Process Control Chapter 1

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UNDERSTANDING THE INDUSTRIAL PROCESSES &

PROCESS CONTROL



Understand typical type of processes



General concepts and terminology



Type of process variables

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1.1. THE INDUSTRIAL PROCESSES

Process as used in the terms process control

and process industry, refers to the methods of changing or refining raw materials to create end products. The raw materials, which either pass through or remain in a liquid, gaseous, or slurry (a mix of solids and liquids) state during the process, are transferred, measured, mixed, heated or cooled, filtered, stored, or handled in some other way to produce the end product. Industrial manufacturing processes can be generally be classified as continuous, discrete, or batch process. How a process is classified depends on whether the output product from the process appears in a continuous flow or in

discrete batches or quantities.

In a continuous process, product is made by passing materials through different pieces of specialized equipment; each of pieces of equipment ideally operates in a single steady state and performs one dedicated processing function. The output product from a continuous process appears in a continuous flow.

In discrete manufacturing, products having are traditionally manufactured in production lots (a group of products having common raw materials and production histories). In a discrete manufacturing process, a specified quantity of product moves as a unit (group of parts) between workstations; each part maintains its unique identity. One of its useful features is the ability to audit quality economically and answer the growing demand from customers for traceability. What is the process? ____________________________________ ____________________________________ ____________________________________ ____________________________________ ____________________________________ ____________________________________ Industrial manufacturing process can be classified as how many process? List them down: 1. _____________________________ 2. _____________________________ 3. _____________________________ List down two industries are classified as continuous process: 1. _____________________________ 2. _____________________________ List down two industries are classified as discrete process: 1. _____________________________ 2. _____________________________

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So what is a batch process? Shaw gives the following definition of a batch process:

A process is considered to be batch in nature if, due to physical structuring of the process equipment or due to other factors, the process consists of a sequence of one or more steps (or phases) that must be performed in a defined order. The completion of this sequence of steps creates a finite quantity of the finished product. If more of the product is to be created, the sequence must be repeated.

Batch processes are discontinuous processes.

Ingredients are sequentially prepared, e.g., mixed, cooked, reacted, finished, and the packaged. When the ingredients are combined in the proper proportions and exposed to the required process conditions for necessary times, the process usually results in an acceptable product. Batch processes are neither discrete nor continuous; however, they have characteristic of both. From a control standpoint, a major distinction between the types of processes (continuous, discrete, and batch) is the frequency of starting the manufacture of a new product.

List down two industries are classified as

batch process:

1. _____________________________ 2. _____________________________

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Any study of process control must begin by investigating the concept of a PROCESS. It is generally thought of as a place where materials and most often, energy come together to produce a desired product.

A process is identified as leaving one or more variables associated with it that are important enough for their values to be known and for them to be controlled.

In order to understand the concept, let us consider an example of a heat exchanger in which a process stream is being heated by condensing steam. The process is sketched in figure below.

One way to accomplish this objective is to

measure the outlet temperature T(t), compare

this value to the desired value, and, based on this comparison, decide what to do to minimize the deviation. The steam flow rate can be used to correct for deviation. That is, if the temperature is above the desired value, the steam valve can be closed a little bit to cut the steam flow (energy) to the heat exchanger. If the temperature is below the desired value, the steam valve could be opened a little bit to increase the steam flow (energy) to the exchanger.

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1.2. TYPICAL PROCESSES

There are several typical types of processes with its owned process behaviour and features in the process industries: I. Typical Process I II. Typical process II III. Typical Process III I. Typical Process I  Process with negligible multicapacity  Process without DEAD TIME  Process with negligible LAG The typical process generally found in:  Fluid Flow process  Gas Flow process  Fluid Pressure process

These processes generally noisy and fast response.

Liquid flow process is a noisy process without DEAD time and negligible LAG (time constant) FCV101 : Level Pneumatic Control Valve FT101 : Flow transmitter

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II. Typical Process II  Process with single capacity  Process without DEAD TIME  Process with LAG The typical process generally found in:  Liquid Level process  Gas Pressure process LCV101 : Pneumatic level control valve LT101 : Level transmitter MV101 : Manual valve The liquid level process is Self Regulating (SR) process with single capacity and LAG (time constant) response. The outflow is dependent on h. Time Constant (LAG) is a time to achieve the level response reached to 63.2% of the steady state level. The process Time Constant (TC) or LAG is dependent on the h height of the tank level.

