chapter 24 process control

7/29/2019 Chapter 24 Process Control

1/36

Handbook for Pulp &

Technologists

Chapter 24: Process Control


2/36

Introduction

All industrial processes manifest inherent variability which must be

minimized it the plant operation is to yield uniform, high-quality product

with efficient utilization of raw materials, manpower, and energy. Theobjective of process control is to maintain the process (and each element

of the process) within well-defined limits of variation.

A time plot for a process variable (shown in Figure 24-1) illustrates

acceptable control, except for one period of operation that is "out of control".

For a process under manual control, the operator watches the trend of the datapoints, and makes adjustments to keep the process within limits. But

sometimes, depending on the degree of process stability, accuracy of the data

points, and narrowness of the limit lines, the operator may only be able to take

corrective action after the variable has strayed outside the process limits.

Very few processes or process "elements" within the pulp and paper mill are

stilt under manual control. Most processing steps are automatically controlled

utilizing digital or analog equipment. Increasingly, computers are utilized to

coordinate control and record-keeping aspects for the entire manufacturing

sequence.


3/36

Introduction


4/36

I. Measurement and Control

A typical automatic control loop consists of three basic components:

sensor (with transmitter), controller, and control element, The interaction

of these components is shown schematically in Figure 24-2.An example of a control loop common to all pulp and paper operations

is the consistency control loop shown in Figure 24-3. Here the sensor

measures consistency in the stock line and transmits an appropriate

signal to the controller; the controller compares the incoming signal tothe set point and transmits an error signal to the dilution valve; the

dilution valve opening is then attenuated in the direction of correcting the

error.

Perhaps the most common control application with respect to all

process industries is temperature regulation utilizing a steam valve as the

control element. Other common applications are control of flow rate and

tank level using a flow control valve and control of gas pressure using a

pressure relief valve.


5/36



6/36



7/36

I.1 Sensor

The sensor is at the heart of any control loop. Automation cannot be properlyimplemented without a reliable indication of dependent variable behavior; andsome operations within the pulp and paper industry have only recently becomeamenable to automatic control by virtue of new measurement techniques. Somecontrol strategies must still rely on manual testing results for input into thecontrol system.

In general, few variables can be measured directly with process instrumentsand dependence must be placed on measurement of related properties. For

example, the moisture content of paper can only be measured directly bylaboratory methods; but it can be accurately inferred from measurement of suchrelated properties as:

resistance to electric current,

dielectric resistance,

absorption of microwave energy,

absorption of infrared energy.

The principles of measurement for some of the variables commonlymonitored or controlled in the pulp and paper industry are summarized in Table

24-1.


8/36

I.1 Sensor


9/36

I.2 Controller

In almost every process, certain variables must be rigidly controlled atspecified values in order to maintain efficient operation. The controllers

are given these desired values in the form of set points. For a basiccontrol loop, the set point may be keyed in at a control console, orentered by means of a set point knob on the controller itself. In eithercase, the setting corresponds to a dc electrical signal within the 4-20 mArange (or to a pneumatic signal within the 3-15 psig range).

The measured value is also converted to a 4-20 mA DC (or 3-15 psig)signal and is transmitted from the sensor to the controller. Comparison ofthe set point and input signals results in a difference, known as the error,which is the key actuating signal in the controller. The respondingcontrol action will depend on the direction and magnitude of the error,

and in some cases on how fast the error is changing. The controller actsto reduce the error and bring the actual value toward the desired value bymeans of the output signal. The output signal is generally of the sametype as the set point and input signals, namely 4-20 mA do (or 3-15

psig).


10/36

I.3 Analog model

If the Controller is to carry out its function to automatically regulate a

process variable at a desired value, the control loop must be adapted and

tuned to the characteristics of the process. Three common modes ofcontrol are used in analog systems for the majority of process

applications depending on response time and lag-lead requirements. The

respective adjustable settings are known as proportional band or gain (P),

integral or reset (I), and derivative or rate (D).

