Patent GB-744566-A (GB)
Title: Improvements in or relating to fluid pressure relays
IPC: F22G7/10
Filed: 1953-11-06
Abstract: 744,566. Fluid-pressure servomotor-control systems. BAILEY METERS & CONTROLS, Ltd. Nov. 6, 1953 [Nov. 6, 1952], No. 30826/53. Addition to 739,501. Class 135. In the fluid-pressure relay described in the parent Specification, the means associated with the cooperating baffle and nozzle elements 20, 21 for adjusting the relationship between the angular movement of the first force-balance beam 17 and the consequential angular movement of the second balance beam 18, i.e. the sensitivity or " proportional band " of the relay are actuated by means such as the bellows 30 which can be remotely controlled. The invention is applicable to boiler feed control, to fluid flow ratio control, to the establishment of a function of a variable such as its square or its square root, and to convert the non- linear readings of a flowmeter functioning according to the pressure difference across an orifice into linear readings of fluid flow. Fig. 1 shows the relay 16 applied to control the supply of feed water through a pipe 2 to a steam drum 1. The demand or output rating of the boiler represented by steam flow in the pipe 6 and measured by the pressure difference across an orifice 5 and converted by meter 8 having a relay as described in Specification 736,731 to a varying loading pressure in a pipe 10 is compared with the water flow in the pipe 2 as measured by the pressure drop across an orifice 4 and converted by the meter 7 to a loading pressure in a pipe 11. These loading pressures are shown on an indicator 12 and applied to the bellows A, B of the relay. If the pressures are out of balance, the output control pressure is in the bellows D and in an output pipe 19 is varied by a relay 22. This latter pressure is then compared by a further relay 35, generally similar to the relay 16, but having no or only manually actuated sensitivity adjusting means, with a loading pressure in a pipe 13 produced by a meter 9 and representative of the actual water level in the drum 1. Any disparity in these latter pressures varies the pressure in a pipe 36 connected through a manual-automatic selector station 38 to a positioner 40 as described in Specification 683,696. The latter adjusts a regulating valve 3 in the pipe 2 so as to correct the liquid level in the drum 1. As the meters 7, 8 are " head meters " responding to pressure drop, they are, due to the quadratic relation between pressure head and flow, much less sensitive at low output rates than at high ones and to compensate for this the relay 16 is made progressively less sensitive as the rating increases by varying the proportional band. As in the parent Specification, the proportional band is adjusted by movement of a sector 23 about a fixed axis 24. The sector 23 carries a pivot 25 for a link 28 having a floating pivot on the baffle 20. The pivot 27 and the axis 24 coincide in the normal balanced position of the beams. The sector 23 is adjusted through a linkage 32 by the bellows 30 which is subject to a pressure representative of the current boiler rating, in the case of Fig. 1, the pressure in the pipe 10 which is itself representative of the steam outflow rate. The sector 23 may, however, be positioned by some other index of boiler rating. Thus, it may be positioned by a linkage connected to the stem of the regulating valve 3, or by a bellows subject to pressure acting on the diaphragm 45 of the valve 3. The output pressure in the pipe 36 acts directly on the bellows D of the relay 35 and through a restriction 37 on the bellows C so that this relay functions in a manner similar to that described in Specification 461,389. Fig. 5 shows the relay 16 applied to a flow ratio control. The flow in conduit 51 (flow No. 2) is regulated by a valve 52 so that it bears a predetermined ratio to the master flow (flow No. 1) in a conduit 50. The meters 53, 55 register flow rate in equal increments and produce loading pressures in conduits 54, 56 which act on the bellows A, B of the relay 16. The output pressure in a line 57 acts on the valve 52 and also on the bellows C, D. If a 1 : 1 ratio were desired, the proportional band would remain fixed at 100 per cent and no adjustment thereof would be necessary. If the ratio of flow No. 2 to flow No. 1 is to be greater than unity, the proportional band is continuously adjusted toward the narrow end (i.e. greatest sensitivity) by subjecting the bellows 30 to the loading pressure in the conduit 54. If this ratio is to be less than unity, the bellows 30 moves the sector 23 in the opposite direction through a reversing linkage so that the proportional band is adjusted towards the wide end (i.e. least sensitivity). If the meters 53, 55 register head and their responses are therefore non-linear, compensation for the quadratic relation is effected by connecting the linkage 32A, Fig. 5, or the reversing linkage to the sector through a cam mechanism comprising a shaped slot in the sector 23 guiding a roller on the link 32A. In some cases, e.g. in chemical treatment processes, it may be necessary to adjust the ratio of two flows in accordance with a third variable such as temperature, pressure, hydrogen ion concentration, viscosity, &c., and for these cases a loading pressure continuously representative of this third variable is applied to the bellows 30. In Fig. 8, a pneumatic loading pressure representative of a single variable is applied to the bellows A of the relay 16 by a pipe 66 and its characteristic is changed in accordance with a second variable by applying a pressure representative of this second variable to the bellows 30, thereby modifying the relation between the input pressure in pipe 66 and the output pressure in the pipe 68. Such an arrangement may be utilized to maintain a uniform superheat temperature over the whole range of boiler rating in plant where radiant and convection superheaters are used. Fig. 10 shows the relay 16 used as a square root extractor more particularly for converting the non-linear responses of a " head " meter 71 so as to give linear responses on a flow indicator 75 connected to the output of the relay 16. In this case the bellows 30 is subject to the loading pressure from the meter 71 and adjusts the sector 23 through a reversing linkage 32. In a further modification, the same result is achieved by applying the output pressure in the line 74 to the bellows 30. The relay 16 may be utilized to obtain other functional relationships, e.g. a five halves power law.