C. CHEMICAL PROCESS CONTROL INSTRUMENTATION 7. Instrumentation Systems, by P. S. Buckley and J. M. Mozley
C
CHEMICAL PROCESS CONTROL INSTRUMENTATION Chapter7
Instrumentation Systems
P. S. Buckley and J. M. Mozley
1. Trends and Limitations in Systems Engineering 2. Control Functions
3. Pneumatic Control Systems
4. Electric and Electronic Control Systems 5. Hydraulic Control Systems
6. Pneumatic Components
7. Electric and Electronic Components 8. Self·Actuated Controllers 9. Control Panels
References
1. TRENDS AND LIMITATIONS IN SYSTEMS ENGINEERING
7-01 7-03 7-10 7-17 7-18 7-18 7-60 7-75 7-79 7-81
In the process industries, as exemplified by the chemical and petroleum industries, the use of instrumentation and automatic controls has tended to follow certain typical patterns (Ref. 4). The best practice today con-sists of transmitting process data from the plant to a central control room where there are data recorders, indicators, controllers, and command de-vices for setting the level of those process variables which are to be con-trolled. From the control room command signals are sent back out to the plant to final control elements, usually control valves. Usually, the process variables are controlled separately and individually; in a plant with a high degree of automatic control this sometimes leads to interactions be-tween control systems.
This chapter has been organized and presented for use in a systems
en-7·01
7·02 CHEMICAL PROCESS CONTROL INSTRUMENTATION
gineering approach to the design of process control systems. The design methods and principles are covered in Chap. 1, Systems Design, and in Vol.
1, Part E, Feedback Control. To use these procedures, the engineer must have certain data on the static and dynamic behavior of processes as well as data on the static and dynamic characteristics of instruments and con-trol components. Equations for and data on process dynamics are now becoming available (see Ref. 16). In using the material in this section one must keep in mind certain facts about present day process control equip-ment and its applications.
1. The quantitative techniques of designing control systems developed since about 1940 for military purposes are just now beginning to pene-trate into the process industries where control system design has usually been qualitative.
2. Quantitative methods of designing control system components for specific dynamic behavior have not been widely used in the design of
typical process instruments.
3. The commonest process control instruments are pneumatic. As a result of (2), they are often less than optimum with respect to such factors as impedance matching, power supply, saturation, and dynamic nonlinearity.
4. Measurement problems are much more severe in chemical process operations than in standard electrical and mechanical operations. Highly corrosive fluids, fluids containing solids and gummy materials, high tem-peratures, and high pressures often require that measurement devices be protected from the environment whose properties they are trying to measure. Both static and dynamic accuracy may suffer, and the ques-tions of reliability and maintenance may be serious.
5. Partly as a result of (4), process control systems are almost always designed with provision for manual control in case of emergencies.
6. A process control system is usually a single variable control system (such as temperature or pressure control), and it is rare for systems to have numerically identical parameters. This means that it is hard to justify for each problem the extensive research and engineering that goes into mass-produced, identical control systems.
7. The objectives of process control system design are quite different from those of servomechanism design. In typical servo designs, perform-ance is maximized, size and weight are limited, and cost is usually not a major consideration. In typical process controlsystems, cost is minimized for a certain lower limit on performance, and size and weight are usually not important. The process control system is usually a regulator, while the servomechanism is a followup system. It should be noted that the same control system may be required to function both as a regulator and
INSTRUMENTATION SYSTEMS 7-03 as a servomechanism. The distinction is that a regulator keeps the value of the controlled variable constant in the face of disturbances, while a servomechanism makes the output of the controlled system follow the input.
In view of the above, although special purpose control systems are sometimes designed, application of the systems engineering philosophy is usually directed toward effective utilization of commercially available process instruments. By providing better power supplies, by improving the impedance match between components (see Sect. 2), and by careful attention to installation practices, it is sometimes possible to achieve phenomenal improvement in system dynamics. Often, too, a simpler, less expensive, and more readily maintainable system results.
In the sections which follow, the term "system" is usually used in the restricted sense of applying only to instrument components; the process is not included except in the early part of Sect. 2. A comprehensive dis-cussion of available components was not possible, and the ones chosen for discussion are typical only. Neither approbation nor condemnation of any manufacturer's equipment is intended or implied.
