Automatic control in the chemical industry

AUTOMATIC CONTROL IN THE CHEMICAL INDUSTRY*By J. W. BROADHURST,t T. C. BRODRICK5f A. W. FOSTER,f and G. E. WHEELDON.f

With Appendices by A. W. FOSTER and J. ATKINSON; A. W. FOSTER; A. W. FOSTER and T A RADGE- andJ. D. TALLANTIRE.

(The paper was first received llth March, and in revised form 18/A April, 1947. It was read at the CONVENTION ON AUTOMATIC REC;I J UORS \ \ D

SERVO MECHANISMS 22nd May, 1947.)SUMMARY

An attempt is made to summarize the present position of automaticcontrol in the chemical industry.

After discussing the advantages to be gained by automatic control,it is shown that at present the chief difficulty in the chemical industrylies in the measuring element. These difficulties are discussed, andtypical examples are given.

A resume is given of basic types of control equipment at presentin use in the chemical industry. These are, in the main, pneumaticor hydraulically operated; the reasons for this are discussed.

The controlled process is briefly considered, and the lines of possiblefuture development are discussed, with particular reference to processanalysis and the design of complete electrical control systems capableof working under chemical plant conditions.

(1) INTRODUCTIONThis paper does not aim at extending the science of automatic

control but attempts to introduce the reader to some of the toolsof the trade of the instrument engineer in the chemical industryand to present some of his unsolved problems. It is fitting thatthis paper should be read before The Institution of ElectricalEngineers, among whose members are counted so many whosenames are familiar to us by virtue of their achievements inelectrical control and servo position control, particularly duringthe late war.

In the authors' opinion, which it is believed is widely sharedby those who direct and operate the chemical industry, one of themost certain ways to a greatly increased industrial efficiency liesin the development and application of automatic control.

The major purpose of the paper is to bring to the notice ofthose working in allied fields the problems which must be solvedbefore the potentialities of automatic control can be fullyexploited to the advantage of all industries. It is hoped that, bymaking the problems known, progress in solving them will beexpedited.

The paper is divided into four sections. The first indicatesthe present difficulties in the chemical industry, particularlywith reference to the primary measuring element. The secondattempts to show those who are not versed in the application ofautomatic control in the chemical industry what equipment isavailable and what can be done with it. Particular reference ismade to the pneumatic and hydraulic systems used, since it isrealized that the majority of those attending this Convention areconversant with the available electrical equipment. The thirdsection briefly considers the controlled process itself, whilst thelast section endeavours to put before this conference of electricalengineers what the chemical industry would like, and how thosewho have specialized in servo systems and allied subjects canhelp us.

No attempt is made to treat the theory of control as appliedin industry, since this can be found in the literature and isfamiliar to the majority. A brief resume of the basic principlesof available equipment has been included for the sake ofcompleteness.

• Measurements Section paper. t Imperial Chemical Industries, Ltd.

ExamplesAppendices.

of various types of controller are given in

[ 7 9 ]

(1.1) Present Position of Automatic Control in the ChemicalIndustry

The main body of chemical plant was designed at a time whenautomatic controls were comparatively trivial. To-day it isprobably true to say that, on old and new plant, each point isassessed for its suitability for automatic control, and such controlsare installed wherever a well-tried design is available. It wasnot until 1937 that the air-operated, proportional-plus-floatingcontroller began to make its appearance on plants in thiscountry.J Such controls had been in successful operationpreviously in the U.S.A. and a theoretical treatment—which isstill the fundamental basis for process control developments—had been worked out and published in England in 1936.$

Before the modern industrial controller was available, methodsof utilizing existing equipment were very often improvised in aneffort to obtain better control. It is interesting to compare thetwo methods of control described in the paper by J. D. Tallantire(see Appendix 7.4) and to note that they were designed for thetypes of controllers currently available. The example illustrates,as the author points out, the way in which control systems ha\ehad to be designed to suit available controllers.

More recently, however, more satisfactory controllers ha\ebeen available, and in the last ten years they have been installedon many plants originally designed for manual control. Mostof these installations have been successful in controlling thegiven variables within the specified limits, and by so doing haveproved economic by effecting savings in operating costs, byimproving the quality of the product or by increasing the output.Many of the installations were straightforward, but others con-fronted the instrument engineer with difficulties which requiredmuch ingenuity and patience to overcome Some of the diffi-culties remain on up-to-date plants, and will be discussed later,while others no longer occur because modern plants are designedwithatleast the knowledge thatautomaticcontrols will beemployed.

One of the difficulties in older plants is to find a suitableposition for the measuring element. Another is the absence ofsuitable locations for the controllers themselves, and frequentlyit is not possible to group them without using long instrumentlines. A scattered distribution of a number of controllers isinconvenient and time-wasting for both process and maintenance.The controlled devices, e.g. valves, cannot always be placed in thebest positions or arranged to operate in optimum conditions forefficiency, because of the layout of pipe lines, vessels, etc.

Plants are now designed to eliminate these faults as far aspossible, and among those erected within the last seven or eightyears are plants whose processes are so complex, or demandsuch close and accurate correlation of variables, that they couldnot be run without difficulty by manual control alone and are

t CLARK, W. J.: "Automatic Control of Chemical Processes," Society ot ChemimlIndustry, Proceedings of the Chemical Engineering Croup, 1936, i 8 , p. 125.

§ CALLFNDER, A., HARTREE, D. R., and PORTER, A.: "Time Lag in a ControlSystem," Philosophical Transactions oj the Royal Society, A, 1936, 235, p. 415.

BROADHURST, BRODRICK, FOSTER AND WHEELDON:

almost completely automatic. In spite of this no study has yetbeen made of the improved results which would be obtained bydesigning plant to take full advantage of automatic controls.The designer cannot yet make complete use of automatic controlto reduce the size and hence the cost of a plant, because thebasic information is not available. He does not know how muchoutput and quality could be improved; all he can do at thepresent time is to ensure that the controls installed are given thebest working conditions as indicated by present practice. Thedesign of plants continues, therefore, to be conservative, althoughthis is the result of lack of knowledge in other spheres ofchemical engineering as well as in automatic control.

(1.2) Guiding Principles in Plant ControlEnthusiasm for automatic control and protective devices

sometimes leads to a disregard of the ultimate objectives. Theseare:-

ia) Smooth running under all conditions—in particular aprocess should start up and shut down in an easy, orderlymanner and be undisturbed by changes in load.

(b) Maximum economy of power, e.g. small pressure-dropsacross the controlling valves.

(c) Simplicity both in control equipment and arrangement, e.g.if a simple self-acting regulator will control the variable within thelimits required there is no point in installing a servo-operatedsystem at ten times the capital cost.

(d) Standardization of equipment to simplify maintenance andthe provision of spares.

(<') Provision for automatic shut-down in case of an emergency,with arrangements to ensure that any failure of power, air supply,etc., operating the controllers should cause the controlled valves,etc., to move to "safe" positions.

(1.3) Advantages to be gained by Automatic ControlThe advantages of automatic control are, in general:(a) Greater productivity from available labour by making less

demand on the intelligence and physical effort of the operator.(h) A considerable amount of the monotonous or dirty part of

the process worker's duties can be eliminated by automatic con-trol, so that his conditions of working become more attractive,with consequent improvement in his well-being and resultingincrease in his efficiency.

(c) Increased efficiency of plant by increasing throughput,constancy of running conditions, and improved quality of product.

(d) The possibility of running processes which would bedifficult to control manually without a large amount of skilledlabour.

(1.4) The Elements of the Control LoopIn considering the application of an automatic controller to a

process, the complete system, consisting of the process and thecontroller, must be taken into account, i.e.,

{a) The measuring element.(/>) The control mechanism—operating on information passed

to it by (a).(c) The controlled device, e.g. a valve, operated by (b) on

information supplied by (a).(d) The process which, consequent upon the variation in (c), is

restored to balance.The system is termed an "error-actuated" or a "closed-loop"

control system, since it requires some small deviation of thecontrolled variable from the desired value or "set-point" beforeany corrective movement can be applied to the controlledmember. The process completes the loop between the controlledmember and the measuring element.

The four elements of the control loop will now be consideredin turn from the viewpoint of the chemical industry.

(2) THE MEASURING ELEMENTThe selection of a measuring element is the greatest difficulty

in applying automatic control in the chemical industry at thepresent time. The process of selection normally followed is:—

(a) To find the physical measurement which most clearlydefines the variable to be controlled.

(b) To develop apparatus of minimum complexity to measurethe variable on a standard instrument.

(c) To have the output of (/>) in such a form that it can be fedinto a standard control mechanism with suitable characteristics.

(d) To design the necessary apparatus to function reliably andcontinuously in the working conditions of the plant.

A typical problem may bring out the relevance of these points.It is desired to control the concentration of butane in propane,which is the main product of a distillation column. The prefer-ential absorption of infra-red rays of a given wavelength bybutane gas provides a suitable physical measurement of sufficientspeed, sensitivity and reliability. Instruments which will carryout the measurement and give an electrical output suitable foroperating control equipment have already been developed forlaboratory use, so that they will satisfy conditions {a), (b) and (c).Condition {d), however, will involve difficulties, such as protectingthe apparatus from harmful effects met with in plant use (tem-perature-changes, vibration, dirt or oil in the gas, overload due tomishandling of the gas sample), obtaining a truly representativegas sample from the liquid propane stream and allowing foroccasional contamination with (say) water, reducing the samplinglag to a value which allows good automatic control. Solutionof these problems demands close attention by experienced tech-nically efficient personnel, careful plant measurement andpersistent experiment.

