Patent GB-895792-A (GB)
Title: Method and apparatus for summing pneumatic signals
IPC: G01D5/12
Filed: 1960-07-22
Abstract: 895,792. Fluid-pressure servomotor-control systems. SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ NV. July 22, 1960 [July 24, 1959], No. 25595/60, Class 135. [Also in Groups XI and XIX]  Apparatus for summing two or more entities F 1 , F 2  comprises a vessel G, in which a constant pressure prevails, the vessel being supplied by two or more lines, each connected to a source of pneumatic pressure signals governed by a measuring instrument M 1 , M 2  ... and vessel discharging, through a line L, the total amount of gas supplied by the sources, so that an instrument, e.g. a heat-conductivity meter, measuring the magnitude of this gas stream indicates the required sum. Where the entity F 1  say is a flow in a line L 1  measured by the instrument M 1  by the pressure difference over an orifice plate in the line L 1  there will be a square relationhip between the flow F 1  and the pneumatic pressure signals governed by the instrument M 1  to represent the flow F 1  and the supply line to the vessel G will have to incorporate a resistance R 1  with a square resistance characteristic If one of the entities is a flow measured by a volumetric flowmeter the supply line to the vessel G will have to have a linear resistance characteristic. In each case the amounts of gas # 1  . . . supplied to the vessel G will be directly proportional to the entities F 1 ... By suitably choosing the resistances R 1  . . . and the levels of the pneumatic signal sources, a weighted sum may be obtained representing, e.g. the total combustion value of various streams of fuels having different specific combustion values. In the embodiment of Fig. 2, a pressure controller PC affecting the position of a control valve V maintains the pressure in the vessel G constant so that the sum ft of the gas streams # 1 , # 2 , # 3  is drawn off through the line L. A pressure difference depending on the magnitude of ft develops over a resistance R 0  and the pressure at a point S between the resistance R 0  and valve V is a measure of the sum. In another embodiment, Fig. 3, the pressure controller PC and vessel G are housed in a box D separated from one another by a diaphragm E and the valve V comprises a jet pipe H and cover-plate K connected to the diaphragm E by a rod N passing through a very flexible membrane E<SP>1</SP>. A constant pressure is maintained above the diaphragm E by compressedair supplied or drawn off via a reducing valve Q and the arrangement automatically ensures a constant pressure in the vessel G. A pneumatic signal representative of the sum ft can be derived from a branch S between a constriction R 0  and the valve V. In the process of firing a furnace 1, Fig. 4, with gas and oil, summing apparatus 21 is used to control the supply of oil over line 3 in accordance with the variable supply of refinery gas over line 2. The pressure of the gas in the main line 2 is measured by a meter 10, which adjusts the set point of a controller 13 and in turn adjusts a valve 11 in the supply line 2<SP>1</SP> to the gas burner 4, after comparing the gas flow in line 2<SP>1</SP>, as measured by a meter 12, with the set point value of controller 13. A valve 14 in the supply line 3 to the oil burner 5 is similarly adjusted by a controller 16 after comparing the oil flow in line 3 as measured by a meter 15 with the set point of the controller 16. The total combustion value of the gas and oil supplied is derived by the apparatus 21 from the quantities measured by the meters 12 and 15 weighted by assumed average specific combustion values of the gas and oil respectively and the resulting signal is compared with the set point value of a controller 22 which has been preset to the value required for normal operation of the furnace. The controller 22 adjusts the set point of the controller 16 as as result of this comparison. A further correction may be applied by a meter-controller 17 if the end product, e.g. the pressure of steam in line 9 deviates from a set value, by arranging for the controller 17 to adjust the set point of controller 22. If the furnace is under a variable load it is desirable to control the air supply from line 6 by a valve 18 controlled by a controller 20 whose set point is controlled by the signal from the summing apparatus 21. To prevent the oil burner 5 from being underloaded a limit may be applied to the extent to which the set point value of 16 (via 22) may be reduced, as well as a limit to the extent to which the set point of 13 (via 10) may be increased.