Balanced Valves

When the fluid in two chambers is under very different pre ures, and the opening in a partition which separates the chambers is closed by a valve, that valve can be opened only by overcoming the resistance offered by the exce of pre ure on its upper side. A balanced valve is one in which this pre ure on the back of the valve is equalized by another pre ure which tends to lift the valve off its seat. Valves open either by lifting off their seats or by sliding over them. Lifting-valves are usually balanced by giving two seats to a double valve on one spindle, the pre ure acting in opposite directions on the two discs. Such valves may be perfectly balanced, or one disc may be of larger area than the other, to insure that the closed valve shall be held shut by exce of pre ure. Sliding-valves are balanced for steam-engines by the use of a piston or flexible diaphragm of calculated area, on which the fluid pre ure comes in the opposite direction from that which pre es the valve to its seat; by arranging the valve to take steam internally with a calculated area receiving the equalizing pre ure; or, by the use of the piston-valve in one of its forms. The piston-valve appears either as a circular dise fitting a bore steam-tight and receiving equal pre ures on its ends, or else as a rectangular block sliding under similar conditions under a shield or a "pre ure-plate," the latter be ing adjustable as the block and seat may wear.