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The Gas Pressure process above is a Single Capacity process with LAG (Time constant) PCV101 : Pneumatic Pressure Control Valve PT101 : Pressure Transmitter

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III. Typical Process III  Process with single or multi capacities  Process with DEAD TIME  Process with LAG The typical process generally found in:  Liquid Temperature process  Multi Pressure Vessel process

The Shell & Tube Heat Exchanger is a good example of

typical Process type III. The temperature is a slow response

process with large LAG time or Time Constant, and large Dead Time (DT). What is the process Dead Time ? ____________________________________ ____________________________________ ____________________________________ ____________________________________

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WHAT IS DEAD TIME (Dt)

As mentioned in the early section, there are three (3) different typical processes. Typical process with Dead Time (Dt) is generally response slow to the input and disturbances. In process control, Dead time is the delay from when a controller output (OUT or MV) signal is issued until when the measured process variable (PV) first begins to respond. The presence of Dead Time, is never a good thing in a control loop.

Shell & Tube Heat Exchanger temperature control is a good example of the Dead Time process.

Definition:

Dead Time is the time between the change in the controller Output (MV or OUT) and the beginning of its effect on the measurement (PV)

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It also defined as Transport LAG, Distance‐

Velocity LAG, or TIME DELAY.

Another good example of Pure Dead Time

Process is Conveyor process as shown below: Dead Time (Dt) = L /Speed = 100 / 10 = 10 min Process response (PV process variable) is similar to the output signal from the controller to the process. The trending shown above, both PV and OUTPUT trending and amplitudes are similar. Therefore; Process GAIN : 1.0 Process Phase Shift : ‐DT/To x 360o Other examples of Dead Time processes  Analyser sampling system  Multicapacity process plant  Mixing process

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WHAT IS TIME CONSTANT (Tc)

Time constant of the system is the time taken to reach 63.2% of the final value or steady state value after a step change has been applied to the system.

After four Time Constants the output response has reached 98% of the final value that it will settle at steady state value. Response of the 1st order lag system to a STEP input gives as exponential output (1 ‐ e‐1/TC) Time Constant Output (1 – e ‐1/TC)_ 0 TC 0 % 1 TC 63.2% 2 TC 86.5% 3 TC 95.0% 4 TC 98.2% 5 TC 99.5%

The output response due to STEP change of the input (controller output change signal, ∆ OUT) to the process can be estimated or defined

Kp

= ∆ _∆

Kp,

is the steady state gain The Process model shall be: Process Model = . . .

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TYPICAL PROCESSES & ITS CHARACTERISTICS

Every type of the processes has its own characteristics as shown in the table below: Liquid Flow process: Dead Time : No dead time Capacity : Very minimum, it can consider negligible Noise : Liquid flow is a noisy process

Linearity : Require square root to calculate the actual

flow measurement.

Period : It is a fast response process. The period cycle time is small. It varies from 1 ~ 10 seconds Gain : It is a fast response process; the process Gain is high Liquid Pressure process: Dead Time : No dead time or minimum Capacity : Single capacity Noise : Liquid Level is a noisy process Linearity : Linear.

Period : It is a fast response process. The period cycle time is small. It varies from 1 ~ 10 seconds Gain : It is a fast response process; the process Gain is high Gas Pressure process: Dead Time : No dead time Capacity : Single or Multi capacity Noise : Very minimum or can consider negligible Linearity : Linear.

Period : It is depends whether it is single or multi capacity system. It varies from 2 min or above

Gain : It is depends whether it is single or multi capacity system. Gain is high for Single capacity pressure process, and Gain is low for Multiple capacity. Liquid Level process: Dead Time : No dead time Capacity : Single capacity Noise : Liquid Level is a noisy process Linearity : Linear. Period : The period cycle time is small. It varies from 1 ~ 30 seconds or more. Gain : It is a fast response process; the process Gain is high

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Temperature process: Dead Time : Yes, with Dead Time Capacity : Multiple capacities Noise : No Linearity : Linear. Period : It is a slow response process. The period cycle time is large. It varies from minutes till hours Gain : It is a slow response process; the process Gain is low Composition process:

This composition process normally related to chemical reaction process etc. Dead Time : Large dead time Capacity : Multiple capacities Noise : Noise process Linearity : Linear. Period : It is a slow response process. The period cycle time is large. It varies from minutes till hours Gain : It is a slow response process; the process Gain is low.

