by Maryam Hussain
When to operate a controller on MANUAL and AUTOMATIC
control modes?
To illustrate the need to operate
a controller on MANUAL and AUTOMATIC control modes for controlling a process,
let us consider an example of a shell and tube heat exchanger in which a
process stream, on the tube side, is heated by condensing steam on the shell
side.
The objective of this heater
is to supply the process fluid, at a specific temperature, to the downstream
process otherwise the whole production will be upset. If this supply
temperature deviates due to some reasons (there may be several
causes/disturbing variables), then some action must be taken to correct the
deviation.
One way to achieve this objective is through the “monitor and control” job performed by the process plant operator. The operator observes the supply temperature of process stream Ta, compares it with the desired temperature Ts, and, on the basis of this comparison, decides whether to throttle or un-change the opening degree of the steam inlet control valve to the heat exchanger. This mode of operation is called MANUAL control mode and is very useful during a start-up, shutdown, maintenance, or emergency situation.
After the
performance of such operations in manual control mode, the control
room operator is used to shift the control mode to AUTOMATIC control mode.
In automatic control mode,
the process controller receives the continuous input signal of
process stream temperature from the temperature transmitter at outlet of the
heat exchanger, compares it with the desired value, and depending on the result
of this comparison (positive error or negative error), the process controller
decides whether to throttle or un-change the opening degree of the
steam inlet control valve to the heat exchanger. On the basis of this decision,
the process controller sends an output signal to the steam inlet control valve
to the heat exchanger, which in turn, manipulates the steam flow.
To accomplish this, a
manual/automatic switch, or the software equivalent, is used to transfer the
controller from the manual mode to the automatic mode, and vice versa.
To execute the shift commands
from manual to automatic mode or vice versa, the controller is usually
provided with a set point tracking feature.
When a process plant is started
up, a common procedure is to start the equipment and the process through
manual control mode. In this case, the operator adjusts the output of the
controller until the process variable comes to a desired steady state. When
the process controller lacks the set point tracking feature, the set point
must be manually adjusted until it equals the process variable before the
controller is transferred to automatic; the process then continues running
smoothly.
If the operator adjusts the set
point to the process variable after switching to automatic mode, there may be a
temporary disturbance in the process variable. This disturbance is called a
bump. With the set point tracking feature, the operator does
not need to think about adjusting the set point to the process variable,
because it is done automatically. In other words, set point tracking
provides bump-less transfer when switching from manual to automatic control
mode.
Which equipment failures protect the facility, employees,
the public, & environment?
The identification of undesired
events that lead to the materialization of a hazard & the mechanisms by
which these undesired events could occur are numerous. In such a process, all
known failure modes of components or features of a system are considered and
undesired outcomes are noted. To provide the audience with the flavor of
the subject, some failures, and fail-safe conditions are elaborated on here.
Pneumatic control valves,
either regulating or fully open or fully closed type, in process plants, power
plants, water demineralization plants, etc. consist of a diaphragm actuator
that is actuated by compressed instrument air.
Although numerous advantages such
as reliability, economy, and so on, are included in design & manufacture, the
major advantage of the diaphragm actuator is its behavior on the loss of supply
air. On one side of diaphragm, air exerts pressure against the
compression-expansion force of the spring. Two mechanical configurations are
available.
One configuration that
drives the valve toward a fully closed state upon loss of compressed instrument
air; this configuration is called a fail close (FC) pneumatic control valve,
and such type of valve opens when air pressure is increased against the spring
force, therefore, referred as air to open (AO) pneumatic control valve.
The other configuration is
that drives the valve toward a fully open state upon loss of compressed
instrument air; this configuration is called a fail open (FO) pneumatic
control valve and such type of valve opens when air pressure is decreased
against the spring force, therefore, referred as air to close (AC) pneumatic
control valve.
Now let us know to decide on FO
or FC types of pneumatic control valves. After the development of any process
or process system for the facility, hazards are identified for the operation
of the developed process and these hazards are identified and analyzed by a
number of tools. One of these tools is called hazards and operability analysis
(HAZOP). It is best used as late as possible with a new design, in
order to be as complete as possible. With an existing facility, it can be used
at any time. HAZOP can also be used for analyzing
operating procedures so that sources of human error can be
identified. HAZOP Simulates abnormal situations by using guide words
applied to parameters and operations to create deviations. Along with many
other recommendations of a HAZOP report, the fail open/fail closed choice for a
control valve is also made by those responsible for process safety.
Illustation-1:
In order to give you a flavor
that how pneumatic control valves are specified as FO and FC, let us see the
following simple examples. Specify the fail safe modes of the pneumatic
control valves that are listed in the applications below. State whether a FO or
FC valve should be specified for the following manipulated variables. Give
reason(s) in support of your answer:
(a)
Steam pressure in a shell of shell & tube heat exchanger.
(b) Flow
of effluent from a wastewater treatment plant hold-up tank into a sea.
(c) Flow
rate of reactants into a polymerization reactor.
(d) Flow
of cooling water to a reflux condenser of a distillation column.
Solution:
(a) FC,
to make sure that a shell steam pressure transmitter failure (or fail low) or
power failure to the solenoid valve or instrument air supply failure, will not
cause the over-pressurization & over-heat of the shell of the heat
exchanger.
(b) FC,
to make sure that a level transmitter failure (or fail high) or power failure
to the solenoid valve or instrument air supply failure, will not cause
excessive and perhaps untreated waste from entering the sea.
(c) FC,
to make sure that a reactor level transmitter failure (or fail low) or reactor
pressure transmitter failure (or fail low), or power failure to the solenoid
valve or instrument air supply failure, will not cause the reactor to be
flooded with excessive reactants.
(d) FO, to make sure that a top plate pressure transmitter failure (or fail high in case of non-vacuum distillation) or power failure to the solenoid valve or instrument air supply failure, will not cause the over-pressurization of the distillation column.
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