For probably 90 % of applications, standard measuring equip-ment—although it suffers to a greater or lesser extent from theimperfections discussed below—can be employed with resultswhich are satisfactory by present standards. In general, tem-perature, pressure, flow, depth, thickness, and a considerablerange of analysis measurements are provided for by this equip-ment—but difficulties arise in special cases, as discussed in thefollowing Sections. It can be stated at once that the chiefdifficulties are due to:—

{a) Measuring lag.{b) Unsuitable design for the working conditions to be met in

the factory.(2.1) Measuring Lag

Slow response of the measuring element—usually referred to as"lag"—throws an added burden on the control equipment. It isespecially difficult to reduce in temperature measurement, and alsoin analysis where a sampling method is often the only one possible.

Measurement lag is always present to a greater or lesser extentin all process control systems, and in spite of the great deal ofwork which has been done it still remains a frequent limitation*When this is so two courses of action are open. The first is touse a more sensitive measuring instrument which will demandaction from the controller for a very small deviation from set-point. The alternative is to compensate for lag in the measuringelement by a suitable system in the controller. This leads to theintroduction of a first derivative term into the control equationand also the system of one controller resetting the control pointof a second controller as discussed in Porter's recent paper.*

(2.2) Unsuitable Design of Measuring Elements for WorkingConditions

It often happens that the measuring element which wouldnormally be chosen for a given application cannot be used,

* "Basic Principles of Automatic Control Systems," read before The Institution ofMechanical Engineers 7th February, 1947.

AUTOMATIC CONTROL IN THE CHEMICAL INDUSTRY 81

owing to the particular plant working conditions which have tobe met.

The most usual problems are set by corrosion, the necessity forrobustness with sensitivity and low lag at the same time, andextraneous interference from changes in ambient temperature,breakdown of insulation due to surface films in humid and acidatmospheres or electrical disturbances. In addition, the dimen-sions of the element are often too large to allow of its use.Numerous specific examples of such difficulties could be quoted byany plant instrument engineer. /

The most interesting example to the electrical engineer isprobably that of pH measurement in which it is often necessaryto use a glass electrode—with a resistance of order 100 megohmsand an output of order 50 mV per pH unit. Apart from theelectrode's fragility, errors introduced by large concentrations ofsodium and potassium ions, and the difficulty of using it athigher temperatures, the problem of leading the output of theelectrode to the instrument without leakage, and thereafteramplifying it, is constantly present where the plant atmosphereis unfavourable.

(2.3) No Suitable Physical Variable Available as Basis for ControlThe worst problem is presented by a process in which no

physical variable can be found on which the process can becontrolled. As an example, the problem usually arises when it isdesired to convert a batch process to a continuous process. Ifthe original batch process has been run on the results of chemicalanalysis carried out during the batch to determine the progressof the reaction, when the decision is made to convert to a con-tinuous process it becomes necessary to replace the chemicalanalyses by some representative physical measurement which canbe made continuously. Sometimes electrical conductivity, pH ordensity is of value as the physical variable, but cases do occurwhere the normal plant control is by purely chemical tests whichhave not at present been adapted to a continuous form ofmeasurement.

(2.4) Difficulties in Measurement of the Required VariableThe versatility of control instruments is such that there are

relatively few processes which cannot be controlled within limits,provided the variables are measurable. It is inevitable, therefore,that the usual difficulty encountered is with the measuring ele-ments. Typical difficulties which arise are enumerated below.

(2.4.1) Flow.Although the technique of flow measurement is so advanced,

there are still some unsolved and difficult problems. It is still notpossible to measure the flow of a boiling liquid or a liquid whichis releasing entrained gases, and there is no satisfactory means ofmeasuring the flow of powdered materials, such as pulverizedfuels, along pipe lines except on those rare occasions where atotalizing type of weighing machine can be used.

Pulsating flows also present difficulties which are only nowbeing overcome for low flow rates by the use of special types ofrotameters.

(2.4.2) Temperature.The difficulty of thermometric lag has already been dealt with

in Section 2.1. Determination of the temperature of smallflows of gas presents difficulty, owing to the necessity of obtainingmeasuring elements of low thermal capacity compared with thatof the gas flow.

Another typical problem is the measurement of the reactiontemperature of catalytic-vapour phase reactions in which thereacting gases pass through a nest of tubes filled with pelletedcatalyst. A reaction vessel may have several hundred such tubes,and it is obviously not practical to insert sheaths in all. When

VOL. 94, PART IIA.

the tubes are small (say Hin bore or less) with respect to theminimum outside diameter (about 0-1 in) of a sheath enclosing afine thermocouple, the insertion of such a sheath axially downthe tube can so disturb the flow of gas through the tube as to makethe temperature non-representative of the remaining tubes.Again, the heat exchange between thermocouple sheath andcatalyst or gas varies along the sheath as "hot spots" develop inthe tubes, and this leads to considerable inaccuracies in reading.

(2.4.3) Level.The measurement of liquid levels frequently involves the

measurement of small pressure differences at high static pressures,and this can, and does, involve difficulties. Probably, however,the measurement of the level of solid materials in bins is themost outstanding problem.

(2.4.4) Analysis.The most frequent variables used in analysis measurements are

thermal conductivity and absorption of light in the visible, ultra-violet or infra-red parts of the spectrum. The question ofmeasurement lag on such systems has already been referred toin Section 2.1. An additional difficulty is the deposition of dirton measuring filaments and sight glasses, which gives rise to errors-that are difficult to compensate for or eliminate.

The question of controlling the acidity or alkalinity of asolution by means of its pH-value or hydrogen ion concentrationgives rise to problems, particularly when measurements have tobe taken in the highly alkaline region near pH 14 in the presenceof large concentrations of sodium salts. It is difficult to get ameasuring element which will stand up continuously to suchconditions, particularly if, as in batch processes, the element isexposed at times to high acidities as well as high alkalinities

(2.5) Conditions of Measurement on the PlantExternal corrosion and dirt caused by chemical plant conditions

have always been a source of difficulty, particularly where lightinstrument mechanisms are concerned. Improvements in plantdesign are effectively reducing this trouble, whilst the incor-poration of transmitting devices enables the more vulnerablepart of the mechanisms to be grouped in comparatively cleanareas or in air-conditioned control rooms, where adequatecleaning and lighting is possible. Even so, it is desirable thatall standard instruments should be constructed of materialswhich do not readily corrode or are well protected by enamel orplating. Corrosive atmospheres cause trouble chiefly by attack-ing instrument pivots, by causing electrical leakages in high-resistance circuits and by causing faulty contacts. It is extremelydifficult to seal completely any instrument case so that breathingdue to changes in ambient temperature will not give trouble;the only remedy is to purge the case with dry air.

The necessity for flameproofing equipment in areas whereinflammable or explosive materials may be present can beeliminated to some extent by suitably locating the equipmentoutside the danger areas—or in air-conditioned control rooms asmentioned in the previous paragraph. It is clearly in the interestsof those concerned with the development of electrical measuringand transmitting mechanisms to provide compact and lightflameproof equipment.

(2.6) The Linkage between Measuring System and ControllerHaving measured the required variable, it is essential that the

output of the measuring system should be in such a form thatit can be fed into a standard controller. Thus, to take anextreme example, if it is required to control a surface temperatureit is no good proposing the use of a disappearing-filamentpyrometer unless automatic setting and transmission of thereading to the controller is available.

6

82 BROADHURST, BRODRICK, FOSTER AND WHEELDON:

Any measuring system employed should be such that itdevelops sufficient operating force to operate a controller eitherdirectly or through a simple servo system. Thus a measuringsystem which has sufficient power to move an indicating pointermay have insufficient power to operate a desired controllingmechanism, e.g. a microammeter cannot be used to operate a"flapper and nozzle" control system directly.

(3) THE CONTROL MECHANISMThe chemical industry uses hydraulic, pneumatic and electrical

types of control equipment as well as the direct-operated forms.Influenced largely by the preponderant use of pneumatic controlin the oil industry, the pneumatic designs have made most head-way in recent years. Unhampered by flameproofing problems,with adequate power to operate all except very large or high-pressure valves, with fittings which can be neatly and safely builtinto standard panels, pneumatic controllers have proved capableof meeting most control requirements.

Hydraulic mechanisms have not developed so rapidly, andtheir use is chiefly confined to boiler control and to the operationof large valves or dampers and certain classes of heavy machines.Designs employing oil as the control medium have been foundparticularly reliable and easy to maintain.

The chemical industry has largely confined its use of theelectrical controller to plant such as electrically-heated furnacesand electrically-driven machinery, leaving pneumatic apparatusto gain the popular place in control of flow, pressure and, to alarge extent, process temperatures.

The types of control mechanism which are available will nowbe considered, and particular reference will be made to pneumaticand hydraulic types. A great deal of confusion is caused to thenewcomer to the field of control by the diversity of types ofcontrol mechanism which are available and the various tradenames under which they are advertised. A good example atthe present is the type of controller which includes a controlterm directly proportional to the rate of change of the controlledvariable. These controllers are advertised under such apparentlyunrelated terms as "anticipatory control," "pre-act control,"and "derivative control."

All controllers at present in use in the chemical industryemploy one or more of the following control terms:—

(a) Multiposition control, including simple "on/off."(b) Proportional control.(c) Floating control.(d) First-derivative control.

