VALVES (MECHANICAL) are devices for controlling the flow of air, gases, or liquids. They vary from the small air-valve in the auto mobile tyre to huge valves for controlling the flow of large volumes of water in such engineering developments as the locks of the Panama Canal.
One of the simplest types is the valve shown in fig. I. This is made in all sizes up to about so ft. in diameter in water-pipes and is known as the butterfly-valve. This valve is similar to the damper used in stove-pipes. Another simple type is the flap-valve which is used as a check-valve in hand-pump pistons, to allow water to pass through the plunger on the down stroke and to prevent its return on the lift stroke. A similar valve (fig. 2) is also used between steam boilers and the pump or other method of supplying the boiler with water. When the water pressure is withdrawn, the steam pressure in the boiler forces the valve to its seat and retains the water in the boiler. In another form of check valve the stem guides the valve to its seat.
It is but a step from this to the globe- 'valve in fig. 3 where the valve is closed by screwing down on the valve stem. This type of valve is probably used more largely than any other in controlling water and steam. The valve seat in fig. 3 is usually separate from the valve - stem. In most cases the seat can be easily removed when worn and replaced. In some designs of simple globe-valves, the seat can also be replaced when worn.
In a later design of globe-valve, which has replaced the valve in fig. 3 to a con siderable extent, the valve acts as a gate in the line of flow, and does not compel the liquid or gas to take such a circuitous route as in the globe-valve. Gate-valves are made with both straight and angular seats. When the angular seat is used, the gate acts as a taper wedge between the seats. When the seats are parallel, the wedging action is secured by a tapered piece on the valve stem being forced between the two discs. The valve openings in both types are usually round, as when placed in a pipe line and forming part of it. In exceptional cases, how ever, the valves are rectangular, as in some of the gates of the intakes of the Panama Canal locks.
A valve used largely in high pressure work and for securing fine adjustments of the opening, as in spraying or atomizing, is known as the needle-valve (fig. 4). The area of the opening can be
varied very slightly as the needle is moved in its seat. By varying the angle of the needle point, the opening per unit of movement can be varied between wide limits. This type of valve is used in spraying fuel into Diesel engine cylinders.
Valves for admitting steam or gas to engines are either sliding, rotary or poppet. The slide-valve is among the oldest types used for this purpose, but has largely disappeared, owing to the power required to move them under high pressure. The old steam engine D-valve admits steam first to one end of the cylinder and then the other, the exhaust taking place in the central cavity. This type of valve has now been largely supplanted by the piston-valve, particularly in locomotive use. A piston-valve (fig. 5) acts in the same way as the D-valve, the difference being that the valve is balanced by the steam at each end or in the centre. In some cases, the admission of the steam takes place at the centre of the valve • and the exhaust from each end.
Rocking- or rotary-valves are also used for similar purposes. Such valves admit steam in one position and block its flow when turned so as to close the port. Valves of this kind were used on steam engines of the Corlis type. Similar valves are also used to direct the flow of air, water, or gas in one or more directions at will. They are frequently known as three way or four-way valves. A four-way valve is shown in fig. 6.
When moved from one position to another, the flow is directed into any of the pipes shown. The simplest form of rotary-valve is the plug cock in fig. 7. (For gasolene engines of the poppet and sleeve types see MOTOR CAR, Valves.) Safety-valves, for relieving the pressure of boilers, pressure tanks, or pipe lines, may be said to be check-valves held to their seats by weights or springs set so as to open when the desired blowing-off pressure has been reached. Safety-valves are made in a variety of forms, the older type hav ing a lever with a weight placed at the proper point to give the desired pressure. Loaded valves have springs over the valve seat instead of the lever. (F. H. C.)