 2160 wireless vibrating fork liquid level switch was installed to detect when the water level reached overfill conditions when the vessel was full and was ready to be drained.  On detection of an overfill condition, the control sends an “open” command to the wireless positioner to open the drain valve. After 30 seconds, it sends a “close” command to the valve positioner to complete the draining cycle

Wireless Control of Tank Temperature  At the AkzoNobel surfactants processing plant in Belgium, the fatty nitriles and amines products produced by the plant are stored in 40 tanks before they are shipped to customers. To facilitate loading the products into road tankers, the fatty nitriles and amines must be maintained within specific temperature parameters.  Four WirelessHART temperature transmitters have been installed so far (at the time of this writing) to control the temperature of four storage tanks.  Temperature is transmitted every minute to a wireless gateway and is then integrated into the existing DCS. The DCS automatically controls a simple On-Off steam valve that heats the tanks. The temperature of the tanks can now be maintained using this wireless closed-loop control, enabling the final product to be delivered at the appropriate temperature.

Pulse Duration Modulation  In many areas of the process industry, pulse duration modulation capability may be required to regulate field devices such as on-off valves.  This output capability may be applied to an on-off valve to be turned on for a precise period of time. The maximum time that the pulse output may be turned on in one request is determined by the length of the duty cycle.  The time over the duty cycle that the output is turned on determines the pulse width (% time on).

Increase-Decrease Control for Motorized Actuator  Motorized actuators may be used for applications that require larger valves may be preferred in some applications that require precise positioning of the valve. The motor is designed to run in a forward or reverse direction to open or close a valve.  The length of time the motor remains on and its direction of rotation determine how much the valve is closed or opened. This type of regulation is known as increase-decrease control  In these types of applications, the time-to-apply command can be used to tell the motor when to turn on and off..

Exercise: Discrete Control Using Wireless Field Device This workshop provides several exercises that can be used to further explore wireless discrete control. In the workshop a module is used that addresses the tank temperature control using a wireless temperature transmitter and wireless on-off valve. The tank heat loss is a disturbance to control operation that can be adjusted in the workshop.  Step 1: Open the module that will be used in this workshop and observe the control and simulated processes.  Step 2: Change the mode of the discrete control block, TC206, from Manual to Auto and observe how the steam valve is regulated to raise the tank temperature. Does the tank temperature control allow the temperature to overshoot setpoint.  Step 3: Observe the variation in temperature and the frequency at which the valve must be opened and closed to maintain the temperature. Change the tank heat loss rate and observe the impact on the frequency at which the valve must be opened.  Step 4: Change the value of the Deviation parameter and examine the impact on temperature variation and the frequency at which the valve must open to maintain temperature.

Process: Discrete Control Using Wireless Field Device A simulated storage tank is used to demonstrate discrete control of tank temperature using a wireless transmitter and a wireless discrete on-off steam valve.