Patent GB-737891-A (GB)
Title: Improved methods of and apparatus for the regulation of final temperature of superheated vapour
IPC: F22G5/20
Filed: 1952-11-06
Abstract: 737,891. Fluid-pressure servomotor-control systems. BAILEY METERS & CONTROLS, Ltd. Nov. 6, 1952 Nov. 6, 1951], No. 27943/52. Class 135. In a vapour generating and vapour heating unit in which the vapour temperature constitutes a controlling factor giving an integral control over a normal range of operation, a further impulse is added to the control factor when the unit is operating outside the normal range to adjust the unit before, or to hasten the adjustment of the unit upon, entry into the normal control range. Fig. 1 shows the application of the invention to a steam generator with a convection type superheater. The feed water supply 5 to the boiler drum 1 can flow through parallel paths 6, 7, path 6 including an economiser 8 and path 7 including an attemperator 3 for desuperheating steam in the superheater elements 4. The proportioning of the flows through the paths 6, 7 and, hence, the. control of the steam temperature, is effected by valves 24, 25, respectively, these being jointly controlled by an air flow meter 10 to give a coarse control in accordance with the load or demand on the unit and a meter 13 responsive. to final steam temperature to give a fine control. An adjustable valve 9 by-passing valve 24 ensures that, at all times, there is sufficient flow of water through the economizer to prevent steaming. Meter 10 positions a pilot valve 11- described in Specification 425,507, [Group XII],-to give an output pressure which is. applied directly to chamber C and, through an adjustable bleed 17, to chamber B of a relay 16 -as described in Specification 461,389. The output from the relay includes proportional and integral components and it is applied, through a manual-automatic selector 19, to adjust valve 25 and, through a reversing relay 21 and a manual-automatic selector 22, to adjust valve 24. Meter 13 actuates a pilot valve 14 to give an output pressure which is applied to a relay 28. The output from this latter relay includes proportional and integral components and it is applied through a relay 29 as a controlling input pressure to relay 16. Relay 29 transmits a proportional and second derivative control effect. The output pressure of pilot valve 14 also acts on bellows-actuated valves 32, 26 and keeps them closed during operation over the normal range. When the steam temperature is below the control range, e.g. on start-up of the unit, valve 32 opens and the output pressure' from pilot valve 14, acting through a relay 36, adds an additional impulse to relay 28 giving an enhanced output with the integral action removed. At the same time, valve 26 is opened and the output pressure from pilot valve 11 is also applied to chamber B of relay 16. This has the effect of removing the control by the air flow (demand) through the unit. The various relays are all similar to relay 16 but with the connections modified as shown. The devices positioning valves 24, 25 are as described in Specification 683,696. Valves 26, 32 may be actuated electrically from meter 13. The reversing relay 21 is omitted if valves 24, 25 are to open in phase but by differing amounts. Control by the feed water flow and by the water level in the drum may be added. The invention is also applicable to a unit with the attemperator located between primary and secondary superheaters and with the attemperator water supply in series with the single supply to the drum through the economiser. Specification 730,589 also is referred to.