The control itself can be no more accurate than its metering impulses. Since the power a v a i l a b l e from a n y m e t e r i n g impulse t a k e n f r o m an orifice or a Venturi tube varies as the second power of the flow or velocity, at low flows this power is a minute value; therefore, to cover an appreciable flow range it is essential that any flow or ratio control using this force as an actuating means should consist of a power relay that o p e r a t e s without weight or friction. It must be appreciated that an orifice producing a differential of 1-inch water column a t H. J. VELTEN 100 Der cent of flow creates a Askania Regulator Company, Chicago, Ill. differential of only 0.01-inch water column at 10 per cent of flow. Obviously, therefore, if the control is to be successful, H E proportional mixing of gases and liquids has long it must respond as readily to this minute impulse as to the been one of the perplexingproblems of engineersin many greater one. industries. Various means have been employed-the This illustrates why mechanical devices which are actuated manual positioning of valves, the manual adjustment of valves directly by metering impulses cannot operate within the same to balance pressures as indicated by pressure gages, and the degree of accuracy through an appreciable range. For example, assume a valve operated directly by a metering diamanual adjustment of valves to maintain constant flows as indicated by flowmeters. phragm; the valve has a mechanical friction of l pound. If the valve is to be moved by an impulse change of 0.01-inch Each of these manual means was an advance step, but each water column or 10 per cent of Bow, it must have a diaphragm fell short of perfection since the resistance to flows due to dirt area of approximately 20 square feet. accumulations, etc., or other physical inequalities in the piping systems, introduced errors whose magnitude could not Principle of the Jet Pipe readily be determined or compensated. For these same reasons the attempts to use various types of volumetric proporSeveral highly sensitive control relhys have been developed tioning valves have been only relatively successful. for transforming these minute metering impulses into operatThe true measurement of gas or liquid flow is determined by ing forces. The jet pipe is one of these, and its principle will actual area of a given resistance and the velocity. If either be used for illustration in this paper. It consists of a jet pipe of these two factors is maintained constant, the variation of which swings on a hollow, vertical pivot, as shown in Figure 1. the other can be used as a true measurement of the flow. Through this hollow pivot the operating medium (oil) is supplied to the jet pipe at high velocity. As the oil stream leaves The m o s t c o m monly known method the noBzle of the jet pipe, it strikes at two closely adjacent of measuring the flow COUNTERACTING orifices in a distributor which of gases and liquids is communicate through c o p p e r the measurement of tubing to the two sides of a resistance created by d o u b l e - a c t i n g piston. The a known restriction slightest change in the controls u c h a s a n orifice ling impulse acts on a diaphragm plate or a V e n t u r i which moves the jet pipe, camtube. These means JET PIPE ing the oil to strike either the are capable of being right orifice, the left orifice, or calculated accurately, between the two. As a result are relatively simple the piston is moved to the right to i n s t a l l , and are, or left, or is held stationary to FIUURE 1. JET PIPEPRINCIPLE therefore, most comsatisfy the control requirements. monly used by manuFigure 2 shows a jet pipe facturers of metering equipment. Other means of volumetric regulator for maintaining a conmeasurement are available and may be used in cases where stant pressure in the pipe line. the physical arrangement of piping does not permit the proper Jet pipe 1 is balanced between 2. APPLICAarrangement of orifices or Venturi tubes. d i a p h r a g m 3 and measuring TION OF A PRESSURB ' The purpose of this paper is to discuss principles of metering spring 2. An increase in presREGULATOR Qnlyas a means to illustrate the problems to be met in the apsure deflects the jet pipe counterblication of automatic proportioning control. The performclockwise and thereby increases the pressure on the left side of ftnce of automatic volumetric control or volumetric proportionpiston 5. This opens butterfly 6 until the pressure is reing control depends upon the correctness and accuracy of stored. Different settings of the spring result in correspondthe metering principle on which it is based. I:
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Automatic Mixing and Proportioning
of Gases and Liquids
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ing controlled pressures. This regulator may also be used as a flow controller for maintaining a constant differential pressure. If, for example, an orifice is installed in the pipe line end the differential pressure across this orifice actuates upon both sides of the diaphragm, the regulator will maintain a constant flow through the line. The adjusting spring on the pressure regulator may be replaced by a second impulse system, and the regulator will then control the secondary impulse in relation to the primary impulse or it will act as a proportioning regulator. This double-impulse or proportioning regulator is explained in Figure 3.