Proportional hand is defined as the range of values of the process

variable in which there is proportional control action, expressed as a

percentage of the full scale range of the controller. For example, if the

full scale range of a temperature controller is 0 to 200 C and the

proportional control action is over a 50 C range, the proportional hand is

then 25%. If the proportional control action is over a 2000 C range, the

proportional hand is then 100%. The width of the proportional band

determines the amount of valve motion for any given change in the

controlled variable.


11/36

I.3 Analog model

A process may stabilize at a point other than the set point when using a

simple proportional controller. The difference between the stabilization point

and the set point is known as offset. Sometimes the offset is small and can betolerated. When offset is undesirable, proportional plus reset (two-mode

control) is required to modify the output signal and eliminate the offset. Reset

action can be visualized as a shifting of the proportional hand. The reset

adjustment (tuning) is in the form of timing, and the reset rate is commonly

expressed as repeats per minute. Reset is proportional to the magnitude of errorand to the duration of error; its action accumulates as long as there is an error,

but ceases when the error is reduced to zero.

Occasionally, a particular process will have a dead time and transfer lag in

such a way as to cause unsatisfactory control with single or two-mode control.

It is then necessary to add the third mode of control known as derivative or rate

action. This third mode provides a continuous relationship between the rate of

change of the process variable and the output signal. It provides an initially

large over-correction to compensate for an unfavorable process lag. Derivative

time is the interval by which derivative action advances the time of proportional

action, and the derivative mode adjustment (tuning) is in a timing form.


12/36

I.4 Digital Controller

In the 1970's, instrument loops requiring the PID control function werebest served by analog equipment. Computer control techniques such as

time sharing and point sampling introduced deadtime and phase lag intothe PID function which diminished the responsiveness and stability ofthe control system. These problems were overcome in the distributedcontrol systems of the 1950's, through development of new algorithmsand nonlinear compensation methods. The majority of controllers today

are micro-processor based.Modern digital controllers provide some significant advantages over

analog systems. Firstly, the same control functions can he implementedwith less hardware and with more reliable components. Perhaps thegreatest advantage is that the control functions are not tied to hardware,

but can be configured to implement a variety of functions. Moresophisticated and better control strategies can be implemented at littlecost once the basic controller hardware is installed. Also, the highaccuracy and drift-free nature of digital control allows the user to operatehis process nearer to its limits.


13/36

I.5 Smart Sensors

In the context of microprocessor-based controllers, the term "smart

sensor" is often used. Generally a product with the designation "smart"

will have certain capabilities of self calibration, internal diagnostics,setting of parameters from remote locations. compensation for variations

in other variables, linearization, and simplified tuning of coefficients.


14/36

I.6 Programmable Logic Controller (PLC)

A programmable controller is a digitally operating electronic apparatus

that uses a programmable memory for the internal storage of instructions

that implement specific functions such as logic, sequence, timing,counting. and arithmetic to control machines and processes. This type of

controller was developed in the 1970's and has a wide application in

certain manufacturing operations. In the pulp and paper industry, these

controllers are utilized mainly for discrete logic, safety interlocks and

other sequencing operations.

While modern PLC's have the capability to control certain analog

loops, they tend to interface poorly with computers because their design

emphasis has not included algorithmic and configuration requirements.

The inability to integrate with computer software has thus far severely

limited their application to process control in the pulp and paper industry.


15/36

I.7 Feedback and Feed-Forward Control

A typical control loop utilizes feedback control. With this node of

control, the controller cannot act until an error has developed. In most

control applications, the elapsed time between controller action andfeedback signal is short, and satisfactory control is achieved.

For slow processes with long time delays, change in load or

composition may show up too late to enable the controller to hold the

controlled variable within the desired limits. By adding feed-forwardcontrol, a correction can be made for anticipate,) changes of load or

composition, thereby minimizing the transient error that would otherwise

result, Feed-forward control by itself is functionally limited, and

must be used in conjunction with a feedback loop to provide full control.


16/36

I.8 Control Valves

In the majority of control applications, an automatic valve is employed

as the control element. The control valve is essentially a variable orifice

used to regulate the flow of a process fluid in accordance with therequirements of the process. Proper selection of the valve with respect to

design, size and materials of construction is critical to satisfactory

performance and service.

