Discrete Control Using Wireless Field Devices

1
Chapter 7

• Discrete Control Using Wireless Field Devices

2
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.

3
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.

4
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.

5
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.

6
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.

7
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.

8
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

9
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.

10
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).

11
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..

12
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.

13
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.