Other important aspects of process control and process control hard-ware are discussed elsewhere in this handbook. To handle most effectively the mass of data from a big refinery or chemical plant, data loggers, which include scanning, monitoring, and interlock functions, are being used to an increasing extent. These are discussed in Chap. 14, Data Proc-essing. To tie together local or individual control loops into an overall process control system, process control computers have been developed (see Chap. 13, Computer Control). These computers are making it possi-ble to optimize automatically process economics.
2. CONTROL FUNCTIONS Introduction
A simplified schematic diagram of a typical process control loop is given in Fig. 1. For purposes of clarity, none of the normally provided subsidiary features, such as manual-automatic transfer stations, fail-safe devices, safety interlocks and alarms, and indicating and recording equip-ment, have been included. The operation of the control loop may be qualitatively described as follows. When disturbances act upon the process, they cause a change in the measured variable, which has been selected to be most representative of the desired process condition. The measured variable actuates the transmitter which relays a signal repre-sentative of the magnitude of the measured variable to the controller. As indicated in Fig. 2, the controller compares the transmitted value of the
7-04 CHEMICAL PROCESS CONTROL INSTRUMENTATION
Measured variable
Set point
Disturbances
Actuating signal
Final control element
FIG. 1. Simplified process control loop.
Manipulated variable
measured variable () to the desired value of the measured variable ()s which is stored in the controller as a set-point adjustment, and produces an error signal ()E, equivalent to the difference between the transmitted and desired values of the measured variable. The error signal is operated on by the controller mechanism to produce the controller output P, an actuating signal of sufficient power to operate the final control element.
The final control element adjusts the flow of energy or material (manipu-lated variable) entering or leaving the process in the proper direction so as to force the error to zero. The functional relationships developed be-tween P and ()E by the controller mechanism are known as the control functions· or control modes. The control functions may be continuous or discontinuous.
()-~)>()---~
FIG. 2. Generalized controller block diagram.
INSTRUMENTATION SYSTEMS 7-05 Continuous Control Functions
Although the number of possible continuous control functions which might be used is very large, only three are used to any great extent in process control. These are (a) proportional action, (b) ~utomatic reset or proportional plus integral (floating) action, and (c) rate or derivative action.
Proportional Control. In the proportional control mode,. the con-t.roller output P is proportional to the control error OJi):
where K
=
proportional gain.In process control the more common expression for the proportional factor is proportional band or throttling range, defined by' the following:
Per cent proportional band = - . 100 K
The frequency response amplitude characteristic of a proportional con-troller is not perfectly flat as indicated by the defining equation above, but has some dynamic features dependent upon the particular controller design and the controller load.
Reset Action. A controller having only integral or floating action is not too common. Usually proportional and floating action are combined.
Ideal proportional plus automatic reset action may be defin~d in the time domain as
or
P = KOE
:+
KR fOE dt.Laplace transforming leads to the equation P(s)
= (K + ~R)
OE(S)= (}E(S) KR
(~S + 1)
S KR
= (}E(S) K
(~S + 1),
s(K/KR) KR
pes) KR K
- - = - (TRS
+
1) = - (TRS+
1)(}E(S) S TRS
where TR = K/ KR = Automatic reset time constant.
The frequency response plot of this idealized control function is given in Fig. 3. The effect of automatic reset is to give greater controller gain
7-06 CHEMICAL PROCESS CONTROL INSTRUMENTATION
FIG. 3. Frequency response, ideal proportional reset action.
at the low frequencies, starting at the corner frequency, l/TR, and increas-ing at a rate of 6 db/octave as frequency decreases. An increased phase lag is also associated with this action. It is common in process control to refer to the amount of automatic reset action by the magnitude of l/TR expressed as repeats/unit time (equivalent to radians/unit time).
The idealized automatic reset action described above is never actually obtained in a practical controller because of the expense involved in its mechanization. The reset action most often realized is similar in per-formance to a lag network having the transfer function
peS) TRS
+
1 desirable but cannot be avoided in certain controller designs.Rate Action. The rate or derivative control mode is never used alone in a process controller. It most commonly appears in conjunction with proportional or with proportional-reset action. Ideal proportional rate action may be characterized in the time domain by
. dBE P = KBE
+
KD - ·dt
en
FIG. 4. Frequency response, realistic proportional reset action.
By Laplace transformation,
P(s) =
K
(Kn s+ 1)
= K(TnS+
1)(}E(S) K
where Tn = KD/ K = Derivative time constant.
7-07
The frequency response plot of this idealized proportional rate control function is given in Fig. 5. The effect of derivative action is to give phase
--FIG. 5. Frequency response, ideal proportional rate action.
