Unit 4 Process Control Principles

• Recent explosion in Andhra Pradesh, India, caused by a chemical leak at an LG Polymer plant. As many as 12 people died, and 1000 were affected by the styrene vapour leak. NDTV further reports that 200 people were hospitalised. The scientist said styrene has a high boiling point of 145 degree centigrade. Owing to its nature, it needs to be stored in a temperature-controlled tank. Ambient temperatures at this time of the year in India are quite elevated and if the controlled environment is disturbed, it could set off a reaction that could rupture the storage tank.

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• Another gas leak at a paper mill in the industrial town of Raigarh in Chhattisgarh sent seven workers to the hospital, and a boiler blast at a thermal power station in Tamil Nadu injured three.

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Process Control

• Organized activity to monitor and control process parameters
• Process parameters – temperature, pressure, pH, moisture, levels, flow rates etc.,
• The objectives of process control are, among others:
• To enhance the reliability of the process
• To improve process safety
• To increase production
• To reduce the proportion of production units which do not meet the specifications
• To improve process economics, by reducing production cost.

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Process Control Systems

• Pneumatic systems
• Hydraulic systems
• Pressure systems

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Process Control Loop

• A control loop is the fundamental building block of industrial control systems.
• It consists of all the physical components and control functions necessary to automatically adjust the value of a measured process variable (PV) to equal the value of a desired set-point (SP).
• It includes the process sensor, the controller function, and the final control element (FCE) which are all required for automatic control.

Components of Process Control

Set Point

• Set point (also set point or set-point) is the desired or target value for an essential variable, or process value of a system.
• Departure of such a variable from its set point is one basis for error-controlled regulation using negative feedback for automatic control. The set point is usually abbreviated to SP.

Process Variable

• Process variable, process value or process parameter is the current measured value of a particular part of a process which is being monitored or controlled.
• Example - temperature of a furnace. The current temperature is called the process variable, while the desired temperature is known as the set-point.
• process variable is usually abbreviated to PV.

Final Control Element

• A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller.
• The opening or closing of automatic control valves is usually done by electrical, hydraulic or pneumatic actuators.
• The pneumatic control signals are traditionally based on a pressure range of 3-15psi (0.2-1.0 bar), or more commonly now, an electrical signal of 4-20mA for industry, or 0-10V for HVAC systems

Control Valve

• An automatic control valve consists of three main parts in which each part exist in several types and designs:
• Valve actuator - which moves the valve's modulating element, such as ball or butterfly.
• Valve positioner - Which ensures the valve has reached the desired degree of opening. This overcomes the problems of friction and wear.
• Valve body - in which the modulating element, a plug, globe, ball or butterfly, is contained.

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Open Loop Control

• Open-loop system, also referred to as non-feedback system, is a type of continuous control system in which the output has no influence or effect on the control action of the input signal.
• In other words, in an open-loop control system the output is neither measured nor “fed back” for comparison with the input
• Example: Washing machines

Closed Loop Control

• Closed-loop Systems use feedback where a portion of the output signal is fed back to the input to reduce errors and improve stability
• The quantity of the output being measured is called the “feedback signal”, and the type of control system which uses feedback signals to both control and adjust itself is called a Close-loop System
• A Closed-loop Control System, also known as a feedback control system

Basic terms

• Lag time is the time required for a control system to return a measured variable to its set point after there is a change in the measured variable, which could be the result of a loading change or set point change and so on.
• Dead time is the elapse time between the instant an error occurs and when the corrective action first occurs.
• Set point is the desired amplitude of an outpoint variable from a process.
• Error signal is the difference between a set reference point and the amplitude of the measured variable.
• Transient is a temporary variation of a load parameter after which the parameter returns to its nominal level.
• Measured variable is an output process variable that must be held within given limits.

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Basic terms

• Controlled variable is an input variable to a process that is varied by a valve to keep the output variable (measured variable) within its set limits.
• Variable range is the acceptable limits within which the measured variable must be held and can be expressed as a minimum and a maximum value, or a nominal value (set point) with ± spread (percent).
• Control parameter range is the range of the controller output required to control the input variable to keep the measured variable within its acceptable range.
• Offset is the difference between the measured variable and the set point after a new controlled variable level has been reached. It is that portion of the error signal which is amplified to produce the new correction signal and produces an “Offset” in the measured variable.

