LANTS INSTRUMENTATION FOR PIL PLANTS Operation and Application Fundamentals E. F. POLLARD, R. 31. PERSELL, H. J. 3IOL=IISON, AXD E. A. GAS'TKOCK Southern Regional Research Laboratory, New Orleans, La.
Instrumentation fundamentals are presented as a guide to chemical engineers in the application of instrumentation in pilot plants. The elements of control systems and the control characteristics of processes are defined. The operation and application of simple control systems are illustrated and discussed. Several automatic controller actions in industrial use are described in detail. The terminology, definitions, and illustrations used here are fa niliar to chemical engineers and will give them an initial 0 1 entation in this field.
Figure l.
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HE work of chemical engineers in pilot plants (Figure 1) requires an accurate analj-sis of cause and effect to develop a process concept into a practical working plan. A knowledge of the fundamentals of instrumentation is essential to the accomplishment of this objective. Process instrumentation is too often t,hought of as merely the application of indicating, recording, and controlling instruments to the control, manual or automatic, of some process or machine. Although this approach may be satisfactory for the solution of industrial control problems, it invariably gives an inadequate picture of the control characterist'ics of the process being de-
Pilot Plant a t Southern Regional Research Laboratory
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hydrogen ion concentration. Process variables are those physical or chemical conditions which vary with time. Process control is the maintaining of a selected process variable (or variables) at a constant value, or within predetermined limits, or the altering of such a variable in a predetermined manner. I n considering the regulation of a process i t is necessary to determine the process variable (or variables), the control of which will accomplish the desired results. The value of the controlled variable may be varied directly or by varying the value of a related variable called the manipulated variable. The magnitude a t which the variable or variables are to be maintained must lie within the capabilities of the process and the degree of regulation required must be within the range determined by the inherent, or built-in, self regulation of the system. Such a n examination may indicate that a modification of the process or equipment is necessary to ensure economical and adequate regulation (6, 15). FROM PIL o
r
VAL V E
Figure 2. Mechanical Vacuum Regulator (upper r i g h t ) i n Steam Supply Line to Stripping Column DIA PHR A GM SPRING
veloped and of the instrumentation required. It results also in unnecessarily complex control equipment which inevitably breaks down when the inherent self regulation (6) of the process to be controlled is out of line with the performance required. Although the thorough mathematical analysis of a control system often becomes complex ( 3 , 8, 9, 12, 16) and the solution of certain control applications is extremely difficult, the fundamentals of control systems and of the control characteristics of processes are comparatively simple. The consideration of instrumentation problems in terms of these fundamentals a t the Southern Regional Research Laboratory not only facilitates the solution of control problems but also increases the reliability and usefulness of the simplest instrument applications.
TO C O N T R O L L E D PILOT VALVE
Figure 3.
Pneumatic Control Mechanism
T o chemical engineers a process is the method by which a change takes place within the mass and energy that are confined by known boundaries (10). Such a process can be defined in terms of physical quantities such as temperature, pressure and vacuum, fluid flow, displacement, velocity, acceleration, density, humidity, viscosity, voltage, amperage, power, and frequency and in terms of chemical quantities such as composition and
FLOW_
Figure 4. Direct-Acting Single-Seated Pneumatic Diaphragm Valve
The control system itself is an arrangement of elements so connected t h a t the control requirement set into the system results in an appropriate positioning of the final control element. These elements may be mechanical, electrical, or human, and their selection and arrangement may determine the method of control. The control requirement is the value or schedule of values of the controlled variable (or variables) which i t is desired to maintain. The final control element is that portion of the controlling means which directly changes the value of the manipulated variable (16). A classification of control systems according to the method of control is: (1) manual or automatic unmonitored systems; and (2) manual or automatic deviation-actuated systems. In a n unmonitored control system the position of the final control element is independent of the instantaneous value of the controlled variable. An example of the manual system of this class would be a n operator opening and closing a valve in accordance with a predetermined schedule. But if the operator were replaced by a cycle controller the system would become automatic. I n a deviation-actuated control system the difference between the instantaneous value of the controlled variable and the value of the controlled variable corresponding with the set point initiates a predetermined variation of the manipulated value which operates to correct or limit this deviation. Here the set point is the position to which the control point-setting mechanism is set. An example of the manual control system of this class would be a steam-heated oven, the temperature of which is indicated by a mercury thermometer and controlled by a n operator adjusting a valve in the steam supply line to correct or limit the deviation of the temperature of the oven from the desired value,
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INDICA r-oR ACTUATED BY MEASUR/NG M E A N S
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COMMON LOW
Figure 5A. Electric Control Slechanism
In the automatic system of this lass the operator and thermometer are replaced by a mechanism (an automatic controller) which includes both the measuring means and the controlling means. Such controllers are self-operated when all the energy necessary to position the final control element is derived from the process through the measuring means. They are relay-operated when this energy is amplified or supplemented by energy from any other source. The measuring means of a n automatic controller includes a primary element and a measuring element. The primary element is responsive to changes in the magnitude of the controlled variable, and transmits the response to the measuring element which then converts the response into a n indication of the magnitude or the deviation of the controlled variable. Pressure thermometer bulbs, thermocouples, orifices, and pH electrodes are examples of primary elements. Pressure responsive spirals or helices, potentiometers or millivoltmeters, differential pressure manometers, and potentiometers are the respective measuring elements (2j.