7-08 CHEMICAL PROCESS CONTROL INSTRUMENTATION
lead. Also associated with this desired phase lead is an inescapable in-crease in controller gain at the higher frequencies. In process control, the amount ~f derivative action is expressed as rate time in units of time, equivalent to the magnitude of Tn.
In a practical controller, it is physically impossible to achieve ideal derivative action. In fact, such action would· ren~er the controller use-less, since it w,ould amplify process "noise" (which usually predominates 'at' the higher frequencies) and would saturate the controller output.
Therefore, the practical implementation of rate action is very similar to a lead network having the transfer function
pes) [TDS
+
1 ]FIG. 6. Frequency response, realistic proportional rate action.
from 0 to 50 depending upon the controller design and sometimes upon the value of the proportional gain.
Controller Mechanisms. :Mechanization of control functions can be accomplished in a variety of different ways, electronically, pneumatically, hydraulically, mechanically, or by any combination thereof. The task of quantitatively analyzing all these specific devices, indeed even of qualitatively describing most of them, is next to impossible. Therefore, the reader is referred to the voluminous controller manufacturers' litera-ture for these specific details.
INSTRUMENTATION :SYSTEMS 7-09 However, two important types of controllers will be discussed by means of selecting examples-the electronic controller (see Sect. 7, Elec- . tric and Electronic Components) and the pneuinatic controller (see Sect.
6, Pneumatic Components). The latter is important since pneumatic con-trollers are by far the most commonly used type in the chemical and petro-leum industries. The former is important because it represents a new trend and is being applied more and more frequently in industrial process control systems.
Discontinuous Control Functions
A great many types of controllers operate in a discontinuous fashion.
In one class of discontinuous controllers, the corrective action is a discon-tinuous function of the measured variable. In this class are the off-on or two-position controllers, which are used widely in industrial process con-trol and which will be described briefly here. In another class of discon-tinuous controllers, the corrective action and/or the error sampling are discontinuous functions of tlflle. Members of this class are discussed in Chap. 12, Sampled-Data Control, and Vol. 1, Chap. 26, Sampled-Data Systems and Periodic Controllers, and will not be treated here.
The off-on controller is used primarily because of its simplicity of de-sign and construction and its correspondingly low cost. Its successful use is restricted to processes which are characterized by one predomi-nantly large first-order time constant and small dead time.
The off-on controller action is given in Fig. 7. \Vhen the controlled variable is outside the differential gap in one direction, the corrective action is maximum or on; when the controlled variable is outside the
On
~
Corrective action
~ Differential gap
Off
Measured variable
FIG. 7. Off-on controller action.
7 -10 CHEMICAL PROCESS CONTROL INSTRUMENTATION
differential gap in the other direction, the corrective action.is minimum or off. The purpose of the differential gap is to increase the switching period so as to reduce wear.
Analysis of the off-on controller alone is fruitless and must be done in conjunction with the process which it is to control. Use of analog com-puters is highly recommended for this type of problem. However, some powerful analytical methods are available where access to an analog com-puter is inconvenient. Oldenbourg and Sartorius (Ref. 9) have analyzed the off-on controller with processes having dead time and a single first-order lag, and have developed charts for predicting the period and maxi-mum amplitude of the controlled variable. Kochenburger has developed a describing function technique suitable for analysis and synthesis of off-on control systems (Ref. 8). The powerful phase plane technique, useful in analyzing systems of lower than third order, has been applied to off-on control systems by Eckman (Ref. 5).
3. PNEUMATIC CONTROL SYSTEMS
Historically, pneumatic and hydraulic devices antedated the develop-ment of electronics. For reasons of cheapness and safety (no spark hazard, no combustible hydraulic fluid), pneumatic equipment took and maintained an early lead in the petroleum industry, which until World War II was more advanced than any other process industry in its use of automatic control.
Board-Mounted Controller
The commonest arrangement of pneumatic devices in a pneumatic control system has both the process variable transmitter and the final control element in the plant with other equipment located on or behind an instrument panel (control board) in the central control room. As shown by Fig. 8, this system may be cut into three noninteracting seg-ments for testing or for system design.
Transmitter Input to Controller Input. It is assumed that the tram;-mitter input impedance is high in comparison with the signal source im-pedance. This is sometimes not true, however, as for example when a pressure or differential pressure transmitter is connected by long, small-diameter impulse lines to the signal source. Displacement type level transmitters also have a low input impedance. This necessitates con-sideration of the interaction between the process and the transmitter. The frequency response of a typical transmitter plus 250 ft of ~ -in. o.d.