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1.3. PROCESS CONTROL

There are two (2) basic Laws of Process Control you must understand before exploring Process Control. i. GENERAL PRINCIPLES OF PROCESS CONTROL Step 1: Assess the process and define control objectives. The issues to be resolved in this step include the following: (a) Why is there a need for control? (b) Can the problem be solved only by control, or is there another alternative (such as redesigning part of the process)? (c) What do we expect the control system to achieve? Step 2: Select the process variables to be used in achieving the control objective articulated in Step 1. Here we must answer the following questions: (a) Which output variables are crucial and therefore must be measured in order to facilitate efficient monitoring of process conditions? (b) Which disturbances are most serious? Which ones can be measured? (c) Which input variables can be manipulated for effective regulation of the process? First Law: The simplest control system that will do the job is the best. Complex elegant process control system look great on paper but soon end up on “Manual” in an industrial environment. Bigger is definitely not better in control system design.

Second Law:

You MUST understand the process before you control it.

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Step 3: Select control structure.

What control configuration is chosen depends on the nature of the control problem posed by the process system. The usual alternatives are: Feedback, Feedforward, Open Loop (manual), Cascade, and others, which we shall discuss later.

Step 4: Design the controller.

This step can be carried out using varying degrees of sophistication, but it essentially involves the following: Obtain a control law (By a control law we mean a set of rules whereby the input to the process is transformed to its output) by which, given information about the process (current and past outputs, past inputs and disturbances, and sometimes even future predictions of the system output), a control decision is determined which the controller implements on process by adjusting the appropriate manipulated variables accordingly.

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ii. THE CONCEPT OF CONTROL

a) Motivation

The process control system is the entity that is charged with the responsibility for monitoring outputs, making decisions about how best to manipulate inputs so as to obtain desired output behaviour, and effectively implement such decisions on the process. It is therefore convenient to break down the responsibility of the control system into the following three major tasks:

 Monitoring process output variables by

measurement

 Making rational decisions regarding what corrective action is needed on the basis of the information about the past (Integral action takes into account the past state of the process), current and desired state of the process

 Effectively implementing these

decisions on the process

When these tasks are carried out manually by a human operator, we have a manual control system. A control system in which these tasks are carried out automatically by a machine is known as an automatic control system; in particular, when the machine involved is a computer, we have a computer control system.

b) Why process control

i. Suppressing the influence of external disturbances:

Disturbances are usually out of the

reach of the human operator.

Consequently, we need to introduce a control mechanism that will make the proper changes on the process to cancel

the negative impact that such

disturbances may have on the desired operation of a plant.

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ii. Ensuring the stability of a process:

To ensure that, for every bounded input, a dynamic system produces a bounded output, regardless of its initial state.

iii. Setpoint tracking:

The control mechanism should be capable of making the process output track exactly any changes in the set point.

iv. Optimizing the performance of a plant:

It is desirable that a plant should always operate, at the point of minimum production cost or maximum profit. This can be achieved by an optimizing control strategy which:  Identifies when the plant must be moved to a new operating point in order to reduce the operating cost.  Make the appropriate set point changes to bring the plant to the new optimum operating point. Diagram above shows the concept of the process control. The above concepts can be explained using various examples in real life. System Control Variable (CV) Manipulated

Variable (MV) Sensor Actuator

Car Direction Speed Axle Position Fuel Flow Human Speedometer Steering Wheel Gas Pedal Reactor Product Quality Production Temperature Feed rate Meters Valves Home Lighting Temperature Voltage Air Flow, Mix Photoelectrics Thermostats Dimmers AC Controls DC Motor Speed Armature Current: Field Current Tachometer Armature Resistance

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iii. CONTROL LOOP COMPONENTS

There are four (4) components of the entire feedback control loop. a) Sensor  Also often called as Primary Element.  Acquires information about the status of the process variables.  Typical examples: thermocouples (for temperature measurements), differential pressure cells (for liquid level measurements), gas/liquid chromatographs (for composition measurement), etc. b) Controller

 The brain or heart of the control

system (the decision maker).  It is the hardware element with built‐ in capacity for performing the only task requiring some form of intelligence.  Typical examples: Pneumatic controller, Electronic controllers, digital computers used as controller. c) Transmitter  Secondary Element.