(3.1) Multiposition Control

[n two-position control, the control system causes the valveto take up one of two positions according to whether themeasured variable is above or below the set-point. In oneposition of the controlled member the corrective action isgreater, and in the other position less, than is required to main-tain the measured variable at the set-point, and the systemtherefore hunts between the conditions of slightly too much andslightly too little corrective action. On/off control is a specifictype of two-position control in which the valve, for example, iseither full open or shut. The amplitude of the oscillation, orhunt, resulting from two-position control can be reduced byincreasing the number of positions which the valve can assume;hence the use of multiposition control. Theoretically, two-position control can be regarded as a particular case of propor-tional control (see Section 3.2), in which the proportional band isinfinitely small. Considering the case of temperature control,two-position control is applied to processes where the heating orcooling rate is small compared with the thermal capacity of thesystem.

(3.1.1) Multiposition Control Mechanisms.Using electrically-operated control valves, two-position control

is easily obtained by arranging for the pointer of the measuringinstrument to operate a contact or tr ip switch at the requiredset-point.

The corresponding systems using air, water or oil as the controlmedia are shown in Figs. 1 and 2. The principle of operation isthat air is fed through restrictor R to jet J ; so long as the jet isuncovered the pressure in the valve diaphragm chamber D ,Fig. l(a), or the capsule C, Fig. 2, is low. The pointer of themeasuring instrument is connected to a flapper F . When theset-point is reached, F restricts the flow of air from the jet and sosets u p a back-pressure in D or C.

Thus in Fig. 1 the effect would be to open the controlled valve,whilst in Fig. 2 the small pilot-valve relay P would first operateand then, in the case illustrated, the controlled valve would closeowing to the air pressure being released from its diaphragmchamber.

o measuring)j system j

Fig. 1.—Simple open-shut mechanism.

[TJnlsage frora| '•! measuring[_ system...

T to 1 Kf t i rc o n t r o l l e d ! $ j J P

I vjilve ! I

Position of pilot valve whenflapper is in position Fl

Fig. 2.—Open/shut mechanism using a pilot valve.

AUTOMATIC CONTROL IN THE CHEMICAL INDUSTRY S3

In the above a fixed nozzle has been shown with a movableflapper to restrict the air leakage from the nozzle. Commercialcontrollers usually employ either this system or one in which ablade comes between two opposing nozzles as shown in Fig. 1 (b).This latter system has the advantage of putting less loading onthe flapper and hence on the movement of the measuring device.

(3.2) Proportional Control

With this form of control the valve opening is proportional tothe deviation of the controlled variable from the set-point andhence the system can be represented by the equation

Movable fulcrum to—7

where 6 denotes the valve position; k2 the displacement of thevalve from the normal position per unit of error; e the error ordeviation from set-point; 90 the valve position which gave controlunder initial load conditions.

The deviation from set-point necessary to cause full movementof the valve is termed the "proportional band" or "throttlingrange" of the controller. It is usually expressed as a percentageof the chart width or measuring range of the instrument. k2 isinversely proportional to the throttling range. Once the pro-portional or throttling band has been set for a given process itshould not require further adjustment unless the method ofrunning the process is seriously altered, but k2 must be adjustableso that it can be set initially to suit the process. Thus oneprocess might require 6 to go from its minimum to its maximumvalue for a change of e corresponding to 10% of the chartrange, whereas another application would require a change of ecorresponding to 200% of the chart range, i.e. movement of thepointer of the measuring instrument from zero to full scale wouldcause a change in valve position corresponding to only one-halfits maximum travel.

At this stage it is instructive to consider what happens in aprocess using a proportional-control system when a load changeoccurs. For example, if a proportional-control system is con-trolling the temperature of water flowing through a tank byoperating a valve controlling a steam supply, and the flow ofcold water to the tank is suddenly increased, then the steam inputto heat the water must also be increased. But from eqn. (1) thevalve opening at any time is dictated by the deviation of thewater temperature from set-point at that time. Therefore theonly way in which the valve opening can be increased to providefor greater flow of steam is for the controller to settle out at anew temperature below the fixed point. The amount of thisdeviation from the control point is called "droop" and will bereferred to again in Section 3.4. The effect is illustrated inFig. 8(a), and it should be noted that for a given load changethe wider the throttling range the greater is the droop.

(3.2.1) Proportional-Control Mechanisms—Low Input Power.The following mechanisms are used in controllers where only

a relatively small load can be imposed on the measuring system.In this category come controllers having as their primarymeasuring elements such systems as vapour-tension thermo-meters and low-range pressure gauges. Use is made of the factthat with the systems shown in Figs. 1 and 2 the flapper does notsuddenly cut off the flow of air or liquid from the jet, but passesthrough a region where the back-pressure is a function of thedistance between flapper and jet. A region of linearity exists ofthe order of 0-001 in, and various mechanisms are employed bymanufacturers to increase the effective width of this region so asto obtain a proportional band of the required width.

The simplest method of achieving this amplification of theproportional band of the nozzle flapper assembly is shown inFig. 3. The flapper is connected through a lever system having

vary the mechanicaladvantage

: Linkage frommeasuring

L_ system

Air

supply

JTb controlled!valve

Fig. 3.—Simple proportional-control mechanism.

a large and adjustable mechanical advantage, instead of directlyto the indicating pointer, so that a large movement of the pointeris required to move the flapper through its proportional band.This system is satisfactory in theory, but in practice it is difficultto avoid lost motion due to slight play in the connectingleakages.

Using the opposed-nozzle system, an alternative to a leverageof large mechanical advantage can be obtained by the use of ashaped vane (see Fig. 4). The arc of the vane is cut to have the

Linkage frommeasuring

[_system_

Fig. 4.—Proportional-control mechanism using a shaped vane.

same radius as the distance from the nozzle to the axis of rotationAA of the nozzles. The vane is pivoted on axis BB. Thus fora given measuring-pointer movement the amount of restrictionto the air escaping from the nozzle varies according to theposition of the nozzles. If the nozzles are set at a position nearto C or D, a small pointer movement causes a large valve travel,whilst if the nozzles are set at a position near axis BB the samepointer movement causes a very small valve travel.

A method of more general use to avoid levers of large mech-anical advantage, and one which has decided advantages whenused in conjunction with floating and first-derivative controlsystems, employs a feedback system (see Fig. 5). The jet,instead of being fixed, is connected to a bellows B which isopposed by a spring S, so that the extension of the bellows is

Movahle platformCo-axial pilots •

Linkage from jmeasuring •

system I

L J - 1 -^Fixed- / [To controlled]

[_ valve _|Fig. 5.—Proportional-control mechanism using mechanical feedback.

BROADHURST, BRODR1CK, FOSTER AND WHEELDON:

proportional to the air pressure inside the bellows. The airpressure fed to the controlled valve is also fed back to bellows B.The linkage between bellows and flapper is so arranged that asthe flapper approaches the jet the bellows introduces an opposingmotion which causes the jet to recede from the flapper. The netresult is that a large indicating-pointer movement is required totake the valve over its full travel. The variation in proportionalband is achieved by making the linkage between jet and bellowsof adjustable mechanical advantage. Since the maximum move-ment of the bellows is appreciable (or the order of 4 in) loosenessin the connecting linkages, whilst still not desirable, is of far lessconsequence than when a flapper movement of a fraction of athousandth of an inch is being magnified.

(3.2.2) Proportional-Control Mechanisms—High Input Power.There is a further class of mechanism which is used when an

appreciable load can be imposed on the measuring system,without causing errors. In this class are controllers having astheir primary measuring elements pressure gauges with largeavailable positioning power and instruments employing for theiroperation subsidiary servo systems, e.g. self-balancing potentio-meters. Since more positioning power is available, the jet/flapperassembly is replaced by a small pilot valve having an output pres-sure characteristic which varies linearly with the valve lift. Themeasuring element is made to rotate a shaped cam (see Fig. 6); the

!~ Cam rotated by j[ measuring system j Air

fib controlled!! valve !

supply

Fig. 6-Proportional-control mechanism: high input power.

variation in lift of the cam, which may be as much as 1 in, istransmitted through a lever system of variable mechanicaladvantage to the pilot valve. Owing to the greater poweravailable, this type of system lends itself to more robustconstruction.

Before leaving thequestion of proportional-control mechanismsit should be pointed out that the normal spring-loaded air- orsteam-reducing valve is in effect a self-actuated regulator using afixed proportional band. A deviation of the downstream pres-sure from the required value causes a diaphragm to move againsta loading spring and so adjust the position of the valve seat (seeFig. 7). In this category can also be included those so-calledself-operating temperature regulators which use the expansionor the vapour pressure of a liquid to position the valve seat.

(3.3) Floating Control

With floating control the position of the controlled valve bearsno relation to the value of the controlled variable. As soon asthe controlled variable departs from the required set-point thevalve moves in such a direction as to reduce the error. Ingeneral the speed of valve movement is low and is such that thevalve will not have reached the limit of its travel before the errorhas been corrected and the valve movement stopped. The valvespeed can be constant, two-speed, or proportional to the devia-tion of the variable from the set-point. Mathematically :—

Ct (2)

(3)

Constant-speed floating control: 6 6Q

Proportional-speed floating control: tlti/dt kle

where / is the time that the system is away from the set point;ATJ the rate of valve opening per unit error, or deviation, fromset-point.

Equation (3) gives, on integration,

(4)

where 6'0 is the constant of integration and represents the initialvalve position when the controlled variable was at the set-point.