Three features of control valves should be evaluated to ensure correctcontrol response: capacity, characteristic and rangeability. The

characteristic is the flow response to valve opening (percent of stem

travel) over the full range. Different valve characteristics are illustrated

in Figure 24-4. For most control applications, a linear-type response is

desired. The rangeability is the ratio of flows (high to low) through

which the control valve can give stable operation: a relatively low figure

indicates that the valve characteristic is non-linear.


17/36

I.8 Control Valves


18/36

I.8 Control Valves

It is also important to select the proper type and structure of valve to

avoid such service-related problems as corrosion, flashing, plugging,

cavitation, noise, vibration and seat leakage. Some common types ofvalves are depicted in Figure 24-5. A typical automatic flow control

valve with pneumatic actuator is shown in Figure 24-6.

The actuator on a control valve is normally an air-operated diaphragm

or piston, which translates it signal from the controller into stem or vanemovement. Springs oppose the force of air pressure to hold the plug or

vane in position against the forces of fluid flow, and act to return the

valve to a closed position when the air pressure is reduced.


19/36

I.8 Control Valves


20/36

I.8 Control Valves

9 A C C


21/36

I.9 Advanced Control Concepts

More complex control systems often use two or more measurements in

a control loop. The basic form for multiple-variable control is cascade

control. Other common systems are ratio control and cutback control.With a cascade arrangement, the objective is to improve control of the

primary variable by interlocking the primary controller with the

controller for a secondary variable which influences the value of the

primary variable, The output from the primary controller adjusts the set

point of the secondary controller, thus helping to manipulate the primary

variable. The secondary loop is typically introduced to improve control

of processes marked by upsets and long time constants, and which

cannot be satisfactorily controlled by it single three-mode controller. The

secondary variable is not controlled in the usual sense, but ismanipulated.

I 9 Ad d C l C


22/36

I.9 Advanced Control Concepts

Ratio control is used when it is desired to control one variable in a

fixed ratio to a second variable. In practice, a change in set point for a

primary controller will actuate the ratio mechanism in the secondcontroller, which adjusts the set point of the secondary variable to

maintain the fixed ratio. This control principle is commonly used for

paper mill stock proportioning systems.

Cutback (or auto selector) control is used when a single, final controlelement is to be manipulated to prevent a process variable from

exceeding a preset limit. This control mode may be provided to ensure

operator safety or protect equipment in case of process system

malfunction.

I 10 El t i P ti I t t ti


23/36

I.10 Electronic vs. Pneumatic Instrumentation

Electronic and pneumatic instrumentation systems are both widely

used, and each type provides good performance. Electronic systems

cover virtually all industrial processes. Pneumatic systems can measuremost variables, with a few exceptions such as pH and chemical

composition. In many applications, there appears to be little difference in

sensitivity and reliability between the two types of systems. However,

electronic systems have the advantage of instantaneous response; and

there is no limit to the distance that an electronic signal can be

transmitted.

Generally, costs for electronic instrumentation are higher than for the

pneumatic counterpart. For computer and microprocessor-based systems,

electronic signals are required. While a pneumatic signal can be easilyconverted to an electrical signal using a transducer (and vice versa), the

conversion imposes additional limits on control accuracy.

II P C t l C t


24/36

II. Process Control Computers

In the 1960's the digital computer was introduced into process control, but

was slow to gain acceptance in pulp and paper mills because of high capital

investment and because measuring techniques were not available to providemany of the inputs required for overall control strategy. The utilization of

computers accelerated in the 1970's when their relative cost declined markedly

at the same time that many new sensing techniques were introduced.

In the final analysis, the major reason for installing a computer is to be able to

operate the process more efficiently and more profitably. Other benefit, arethe centralized interface which increases operator awareness of the process, and

the provision of reporting documentation for management purposes.

Today's process computers and microprocessors utilize concepts of logic and

handle complex control schemes including niultiloop cascade system,

compensation for time delays, and non-linear and variable instrument

parameters. They also provide for data acquisition and display, statistical

analysis and establishment of mathematical models.

