Instrumentation is defined as the art and science of measurement and control. 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.
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. Page 104 CHAPTER 7 INSTRUMENTATION AND CONTROL 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 Page 105 CHAPTER 7 INSTRUMENTATION AND CONTROL 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. Page 106 CHAPTER 7 INSTRUMENTATION AND CONTROL 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). Page 107 CHAPTER 7 INSTRUMENTATION AND CONTROL 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 Page 108 CHAPTER 7 INSTRUMENTATION AND CONTROL 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. Page 109 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. Page 110 CHAPTER 7 INSTRUMENTATION AND CONTROL 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 Page 111 CHAPTER 7 INSTRUMENTATION AND CONTROL 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 Page 112 CHAPTER 7 INSTRUMENTATION AND CONTROL 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. Page 113 CHAPTER 7 INSTRUMENTATION AND CONTROL CSTR CONTROL CONFIGURATION Figure: Control Scheme Control Loop around Heat Exchanger Single Loop Page 114 CHAPTER 7 INSTRUMENTATION AND CONTROL Double Loop Cascade Control Page 115
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