I/EC
Instrumentation
Pneumatic Black Boxes . . . Simple
Computers
for Process
Control
About to get your feet wet in computerization? offer advantages you will want to look into by P. H. Stirling
^DETERMINING process variables that cannot be measured directly by on-line computing is becoming more widespread. Systems engineers and control specialists today use on-line computing in the application of modern energy and material balance control concepts. Basic process variables such as mass flow or enthalpy flow are computed from directly measurable and derived variables used for control. Often merely adding a simple computed mass balance or energy balance display to a control panel enables an operator to adjust his operating variables so as to optimize his process because of the clearer understanding gained from the derived display. Such displays can be of great assistance during start-ups and process upsets. Data logging ideas have also become more advanced and the old ideas of recording almost every process variable are giving way to recording a smaller number of derived process variables of more immediate significance to the process engineer. Use of derived process variables is also gaining ground where management is considering the future use of a special purpose digital or analog computer for plant process control. Studies for larger systems depend on understanding the smaller process blocks or unit operations. Role of Small Computers
Both large and small companies are looking hard at the benefits to be gained from on-line computing. The valuable experience and orientation of personnel to be gained from applying a small specialized computer to a unit operation as a preliminary step is encouraging many to "get their feet wet" in computer control, by this route. An elegant example of small computer application, recently described by the 52 A
and Henry Ho, Canadian
Industries
Ltd.
Humble Oil Co., is the utilization of the small Southwestern Industrial Electronics solid state electronic analog computer to monitor the efficiency of a fractionating column and thereby optimize its operation. This pioneering development may be regarded as a typical stepping stone to the use of large scale computing aids and systems engineering. Improvement of automatic process control, which is the next logical step after acceptance of a monitoring display, has been accomplished in many places usually with electronic analogs. Emphasis on the use of high speed electronic computers has obscured the fact that there arc many cases where the slower response of pneumatics is adequate. Many simple computations can be carried out using available pneumatic devices. All the basic computing operations are available in pneumatic guise. Algebraic calculations are especially suited to pneumatic computation since the cost of an accurate multiplication or devision operation is comparable to the cost of integration. Similar electronic operations are usually several times the cost of integration. The cost of multiple pneumatic additions or subtractions is slightly below the cost of integration similar to electronic costs. Multiplication by a constant less than one is relatively expensive with pneumatic devices and costs range from one quarter to the same as integration. Electrically, this operation only costs one twentieth of an integration or less. Pneumatic multiplication by a constant greater than one but less than ten costs almost the same as multiplying by a constant less than one. Eor this case electronic costs roughly equal the cost of integration. Pneumatic computing devices have been slowly evolved, for special applications here and there, over the
INDUSTRIAL AND ENGINEERING CHEMISTRY
Pneumatic systems
last 30 years or so. They are generally inexpensive, reliable, simple in construction and use, familiar to instrument mechanics, and generally possess adequate accuracy ( ± 1%) for most industrial purposes. They offer the real advantage of being directly insertablc into standard industrial pneumatic control loops without the necessity of signal conversion. Combination of basic pneumatic blocks to generate a range of computing functions capable of satisfying most present day process needs is possible. Arithmetic operations have been carried out pneumatically in the process industries for a great number of specific applications. Square root elements are commonly used to linearize flow orifice signals. Multipliers are used for obtaining mass flows from linearized orifice signals and for correcting variations in temperature, pressure, and composition of the stream—for example, the Bristol Series 500 pressure-compensated flow meter of the Bailey Computing Relay. Averaging pneumatic relays are used for totalizing the flows from several orifices, ratio relays for controlling the ratio of two flows, and a load sharing type for distributing weighted control signals to different regulators from a common controller output. Differencing pneumatic relays are used to control level, pressure, and temperature. Simple pneumatic logic elements are also widely used in process applications when simple automatic decision making is required. Splitranging types are used when different control actions are required as the control variable changes from one range to the other. High-low limiters and high or low signal selectors are often used for pressure, temperature, or speed. The familiar three term controller performs the simultaneous operations
INSTRUMENTATION
These schematic diagrams show you what functions can be performed on pneumatic computers and the operations on two of the leading types. The last representation on the bottom line is the fluid tube, most recent innovation to this t y p e of control
of multiplication by a constant, integration, and differentiation. Various combinations of these can be m a d e u p . T h e stack controller con struction makes it possible to gener ate the inverse of these functions a n d the useful action of "inverse deriva tive" can be helpful in stabilizing p n e u m a t i c computing circuits. D. E. Lupfer and D. E. Berger of Phillips Petroleum Co. reported [ISA Journal 6, No. 6, 34-9 (1959)] a suc cessful application of pneumatic com puters employing the new concepts of material a n d energy balance control on fractionating towers equipped with overhead air-cooled condensers. A pneumatic square root extractor and a multiplier were used in con junction with an e.m.f.-pneumatic converter to c o m p u t e the actual in ternal reflux rate. Some Basic Computing Blocks
