VALVE

in music, a device, first used in 1815 by musicians Heinrich Stlzel and Friedrich Blhmel of Berlin, that alters the length of the vibrating air column in brass wind instruments by allowing air to pass through a small piece of metal tubing, or crook, permanently attached to the instrument. Descending valves switch in extra tubing, lowering the fundamental pitch; the less common ascending valves cut air off from the tubing, raising the pitch. Valves enable players to produce notes lying outside the harmonic series of an air column the length of the original tubing (in relative pitch, Ccgcegb [approximately]cde, etc). Brass instruments normally have three descending valves; used in combination they can lower the pitch of the instrument six semitones. Two principal switching methods are used: piston and rotary mechanisms. in anatomy, any of various membranous structures, especially in the heart, veins, and lymph ducts, that function to close temporarily a passage or orifice, permitting movement of a fluid in one direction only. A valve may consist of a sphincter muscle or two or three membranous flaps or folds. In the heart there are two valves that prevent backflow of blood from the ventricles into the atria. On the right side of the heart is the tricuspid valve, composed of three flaps of tissue; on the left is the two-piece mitral valve. Once blood has left the heart and entered the aorta, its return is prevented by the semilunar valves, which consist of membranous saclike flaps that open away from the heart. If the flow of blood reverses, the flaps fill and are pressed against each other, thus blocking the reentry of blood into the aorta. The valves in the venous system are of this same type. A valve unique to the lower vertebrates is the renal portal valve, which closes to shunt blood past the kidneys, increasing its supply elsewhere when necessary. In the digestive system of mammals the ileocecal valve, controlled by a sphincter muscle, prevents the return of the contents of the small intestine after they have passed into the colon. in mechanical engineering, device for controlling the flow of fluids (liquids, gases, slurries) in a pipe or other enclosure. Control is by means of a movable element that opens, shuts, or partially obstructs an opening in a passageway. Valves are of seven main types: globe, gate, needle, plug (cock), butterfly, poppet, and spool. In the globe valve shown in the Figure (far left), the movable element M may be a tapered plug or a disk that fits a seat on the valve body; the disk may carry a replaceable rubber or leather washer, as in a household water faucet. In a gate valve, the movable element is a wedge-shaped disk that seats against two tapered faces in the valve body. A needle valve has a long tapered needle fitting in a tapered seat. A plug valve, or cock, is a conical plug with a hole perpendicular to its axis fitting in a conical seat in the valve body at right angles to the pipe. By turning the plug the hole is either lined up with the pipe to permit flow or set at right angles to block the passage. A butterfly valve is a circular disk pivoted along one diameter; the solid lines in the Figure (left centre), show one in the closed position. In the fully open position, shown dotted, the disk is parallel to the direction of flow. The damper in a stovepipe or a warm-air heating system is of this type, which is also used in the intake passage to carburetors on gasoline engines. On hydraulic turbines such valves may be 20 feet or more in diameter. Some valves operate automatically; check (or nonreturn) valves, for example, are self-acting and permit flow in one direction only. They are made in several types. If the movable element in the globe valve in the Figure were kept on its seat by gravity or a spring, it would permit flow from left to right but not from right to left. Safety valves, which are usually of the poppet type, open at a predetermined pressure. The movable element may be kept on its seat by a weighted lever or a spring strong enough to hold the valve closed until the pressure is reached at which safe operation requires opening. On gasoline engines, poppet valves are used to control the admission and rejection of the intake and exhaust gases to the cylinders. In the Figure (right centre), the valve, which consists of a disk with a tapered edge attached to a shank, is held against the tapered seat C by a compressed spring. The valve is raised from its seat by the action of a rotating cam that pushes on the bottom of the shank, permitting gas flow between region A, which leads to the intake or exhaust pipes, and region B, which leads to the cylinder. In hydrostatic fluid-power systems, in which the working medium is usually pressurized oil, spool valves are employed to regulate the oil flow. The valve shown in the Figure provides two flow paths for the output from a pump. In the extreme upper position, as shown, active flow is from the pump port P to the working, or load, port B; discarded fluid from the load passes from port A to the tank or sump port T. In the extreme lower position, the functions of ports A and B are reversed. In the mid or neutral position of the spool, ports A and B are blocked. The movement of the spool may be manually or electrically controlled.