II 1 E l ti f D l t


25/36

II.1 Evolution of Development

The first computer technology available for process control was the

large main frame computer. To justify installation of this expensive unit,

extensive (and often unrealistic) project objectives were developed. Thepurchasers were often unaware of the time and effort that was necessary

to program their computer.

The earliest installations were used primarily to gather and record

process data (i.e., monitor the process and act as data loggers), set offalarms (if necessary) and make calculations. Initially, these computers

were not used to control process instruments directly, but they supplied

the operator with the information lie needed to properly adjust the set

points of various control devices.

II 2 S i Di it l C t l (SDC)


26/36

II.2 Supervisory Digital Control (SDC)

Supervisory control was a logical progressive step for a mill that was

fully automated with analog control loops. In this concept, the computer

receives the same sensor signals as the controllers and is able to providea picture of the current status of the process variables. The computer

periodically calculates (on the basis of input signals and programmed

instructions) what the optimum values should be for selected variables

(e.g., to produce maximum yield of product at the required quality level).

It then changes the set points of the controllers to bring the process

operation in line with the newly calculated conditions, Without the

computer, these calculations were handled manually, usually with less

frequency and with far less precision.

In a mill with existing controllers that are capable of accepting remotesetpoints, SDC may still be it cost-effective approach to adding it

computer control system. However, if the existing controllers must be

replaced. a distributed control system will typically be the most viable

option.

II 3 Di t Di it l C t l (DDC)


27/36

II.3 Direct Digital Control (DDC)

The next development was direct digital control where the computer

replaced the functions of the analog controllers in the control loops, and

added a level of supervisory control. With DDC, the loop concept isretained, but all loop variables feed to the computer. In each case, the

computer calculates the necessary corrective action and transmits it

directly to the control element. The major problem with DDC is total

dependency on the computer, and it is necessary to provide backup

options to ensure continuity of operation during servicing of the

computer. The most satisfactory strategy has been to employ two digital

computers, each with the full facility to control the process should a

failure occur in the other.

The popularity of DDC was short-lived, and this concept is nowseldom used. Dual computers are costly and quickly require upgrading or

the addition of more dual computers as the control and programming

tasks increase.

II 4 Mi i t


28/36

II.4 Minicomputer

With the development of smaller, lower-cost computers in the early

1970's, it was no longer necessary to put together an extensive control

project to justify, the purchase and installation of a system. Theminicomputer was economically more suited than the mainframe to

simple control strategies involving a few key loops. The special-purpose

computer along with improved sensor technology was utilized most

successfully around the paper machine. In spite of shortcomings,

packaged control systems were successful because they concentrated on

the key variables of any papermaking operation and controlled them.

However, by the late 1970's, the dedicated minicomputer approach

was already at its limits. As the mini became more powerful, functions

and features were added; and this brought to the minicomputers theproblems of complexity and unreliability formerly associated with large

computer projects, Expansions and retrofits were complicated, and inter-

system communications were difficult and costly.

II 5 Di t ib t d C t l S t (DCS)


29/36

II.5 Distributed Control Systems (DCS)

Today's preferred mode of computer control is by deployment of

numerous smaller computers in place of the large general-purpose

computer. The key to DCS is to spread computing and controllingfunctions throughout the plant. This distribution puts the controlling

"intelligence" close to the unit operations. Generally, smaller computers

or microprocessors (designed and programmed for specific tasks) are

easier to justify both technically and economically.

Many distributed systems utilize a hierarchy of controllers from single-

loop microprocessor-based controllers tip to multi-loop or interactive

microprocessor-based controllers. This ability to select the appropriate

sophistication of control technique for each specific process unit

provides for the functional distribution of control throughout the entireprocess.

II 5 Distributed Control Systems (DCS)


30/36


In a truly distributed system, the various process control units (which

may be geographically separated) are linked together on it data bus or

data highway, allowing speedy, efficient transfer of data back and forthbetween local computers (and with a central-operations computer, if

used). This communication technique usually results in, significant

reduction in field wiring casts (compared to it central-control computer)

since a single coaxial cable can link together as many as four or five

hundred single control loops. In DCS, redundant communications are

common, allowing for a back-up communications link to take over

should the primary one fail.