From (4) we see that the valve opening is proportional to theintegral with respect to time of the deviation from set-point,hence this form of control is also termed "integrated errorcontrol."

Time

(6)

A S~\ -

V \y

Fij>. 7.—Self-actuated regulator.

Fig. 8.—Process response curves.(o) Proportional control.(b) Proportional and floating control (optimum setting).(c) Proportional and floating (non-optimum setting).(d) Proportional and floating and derivative (hunting damped out).

The advantage of floating control over proportional control isthat it always brings the controlled variable back to the set-point,provided the load change is within the capacity of the controlledvalve. With normal plant lags the valve should be made tomove slowly, i.e. the floating rate should be low, in order toprevent overshooting. This type of controller is useful whereload changes occur, provided these are slow changes. It is oftenused for pressure control, and for systems where the lag is small.


(3.3.1) Floating-Control Mechanism.Constant-speed floating control can be achieved electrically by

high and low contacts on the dial of the measuring instrument,such that when the high contact is made the valve closes slowlyat a constant speed, whilst when the low contact is made thevalve opens slowly at a constant speed (or vice versa). Thecontacts are separated slightly so that a dead zone exists betweenthem. As soon as one or other contact is broken, movementof the valve is arrested and the valve position fixed until thecontrolled variable travels through the dead zone and makes theother contact. A dead zone is usually introduced to avoidcontinuous hunting of the controlled variable.

Two-speed floating control uses one valve speed when thedeviation from control point is small, and a higher speed whenthe deviation is large. It can be achieved by a system similar tothe single-speed system, but using two high and two low contacts.

A proportional-speed floating control is shown in Fig. 9. Here

Pivot

[Mechanical linkqge to!•ontrollud valve

jCoonection to measuring!j syatem_ I

Fig. 9.—Floating-control mechanism.

oil under pressure is used to operate the valve via a jet-and-pistonarrangement. When the controlled variable is at its requiredvalue the jet is in its mid-position and the controlled valve isstationary. When an error in the variable occurs, the jet isdisplaced slightly to one side of the centre division, thus increasingthe pressure on one side of the piston, so moving the piston andhence the controlled valve. As the error increases, so the jetmoves further across the centre division, thus increasing thepressure difference across the piston and increasing the speed ofvalve movement. With this system there is no dead zone, as thejet plays equally upon both sides of the centre division when thereis no error in the controlled variable.

(3.4) Proportional-pIus-Floating ControlIn Section 3.2 reference was made to the "droop" which occurs

with a proportional controller following a load change. If nowa floating control action is combined with proportional control,then so long as droop is present a rate of valve movement willoccur which is proportional to the droop. Once the set-point isregained this component vanishes. As stated in Section 3.3, thefloating rate should be low to prevent overshooting. In practice,when combined with a proportional controller the adjustment ofthe floating term is not critical, provided the floating rate issufficiently low to prevent its causing hunting. If it is set lowerthan this optimum value it merely means that the controlledvariable takes longer to regain the set-point. Higher floatingrates may be employed in conjunction with proportional controlwithout causing hunting, than with floating control alone.

The equation for proportional-plus-floating control is obtainedby combining equations (1) and (4) thus:—

Curves comparable with those in Fig. 8(0) are given in Fig. 8{b)for a proportional-plus-floating controller when subjected to theunit-function load change.

The easiest way to adjust such a controller to a given processis to start with the lowest floating rate and to decrease theproportional band until damped oscillations of the controlledvariable are obtained; then to increase the floating rate until theoptimum recovery curve is obtained.

Proportional-plus-floating controllers are frequently referredto by manufacturers as proportional controllers with automaticreset, as opposed to straight proportional controllers where thecontrol point has to be "reset" manually to eliminate the droopconsequent upon load change. The use of the term "automaticreset" in this connection is to be deprecated, since it leads to aconfused terminology.

(3.4.1) Proportional-plus-Floating Control Mechanisms.All pneumatically-operated proportional-control systems apply

to the controlled valve a pressure which is proportional to thedeviation of the controlled variable from the set-point. If nowthis pressure is also fed, through a restriction, to a bellows whoseelongation is opposed by a spring, then the rate of increase ofpressure in this bellows, and hence its rate of movement, isproportional to the deviation of the controlled variable from therequired set-point. (This statement is strictly correct only whenthe increase in volume of the bellows is small compared with itstotal volume.) If this bellows is made to act on a pilot valve, ornozzle flapper assembly, a rate of change of valve movementand hence a rate of change of valve position can be obtainedwhich is proportional to the deviation of the controlled variablefrom the set-point. Fig. 10 shows a method of applying a

sTo controlled! Secondaryi valve ! pilot valvel/'" >̂

Air

j Proportionalj controller j[_as Figs 3 - 6 j

"1

d d0 -- krfedt - k2e (5)

Supply

Fig. 10.—Combination of floating and proportional control, usingpneumatic balance.

floating component to any proportional controller by the uso ofa pneumatic balance. Assume that the air pressure applied toboth the balance and the proportioning controller is 2.v lb/in»,then the proportional controller is arranged to give an outputof x Ib/in2 when at control point. Then bellows A and D holdthe arm in balance, since D has half the leverage of A. Atequilibrium the beam can remain in balance for any secondarypilot-valve pressure, since B and C are at the same pressure andhave equal moments. Suppose, now, that a load change takesplace. Let the pressure in A increase. The balance deflects andthe secondary pilot valve readjusts itself until the pressure in B


equals that at A. This is only a transient state, however, sincethe pressure in B begins to leak through the needle valve into Cso as to equalize the pressures in B and C. Therefore thesecondary pilot moves to increase still further the pressureapplied to the controlled valve and to B. Finally, when thesystem has settled to equilibrium at the new load, the value of thecontrolled variable is again at the set-point, the pressure at Ahas returned to .v lb, while the pressures in B and C are equalbut at some higher pressure than that which they had before theload change.

With this arrangement, the needle valve is the restrictor andC' the bellows, which add to the valve movement the integratederror term described above. The value of the constant k{ isvaried to suit process conditions by varying the opening of theneedle valve.

The above is probably the easiest method to understand ofapplying the floating component. Different manufacturersapply different variations; thus the bellows might be replaced byBourdon tube elements, or the restrictor valve by interchangeablecoils of capillary tubing. Instead of using two pilot valves asystem similar to that shown in Fig. 5 might be used but, insteadof the proportional bellows B linking directly back on to theno/zle, this bellows is opposed by the floating-term bellows Cand the difference in pressure linked back on to the flapper—inthe same way as the difference in pressure between B and C inFig. 10 is linked to the secondary pilot valve. This system isshown in Fig. 1 1A. Either capillary tubing, as shown in Fig. 1 1A,or a restrictor valve, as shown in Fig. 11B, can be used for therestrictor. Fig. 11B also shows the bellows for the floating

Movable platform

B - '["Linkage from"!

measuring Isystem j

— r'ixcd —' JTo controlledj valve

Fij?. 11 A. Combination of floating and proportional control, usingmechanical feedback: system A.

Connection to nozzle vialinkage of adjustable

mechanical advantage

Air from primary pilot valve

Fin- l l». -Combination of floating and proportional control, usingmechanical feedback: system B.

component mounted slightly differently in relation to the pro-portioning bellows B. Longer delay terms are sometimesintroduced by putting a capacity tank in parallel with the floating-component bellows.

A further variation is shown in Fig. 1 lc. Instead of feedingthe proportional pressure through a restrictor into the floating-component bellows, the proportional pressure is applied to

Air•To controlledI valve j

Fig. lie.—Combination of floating and proportional control, usingmechanical feedback: system C.

chamber B and so compresses a subsidiary bellows D connectedto a further bellows E which acts back on the nozzle. The float-ing component is then added by the leak valve which allowsthe pressure in D and E to vent to atmosphere. Thus at equili-brium the pressure in D and E is atmospheric, although it maybe above or below this pressure when load changes are occurring.

A system employing oil, instead of air, as the operative fluidfor the floating component is shown in Fig. 1 ID. The oil system

Air from primary _

pilot valve

Fig. 11D.-

i measuring system!

Connection to flapper via ilinkage of adjustable •

! _mechanical_ advantage j

-Combination of floating and proportional control, usingmechanical feedback: system D.

is filled during manufacture and then sealed. The advantages ofsuch a system are that the restriction has not to be made so small,and small leaks in the system are more easily detected than whenusing air as the operative fluid. One disadvantage is that thefloating rate depends on the viscosity of the oil, and therefore itwill vary with the temperature of the bellows system. It shouldbe noted that a fixed nozzle is employed, and both the primarymeasuring element and the control bellows act on the flapper.

(3.5) First-Derivative Control

The control function most recently applied in commercialpneumatic controllers is first-derivative control, which gives anopposing valve displacement proportional to the rate of changeof the controlled variable from the set-point. It cannot be usedalone, since it would permit the controlled variable to balanceout at any steady value. The equation for first-derivative con-trol is

6 :--• k3tieldt (6)

where k3 is the displacement of the valve per unit rate of changeof error.

(3.6) Proportional-pIus-Floating-plus-First-Derivative ControlIf a process is subject to wide load changes and considerable

measuring lag, then proportional-plus-floating control may beunable to prevent considerable departure from the set-pointfollowed by either a very slow return to set-point or a rapidreturn with considerable hunting. If a first-derivative control


term is added, then immediately e changes at a finite rate,corrective action is applied to reduce this rate. Similar actionof this term slows up return to the set-point after a disturbance.