Ratio Regulators Figure 3 shows a simple form of ratio regulator where two gases are to be mixed and maintained a t a constant proportion. Since all proportioning controls are based on this principle, it may be described in some detail: Main 1 re resents the gas line of gas I; main 2 represents the line for gas PI, the flow through which has to be controlled by butterfly 3 in accordance with the variation of the flow through 1. The differential pressure created by orifice 4 is applied to both sides of measuring diaphragm 9 and thereby creates a force acting from right to left. The controlled flow in line 2 creates another
Correct proportional mixing of gases and liquids has long been a problem for engineers in various industries. I t has been solved by automatic means, most of which were more or less incomplete or unsuccessful in the past. Since perfect maintenance of volumetrically proportional flows and accurate mixing must be based on correct metering of the individual flows, the present paper deals with such automatic control which uses approved and reliable metering devices and maintains a correctly metered proportion. A number of proportioning and mixing problems are described as they have been solved in recent years, and actual installations are shown and discussed as they were made in different industries for various purposes. New metering devices are described which have been developed for the purpose of automatic proportion1. ing of gases and liquids. Special attention is paid to automatic proportioning over ranges wider than are with ordinary flowmetering d monly used for industrial applications. The tendency towards greater flowmetering ranges necessitates fupther research and development work which can most profitably be furt ered by the cooperation of manufa rers and operating engineers of the industries.
OF A RATIO REGULATOR FIGURE 3. APPLICATION
force in the same manner on diaphragm 10 which acts from left to right. Both forces vary with the second power of the flow. The ratio of the two flows will, therefore, be correct as long as both opposing forces are in balance. The figure also shows jet pipe 13, counterlever 17, and ratio-adjusting slider 18-19 between the two diaphragms. The ratio slider is used to vary the proportion between the two flows and to change the ratio setting, which is shown on a somewhat larger scale in Figure 4. With the ratio slider in the center position, the ratio of the two forces opposing each other is 1to 1. This is shown in the center diagram of Figure 4. It is assumed that the force applied from below equals 1 pound, which would call for a force of 1 pound from the top to restore balance. With the ratio slider in its innermost position, the action of the force of 1 pound acting from below is increased to such an extent that 2.5 pounds are necessary from the top to obtain equilibrium. I n the right-hand sketch the other extreme is shown where 0.4pound will be sufficient to restore the balance. It is evident that, by different settings of the ratio slider, any ratio between these two limits can be obtained and, therefore, the proportion between the two forces varied. The ratio slider is equipped with a scale indicating the proportion for which the control is set. The orifice plate may be replaced by a Venturi tube, but in many cases neither can be applied for measuring the flow, and other means must be used. A Pitot tube, for example, or a flow nozzle is frequently applied where the space is not sufficient for the installation of an orifice plate or a Venturi tube, or where the piping layout makes the installation of a nozzle rhore desirable. Figure 5 shows a ratio control which proportions air t o either gas or oil. The gas and air are measured by means of orifice plates, and the oil is measured by a positive displacement meter; the latter drives a transformer and thus transfprms the speed of the meter into a pneumatic impulse. The pneumatic impulse, which also changes as the square of the oil flow, can be used as a metering impulse for the regulator in the same way as the differential impulse created by an orifice. ?he gas orifice and the oil-metering device are in this case so designed that the impulse transmitted to the diaphragm of the
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FIGURE 4. PRINCIPLE OF
regulator is of the same magnitude for the corresponding flows. The regulator will supply the correct amount of air no matter whether oil or gas is the primary flow. The transformer or Transometer (Figure 6) consists of a flyball governor driven by the positive displacement meter. The vertical force in the upper direction exerted by the governor is proportional to the square of the speed and actuates a small jet pipe of the same design as previously described. In this case, however, there is only one receiving nozzle provided in
HEATINPUT
I
METER FOR~ S N I D O I L
FIGURE 5.
