Although many different types of valves are used to control the flow of fluids, the basic valve types can be divided into two general groups: stop valves and check valves. Besides the basic types of valves, many special valves, which cannot really be classified as either stop valves or check valves, are found in the engineering spaces. Many of these valves serve to control the pressure of fluids and are known as pressure-control valves. Other valves are identified by names that indicate their general function, such as thermostatic recirculating valves.
Stop Valves :Stop valves are used to shut off or, in some cases, partially shut off the flow of fluid. Stop valves are controlled by the movement of the valve stem. Stop valves can be divided into four general categories: globe, gate, butterfly, and ball valves. Plug valves and needle valves may also be considered stop valves.
GLOBE VALVES: Globe valves are probably the most common valves in existence. The globe valve is named after its shape. Its body has an interior partition, and the valve inlet and outlet centers are inline. This configuration forces a change in the direction of flow in the form of an S. A Globe valve is a device for regulating flow in a pipeline, consisting of a movable disk-type element and a stationary ring seat in a generally spherical body. Generally speaking, there are 3 styles of globe valves: straight pattern, angle pattern and Y-pattern. These are determined by the geometry of the end connection and the stem. In a straight pattern globe, the end connections for a straight line with the stem pointing vertically from this line. A Y-pattern is similar but the stem is angled by 45 degrees from the inlet-outlet path. An angle valve has the inlet and outlet oriented 90 degrees from each other with the stem perpendicular from one and in line with the other. Globe valves are used extensively throughout the engineering plant and other parts of the ship in a variety of systems
GATE VALVES: A Gate Valve, or Sluice Valve, as it is sometimes known, is a valve that opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar. The gate faces can form a wedge shape or they can be parallel. Gate valves are sometimes used for regulating flow, but many are not suited for that purpose, having been designed to be fully opened or closed. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low friction loss. Gate valves are characterised as having either a rising or a nonrising stem. Rising stems provide a visual indication of valve position. Nonrising stems are used where vertical space is limited or underground
BUTTERFLY VALVES: A butterfly valve is a type of flow control device, typically used to regulate a fluid flowing through a section of pipe. The butterfly valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle. The resilient seat is under compression when it is mounted in the valve body, thus making a seal around the periphery of the disk and both upper and lower points where the stem passes through the seat. Packing is provided to form a positive seal around the stem for added protection in case the seal formed by the seat should become damaged. There are different kinds of butterfly valves, each adapted for different pressures and different usage. The resilient butterfly valve, which uses the flexibility of rubber, has the lowest pressure rating. The high performance butterfly valve, used in slightly higher-pressure systems, features a slight offset in the way the disc is positioned, which increases the valve's sealing ability and decreases its tendency to wear. The valve best suited for high-pressure systems is the tricentric butterfly valve, which makes use of a metal seat, and is therefore able to withstand a greater amount of pressure.
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.
REMOTE-OPERATING VALVES: Remote operating gear is installed to provide a means of operating certain valves from distant stations. Remote-operating gear may be mechanical, hy¬draulic, pneumatic, or electric. Some remote-operating gear for valves is used in the normal operation of valves. For example, the main drain system manual valves are opened and closed by a reach rod or a series of reach rods and gears. Reach rods may be used to operate engine-room valves in instances where the valves are difficult to reach from the operating stations. Other remote-operating gear is installed as emergency equipment. Some of the main drain and almost all of the secondary drain system valves are equipped with remote-operating gears. You can operate these valves locally, or in an emergency, you can operate them from remote stations. Remote-operating gear also includes a valve position indicator to show whether the valve is open or closed.
PRESSURE-REDUCING VALVES: Pressure reducing valves are automatic valves that provide a steady pressure into a system that is at a lower pressure than the supply system. Reducing valves of one type or another are found, for example, in fire main, seawater, and other systems. A reducing valve can normally be set for any desired downstream pressure within the design limits of the valve. Once the valve is set, the reduced pressure will be maintained regardless of changes in the supply pressure (as long as the supply pressure is at least as high as the reduced pressure desired) and regardless of the amount of reduced pressure fluid that is used. Various designs of pressure-reducing valves are in use. Two of the types most commonly found on gas turbine ships are the spring-loaded reducing valve and the air-pilot operated diaphragm reducing valve. Air-pilot operated diaphragm control valves are used extensively on naval ships. The valves and pilots are available in several designs to meet different requirements. They may be used to reduce pressure, to increase pressure, as unloading valves, or to provide continuous regulation of pressure. Valves and pilots of very similar design can also be used for other services, such as liquid-level control and temperature control.
PRIORITY VALVES: In systems with two or more circuits, it is sometimes necessary to have some means of supplying all available fluid to one particular circuit in case of a pressure drop in the system. A priority valve is often incorporated in the system to ensure a supply of fluid to the critical/vital circuit. The components of the system are arranged so the fluid to operate each circuit, except the one critical/vital circuit, must flow through the priority valve. A priority valve may also be used within a subsystem containing two or more actuating units to ensure a supply of fluid to one of the actuating units. In this case, the priority valve is incorporated in the subsystem in such a location that the fluid to each actuating unit, except the critical/vital unit, must flow through the valve.

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