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Basic terms

• Servo control - The set-point signal is changed and the manipulated variable is adjusted appropriately to achieve the new operating conditions.
• Regulatory control – The set-point is fixed at a constant value. When any disturbance enters the system, the manipulated variable is adjusted to drive the controlled variable back to its fixed set-point.

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Control Modes

Two basic modes of process control

On/off

Continuous control

In either case the purpose of the control is to hold the measured variable output from a process within set limits by varying the controlled input variable to the process.

• Discrete control or two position control, the output of the controller changes from one fixed condition (ON) to another fixed position (OFF)

• Modulating control action the feedback controller determines the error between a set point and a measured variable
• The error signal is then used to produce an actuator control signal to operate a valve and reduce the error signal.
• This type of control continuously monitors the measured variable and has three modes of operation which are proportional, integral, and derivative. Controllers can use one of the functions, two, or all three of the functions as required.

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ON/OFF action

• The measured variable is compared to a set reference.
• When the variable is above the reference the system is turned ON and when below the reference the system is turned OFF or vice versa, depending upon the system design

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Proportional Control Mode

• Continuous linear relation between value of the controlled variable and position of the final control element
• the output of the controller is proportional to error e(t)
• relation between the error e(t) and the controller output p is determined by a constant called proportional gain constant denoted as K_p
• p(t) = K_pe(t) + p(0) 

Proportional gain

percentage change of the controller output relative to the percentage change in controller input

Proportional band

Proportional band, (also called throttling range), is the change in value of the controlled variable that causes full travel of the final control element

Offset

Deviation in process variable that remains after a process has stabilized

Proportional Integral Control

• The controller output is proportional to the amount of time the error is present
• Integral action eliminates offset that remains
• the value of the controller output p(t) is changed at a rate which is proportional to the actuating error signal e(t)

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Proportional Derivative Control�

• Output of the controller depends on the rate of change of error
• Also called rate action mode or anticipatory action mode
• Derivative gain constant indicates by how much % the controller output must change for every % per second rate of change of the error
• Derivative control thus anticipates the actuating error, initiates an early corrective action and tends to increase stability of the system, improving the transient response
• The derivative or differential controller is never used alone because when error is zero or constant, the controller has either no output or the nominal output for zero error.

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PID control mode

Advantages�

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• This mode eliminates the offset of proportional mode. 
• It provides the most accurate and stable control of the three controller types.
• It is recommended in systems where compensation is required for frequent changes in load, set point, and available energy.
• It can help achieve the fastest response time and smallest overshoot.

Summary

• Term P is proportional to the current value of the SP − PV error e(t). For example, if the error is large and positive, the control output will be proportionately large and positive, taking into account the gain factor "K“

Summary

• Term I accounts for past values of the SP − PV error and integrates them over time to produce the I term. For example, if there is a residual SP − PV error after the application of proportional control, the integral term seeks to eliminate the residual error by adding a control effect due to the historic cumulative value of the error. When the error is eliminated, the integral term will cease to grow.

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Summary

• Term D is a best estimate of the future trend of the SP − PV error, based on its current rate of change. It is sometimes called "anticipatory control", as it is effectively seeking to reduce the effect of the SP − PV error by exerting a control influence generated by the rate of error change. The more rapid the change, the greater the controlling or dampening effect

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Codes, Symbols and Standards

• Why these?
• Authorities
• ANSI (American National Standards Institute)
• IEEE (Institute of Electrical and Electronics Engineers
• ISA (Instrument society of America)
• NFPA (National Fire Protection Association)
• SAE (Society of Automotive Engineers)
• SME (Society of Manufacturing Engineers)
• OSHA (Occupational safety and Health Act)
• BIS (Bureau of Indian Standards)
• FSSAI (--------------------------)
• What they do? And Why? – Fix, Publish and Review

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• Process flow diagram – Drawing that shows the general process flow between major pieces of equipment of a plant and the expected operating conditions at the target production rate.
• Control Loop - One segment of a process control system.
• Piping and Instrumentation Diagram (P&ID) - Drawing that shows the instrumentation and piping details for plant equipment.
• Instrumentation department of an engineering firm is responsible for the selection of field devices that best matches the process design requirements.
• Tag number – Unique identifier that is assigned to a field device.
• Loop Diagram – Drawing that shows field device installation details including wiring and the junction box (if one is used) that connects the field device to the control system.

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