The controlling means can be mechanical (Figure 2), pneumatic, electric, or can be based on several other operating principles. A simple pneumatic control mechanism (Figure 3) consists of a nozzle connected directly to a relay valve and through a restriction to a source of air. A flapper, whose position is varied by the measuring means in accordance with the magnitude of the controlled variable, offers a varying obstruction to the flow of air from the nozzle. A varying back pressure caused by the obstruction actuates the relay valve controlling the supply of air to the final control element. The set point can be changed by varying the position of the nozzle Ti-ith r e q e c t to the flapper. A pneumatic diaphragm valve (Figure 4)can be used as a final control element with pneumatic control mechanism. I t consists of a pneumatic diaphragm working against a spring to control the position of a valve. A diaphragm valve is direct-acting when a n increase of air on the diaphragm tends to close the valve and reverse-acting when a n increase tends to open the valve. 4 simple electric control mechanism (Figure 5 ) consists of adjustable high and low contacts and a common contact. The position of the common contact is varied by the measuring means in accordance with the magnitude of the controlled variable. Such a control mechanism can operate a reversible electric motor-driven valve, a solenoid, or an electric relay (8).
-1 SUPPLY
PIL 0 T L IN€
DIAPHRAGM VALVE
Off-On Electric Controller Mounted on Dryer
The controlling means of a n automatic controller includes a control mechanism and a final control element. The control mechanism detects any deviation of the indicated value of the controlled variable from the set point and initiates and sends the proper corrective action to the final control element (the element which directly changes the value of the manipulated variable).
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Figure 6.
Figure 5B.
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Automatic Liquid Level Control
Figure 6 illustrates a n application of automatic control of the deviation-actuated control class. Here water is flowing into a tank through one valve and out of it through another. A liquid level controller operates the upstream valve to keep the level of the water in the tank at a predetermined value. The float and linkage to the instrument are the measuring means. The control mechanism (air-nozzle flapper) in the instrument and the diaphragm valve are the controlling means. The tank, water, and downstream valve are the system being controlled. The water is the controlled medium. The level of the water in the tank is the controlled variable. The value of the liquid level put into the control point-setting mechanism is the set point. When there is an equilibrium between water entering and water leaving the tank the liquid level is at the control point. If the liquid level in the tank drops owing to a drop in the upstream water pressure, a supply disturbance results. If the liquid level rises owing to a restriction in the downstream opening, a demand disturbance results. The action of the control system is always to keep the liquid level (the controlled variable) a t the control point. The control system must be able to maintain a satisfactory relationship between the liquid level and the control point in spite of disturbances. I n any deviation-actuated control system there are certain delays and retardations between the occurrence of a disturbance and the correction of the resulting deviation which are due t o the characteristics of t h e control system and of the process ( 7 , 2 1 , 17).
