13_ Instrumentation & Control

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Instrumentation And Control

INSTRUMENTATION AND CONTROL 13.1 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 can 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 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.

13.2 Instrumentation and Control Objectives: The primary objective of the designer when specifying instrumentation and control schemes are:

1)

Safer Plant Operation

 To keep the process variables within known safe operating limits.  To detect dangerous situations as they develop and to provide alarms and automatic shut-down systems.

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

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2)

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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.

13.3 Component of the 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 cracking of naphtha.

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.

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13.4 Variables to be measured:  Pressure measurements  Temperature measurements  Flow Rate measurements  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.

13.5 Classification of Controllers: 212

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In general the process controllers can be classified as:



Pneumatic controllers



Electronic controllers



Hydraulic controllers

In the ethylene manufacturing from naphtha the controller and the final control element may be pneumatically operated due to the following reasons:

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

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

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

13.6 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:

 On-off Control 213

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 Integral Control  Proportional Control  Rate or Derivative Control In industry purely integral, proportional or derivative modes seldom occur alone in the control system. The On-off controller in 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.

13.7 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 automatic trip systems are:

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

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

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

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A safety trip can be incorporated in control loop; as shown in figure . 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.

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

Temperature Controller Thermocouples are the most commonly used temperature sensing devices. The two dissimilar wires produce a mili volt 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 is balanced by the movement of a spring. The spring position can be related to process pressure. Level Controller 215

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



Following the position of a float, that is lighter them the fluid.



Measuring the apparent weight of a heavy cylinder as it buoyed up more or less

by the liquid (these are called displacement meters).



Measuring the difference in static pressure between two fixed elevations, one in

the vapour 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 (milli volts, 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 open 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.

13.9 Control Schemes Of Distillation Column: General Consideration: Objectives In distillation column control any of following may be the goals to achieve 216

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 Over head composition.  Bottom composition  Constant over head product rate.

.

 Constant bottom product rate.

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

 Steam flow rate to reboiler.  Reflux rate.  Overhead product withdrawn rate.  Bottom product withdrawn rate  Water flow rate to condenser

13.10 Loads or Disturbances: Following are typical disturbances

 Flow rate of feed  Composition of feed.  Temperature of feed.  Pressure drop of steam across reboiler  Inlet temperature of water for condenser.

13.11 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. 217

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

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

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Figure 13.1: Control scheme

REFRENCES 1. G. Stephanopoulos, “Chemical Process Control, Prentice Hall,1995

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2. S.K. Singh, Industrial Instrumentation and Control, Tata 3. www.sciencedirect.com 4.

www.che.cemr.wvu.edu.com

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