RATIOCONTROL APPLIED TO EITHER A GASEOUS OR LIQUIDFUEL
front of the jet pipe nozzle. The receiving nozzle is connected
to the top, and a diaphragm is provided horizontally above the jet pipe and connected with the jet pipe by means of a push pin. The jet pipe is operated by low-pressure air, and the
more closely the jet pipe nozzle registers with the receiving nozzle, the higher will be the air pressure on top of the diaphragm. This air pressure on the diaphragm counteracts the centrifuga1force from below, and these two forces will always be in balance on the jet pipe, If, for example, the oil flow increases, the centrifugal force of the governor increases as the square of this change, the jet pipe registers more closely with the receiving nozzle, and the air pressure actuating on the diaphragm increases until the two forces are in balance. Thus the air pressure on the diaphragm is a square function of the meter speed and can be used as oil meter impulse on the regulator. Figure 7 shows the principle of operation of theTransometer. A positive. displacement meter drives shaft K ,and the action of the centrifugal force of weights G lifts jet pipe I until the pressure in the receiving orifices and on diaphragm H balance the action from below. Figure 8 shows the application of proportioning control for maintaining a fixed ratio between steam load, air supply, and coal supply to a steam generator. The steam load is measured by balancing the pressure in the header against a constant value, shown as a weight. The differential force between the weight on one side of the weigh beam and the header pressure on the other side of the weigh beam is proportional to the
THE
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RATIO SLIDER
pressure drop from the drum to the point where the impulse is taken and is, therefore, a function of the load. This differential pressure on the regulator is in balance with a differential impulse representing the air flow. The latter may be measured by utilizing the pressure drop of the products of combustion across the boiler setting or by providing an orifice in the air supply line to the boiler furnace or by using a flow nozzle a t the inlet of the forced-draft fan. A metering impulse proportional to the stoker speed is obtained from a metering fan which is driven by the variable shaft of the stoker drive and equipped with a small Venturi inlet. The suction a t this Venturi inlet is proportional to the square of the speed, and the impulse obtained from the metering fan can therefore be used as measurement for the speed of the stoker. This impulse device is shown in this connection since it may be applied in the same way for any other speed measurement such as fans, boosters, or pumps. I n many cases a positive displacement blower is used instead of the metering fan; the blower is driven by the stoker drive, and the air delivery of the positive displacement blower is used
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OF FIGURE 7. PRINCIPLE
t o m e a s u r e t h e stoker speed. This latter metering arrangement is preferable wherever the total combustion air can be measured under atmospheric conditions, because in this case both metering impulses transmitted to the regulator are s u b j e c t to the same changes of atmospheric conditions and thus any errors due to changes in atmospheric temperatures are eliminated on the control. All constant resistances for flowmetering, such as orifices, Venturi meters, or nozzles, are limited in their metering ranges. A number of means have been used to increase this range since in many cases it is required to proportion flows of gases and liquids correctly over a range wider than a stationary orifice or Venturi would permit.
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viding the metering orifices close to the mixing point so that pressure changes occur on both * e orifices to the same extent and are, therefore, eliminated. Changes in moisture content and t e m p e r a t u r e may cause a false reading and, therefore, false proportioning. In this case an automatic correction of the volumetric proporK PRINCIPLE OF OPERATION tion is required whenever the problem does not allow such error as this-for example, when mixing gas to a constant B. t. u. content, as is frequently done in the gas distribution system of cities or large industrial plants. Figure 10 shows how a correction of the volu---L metric proportioning is automatically made by OIL FLOW means of a Calorimixer. The Calorimixer continuously takes a sample of mixed gas, measures TRANSOMETER its heat value. and changes the Dosition of the ratio slider whenever the {eat val;e changes from its predetermined value. The Calorimixer has been developed .-.-.-. 1 especially for this purpose. It has only a small time lag and is extremely accurate. The r a t i o slider is actuated by a small electric motor which is started in one direction or the other by the Calorimixer whenever the B. t. u. content of the mixed gas changes. An interrupter is p r o v i d e d to make corrections in small increments to allow for the short time lag of the Calorimixer. Many installations of this kind have been made throughout the country to control city gas supply.
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Proportioning of Mixed Fuels
Proportioning of Two Fluids FIGURE8. BOILERCOMBUSTION CONTROL Figure 9 shows a proportioning control between two fluids. In-this case both flows are measured by means of variable orifices in such a way that a constant differential across each orifice is maintained and the position of each orifice is used as a metering impulse for the flow. A differential pressure regulator is provided for each flow to maintain a constant differential across the movable orifice, and the position of the orifice is used to give a flow impulse to the main proportioning regulator. The impulse systems on the ratio regulator are, therefore, of the mechanical type. The ratio control itself works on the same principle as was previously described except that the metering principle is reversed from that shown on the previous layouts.
