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					CHAPTER 7                          INSTRUMENTATION AND CONTROL

CHAPTER -7

INSTRUMENTATION AND CONTROL

INSTRUMENTS
       Instruments are provided to monitor the key process variables during plant
operation. They may be incorporated in automatic control loops or used for the manual
monitoring of the process operation. They may also be part of an automatic computer
data logging system. Instruments monitoring critical process variables will be fitted
with automatic alarms to alert the operators to critical and hazardous situations.

       It is desirable that the process variable to be monitored be measured directly;
often, however, this is impractical and some dependent variable that is easier to
measure, is monitored in its place. For example, in the control of distillation columns
the continuous on-line, analysis of the over-head product is desirable but it is difficult
and expensive to achieve reliably, so temperature is often monitored as an indication
of composition. The temperature instrument may form part of a control loop
controlling, say, reflux flow; with the composition of the overheads checked
frequently by sampling and laboratory analysis.

INSTRUMENTATION AND CONTROL OBJECTIVE
       The primary objective of the designer when specifying instrumentation and
control schemes are:

1)     Safer Plant Operation

      a) To keep the process variables within known safe operating limits.

      b) To detect dangerous situations as they develop and to provide alarms and
          automatic shut-down systems.

      c) To provide inter locks and alarms to prevent dangerous operating
          procedures.


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2)     Production Rate

       To achieve the design product output.

3)     Product Quality

       To maintain the product composition within the specified quality standards

4)     Cost

       To operate at the lowest production cost, commensurate with the other
objectives.

       These are not separate objectives and must be considered together. The order
in which they are listed is not meant to imply the precedence of any objective over
another, other than that of putting safety first. Product quality, production rate and the
cost of production will be dependent on sales requirements. For example, it may be a
better strategy to produce a better quality product at a higher cost.

       In a typical chemical processing plant these objectives are achieved by a
combination of automatic control, manual monitoring and laboratory analysis.

COMPONENTS OF CONTROL SYSTEM

Process
       Any operation or series of operations that produces a desired final result is a
process. In this discussion the process is the n-butanal production.

Measuring Means
       Of all the parts of the control system the measuring element is perhaps the
most important. If measurements are not made properly the remainder of the system
cannot operate satisfactorily. The measured available is dozen to represent the desired
condition in the process.

ANALYSIS OF MEASUREMENT VARIABLE TO BE MEASURED

Measured
       a) Pressure measurements

       b) Temperature measurements
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       c) Flow Rate measurements

       d) Level measurements

Variables to be Recorded
       Indicated temperature, composition, pressure, etc.

Controller
       The controller is the mechanism that responds to any error indicated by the
error detecting mechanism. The output of the controller is some predetermined
function of the error.

       In the controller there is also and error-detecting mechanism which compares
the measured variables with the desired value of the measured variable, the difference
being the error.

Final Control Element
       The final control element. receives the signal from the controller and by some
predetermined relationships changes the energy input to the process.

CLASSIFICATION OF CONTROLLER
       In general the process controllers can be classified as:

           a) Pneumatic controllers

           b) Electronic controllers

           c) Hydraulic controllers

       In the n-butanal manufacturing from propylene the controller and the final
control element may be pneumatically operated due to thse following reasons:



i)     The pneumatic controller is vary rugged and almost free of maintenance. The
       maintenance men have not had sufficient training and background in
       electronics, so basically pneumatic equipment is simple.




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ii)         The pneumatic controller appears to be safer in a potentially explosive
            atmosphere which is often present in the petro-chemical industry.

iii)        Transmission distances are short.      Pneumatic and electronic transmission
            systems are generally equal upto about 250 to 300 feet. Above this distance,
            electronic systems begin to offer savings.

MODES OF CONTROL
            The various type of control are called "modes" and they determine the type of
response obtained. In other words these describe the action of the controller that is the
relationship of output signal to the input or error signal. It must be noted that it is error
that actuates the controller. The four basic modes of control are:

     i)     On-off Control

     ii)    Integral Control

     iii)   Proportional Control

     iv)    Rate or Derivative Control

            In industry purely integral, proportional or derivative modes seldom occur
alone in the control system.

            The On-off controller is the controller with very high gain. In this case the
error signal at once off the valve or any other parameter upon which it sits or
completely sets the system.

ALARMS AND SAFETY TRIPS AND INTERLOCKS
            Alarms are used to alert operators of serious and potentially hazardous,
deviations in process conditions. Key instruments are fitted with switches and relays to
operate audible and visual alarms on the control panels.

            The basic components of an automatic trip systems are:

i)          A sensor to monitor the control variable and provide an output signal when a
            preset valve is exceeded (the instrument).




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ii)    A link to transfer the signal to the actuator usually consisting of a system of
       pneumatic or electric relays.

iii)   An actuator to carry out the required action; close or open a valve, switch off a
       motor.

       A safety trip can be incorporated in control loop. In this system the high-
temperature alarm operates a solenoid valve, releasing the air on the pneumatic
activator closing the valve on high temperature.

Interlocks
       Where it is necessary to follow the fixed sequence of operations for example,
during a plant start-up and shut-down, or in batch operations-inter-locks are included
to prevent operators departed from the required sequence. They may be incorporated
in the control system design, as pneumatic and electric relays or may be mechanical
interlocks.

DIFFERENT TYPES OF CONTROLLER

Flow Controllers
       These are used to control feed rate into a process unit. Orifice plates are by far
the most common type of flow rate sensor. Normally, orifice plates are designed to
give pressure drops in the range of 20 to 200 inch of water. Venturi tubes and turbine
meters are also used.

Temperature Controller
       Thermocouples are the most commonly used temperature sensing devices. The
two dissimilar wires produce a millivolt emf that varies with the "hot-junction"
temperature. Iron constrictant thermocouples are commonly used over the 0 to 1300°F
temperature range.