 It has the responsibility of passing the information acquired by the sensor to controller and sending the controller decision to the final control element.  Measurement and control signals may

be transmitted as air pressure signals, or as electrical signals.

 Typical examples: Pneumatic

transmitters, Electrical transmitters.

d) Final control element

 Have the task of actually implementing the control command issued by the controller on the process.

 Typical examples: often a control valve

but not always. Other common final control elements are variable speed pumps, conveyors, and electric motors.

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The importance of these components is that they perform the three basic operations that must be present in every feedback control loop. These operations are:

a) Measurement (M): Measuring the process variable to be controlled is usually done by the combination of sensor and transmitter.

b) Decision (D): Based on the measurements and the set point, the controller must then decide what to do to maintain the process variable at its desired value.

c) Action (A): As the result of the controller’s decision, the system must then take an action. This is usually accomplished by the final control element.

LT‐101 is process measurement, a

combination of sensor and transmitter.

LIC‐101 is the PID controller, decides the

manipulated value (MV) or output to the control valve to minimized or reduce the errors between the Set Value (SV) and Process Value (PV).

LCV‐101, is a final control element, where,

it receives the output signal from the PID controller.

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iv. TYPICAL CONTROL CONCEPTS

Before studying the automatic PID process control, it is helpful to explore typical control concepts available. a) Manual Feedback Control

For the Manual Feedback control, all regulation tasks will have to be done manually. For example: to keep constant the temperature of water discharged from an industrial gas‐fired heater, an operator will have to watch a temperature gauge and adjust a fuel gas valve accordingly. If the water temperature becomes too high for some reason, the operator has to close the gas valve a bit, just enough to bring the temperature back to the desired value. If the water becomes too cold, he has to open the gas valve

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b) Automatic Feedback Control To relieve the operator from the tedious task of manual control, we should automate the

feedback control loop. To convert the

Manual Feedback Control to Automatic Feedback Control, the following tasks shall be done:

 Install an electronic temperature

measurement device.

 Automate the gas valve by adding an actuator (and perhaps a positioner) to it so that it can be driven electronically.  Install a PID controller, and connect it to

the electronic temperature measurement and the automated control valve.

A PID controller has a Set Point (SP) that the operator can set to the desired temperature. The Controller’s Output sets the position of the control valve. And the temperature measurement, called the Process Variable (PV) gives the controller its much‐needed feedback. The process variable and controller output are commonly transmitted via 4 – 20mA signals, or via digital commands on a Fieldbus.

The PID controller compares the process variable (PV) to its set point and calculates the difference between the two signals, also called the Error (E).

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v. WHAT IS P‐I‐D

Definition from Shinskey, the PID controller equation is:

“Mathematical purity has no particular value in control. Performance, robustness, ease of operation and tuning are the real issue”

Below is the simplified PID equation for illustration: MV% = [

E

+ + Td ] P I D Proportional Integral Derivative Error, E : SV – PV (reverse action) PB : Proportional Band (PB%) : 0 ~ 100 % Integral/Reset : Seconds, Minutes : Sec‐1, repeat per second : min ‐1, repeat per minute Derivative Td : Seconds, Minutes

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WHAT IS GAIN

Controller tuning is performed to adjust the manner in which a control valve (or other final control element, i.e. pump, heater etc.) responds to a change in error.

In particular, we are interested in adjusting the gain of the controller such that a change in controller input will result in a change in Gain is defined simply as the change in output divided by the change in input. Gain (Kc) = Δ _Δ _% % Examples: Change in Input to Controller ‐ 10% Change in Controller Output ‐ 5% Gain = 5% / 10% = 0.5 Change in Input to Controller ‐ 10% Change in Controller Output ‐ 20% Gain = 20% / 10% = 2

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WHAT IS QUARTER DECAY RATIO (QDR)

Controller tuning is performed to adjust the manner in which a control valve (or other final control element, i.e. pump, heater etc) responds to a change in error