(O.180-in. i.d.) tubing is shown on Fig. 9 (Ref. 4). This is valid for pressure or differential transmitters; for temperature and liquid level the frequency response of the input circuit must be added in. (Note tHat P has the units of Ib/ft2.)
2 ::3
FlO. 8. Typical pneumatic control system.
-- r-...
FlO. 9. Frequency response of typical pneumatic transmitter plus long line.7-12 CHEMICAL PROCESS CONTROL INSTRUMENTATION
Controller Inpui to Valve Input. Controller input volume and mechanical compliance are usually small, which means that the controller usually has a high input impedance. The controller output goes through the manual-automatic station and on to the valve input. By valve input is meant (1) the receiving element of a valve positioner or booster relay,
FIG. 10. Frequency response of typical pneumatic controller plus long line.
Valve Input to Valve Ste-qt Position. It is usually permissible to
0 FIG. 11. Frequency response of typical pneumatic control valve: (1) positioner only;
(2) positioner plus booster.
FIG. 12. Field-mounted controller in pneumatic control system.
7-14 CHEMICAL PROCESS CONTROL INSTRUMENTATION
instrument configuration may be cut into three segments for testing or for sys'tem design.
Transmitter Input to Controller Input. Here the transmitter is close coupled to the controller; the distance is usually less than 25 ft. A branch line, however, goes from the transmitter output back to the con-trol board. This has the effect of lowering the transmitter load imped-ance. The use of a 1: 1 pneumatic relay located in the branch line close to the transmitter output (see Fig. 13) isolates the branch, raises the
To M.-A.
station and
recorder 1: 1 relay
Process transmitter
To controller
8· /.
FIG. 13. Isolation of transmitter output line to main control board.
transmitter load impedance, and speeds up signal transmission to the controller. The frequency response of a typical transmitter plus a short line with and without a branched load is shown on Fig. 14 (Ref. 4).
Controller Input to Valve Input. The controller is close-coupled to the valve input with a branch line from the cutoff relay output back to the main control board. The controller load impedance may be raised by isolating the branch line with a 1: 1 relay (cannot be used if system ever is to be put on manual control), by using a small diameter branch line, or by putting a restriction in the branch line (see Fig. 15). The frequency response of a typical proportional controller plus a short line with and without a branched load is shown on Fig. 16 (Ref. 4). Again, the effect of adding automatic reset and derivative action is shown in Sect. 2, Control Functions.
Valve Input to Valve Stem Position. The same considerations apply here as for the board mounted controller.
Factors Affecting Pneumatic System Performance
Transmission Line Diameter. The most commonly used pneumatic transmission line is
l4
in. o.d. (0.180 in. i.d.). The use of %-in. o.d.(0.305-in. i.d.) tubing improves speed of response but sharply lowers the device load impedance. For some devices with low pilot valve capacity
0
(high internal impedance) this is equivalent to a short circuit, anddistor-tion and poor performance result.
Length of Transmission Line and Branched Loads. For reasons which are discussed earlier it is not easy to draw simple generalizations about the effects of either transmission line length or branched loads. As line length in-creases, the input impedance approaches the characteristic impedance of the line.
For line lengths of 250 ft and greater there is little change in the loading of the device driving the line. As l!ne length approaches zero, the input im-pedance approaches the line termina-tion impedance. Generally, speed of transmission is proportional to line length. See Figs. 20 and 28 in Sect. 6.
7-16 CHEMICAL PROCESS CONTROL INSTRUMENTATION without branched load; (2) with branched load.
I I I "" I I I I I I I I
Branched loads, as shown by Figs. 14 and 16, can have quite a detrimental effect on performance.
Impedance of Transmission ~ine Termination. The termination is almost always a volume~pure capacitance., For very small volumes the transmission line acts as though it is terminated in an open circuit, while for large volumes, such as the dome of a spring-and-diaphragm valve, the line may act as though it is short-circuited.
Signal Level. The Instrument Society of America and the Scientific Apparatus Manufacturers Association have standardized on pneumatic transmission systems with a range of 15 psig. Additional ranges of 3-27 psig and 6-54 psig have recently been made standard. The higher
Signal Level. The Instrument Society of America and the Scientific Apparatus Manufacturers Association have standardized on pneumatic transmission systems with a range of 15 psig. Additional ranges of 3-27 psig and 6-54 psig have recently been made standard. The higher