process control, major issue of chemical engineering
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Industrial Process Control1
It Is defined as:
Physical or chemical change of matter. Energy conversion
e.g., change in pressure, temperature, speed, electrical potential, etc.A process in a collection of vessels, pipes, fittings, gauges etc., is built for the purpose of producing a product or group of products.Process3
The regulation or manipulation of variables influencing the conduct of a process in such a way as to obtain a product of desired quality and quantity in an efficient manner.
Input to Process: Mass or energy applied to the process.
Output of Process: The product delivered by the process. This is a dynamic variable.
Supply: Source of mass or energy input to process.
Control Valve: Consists of the final actuator and final controlling elements. This is the forward controlling element which directly changes the value of the manipulated variable.
Load: Anything that affects the value of the controlled variable under a constant supply input.5
Open Loop: Control without feedback. Open loop can not cope with load upsets. Example of open loop: automatic dishwasher, automatic water sprinkling system, a control loop with the controller in manual.
Primary Element: The measuring element that quantitatively converts the measured variable energy into a form suitable for measurement.
Transmitter: A transducer which responds to a measured variable by means of a sensing element, and converts it to a standardized transmission signal which is a function only of the measured variable.6
Controlled Variable: A variable the value of which is sensed to originate a feedback signal. (Also known as the process variable.)
Controller: A device which operates automatically to regulate a controlled variable.
Controller Algorithm (PID): A mathematical representation of the control action to be performed.
Set Point: An input variable which sets the desired value of the controlled variable.7
Error: In process instrumentation, the algebraic difference between the real value and ideal value of the measured signal. It is the quantity which when algebraically subtracted from the indicated signal gives the ideal value.
Manipulated Variable: A quantity or condition which is varied as a function of the algebraic error signal so as to cause a change to the value of the directly controlled variable.8
Feedback Control: Control action in which a measured variable is compared to its desired value to produce an actuating error signal which is acted upon in such a way as to reduce the magnitude of the error.
Cascade Control: Control in which the output of one controller is introduced as the set point for another controller.
Feedforward Control: Control action in which information concerning one or more conditions that can disturb the controlled variable is converted, outside of any feedback loop, into corrective action to minimize deviations of the controlled variable.9
The operator walked up and down a plant, looking at gauges and opening and closing valves is effective only at the time when the operator moves the valve.
At that instant the loop is closed.
Open loop control works only when the load(s) on the process are constant.
Any load change or supply upset can affect the product quality.Open Loop Control13
Increased productivity: Automatic closed loop control allows the amount of products made in a particular process to be maximized.
On Spec Products: Industrial products are produced to meet certain purity levels.
Energy and Material Conservation: A closed loop control application minimizes the amount of material and energy used in production.
Safety: Closed loop control is the first line of defense before Emergency Shutdown Devices (ESD) override regulatory control devices.Advantages of Closed Loop Control14
Continuous Control is used on continuous processes. A continuous process is one in which process material is continually flowing through the process equipment.
Sequential is often referred to as on/off control. It is a series of discrete control actions performed in a specific order or sequence.
Batch control is a combination of sequential and continuous control. A batch process is a process where the operation is time-dependent and repeatable. Types of Control15
It can be defined as the control action in which the error is reinforced until a limit is eventually reached.
This obviously is not a desirable outcome of control action and should be avoided.
Imagine a tank in which level is being controlled. When the level exceeds the set point, the control action will increase the level further until the tank overflows.
It can be defined as the control action in which the error is minimized, made as small as possible, depending on the algorithm of the controller.
This obviously is a desirable outcome of the control action and should be achieved in all feedback loops.
Direct Acting Element is one in which the value of the output signal increases as the value of the input signal increases.
Reverse Acting Element is one in which the value of the output signal decreases as the value of the input increases.
A control valve consists of a valve connected to an actuator mechanism. The actuator, in response to a signal from the controlling system, can change the position of a flow-controlling element in the control valve.
The action of the final actuator is the first choice and is based on Fail-Safe Control Valve Action. (open, closed, and in place).Control Valves20
It can be set up (calibrated) as either direct acting or reverse acting.
It can be either direct or reverse acting.Most processes are direct acting.
Energy Flow ProcessHeat ExchangerRefrigeration
Mass Flow ProcessLevel TankPipe FlowProcesses24
Rule for Achieving Negative FeedbackTo achieve negative feedback in a control loop you must have an odd number of reverse acting elements in the loop.
The odd number of reverse-acting elements for negative feedback can be determined through an open loop test, conducted in the following manner.
Place the controller in manual (open loop), and step up the output of the controller (5-10%) and observe (record) the output of the transmitter.30
Control Loop Elements AndTheir Contribution To Loop Performance32
Accuracy (Error) = Precision Error + Bias Error
Range: The region between the limits within which a quantity is measured is the range of that measurement.
Span: The measurement span is the algebraic difference between the upper and lower range values.
Minimum Span : The minimum span of measurement that the primary element can be used to measure within its accuracy rating.
Maximum Span: The maximum span of measurement that the primary element can be used to measure within its accuracy rating.
Rangeability (Turndown): In flow applications, rangeability is the ratio of the maximum flow rate to the minimum flow rate within the stated accuracy rating.
Zero Elevation and Suppression: The range at which the zero value of the measured variable is not at the lower range value.35
Response Time: An output expressed as a function of time, resulting from the application of a specified input (step) under specific operating conditions.
Time Constant: This is a specific measure of a response time. It is the time required for a first order system to reach 63.2% of the total change when forced by a step.
Characteristic Curve (Input-Output Relationship): A curve that shows the ideal value of an input-output relationship at steady state.
Reproducibility: There should be a closeness of agreement among repeated measurements of the output for the same value of the input made under the same operating conditions over a period of time, approaching from both directions.
Noise: In process instrumentation noise is an unwanted component of a signal or a process variable.
Flow Coefficient, CV - Is a capacity coefficient which is defined as the number of U.S gpm of 60F water which will flow through a wide-open valve with a constant pressure drop of 1 psi across the valve.39
Current to Pressure Signal Converters, I/P
Analog to Digital, A/D, or Digital to Analog, D/A
A valve positioner is a proportional-only controller whose main function is to eliminate or minimize valve hysteresis
The practical rangeability of a control valve is limited to approximately 100/1 with most valves falling below 50/1. These rangeability values are sufficient for most control applications.
In some applications however, such as pH, the rangeability required may exceed 1000/1 and the control scheme must be designed to satisfy this requirement in order to achieve good control.
In split ranged or sequenced strategies, the controller's output actuates more than one valve, typically two valves. 45Valve Sequencing
Process Modeled ThroughDead Time And Capacity46
The steady state gain of the dead-time element is the ratio of the output amplitude to the input amplitude when both are time invariant.
49Steady State Gain