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waTER
WATER
CY
SSC’
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Figure 7. Heat Exchanger Figure 8. Heat Exchanger with with Large Demand and Small Demand and Large SupSmall Supply Side Capaciply Side Capacities ties C = automatic temperature controller; CY = control valve; TSE =
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LAG RESISTANCE
Figure 9. Heat Exchanger with Small Supply and Large Demand Side Capacities and Transfer Lag
temperature sensitive element; supply side oapacity
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These delays are called, respectively, controller lags and process lags. They tend to produce cycling and drifting of the control point and are usually disadvantageous to control. Controller lags can be reduced to a minimum by the proper selection and adjustment of the excellent control equipment which is available today. Process lags include capacity lag, transfer lag, and velocity-distance lag. Capacity is t h e process characteristic which is usually most important to control. It is the ability of a system to store energy or material and is measured in units of quantity. A singlecapacity system has only one continuous capacity which is affected by the supply or demand. A multiple-capacity system has two or more such capacities. A capacity may be in the supply side or in the demand side. A supply side capacity is always disadvantageous to control, whereas a demand side capacity can be advantageous if it is desired to keep the process variable a t a constant magnitude. Capacity retards the change in magnitude of the process variable when there is a change in the supply or demand. This retardation is called capacity lag. Figure 7 gives a schematic diagram which has frequently been used to illustrate the effect of capacity on control. This represents a heat exchanger in which the temperature of the outlet water is kept a t the desired value by automatically controlling the steam supply. Here the water is the controlled medium, the temperature of the water is the controlled variable, the steam is the control agent, and the temperature of the steam is the manipulated variable. The heat capacity of the steam and steam coils is the supply side capacity. The heat capacity of the water and the tank is the demand side capacity. Here the ratio of the demand side capacity to the supply side capacity is large and favors control. Figure 8 shows the same system where the supply side capacity is large compared to the demand side capacity. This causes “overshooting” of the temperature and gives poor control. When either energy or material passes from one capacity of a system t o another any resistance at the boundary between the two capacities will retard the transfer. This retardation is called transfer lag. Figure 9 is the schematic diagram shown in Figure 7 with transfer lag indicated by a cross-hatched area between the two capacities. Transfer lag is always associated with two or more capacities and increases the total lag. In Figure 10 if the primary element is moved from the origieal position to the position shown in dotted lines, the outlet water will have to travel an additional distance before coming in contact with the primary element. This will cause a definite delay between the occurrence of a disturbance and the sensing of the disturbance by the controller. This delay is a function of the velocity of the water and the distance the water travels and is called velocity-distance lag. Another process characteristic important to control is selfregulation (5, 15). This inherent, or built-in, reaction opposes changes in the equilibrium conditions of a system, with the
Figure 10. Heat Exchanger Showing Velocity-Distance Lag DSC
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demand side capaoity; SSC
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effect of re-establishing the equilibrium at a lower level. A familiar example is an open tank into which water is flowing and from which water is discharged through a n orifice. If the flow of water into the tank is increased the level of the water will rise until a new equilibrium is established between the flow of water entering and water leaving the tank. If the water were discharged from the tank at a constant rate the system yould have no self-regulation characteristic and a n increase in the flow of water into the tank would cause the liquid level to rise until the tank overflowed. Thus self-regulation aids control systems which operate to maintain a variable at a constant magnitude. This characteristic can be used effectively in the design of such a controlled system (5). The dynamic control characteristics of a system are shown b y the response or reaction curve. This can be determined experimentally ( 4 ) by manually positioning the final control element and observing the time and magnitude of the resulting changes in the controlled variable. This curve is useful in the analysis of the system and in the selection of the proper controller response.
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VALVE CLOSES WHEN TEMP RISES TO THIS LINE,
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Figure 11. Two-Position Action
riME
Figure 12. Two-Position Action with Differential Gap
In deviation-actuated automatic control systems the action or response of the final control element can be either a discontinuous or a continuous function of the deviation (8). The first type includes two-position action and multiposition action. The second type includes proportional action, floating actions, proportional plus automatic reset action, and proportional plus automatic reset plus rate action. I n two-position action a deviation of the process variable can cause the final control element to take one of two predetermined positions. An example of this type is a n on-off temperature controller regulating a steam-heated oven. As the temperature reaches the control point the steam valve closes. When the temperature drops below the control point the steam valve opens. As shown in Figure 11, this type of control causes a cycling of the temperature above and below the control point. T o prevent too frequent operation of the control system such a controller can be built with a differential gap (neutral zone) above which the valve is closed and below which the valve is open. I n this case the control point becomes a n average tem-
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Figure 13.
Proportional Action
Narrow band wntrol showing small offset with cycling
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Figure 14.