The control equation is, combining (5) and (6),

krfedt - k2e - k3dd/dt (7)

This damping action is utilized by adjusting kx and k2 so thatacting alone they would produce a rapid return although withsubsequent excessive hunting. The addition of the /r3 termdamps this hunting with the effect shown in Fig. 8(c).

The addition of the third term:—•(a) Reduces the initial departure from set-point.(b) Enables the overall period of oscillation to be reduced.(c) Speeds up the return to steady conditions.If excessive first derivative component is used the system will

approach the set point too slowly after a load change, if insuffi-cient is applied overshooting of the set point will result.

(3.6.1) Proportional-plus-Floating-plus-First-Derivative ControlMechanisms.

When considering a proportional-plus-floating system it wasshown that if capacity C (see Fig. 10) was fed from capacity Bvia a restriction, then if a pressure change p corresponding toan error e occurs in B then the rate of increase in pressure in Cis proportional to p and thus the movement of C correspondsto JW/. Mathematically the bellows C can be regarded asintegrating the pressure changes in B, or conversely we canconsider bellows B as supplying the differential of the pressurevariations in C. If, therefore, we consider air fed from thepilot valve into a bellows "a," thence via a restrictor R1 intobellows "b" and finally via another restrictor R2 into bellows"c,"as shown in Fig. 12, and if further we obtain our proportional

Air frompilot valve

Fig. 12

component by virtue of the extension, or contraction, of "b,"then the movements of "a" can be considered as proportional tothe differential of the pressure changes in "b," whilst the move-ment of "c" is proportional to the integral of the pressure changesin "b." It is assumed that the volumes of the bellows are largecompared with their changes in volume.

A simple method of applying a rate-of-change component to afloating-plus-proportional controller is shown in Fig. I3(a). Inthis case the bellows "a" of Fig. 12 is omitted, use being madeof the fact that, owing to the presence of R,, the diaphragmchamber of thccontrol valve itself acts as bellows "a." As aresult the valve gets a displacement proportional to the rate ofchange of the process variable. Alternatively, one can look uponrestrictor Rt as holding up the supply of air to the proportioningbellows, so that the pressure in the control-valve diaphragm willbe built up more quickly than if R, were not present. Fig. 13(6)shows a similar system with a slightly different arrangement ofbellows.

As an alternative to Fig. 13 a system has been developed byFarrington* utilizing a pneumatic-balance principle similar toFig. 10 to obtain the rate-of-change component. To avoid theuse of very small restrictors, a second pneumatic balance amplifiesthe rate-of-change component and recombines it with the pro-portional component. The mechanical arrangement of this set-up is naturally more complicated, and the reader is referred tothe original paper for further details.

* British Patent No. 568634.

j Connection to nozzle via jlinkage of adjustable

i mechanical advantage

Air supply from primary pilot valve

from primarypilot valve <

,. j zrr[Connection tonoz/leviai

Fig. 13.

linkage of adjustable ! (6)• mechanical advantage I

-Combination of floating, proportional and first-derivativecontrol, using mechanical feedback.

(3.7) The Controlled Valve

The first essential for a controlled valve is that the design ofthe valve body should be in conformity with the rest of theplant both in respect to its maximum working pressure and in itsresistance to chemical attack. It is inevitable that specialdesigns and materials must be demanded, yet a good measure ofstandardization is possible.

The commonest types are probably of the shaped-plug orshaped-skirt designs, which are normally double-seated and givea semi-logarithmic characteristic between the valve position andthe flow through the valve. These valves do not give a tightshut-off; where this is required, in the smaller sizes, a single-seated shaped needle valve is usually employed. For the controlof low-pressure gas and air flows butterfly valves with one ormore louvres are used. These valves have poor controlcharacteristics.

In the chemical industry the nature of the fluids to be controlledoften causes difficulty in the design of the controlled valve. Oneof the most frequent difficulties encountered is in the design of atight yet low-friction gland for the valve stem, and to overcome itspecial bellows-type valves have been developed. In these thecorrosive fluid is prevented from leaking by a bellows sealed atone end to the valve plug and at the other to the valve body.Thus the valve stem is capable of movement without the frictionintroduced by a tightly packed gland.

In the control of pH and other variables, especially on smallexperimental plants, it is sometimes required to control theaddition of corrosive fluids at low rates. The provision of asuitable control valve with the small clearances necessary isdifficult, and blockages are probable. This problem has beenmet by taking advantage of the low ifficbncy of an air lift. Theair lift is used to make the liquid addition, and hence the bore ofthe tube employed can be considerably greater than the clearancesrequired in a control valve of equivalent capacity. Control isaccomplished by adjusting the air supply to the lift.

In specifying a control valve it is usual to state what the valveshould do on failure of the electricity or the air supply to thecontroller. The most usual conditions are that the valve should


either close or open fully, but cases do arise, particularly indistillation processes, where it is better that the valve shouldremain locked in the position it occupies at the time of failure.Similarly, means are sometimes provided for emergency opera-tion of the valve, either manually or from an auxiliary powersupply.

(3.7.1) Valve-Operating Mechanisms.The operat ing medium for a controlled valve is usually,

al though not invariably, determined by tha t of its controller.Automat ic regulators of fixed propor t ional band employ suchmethods as the expansion of a solid or liquid to position thevalve nose. Others use the vapour pressure of the thermometricfluid applied to a bellows, whilst pressure regulators often usethe pressure exerted by the fluid in the main pipe line to positionthe valve (see Fig. 7).

Electrical controllers may be of the solenoid- or motor-operated type. The first operates the valve either directly in thesmaller sizes of valve, or in the larger sizes through a pilot valve,which uses the pipe-line pressure t o operate the main valve.Such valves find limited application, since they can be appliedonly to two-position control , and then only on very clean fluids,since the valves are prone to sticking.

The second type uses a moto r to operate the valve eitherthrough a reduction gear or via a small centrifugal oil p u m p whichpositions the valve by a piston-and-cylinder arrangement.Motor-operated valves can be used for proport ional control .

Pneumatic controllers apply air pressure to a diaphragmchamber mounted on the valve body. Increase in air pressurecauses the diaphragm to expand against a spring and thus movethe valve spindle. As the size of valve increases, so also doesthe size of diaphragm, so that positioning forces of the order ofseveral hundred pounds are applied to large valves. An alter-native method using compressed air, or oil under pressure,incorporates a piston-and-cylinder system. This method gives alonger valve stroke and has the added advantage that the sameforce is available at all times during the stroke.

(3.7.2) Valve Positioners.The available force to move a controlled valve when the output

air pressure of the controller changes by a small amount is oftenrelatively small in comparison with the frictional forces actingon the valve. Where small movements of the valve are im-portant , for example on controllers having large proport ionalbands or using long delay times in the floating term, valvepositioners are used. They are also employed when the con-troller and controlled valve are some distance apart , to eliminatethe time-lag in producing the required pressure change on thevalve diaphragm.

These units are pure positioning servos. Their function is tosee that the valve takes up accurately the position dictated by theoutput air pressure of the controller and thus eliminate theeffects of sticking and hysteresis, that may be caused by a tightlypacked gland, in the controlled valve.

The basic mechanism is shown in Fig. 14. The output airpressure from the controller is fed into a small bellows B whichis opposed by a spring S, so that the bellows has a linear exten-sion/pressure characteristic. A beam is pivoted between thebellows and the valve stem. If the pressure in B is increased bythe controller, the beam moves away from the nozzle and operatesthe relay, causing full air-line pressure to be applied to the valvediaphragm until the valve assumes the required position and thesystem is again in balance. Provision is made by a lever ofvariable mechanical advantage, such as would be obtained bymoving the nozzle sideways along the beam, to obtain differentvalve-stem movements for a given pressure change. Similarly,by the use of valve positioners it is possible to operate two valves

Air

Air fromcontroller

supply

Fig. 14.—Valve-positioner mechanism.

from one controller in such a manner that a pressure changefrom, say, 1 to 8 lb/in2 operates one valve and a change from8-15 lb/in2 operates the other valve.

(3.8) Pneumatic Transmission

In Section 2.5, reference was made to transmitt ing deviceswhereby the reading of a meter could be relayed back to a centralcontrol panel. Using pneumatic systems this is accomplishedby making the transmitt ing instrument a proport ional controllerwith its set-point at the mid-scale position and with a 100% p ro -port ional band. Since the output pressure of a proport ionalcontroller is linear' with deviation from the set-point, it willtransmit the required readings to an indicating or recordingpressure meter. Transmission can be made to distances up to1 000 ft with an accuracy of L 1 % of the scale range.

Scale expansion can be employed by using a smaller propor-tional band. The receiving instrument may also be of thedifferential type, receiving pressure impulses from two trans-mitters in opposition and giving a final indication which is adifference measurement.

(3.9) Pneumatic Setting of a Controller

Cases sometimes arise where the set-point, i.e. control point,of a controller must be varied according to the reading of asecond instrument. T o take a simple example, it may benecessary to mix two liquids continuously in a given rat io. Onthe uncontrolled liquid (independent variable) a flow transmitteris installed. This instrument measures the flow in the pipe lineand transmits its value t o the second instrument which ismeasuring and controlling the flow of the second liquid (de-pendent variable). If now the independent variable increases inmagnitude, the transmitter raises the control point of the secondcontroller by the required amount , so keeping the ratio ofdependent to independent variable constant.