Another problem enters into this kind of proportioningnamely, t o mix two gases, such &S coke-oven and refinery gas, and dilute the latter with air in such a way that the h a 1 mixture when measured by means of an orifice plate will always create a metering impulse proportional to the air requirement for burning the fuel. This has become a common
Proportioning of Gases I n volumetric proportioning of any kind there is a possibility of error due to changes in gravity of the two flows for which no correction is made. Pressure changes, for example, cause an error in the mixing which may be eliminated by pro-
FIQURE9, PROPORTIONING CONTROL OF Two FLUIDB
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and important problem in larger industrial plants where various fuels are available for heating and processing purposes, and where the correct combustion of the fuel is a major function for the operation. I n this case the B. t. u. content is automatically changed in relation to the gravity of the mixed fuel. With automatic proportioning devices it is possible to change from 100 per cent of one fuel to 100 per cent of another or to any mixture of the various fuels and still measure the total mixture by means of orifices; the differential pressure of each is used as the primary impulse for the combustion control a t the point of consumption. The combustion control 1 thus provided has as its primary impulse a measFZ urement proportional to the amount of air required to burn the fuel and as secondary impulse the actual air flow to the furnace. This air flow is being controlled by the combustion regulator, and the combustion is correct for all propbrtioning conditions of the various fuels. The ratio between B. t. u. content and gravity for which the regulator is to be adjusted automatically is set by the Calorimixer, the metering device of which is changed in its adjustment by the position of the ratio slider on the regulator. Thus, the final mixture obtained has a varying B. t. u. content but a cohstant air requirement. I n many cases it is desirable to establish a flow directly proportional to two or more primary flows. This may be accomplished as shown in FIGURE11 (Above). SUMMARIZING OF Two FUELS FIGURE 12 (Below). SUMMARIZING OF THREE DIFFERENT FUELS Figure 11. F1 and Fz represent the primary flows, and transforming regulators A and B are used to establish secondary flows in proportion to primary flows three secondary air flows established are then summarized by means of regulator K which, in turn, controls the valve in Fl and Fe. This is accomplished by taking the differential pressure across the orifice in the primary line as a primary the air supply line to provide the correct amount of air for impulse. The transforming regulators then establish a flow combustion. proportional to the primary flow by controlling valves in lines F A and FB. The two secondary flows established are Conclusions totalized in line Fc and measured bytotalizing meters D or The foregoing shows the Present status in the development E. This flow impulse is transferred to regulator c which, in of automatic ratio controls and indicates the trend for the turn, establishes the flow, Fa. Flow F s is, therefore, in direct future. The tendency towards increase in flow ranges shows proportion to the two primary flows, F1 and Fz. the necessity for combining research work on flow measurement through variable orifiGe plates. Although a number of individual companies have pioneered in this field, it is necessary that a group effort be worked out in a manner similar to that accomplished by research work of the American Society of Mechanical Engineers, on fluid meters and fixed orifices. The successful application of control equipment to any process is dependent upon the following: 1. The control should be based on sound engineering prin-
ciples.
FIGURE10. VOLUMETRIC PROPORTIONING OF Two GASESWITH CORRECTION FOR CHANGES IN HEATVALUE
A further use of a summarizing device may be employed in proportioning air flow to fuel flow where two or more fuels are being bbrned. As outlined in Figure 12, gas, oil, and coal are the fuels to be summarized. A illustrates a transf o r ~ n regulator g which establishes a secondary flow directly proportional to the gas flow. Regulator B establishes an air flow proportional to the amount of oil flowing by, using the Transometer previously described. Blower C is driven by a motor which also drives stoker D. This blower develops a flow of air directly proportional to the speed of the stoker and, therefore, proportional to the amount of coal being fed. The
2. The control manufacturer should be thoroughly reliable. 3. Field service is important towards securing successful results. 4. Sufficient information should be available to specify the type of control correctly.
The last point is without doubt the most important. For example, in specifying a 50 per cent excess flow (on a flow control) over the actual conditions, a 125 per cent increase in range is imposed upon the control equipment, thereby decreasing the accuracy a t actual flow conditions. This phase of specifying the control, therefore, makes it necessary that both the engineer who specses the equipment and the control manufacturer thoroughly analyze the problem SO that the equipment is applied correctly. RECEIVED September 1, 1937.
