790,724. Valves. ROBERTSHAW-FULTON CONTROLS CO. Jan. 25, 1956 [May 19, 1955], No. 2478/56. Class 135. The interior of a bellows 50 is connected to a temperature-sensing device as well as a reservoir bellows 86 by capillary tubing, all of which are filled with a volatile liquid; the sensing device comprises a bulb and bellows which latter is manually adjustable to vary the volume of volatile liquid in a surrounding space. When the closure 24 is unseated steam passing between the valve inlet 12 and outlet 14 contacts the bellows 50, which is attached to the closure 24, and thereby vaporizes the liquid therein as well as the liquid in a bore 56 connected to the interior of the bellows 50 by passages 42 in a disc 40 slidable in the bore 56. A pressure P1, which is thereby generated in the bellows 50, is insufficient to overcome the bellows spring 46 and move the bellows 50; this pressure also acts upwardly on a power bellows 58 in the bore 56, and downwardly in reservoir bellows 86, joined to the power bellows 58 by a stem 62, so that there is a resultant upward force directly proportional to the difference in areas between the bellows 56 and 86, which force is counterbalanced by the spring rate of the bellows and an adjustable spring 126 acting on a lever 112 attached to the stem 62 so that the system is balanced with the closure 24 and bellows 52 fully open. When the temperature of the sensing device increases, additional liquid is forced thereby into the bellows 50 wherein it vaporizes and causes the production of a higher pressure P2 which acts on the area difference between the power and reservoir bellows 58 and 86, thereby destroying the system balance and causing upward movement of the power bellows 58, and hence the reservoir bellows 86, which results in some of the liquid from the latter being forced, via capillary tubing, into the bellows 50 so producing a further pressure increase therein, this regenerative action continuing until the upward Quid force is again balanced by the force produced by the continued expansion of the spring 126 and the spring rate of the bellows. The closure 24 will be seated when the sensing bulb attains the required temperature. Should the bulb temperature fall condensation of the vapour occurs and so the pressure inside the bellows 50 is progressively lowered and the spring 126 forces the bellows 58 and 86 to move downwardly until the system is again balanced, whereupon the closure 24 is opened the requisite amount. The force exerted by the spring 126 may be adjusted by either varying the fulcrum point of a lever 112 on a support 114, the anchor positions of either end of the spring or the position of the pivots 109 on the stem 62.