T h e Sorteberg Force Bridge, byMinneapolis Honeywell, can be used for square root extraction, squaring, multiplication, and division. I n operation, the air motor, con nected to either of the nozzles, positions the roller fulcrums until the two weighbeams are at equi librium when acted on by the forces A, B, C, and D of the load cells. A simple m o m e n t balance of the entire
system reveals that A X C = Β X D when equilibrium is achieved. Re a r r a n g e m e n t of this equation with the possibility of replacing any of the load cells with a calibrated spring permits variety of operations. In square root extraction, for example, the input is connected to D and the o u t p u t is connected to load cells A and C a n d the nozzle ΛΊ. Load cell Β is replaced by a calibrated spring and the air motor is connected to the other nozzle Ar2. Any change in input signal D upsets the equilibrium a n d changes the back pressure of the nozzle ΛΊ. This will cause the pressure in load cells A and C to change which in turn repositions both nozzle flappers. Changes in 1V2 nozzle back pressure cause the air motor to reposition the roller ful crums until equilibrium is attained or (A X C = Β X D). As A and C arc equal and Β is a constant equal to one, A = Λ/Α X C = -y/D and the desired square root is obtained. T h e H a g a n Ratio Totalizer is very similar to the Sorteberg Bridge but performs an entirely different set of functions. I n this device, a beam balance mounted on a flexible strip fulcrum is positioned by two sets of opposing diaphragms. Position of the fulcrum is adjustable to achieve different ratios of the m o m e n t arms. T h r e e of the d i a p h r a g m s are avail
able for input signals while the fourth is used for output. Air supply is connected to the fourth d i a p h r a g m through a pilot valve ar rangement. I n p u t pressure varia tions in any of the three diaphragms is transmitted to the air supply pilot valve causing the output pressure to rise or fall. This device can be used for addition, subtraction, ratio setting, averaging, and reversing of load pressure. T w o ratio totalizers m a y be combined to act as a high or low pressure selector a n d also to give two o u t p u t pressures such that their sum is equal to the input. T h e Bailey Computing Relay, which can multiply, divide, reverse, subtract, average, or totalize pneu matic signals, and the T a y l o r Transet C o m p u t i n g Relay, which can add, average, and ratio pneumatic signals with or without bias, are very similar in construction. Both are force balance-type instruments using two pairs of pneumatic bellows to vary a flapper nozzle back pressure through an adjustable lexer fulcrum arrange ment. In both elements, three input connections are available and the fourth is used for output. An out standing feature is the possibility of function generation by using the cam-follower linkage model—log, ex ponential, gain compensation. {Continued on page 54 A) VOL. 52, NO. 8
·
AUGUST 1960
53 A
INSTRUMENTATION
FRICK ACCEPTS COMPLETE RESPONSIBILITY when we contract to install our air conditioning or refrigeration systems W H E N FRICK designs, manufactures and installs an
air conditioning or refrigeration system of any size . . . it ivorks. We guarantee that by written contract; no excuses, no alibis, no putting the blame on any component manufacturer. Because of our complete and unique engineering service, the pride we take in our work and our 107 years of experience, we have . . . with the cooperation of architects, consultants, and contractors . . . designed, installed and guaranteed air conditioning and refrigeration systems of all types . . . for hotels, restaurants, stores, office buildings, hospitals, processing and industrial plants. Write for estimates. Better yet, a FRICK engineer will be happy to discuss your problem with you. N o obligation.
The Moore Multifunction Relay is another form of versatile pneumatic computing clement in widespread use. It has a stacked diaphragm construction and operates on the force balance principle. Up to five input connections are available in one single relay and it can be used for addition, subtraction, reversing, rationing, and as an on-off device. A N e w Pneumatic Element
Development of a fluid counterpart of the well-known electronic amplifying tube was recently announced by engineers (R. E. Bowles, B. M. Horton, and R. W. Warren) of the U. S. Army Ordinance Corps' Diamond Ordinance Fuze Laboratories. This "fluid tube," which uses low power side streams to direct the main power stream into either of two outlet passages, has been successfully used in building digital and analog circuits and fluid oscillators. It can act as a switch or purely as a flow divider. The possibility of using the process stream itself, either gas or liquid, as the source of power in this device offers seemingly unlimited application in the process industries. The precision and lack of linearity of "resistance" adjustments of pneumatics make them somewhat inferior to present day electronic computing devices, but where only a few computing operations are required and where algebraic operations including a number of multiplications and divisions are involved, pneumatics may serve you well. If precise integrations are required, the incorporation of electro-mechanical integrators, ball and disk, with pneumatic linkages might be considered. Each installation must be treated on its merits but the low installed costs of simple pneumatic computing systems could surprise the electrically minded. As yet there are no reports in the literature of extensive use of pneumatic computation for control but perhaps time will remedy this.
FRICK COMPANY W a y n e s b o r o , Pennsylvania
18 BRANCH OFFICES AND MORE THAN 150 DISTRIBUTORS THROUGHOUT THE WORLD Circle No. 213 on Readers' Service Card
54 A
INDUSTRIAL AND ENGINEERING CHEMISTRY
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly.