Figure 15. Proportional Plns Automatic Reset Action
Proportional Action
Wide band control showing large offset
perature about which the temperature cycles (Figure 12). Offset or drift of the control point due to load changes is an inherent characteristic of such a controller. For certain applications a low rather than a n off position is provided. I n a gas-fired oven, for example, a bypass is placed around the control valve to pro~ gas when the control valve is closed. vide a low f l o of In multiposition action controllers a deviation of the process variable can cause the h a 1 control element to take one of, say, three pred‘etermined positions. These might be off, half open, and fully open positions of a control valve. I n proportional action the value of the controlled variable determines the position of the final control element. That range of values of the controlled variable corresponding to the full operating range of the final control element is called the proportional band. Since under varying loads the magnitude of the controlled variable may swing from one side of the proportional band to the other, offset is a n inherent characteristic of this type of control action. With a narrow proportional band a small change in magnitude of the controlled variable causes a large movement of the final control element. The offset is small, but a large process lag will cause cycling (Figure 13). With a wide proportional band the control is stable, but the offset is large (Figure 14). The rate of change of positibn of the final control element is proportional to the time derivative of the deviation:
This action can be used to compensate partially for supply side capacity lag, but is not satisfactory when there is a long dead time. This action is combined with the proportional plus automatic reset action in the proportional plus automatic reset plus rate action. The rate of movement of the final control element then becomes: =
K 1de &
+ K,re 4- K Sdzez
dt
These instrumentation fundamentals are presented as 3 guide to chemical engineers in the application and use of instrumentation in the pilot plant. Terminology, definitions, and illustrations have been used which are familiar to the chemical engineer and which will give him an initial orientation in this field. Few control problems can be solved mathematically and seldom are two such problems the same, but with a knowledge of instrumentation fundamentals the experience gained with one problem can be applied more intelligently to similar problems ( I , 6,13,14). ACKNOWLEDGMENT
The authors wish to acknowledge the contribution of Joseph L. Hecker and Jack E. Hawkins in the preparation of the drawings. LITERATURE CITED
(1) Bristol, E. S., and Peters, J. C., Trans. Am. SOC.Mech. Engrs.,
where P is the position of the final control element, K , is a constant, e the deviation, and t the time. Floating actions depend on the deviation, not the magnitude,
of the controlled variable. They may be single-speed floating, two-speed floating] and proportional-speed floating or automatic reset. With the last action the rate of movement of the final control element is:
Floating actions tend to produce cycling and are only used by themselves when process lags are small or absent. However, automatic reset action is used with proportional action to compensate for offset. This action, called proportional plus automatic reset action, has the advantage of proportional action without offset (Figure 15). The rate of movement of the final control element is:
where r = reset constant or reset rate. Rate action increases the speed of response of a controller. The rate of movement of the h a 1 control element is given by:
60,641-650 (1938). (2) Chem. & Met. Eng., 50, No. 5 , 97-144 (1943). (3) Dickey, P. S., Zucrow, hl. J., Smith, E. D., Jr., Rolnick, H. A.,
Robinson, C. S., Johnson, E. T., Keppler, P. TI., Salo, E. A., Goetaenberger, R. L., Edwards, A., Hanford, E. F.,Zihlas, E., and Ryan, W. F. (discussions), Trans. Am. Soc. Mech. Engrs., 58, 55-65 (1936). (4) Eckman, D. P., “Principles of Industrial Process Control,” New York, John Wiley R: Sons, Inc., 1945. (5) Grebe, J. J., Am. SOC.Mech. Engrs., Industrial and Regulators Division, Paper KO.48-IRD-1(1948). (6) Grebe, J. J., Boundy, R. H., and Cermak, R. W., Trans. Am. Inst. Chem. Engrs., 29, 211-50 (1933). (7) Hsigler, E. D., Trans. Am. SOC.Mech. Engrs., 60, 633-40 (1938). (8) H a y e s , K. A,, Trans. Inst. Chem. Engrs. (London), 23, 173-9 (1945). (9) Ivanoff, A., J. Inst. Fuel, 7, 117-38 (1934). (10) Kirkbride, C. G., “Chemical Engineering Fundamentals,” New York, McGraw-Hill Book Co., Inc., 1947. (11) hlason, C. E., Trans. Am. SOC.Mech. Engrs., 60, 327-34 (1938). (12) Rlitereff, S. D., Ibid., 57, 159-163 (1935). (13) Peters, J. C., Ibid., 64, 247-255 (1942). (14) Philpot, A. J., Trans. Inst. Chem. Engrs. ( L o n d o n ) , 23, 150-153 (1945). (15) Spitzglass, A. F., Trans. Am. SOC.Mech. Engrs., 60, 665-674 (1938). (16) Ibid., 68, 134-8 (1946). (17) Ziegler, J. G., and Nichols, K. B., Ibid., 65, 433-444 (1943). RBCEIVED January 9, 1950. Work performed a t Southern Regional Research Laboratory, one of the laboratories of the Bureau of Agricultural and I n dustrial Chemistry, Agricultural Research ddministration, U. S . Dept. of Agriculture.