In a recent paper by Porter on "Basic Principles of Automat icControl Systems," a similar technique is discussed which wouldeliminate the droop of a proport ional controller and cut downmeasurement lags.

(3.10) Summary of Available Control Actions and theirCharacteristics

In this Section an at tempt is made to summarize in tabularform the main control actions which are available. The controlequations are given in the Table in the differentiated form whichis sometimes used. Fig. 15 shows diagrammatically some of thepossible combinations of controller and controlled valve whichcan be obtained.

AUTOMATIC CONTROL IN THE CHEMICAL INDUSTRY

Table

Equation

—

dO t/edt 2dt

dOdt l°

dO . , . dedt dt

dd , de , diedt dt ' dfi

dO , . de , d*edt kie lk2dt + k\ft2

Type of control

• Multiposition

Proportional

Proportional-speed floating

Proportional-plus-floating

Proportional-plus-derivative

Proport ional-plus-floating-pl us-derivative

Characteristics

Requires high demand capacity compared with input capacity

Droop occurs with load change

No droop, but permits overshoot; requires low lag forsatisfactory operation

Maintains control point but permits overshoot

Droop occurs with load change, but overshoot is limited

Maintains control point at the expense of some ability tolimit overshoot

(4) THE CONTROLLED PROCESSWhilst in Section 1.4 it was stressed that the controlled process

was equally important a link in the control loop as the measuringelement, controller or controlled value, it is true to say that it isthe link about vvhich least is known. Mathematically thebehaviour of the other components of the loop can be predictedwith some certainty, yet it is only in the simplest of processesthat a fairly accurate estimate of the likely control conditionscan be made when the plant is being designed. This inevitablyleads both to over-designing and to a "cut and try" technique.When the plant is designed, the controls which are thought likelyto be required are chosen, and then when the plant is in produc-tion the controls are modified in the light of experience. Themodifications referred to are not the adjustment of proportionalband, floating rate, etc., of the controllers; these are details which,whilst it would be helpful to predict them before the plant startsup, can be adjusted during the initial stages—although theideal control system would be capable of starting up the plantinitially. The modifications required are more fundamentaland embrace such things as reducing process lags by reposition-ing the measuring element, altering sampling methods, improvingmixing, etc., and frequently go as far as requiring a completeredesign of control equipment based on the actual runningconditions obtained.

Thus at the present time, whilst the major difficulties arelooked for and avoided during the design stage, a great dealremains to be done before the empirical approach to the problemcan be replaced by a more fundamental one which would allow

the requisites of the control system to be accurately predictedin the design stage. The reason for this is, of course, the largenumber of, at present, unpredictable variables which affect thefinal result.

The best that can be done is to consider instrumentation at avery early stage in the preparation of the initial tlow-shee. andplant design. Frequently it is found that, by a re-arrangement ofplant, the installation of a different design of condenser, heatinterchanger, etc., control could be very much simplified and alarge amount of process lag eliminated— whilst in no way up-setting the process conditions specified.

(5) FUTURE DEVELOPMENTSIn the concluding Sections an attempt is made to indicate lines

on which there is need for development. If this paper promotesinterest and development in them, then it will have succeededin its object.

(5.1) Process AnalysisFollowing on the remarks made in Section 4, process analysis

obviously offers a very wide field. A comprehensive mathe-matical treatment already exists for dealing with the problemswhich occur in electrical networks and feedback amplifiers, andit seems likely that these methods might well be applied to theanalysis of chemical processes. If the technique were successful—and in view of the success gained during the war with serv omechanisms it appears likely that it would be—then a powerfultool would be placed in the hands of the chemical plant designer.

Success would enable the practice of automatic control of

(Closes on air i [Stays j [Opens on j1 _fajlure_J L put ; j air failure j jPneumaticj

•_ jettjngj

HuLterfS

Recordingor indicatingproportionalcontroller

JReversej• act ion i

Alura]relav I

Fig. 15.—Block diagram showing some of the combinations of controller and valve which are available in air-operated control schemes.


chemical processes to be pushed still further. Thus it would bepossible to work out a general economic treatment of processcontrol to find :•—

(«) How far the cost of building a plant can be reduced bydesigning the plant to make full use of automatic control.

(/>) How much running costs would be thereby decreased.It must again be stressed that this is a very different matter

from estimating whether or not it is economic to install automaticcontrol in place of manual control on a plant which could beoperated by either. The plant designed for optimum use ofautomatic control would be practically impossible to run onmanual control— the latter could not be relied on to maintainsufficiently closecontrol of the process to make it successful or safe.

In working out the economic balance for any plant, however,allowance must be made, when considering the possible reduc-tion in labour, for the fact that a certain minimum of labour isalways required to deal with emergency conditions.

It should be remembered that the closer plant design comes toutilizing fully the possible performance of the control system,the more it will be necessary to improve the individual controlinstallations.

This problem of plant design can only be solved when sufficientprocess data and control experience are available, and it must bethe responsibility of industry to accumulate this knowledge andmake it available to workers in the control field.

(5.2) Complete Electrical Control SystemAt present in the chemical industry there are a considerable

number of electrically-operated alarms and straight on/off devices,but there are very few controllers.

The almost universal use of the air-operated controller is notfortuitous but is a direct result of several causes.

(a) In the early days the fluid-operated diaphragm valve wasfound to be cheap to install and easy to maintain, and it stillremains superior, except on very large valves, to the electrically-operated valve.

(b) Developments in the controller mechanism were naturallyadapted to use the diaphragm valve.

(<) At the same time, requirements of flameproofing made airoperation preferable to electrical operation in many installations.

(tl) To deal with the long process lags normally encountered inindustry, the air-operated system was simpler to design and tobuild.

So much for the historical side, but with the advent of suchdevices as the electronic self-balancing potentiometer, which willprobably largely replace its mechanical counterpart within thenext decade, an opportunity occurs for the appearance of a com-pletely electrical control system. A large number of the primarymeasuring elements used in the chemical industry are electrical,and electrical measuring instruments and recorders are widelyused. It is suggested that there is possibility here for the elec-trical engineer to develop electrical control equipment. Theintegration, differentiation, etc., of the deviation from set-pointcould be carried out on the output from the measuring element,and then the output, with all the required control terms thusadded electrically, could be fed into a power amplifier to derivethe final signal required to operate the controlled valve. Bystandardizing the output voltage and impedances of all primaryelements at some suitable values, all controllers could be madeto a standard design; this, coupled with the fact that the connec-tion between primary measuring apparatus and controller iselectrical, would result in several advantages—

Ui) The cost of the controller would be decreased owing tostandardization.

(b) On a given works less spare controllers would need becarried.

(c) The conversion from indicator (or recorder) to controllerbecomes a simple matter.

(d) If necessary, a primary measuring system from one manu-facturer could be connected to another manufacturer's controller.

The first three points need no elaboration. In connectionwith the fourth point, the occasion sometimes arises where it isnecessary to purchase the measuring system and the controllerfrom two different manufacturers, and then to combine them onsite. Experience has shown that this is a complicated and costlyprocess so far as the user is concerned, involving him in muchdetailed work before the combined system is finally in operation.

One further advantage of an electrical control system has notyet been mentioned. On batch processes it is frequently neces-sary to get the batch to a given control point as quickly aspossible, hold it there for a given period, and then change toa higher (or lower) control point, again as quickly as possible,and hold it at that point until the end of the batch. All this mayhave to take place in a tile-lined jacketed vessel, which has apoor heat transfer and hence a long thermal lag, under reactionconditions which may be violently exothermic or endothermic atdifferent stages. Thus we have conflicting conditions. For therapid rise to control point a proportional-plus-derivative con-troller is required; once control point has been reached a floatingcomponent is required to correct for the droop which wouldoccur if the reaction becomes exothermic or endothermic;whilst when the second rise to a new control point is required thefloating term would again have to be eliminated. In an electricalsystem it would appear that the switching in and out of thefloating term could be accomplished easily and neatly.

(5.3) Electrically-Operated ValvesUnless a completely new type of electrically-operated valve is

developed, which will be economical to install and which will givesatisfactory service with no more maintenance than is normallygiven to the diaphragm type, it is extremely unlikely that suchvalves will be employed in the chemical industry to any extent.In the design of such a valve, detailed consideration will haveto be given to such points as

(a) Simple design to aid cleaning and maintenance.(6) Resistance to external corrosion by plant atmospheres.(c) Large operating force, combined with rapid action and

accurate positioning.(d) Flameproof certification without the employment of too

heavy a construction.There appears no reason why a design satisfying these con-

ditions should not be produced if sufficient thought is given tothe problem.

An alternative still remains, i.e. to employ a completelyelectrical control system as discussed in Section 5.2, with all itsconsequent • advantages, in conjunction with an air-operatedvalve. The conversion from electrical to pneumatic operationcould take place in the vicinity of the valve, thus taking completeadvantage of the lower transmission lag and higher precision ofthe electrical system, whilst the mechanism employed could besimple and standard for all types of valve. A further advantagewould accrue, in that the final electro-pneumatic positioningservo need only operate at a low power level on the electricalside, thus making the question of flameproofing much easier.

(6) CONCLUSION AND ACKNOWLEDGMENTSAn attempt has been made to indicate the present state of

automatic control in the chemical industry, and to set out theproblems which remain to be solved. It is clear that much workremains to be done and that it lies in two fields, both of whichare familiar to the designers of electrical control systems for otherapplications. These are:—


(a) The analysis of the process so as to make possible thedesign of plants more suited to automatic control.