II 5 Distributed Control Systems (DCS)


31/36


While some weatherproof housing is required for the distributed

controllers, time environmental conditions can be significantly more

extreme than those required for the computer itself. If the computer islost for any reason, the "intelligent controllers continue to control the

process. Thus, it redundant computer does not have to be purchased.

Obviously, should a microprocessor-based controller fail, the degree of

failure in terms of the number of loops affected is dependent on the

design of the control system. Usually, those controllers on the most

critical loops of the distributed system are backed-up by secondary

units (redundant controllers).

Although a distributed system can be configured, if necessary, as it

supervisory or direct digital control system, these control concepts arenot applicable in the majority of cases because of the sophistication

available within the microprocessor-based systern at the single loop

level. Essentially, each microprocessor-based controller has all the

input, output and computing sub-systems as the computer itself.

II 6 Mill Wide Control


32/36

II.6 Mill-Wide Control

While mini-computers and microprocessors have taken over many of the

control functions, interest has re-focused on the central computer as an

information and guidance system, especially for it paper mill or integrated

operation. The objective would be to interconnect all the digital systems into a

mill-wide network to provide effective management decision support tools.

This integration of process control and decision making is sometimes referred

to as the pulp and paper industry's realization of computer-aided manufacture.

In view of the complexity of pulp and paper mill systems and the constantefforts to improve overall mill performance, a sophisticated information and

analytical system can be a valuable tool for both management and operating

personnel. Typically, the central computer (as the final element of a

"hierarchical" control structure) will utilize a high-speed printer to produce a

number of reports on daily, weekly and monthly bases. While graphic displayof an individual process (e.g., digester, lime kiln) is handled in the respective

dedicated computer, the central computer usually provides overview displays. It

also carries out material and energy balances, produces time-based curves

(current and historical), and calculates production optimization strategies.



33/36




34/36


There are many concepts of millwide control. One conceptualization is

illustrated in Figure 24-7 wherein the objectives, implementation

responsibility, and typical response times are laid out (I). The basicbuilding blocks at the bottom of the pyramid (level I) are the

instrumentation systems. Without a well-designed and maintained

instrumentation/electrical system, any attempt at millwide control would

be foolhardy. At the second level is unit process control and supervision

of quality. The third level is production and energy management, which

is usually referred to as the "rnillwide control" level. Finally, the highest

level in this particular model is production scheduling. Here, the major

objective is to schedule orders in a manner to achieve maximum

utilization of the production facilities.In spite of extensive lobbying by the proponents of mill-wide control,

relatively few mill-wide systems had been implemented in North

America by the early 1990s, perhaps because other concerns were

perceived as being more urgent of attention.

II 7 Control room


35/36

II.7 Control room

The move away from conventional analog instrumentation to computer control hasrevolutionized the control room. The most visible aspect is the video-based operatorstation. In the modern control room, the video screen (CRT) is the operator's window to

the process, wherein the logical and functional presentations of process informationpermit a rapid and efficient two-way communication. Although the information on thescreen is much the same as was formerly available on analog instruments, the methodof depiction, the amount of information, and the accuracy are all enhanced.

Typically, the operator runs the process from an air-conditioned and humidity-regulated control center. Ideally the work station should be situated in an environment

where the operator can concentrate on plant status. grid not be distracted by high foottraffic, insufficient work space, or disorganized reference materials (2). All the displaysand controls should be located at a central point within the reach and viewing range ofthe operator. A console should consist of four to eight CRT's installed in a cabinetconfigured for use by a seated operator (for example, see Figure 24-8). The console isusually semicircular in shape with one or two tiers of CRT's.

The keyboard is also a key part of the operator interface. By appropriate input,the operator changes the set points on the basic control loops, and pages throughthe different groups while sitting at the console, analogous to moving along a panelmonitoring various analog instruments. Although most keyboards are standardunits, customized keyboards may be designed for specific operator functions to

simplify the operating system and provide greater security.

II 7 Control room


36/36

II.7 Control room

chapter 24 process control

Documents