The oldest valve is a hinged flap that lets the flow do the work
Picture a flap hanging on a hinge inside a pipe. Push fluid one way and it swings open; try to send it back and the current slams the flap shut. That ancient design, the check valve, needs no handle, no motor and no operator, and the same trick keeps blood moving the right way through your veins.
The word comes from the Latin valva, the moving leaf of a door, which in turn goes back to volvere, to turn or roll. Any valve steers a gas, liquid, slurry or fluidised solid by opening, closing or partly blocking a passage, and in an open valve the fluid always travels from higher pressure toward lower. In air ducts at near-normal pressure the same devices are called dampers instead.
Most valves share a few parts. The body is the outer casing holding the internals, known collectively as the trim, and a bonnet screwed or bolted on top guides the stem, which carries motion from a handle or actuator to the disc that actually blocks the flow. Where the disc meets the body is the seat. Hard seats, cut straight into metal, almost always leak a little but last longer; soft seats of PTFE or rubber compounds seal far more tightly. Gate and globe valves tend to be hard seated, while ball and butterfly valves are usually soft.
Materials follow the job. Brass, bronze, cast iron and stainless steel are everyday choices, but desalination plants turn to duplex steels that resist warm seawater, sulphuric acid works use Alloy 20, and hydrofluoric acid plants rely on monel. Hastelloy handles the heat of nuclear plants, and inconel is favoured for hydrogen. Plastics such as PVC suit gentler pressures and temperatures.
Sizes run from a tenth of a millimetre to 60 centimetres, and special valves top five metres across. Some move themselves: a relief valve stays shut against a spring until excess pressure forces it open. Needle valves use long tapered discs for fine metering, and valves can carry up to 20 ports. The poppet valves in most combustion engines admit the fuel-air mix and release exhaust, while throwaway valves sit inside aerosol cans and pump dispensers.
Source: Valve