(b) The development of the control equipment itself.The former entails an attempt to apply a more rigid mathe-

matical analysis of the process, probably based on the existingmethods of feedback amplifier analysis. The latter includeswork on the measuring system, controller and controlled valve.At present the major portion of the control equipment used bythe chemical industry is of the air-operated type. The extentto which this will be the case in ten to fifteen years' time depends,at least in part, on the amount of effort which is put into thedesign of electrical control equipment. There are indicationsthat some attention is already being paid to the possibilities ofelectrical control systems—one of which is greater flexibility—and it is hoped that interest in these possibilities and theirdevelopments will increase.

In conclusion the authors would like to thank the Directors ofImperial Chemical Industries, Ltd., for permission to read thispaper, and also those many colleagues who have assisted inpreparing it.

(7) APPENDICES

m. (7.1) A Temperature-Gradient ControllerBy A. W. FOSTER and J. ATKINSON.

(This appendix was first received 25th February, and in revised form19//? April, 1947.)

The quality of product from industrial distillation columns isusually controlled by manipulating the reflux/overhead-take

ratio to keep a constant overall temperature gradient. Thus ina column in which the liquid in the case is kept at constanttemperature by regulating the heat directly applied to it, thecolumn top would be kept at a definite temperature by regulatingthe amount of cold distillate returned to the column as reflux.

When the distillation process involves the separation ofcomponents of the feed which boil within a few degrees of eachother, the problem of maintaining the necessary absoluteaccuracy, under industrial conditions, becomes very difficult.On the laboratory scale a differential measurement, using aspecially pure sample of distillate as reference, is frequentlyused. A modification of this has been adopted in the tempera-ture controller described here, in which the temperature differencechosen is that between two points in the column above the feedpoint, i.e. the temperature gradient over a given section of thecolumn is controlled by regulating the cold reflux supply.

The column in question handles about 300 gallons an hour offeed boiling at about 70' C under reduced pressure, with amaximum temperature gradient of about 0-3degC/ft. Twomatched resistance thermometers, with bulbs 10 in long and ofabout 150 ohms fundamental interval, are positioned in sheathsinserted through the side of the column and 6 ft vertically apart.The instrument problem then becomes the provision of a tem-perature controller which will give fully stabilized control of atemperature difference of about 1 • 5 deg C. Other likely develop-ments call for a much higher sensitivity.

None of the established temperature controllers using pneu-matic control mechanism could be obtained with sufficientlynarrow range for this application. Electronic amplification wasstrongly indicated and was most readily available in the Tinsley

StrumAC. mains ' |

D.C. amplifier

Pneumaticmilliammeter

controller

Purge

A,B,CcI*ucecl reaifltorB(about 400 ohms) G»mirror galvanometer Ri>R2-resistance thermometers, 38\5clwu) "vf 65'C

tappings acrosa 38 ohms an C L«lamp S« smoothing circuit

D«3-volt dry battery P« photo-electric cell T»gaa-£illed relay

Fig. 16—Temperature-gradient control of distillation column.

92 BROADHURST, BRODRICK, FOSTER AND WHEELDON:

d.c. amplifier. This apparatus, first described by D. C. Gall,*employs a relatively insensitive reflecting galvanometer andphotocell to control the phase of the grid voltage of a gas-filledrelay acting largely as a rectifier. Negative feedback is intro-duced to stabilize and give more or less null-potentiometercharacteristic to the circuit. The smoothed output, which maybe as high as 30 mA, is measured on a recording milliameter. Inthe present instance the milliammeter is fitted with thestandard Stabilog proportional plus floating pneumatic con-trolling mechanism.

Fig. 16 shows the general layout of the complete controller,including the bridge circuit formed by the two resistance thermo-meters and adjacent fixed resistors. The latter are adjusted tobalance the bridge when the thermometers are at exactly thesame temperature. Current through the bridge is kept at thevalue which gives the required full-scale range on the recordingcontroller. Although for immediate purposes 5 deg C range isfound suitable, ranges as low as 0 1 degC are known to befeasible.

It will be noted from the diagram that the temperature-differ-ence controller does not directly control the reflux rate. Inpractice it is preferable to control the product take-off, and forthis purpose a flow controller is employed to operate a dia-phragm-motor valve in the product line. This will maintain asteady rate of flow of product from the column, independent offluctuations of feed, boil-up rate, column and line pressures,etc., at the value determined by the set control point. Sincechanges are necessary, however, in the product take-off rate toproduce (inverse) changes in the amount of reflux, the tem-perature-difference controller is coupled pneumatically to the

* Journal I.E.I-:., 1942, 89, Part II, p. 434.

flow controller so that the control point of the latter is reset tothe extent needed to give the required constant temperaturedifference.

In spite of its apparent complication, this controlled systemhas proved convenient and capable of keeping a temperaturedifference of 1 -5 deg C constant to 0 05 deg C under industrialplant conditions. Although the precaution was taken ofmounting the amplifier on a main column of the plant structure,there is no evidence that the apparatus was hyper-sensitive tovibration. During 18 months' continuous use the only main-tenance required has been replacement of one galvanometerlamp bulb. •

(7.2) An Electrical Conductivity Level ControllerBy A. W. FOSTER.

{This appendix was first received 25th February, and in /vw.vi'i/ form19th April, 1947.)

In certain products-washing processes employing settling tanksfor separating the product and scrubbing liquid, it is normalpractice to use ball-float type pneumatic level controllers tooperate diaphragm-controlled valves on the tank drain. Ade-quate control of level is usually obtained, provided that theliquids separate satisfactorily and that their specific gravitydifference is large enough to provide sufficient working force onfloats of reasonable size.

The device now to be described was developed to meet casesin which the differences of specific gravity were low, rangingfrom almost nil to 0-2. It relies upon one of the liquids beinga poor electrical conductor and the other relatively good.Although devices employing relays and electronic circuits are

cont iv ;.<_.•

Controlvalve

A B • 21 avcg. Brightray win? (SOiiiiunjCi

C " Insulating link

,D - SpringEi, E2* Steel electrodesG,H = Immiscible fluids (one poor conductor,

the other relatively good conductor)

Fig. 17.—Electrical conductivity type level controller.


more or less well known, in the present example the emphasisis on maintaining the pneumatic type of control system forwhich the plant instrumentation is adapted. The simple "hot-wire" device employed here achieves this aim quite effectively.

An electrode system is so arranged that the change in resistanceduring the first few inches immersion of the electrodes in the goodconductor is sufficient to move the control valve over its fulltravel. Dimensions, shape, material and spacing of the elec-trodes can be varied in ingenious ways to suit particular applica-tions. The present device uses \ in silver-steel rods, about 6 ftlong, kept parallel and 2 in apart by Tufnol spacers and insulatedat the flanged top with Tufnol bushes and Klingerit joint packing.The electrodes are connected in series with the "hot-wire" andthe 2-volt secondary of a small 240-volt, 40-c/s mains transformer.

Although styled "hot-wire," the 21 s.w.g. Brightray wire isnormally not more than 10 deg C above the surroundings. It isrigidly clamped at its ends and can be treated as straight when nocurrent flows through it. Hence the movement of the mid-pointof the wire is roughly proportional to the heating current. Sincethe lever magnifying system used to deflect the pointer is linear,it is possible to arrange for the calibration of the instrument scaleto be roughly linear in interface level over the operating range.ofthe control mechanism.

As shown in Fig. 17, the hot wire is housed in a (iin bore)pipe teed directly on to the case of a standard proportionalpneumatic controller (e.g. a pressure controller with the pressureelement omitted.) Proportional sensitivity is adjustable in thenormal manner; since full-scale movement may involve less than6 in change in level, the throttling zone is usually a large fractionof this scale range.

Using as good conductors such liquids as weak sulphuric acidand caustic soda, interface levels have been controlled reliablyfor long periods to within 1 or 2 in, maintaining an output assteady as the inflow. Electrodes have not given the trouble dueto fouling that was anticipated, presumably because they workcontinuously submerged in liquid. The low voltage used, besidesrendering polarization negligible, greatly minimizes sparkingdangers.

The sensitivity of the device is ample and suggests that it wouldbe applicable to much poorer conductors, using plate electrodesand a finer hot wire. Dielectric-constant differences can be usedin place of electrical conductivity if a suitable high-frequencycapacitance bridge, with amplification, is interposed. Controlof air-liquid interface is clearly possible, although frequentcleaning of the electrodes might be necessary.

It will be noted that this type of controller is not very suitablefor applications in which the controlled level will require to bevaried over a wide range or in which the throttling range is large.Precise calibration of the scale in terms of level depends uponthe constancy of the specific conductivities of the liquids, whichcan rarely be guaranteed in industrial practice. For similarreasons, it is of doubtful value to make the hot wire operate aproportional plus floating controller, although the set-up for thiswould be the same as that described above for the simple pro-portional controller.

(7.3) A Level Controller for Liquids in High-Pressure VesselsBy A. W. FOSTER and T. A. RADGE.

(77/Z.v appendix was first received 25th February, and in revised form\9th April, 1947.)

Several important industrial processes operate at pressuresbetween 3 000 and 10 0001b/in2, in which range the pressuregauge is the sole, fully proved, commercially available instrument.Means of measurement of fluid flow and liquid level have beendeveloped largely by users with the guidance of engineering

principles developed for high-pressure plant construction. Auto-matic control has consisted in substituting these instruments forthe equivalent low-pressure measuring mechanisms in com-mercially-produced automatic controllers. The present case isa good example of this procedure and serves to show how readilyan automatic controller can be built up once a satisfactorymeasuring element and control mechanism have been separatelyproduced.

