BALL VALVES: Ball valves, as the name implies, are stop valves that use a ball to stop or start the flow of fluid. A ball valve is a valve that opens by turning a handle attached to a ball inside the valve. The ball has a hole, or port, through the middle so that when the port is in line with both ends of the valve, flow will occur. When the valve is closed, the hole is perpendicular to the ends of the valve, and flow is blocked. The handle or lever will be inline with the port position letting you "see" the valve's position. A ball valve includes a valve seat assembly having a lower valve seat, an upper valve seat, pickings located in openings formed by apertures in abutting sides of the valve seats, and annular retainers located in central holes through the pickings. Each of the components has a profiled inner surface for slidable contact with the ball controlling communication between the fluid passages in the valve box. The use of such components for the valve seat simplifies manufacture, gives a wider choice of materials to be used and reduces costs of repairs. There are four general types of ball valves: full port, standard port, reduced port, and v port.

Check Valves :Check valves are used to allow fluid flow in a system in only one direction. They are operated by the flow of fluid in the piping. A check valve may be the swing type, lift type, or ball type. As we have seen, most valves can be classified as being either stop valves or check valves. Some valves, however, function either as stop valves or as check valves-depending on the position of the valve stem. These valves are known as STOP¬CHECK VALVES. A stop-check valve is shown in cross section in figure 9-23. This type of valve looks very much like a lift-check valve. However, the valve stem is long enough so when it is screwed all the way down it holds the disk firmly against the seat, thus preventing any flow of fluid. In this position, the valve acts as a stop valve. When the stem is raised, the disk can be opened by pressure on the inlet side. In this position, the valve acts as a check valve, allowing the flow of fluid in only one direction. The maximum lift of the disk is controlled by the position of the valve stem. Therefore, the position of the valve stem limits the amount of fluid passing through the valve even when the valve is operating as a check valve. Stop-check valves are widely used throughout the engineering plant. Stop-check valves are used in many drain lines and on the discharge side of many pumps.

Special-Purpose Valves There are many types of automatic pressure control valves. Some of them merely provide an escape for pressures exceeding the normal pressure; some provide only for the reduction of pressure; and some provide for the regulation of pressure.

RELIEF VALVES: The relief valve is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire. Most relief valves simply lift (open) at a preset pressure and reset (shut) when the pressure drops only slightly below the lifting pressure. System pressure simply acts under the valve disk at the inlet of the valve. When system pressure exceeds the force ex¬erted by the valve spring, the valve disk lifts off its seat, allowing some of the system fluid to escape through the valve outlet until system pressure is reduced to just below the relief set point of the valve. The spring then reseats the valve. An operating lever is provided to allow manual cycling of the relief valve or to gag it open for certain tests. Virtually all relief valves are provided with some type of device to allow manual cycling. Other types of relief valves are the high-pressure air safety relief valve and the bleed air surge relief valve. Both of these types of valves are designed to open completely at a specified lift pressure and to remain open until a specific reset pressure is reached-at which time they shut. Many different designs of these valves are used, but the same result is achieved.

SPRING-LOADED REDUCING VALVES: Spring-loaded reducing valves are used in a wide variety of applications. Low-pressure air reducers and others are of this type. The valve simply uses spring pressure against a diaphragm to open the valve. On the bottom of the diaphragm, the outlet pressure (the pressure in the reduced pressure system) of the valve forces the disk upward to shut the valve. When the outlet pressure drops below the set point of the valve, the spring pressure overcomes the outlet pressure and forces the valve stem downward, opening the valve. As the outlet pressure increases, approaching the desired value, the pressure under the diaphragm begin to overcome spring pressure, forcing the valve stem upwards, shutting the valve. You can adjust the downstream pressure by removing the valve cap and turning the adjusting screw, which varies the spring pressure against the diaphragm. This particular spring-loaded valve will fail in the open position if a diaphragm rupture occurs.For more information,please visit http://www.bossgoo.com