Pressure Controller
       Bourdon tubes, bellows and diaphragms are used to sense pressure and
differential pressure. For example, in a mechanical system the process pressure force


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is balanced by the movement of a spring. The spring position can be related to process
pressure.

Level Controller
       Liquid levels are detected in a variety of ways. The three most common are:

   1. Following the position of a float, that is lighter them the fluid.
   2. Measuring the apparent weight of a heavy cylinder as it buoyed up more or
       less by the liquid (these are called displacement meters).
   3. Measuring the difference between static pressure of two fixed elevation, one
       on the vapor which is above the liquid and the other under the liquid surface.
       The differential pressure between the two level taps is directly related to the
       liquid level in the vessel.

Transmitter
       The transmitter is the interface between the process and its control system. The
job of the transmitter, is to convert the sensor signal (millivolts, mechanical
movement, pressure differential, etc.) into a control signal 3 to 15 psig air-pressure
signal, 1 to 5 or 10 to 50 milliampere electrical signal, etc.

Control Valves
       The interface with the process at the other end of the control loop is made by
the final control element is an automatic control valve which throttles the flow of a
stem that opens or closes an orifice opening as the stem is raised or lowered. The stem
is attached to a diaphragm that is driven by changing air-pressure above the
diaphragm. The force of the air pressure is opposed by a spring.




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CHAPTER 7                         INSTRUMENTATION AND CONTROL


CONTROL SCHEME ON DISTILLATION COLUMN

GENERAL CONSIDERATIONS

Objectives
     In distillation column control any of following may be the goals to achieve

          1. Over head composition.

          2. Bottom composition

          3. Constant over head product rate.    .

          4. Constant bottom product rate.

Manipulated Variables
     Any one or any combination of following may be the manipulated variables

          1. Steam flow rate to reboiler.

          2. Reflux rate.

          3. Overhead product withdrawn rate.

          4. Bottom product withdrawn rate

          5. Water flow rate to condenser.

Loads or Disturbances
     Following are typical disturbances

     1.       Flow rate of feed

     2.       Composition of feed.

     3.       Temperature of feed.

     4.       Pressure drop of steam across reboiler

     5.       Inlet temperature of water for condenser.




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Control Scheme
        Overhead product rate is fixed and any change in feed rate must be absorbed
by changing bottom product rate. The change in product rate is accomplished by direct
level control of the reboiler if the stream rate is fixed feed rate increases then vapor
rate is approximately constant & the internal reflux flows must increase.

Advantage

Since an increase in feed rate increases reflux rate with vapor rate being approximately
constant, purity of top product increases.

Disadvantage

The overhead reflux change depends on the dynamics of level control system that
adjusts it.




                               Figure: Control scheme

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CONTROL SCHEME OF CSTR

GENERAL CONSIDERATION
Objective

In CSTR control any of following may be the goals to achieve

                1. Constant Pressure inside the reactor

                2. Constant Temperature inside the reactor

                3. Constant Level

                4. High quality of Product

Reactor Variable

        The independent variable for the dryer may be divided into two categories

             1. Uncontrolled variables

             2. Manipulated variables

             3. Controlled Variables

Uncontrolled Variables

The variables, which cannot be controlled by controller, are called uncontrolled
variables. The Uncontrolled variables include

             1.Vent gases rate

             2.Temperature of feed, etc.

Manipulated Variables

The independent manipulated inputs are variables, which are adjusted to control the
chemical reaction. Any one or any combination of following may be the manipulated
variables

             1.Flow rate of cooling water

             2.Flow rate of Feed

             3.Flow rate of Product stream



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Controlled Variables

   Any process variable that is selected to be maintained by a control system is called
a controlled variable. Following are the controlled variables

             1.Inside reactor Temperature

             2.Inside reactor Pressure

             3.Level of reacting mixture in reactor

CONTROL SCHEME
Temperature Control

The simplest method of cooling a CSTR is shown in diagram. Here we measure the
reactor temperature and manipulated the flow of cooling water to the jacket. Using a
jacket for cooling has two advantages. First, it minimizes the risk of leaks and thereby
cross contamination between the cooling system and the process. Second, there are no
internals to obstruct an agitator from providing effective mixing.

         A temperature sensor measure the inside reactor temperature and transfer
signal to temperature transducer, transducer convert these signals in other form and the
output of transducer is accepted by controller and controller transfer its signal to final
control element. Final control element takes step to overcome these disturbances.

Pressure Measurement

     Similarly as temperature controller, there is a pressure control loop, which
controls the pressure inside the reactor. This controller takes action on two valves at a
same time. One at the valve of feed stream and other at the valve of product stream. If
pressure is high in the reactor then product stream valve will open and feed valve will
close and vice versa.

Level Measurement

     A sensor measures level of reacting materials inside the reactor and these signals
are transferred to transducer and controller takes action on solvent valve. If inside
level is below the required level then valve will open and vice versa.



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CSTR CONTROL CONFIGURATION




                      Figure: Control Scheme


Control Loop around Heat Exchanger
Single Loop




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Double Loop




Cascade Control




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DOCUMENT INFO
Description: Instrumentation is defined as the art and science of measurement and control.[1] An instrument is a device that measures and/or regulates process variables such as flow, temperature, level, or pressure. Instruments include many varied contrivances which can be as simple as valves and transmitters, and as complex as analyzers. Instruments often comprise control systems of varied processes such as refineries, factories, and vehicles. The control of processes is one of the main branches of applied instrumentation. Instrumentation can also refer to handheld devices that measure some desired variable. Diverse handheld instrumentation is common in laboratories, but can be found in the household as well. For example, a smoke detector is a common instrument found in most western homes.