It is required to control the level of liquid in a high-pressurevessel in which the permissible swing in the level is about 6 in.Greater changes of level can cause emergency conditions inadjoining plant, and within these limits surging of the levelreacts on the system pressure, causing unsteadiness. It followsthat smooth proportional control is needed with automaticcorrection for load error. Automatic controllers which willmeet these conditions adequately under lower pressure con-ditions are available.

Of the various conceivable level-sensitive devices—ball-float,"buoyancy-balance," electrical capacitance, photo-electric obser-vation of level glass, were unsuitable or insufficiently developed—direct measurement of the liquid head above a given point inthe vessel by a high-pressure manometer was adopted. Although1 in of liquid level is less than 0-001 % of the absolute pressurein the vessel, differential manometers of at least this sensitivity

Injection

High-pressuremanometer

Pneumaticcontrciler

Fig. 18.—Liquid level control in high-pressure vessels.

BROADHLRST, BRODR1CK, FOSTER AND WHEELDON:

are commonly used for measurement of h.p. fluid flow by orificeplate. These manometers are usually of the ring-balance typewith a steel-tube (half) ring, containing mercury, and steelcapillary flexible connections to the ring from the sources ofpressure. Fig. 18 shows that these sources of pressure are (a) gasabove the liquid, (/>) gas -f head of liquid. The ring conse-quently swings about its knife-edge by an amount proportionalto the difference between (a) and (b), the liquid head.

The working force obtained is quite sufficient to operate therecording and controlling movements of proprietary pneumaticcontrolling manometers. A suitable controller, minus its (low-pressure) manometer movement, is mounted rigidly on the ring-balance supports (the separation shown in the diagram is merelyfor clarity). Lever linkage is added to give full chart travel ofthe controller pen for full permissible swing of the ring. Anappropriate diaphragm-operated valve must be obtained, ordesigned for throttling on the liquid issuing from the vessel, andcoupled pneumatically, to the controller. The system is handledin the same manner as the equivalent low-pressure installation,whilst observing the special precautions needed for h.p. plant.

It will be noticed that injection of clean gas into the pressurelines to the meter is necessary to make the measurement effective.This bleed of gas also serves to keep these lines free from depositsof dirt and undesirable fluids. If a source of gas at pressurehigher than that in the vessel is available, this can be bled at acontrolled rate, through restrictions composed of lengths of steelcapillary, into each line. Clean liquid can similarly be injected ifthe arrangements customary to sealing practice in low-pressuresystems are adopted. The fluid used for injections must beinnocuous to the liquid in the vessel; it will frequently be possibleto tap off gas or liquid from some point of higher pressure earlierin the process.

The catch-pots inserted at the highest points of the pressure-connecting lines are of greater importance than in low-pressurepractice. Comparatively small pressure surges can cause liquidto be carried well into the pressure lines unless traps of adequatecapacity are interposed.

Controllers of the above type, with gas or liquid injection,have been controlling levels in h.p. vessels for extended periodswith a normal variation of not more than 1 in of liquid level.

(7.4) Automatic Control of Salt Concentration in an EvaporatorBy J. D. TALLANTIRE.

{This appendix was first received 25th February, and in revised form19th April, 1947.)

A common arrangement of chemical plant uses a system ofevaporators in a continuous process to concentrate an incomingweak solution. These are often controlled to give an unsaturatedsolution of constant concentration at a rate which varies tosuit immediate process requirements. This note describes twostages of development of a method which has been used forsome ten years to control such a concentration automatically.

The application has been to evaporators working at pressuresbelow atmospheric, of the order of 200-600 milli-atmospheres(absolute). A steam-heated calandria is used to provide theheat for evaporation, and the concentration is adjusted byvariation of the steam rate. The level of liquor in the evaporatorand the pressure in the vapour space can be kept constant byconventional controllers. The liquors leaving the evaporatorsare nearly saturated aqueous solutions at about 80-100° C.

For manual control of the plant the concentration is usuallymeasured, when required, by taking a sample of the liquor,allowing it to cool slowly, and noting the temperature at whichcrystallization begins. This method is inconvenient, subject tomany sources of error, and involves a time-lag. The difficulty

of developing an automatic control system was almost entirelythat of finding a suitable property of the liquor which could bemeasured as a function of concentration. The measurement ofsuch a property had to be continuous, simple and dependable,and to involve the minimum use of measuring equipment insidethe evaporator (where it would be inaccessible and subject tocorrosive conditions). The measuring mechanism had also to becapable of being used with some available type of control equip-ment. The most suitable properties to measure to achieve theseobjects are specific gravity and boiling-point elevation. The first ofthese involves the use of comparatively complicated equipmentinside the evaporator, and the achievement of sufficient sensi-tivity and dependability in a violently boiling liquor promised tobe difficult, so the methods based on temperature measurementwere used.

The boiling point of a solution depends upon the pressureabove it and upon its concentration. Concentration is thus afunction of boiling point and pressure. This function has to bemeasured and maintained constant. This has been done intwo ways. The first method was devised to suit the controlinstrument mechanism then available; the second is an adaptationto suit what is now standard control equipment.

(7.4.1) First Controller Arrangement.The first controller arrangement to be used is shown in Fig. 19.The bulb pf a mercury-in-steel temperature controller measures

the temperature of the liquor boiling in the evaporator, i.e. boilingpoint at working pressure. A flexible diaphragm is subjected tothe same working pressure, being connected to the vapour spaceabove boiling water in a specially provided "baby evaporator"vented into the plant vessels. An adjustable lever system isused as a mechanical linkage between the pressure diaphragm

Adjustablelinkage

Bourdonmeasuringelement

'Air Co control

Steam '— ^—^=

WaterBaby

evaporatorWater overflow ito lute

Vapour(500 inilli-atmospheres, absdute)

EvaporaturFig. 19.—Automatic control of concentration of an aqueous solution

in an evaporator.


and the thermometer movement. This linkage is so adjustedthat a change in pressure and the boiling-point change whichcorresponds to it produce equal and opposite effects on thecontroller impulse system. This setting is obtained by trial anderror, the record of achieved concentration being used as aguide (see Section 7.4.2). Deviations from the present con-centration, detected as independent temperature or pressurevariations, are corrected through operation of the steam valveby a pneumatic proportional plus floating controller mechanism.

The sensitive element of the temperature controller is a 6-ftbulb mounted horizontally (in a pocket) in the liquor above thecalandria. This thermometer has a range of 90-100° C and issensitive to temperature changes of 0 01 degC. The pressurecompensation in this arrangement is adequate over a range ofI 50 milli-atmospheres from a prescribed setting, and manual

adjustment of the compensating linkage is necessary to coverpressure changes outside this range and significant changes inbarometric pressure. The system gave good results for severalyears with manual control of the pressure and level in theevaporator which permitted short-term pressure fluctuationsover a range of h 20-40 milli-atmospheres.

(7.4.2) Recorder Arrangement.As stated above, concentration is a function of boiling point

and pressure. As the boiling point of pure water is a functionof pressure alone, concentration of an aqueous solution can bemeasured directly in terms of the temperatures of the solutionand of water boiling under the same pressure. If the rate ofincrease of boiling point with pressure is the same for the solutionas for water over the pressure range in question, the differencein temperature between liquor and water measures concentrationdirectly. This difference is measured by a differential resistancethermometer recorder with measuring elements in the evaporatorand "baby evaporator." The "baby evaporator" is providedfor this purpose and is steam-heated and fed with condensedsteam. The arrangement is shown in Fig. 20.

In some cases the variation of boiling point with pressure isappreciably greater for the solution than for water. In suchcases the same kind of resistance-thermometer circuit is used,but the liquor thermometer is so shunted that such proportionaterises in resistance of the two bridge arms as would be producedby a pressure change do not affect the measured difference.

Some ten years of experience, using a concentration record ofthis kind with a liquor-temperature record on the same chart,have shown that it is quite practicable to adjust the linkagebetween the pressure- and temperature-measuring systems bytrial and error to give good control of concentration.

(7.4.3) Later Controller Arrangement.The controller which has been described is an example of the

development of a "measuring element" to fit an availablecontrolling mechanism. At the time when this arrangement was

Self-balancingnull-point avonieror pneumaticcontroller

Vapour

(500 milli -atmospheres, absolute)

I IEvaporator

Fig. 20.—Recorder-controller for automatic control of concentrationof an aqueous solution in an evaporator.

devised, pneumatic potentiometer controllers were not in generaluse, but their availability has made possible a simpler controllingarrangement. This makes use of the differential resistancethermometer arrangement previously used only for recording.A null-point resistance thermometer bridge is incorporated inthe usual type of air-operated proportional-reset potentiometercontroller, which controls the concentration directly by means ofthe steam valve.

This control system has several advantages over its predecessor.It uses standard equipment with little modification, is compactand gives adequate pressure compensation over a wider pressurerange without the need for special running adjustments. Inaddition it is unaffected by barometric pressure changes.

The development of these controllers is a good example of thecommon procedure for arriving at a special automatic controlsystem, in which a suitable physical measurement is selected andarranged to give an impulse suitable for an established type ofautomatic control instrument.

[The discussion on the above paper will be found overleaf.]

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Automatic control in the chemical industry

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