Transcript Chapter 7 Discrete Control Using Wireless Field Devices

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.