All stem seals or packing are subject to wear, which can lead to leakage. Valves with packing require servicing or replacement at regular intervals, although some types of packing, such as the two-piece chevron design, create more effective seals and last longer than others.
Unlike packed valves, diaphragm valves (Figure 2) are packless. They provide rapid shutoff and precise actuation speeds and, in some cases, also may deliver consistent quantities of process fluid. Diaphragm valves typically are employed in high purity applications. They offer the highest cycle life of any valve type, thanks their highly engineered anatomy. Each valve contains a thin metal or plastic diaphragm that flexes up and down, creating a leak-tight seal over the inlet. This robust valve usually is small, with the largest orifice or internal pathway typically less than 2 in.
Bellows valves also are packless, making them a good choice when the seal to atmosphere is critical and access for maintenance is limited. They frequently are specified for the containment area in nuclear power plants. A welded seal divides the lower half of the valve, where the system media resides, from the upper parts of the valve, where actuation is initiated. The stem, which is entirely encased in a metal bellows, moves up and down (without rotating), sealing over the inlet.
Bellows and diaphragm valves are said to have a globe-like flow path. In globe valves, fluid doesn't flow straight through on a level plane as in a ball valve. Instead, it enters the valve under the seat and exits above the seat. Globe valves have lower flow rates than straight-through-flow-path valves of the same orifice size.
FLOW CONTROL VALVES
These enable an operator to increase or decrease flow by rotating a handle. Once adjusted to a desired flow rate, the valve will hold that flow rate reliably. Some flow control valves also provide very reliable shutoff, but many turns of the handle are necessary to move from the fully open to the fully closed position.
The most common flow control valves are needle, fine metering, quarter-turn plug and rising plug.
Needle valves (Figure 3) offer excellent flow control and, depending on design, leak-tight shutoff. They consist of a long stem with a highly engineered stem-tip geometry (e.g., vee- or needle-shaped) that fits precisely into a seat over the inlet. The stem is finely threaded, enabling precise flow control. Stem packing provides the seal to atmosphere.
Some designs feature a metal-to-metal seat seal; consequently, needle valves are a good choice for high-temperature applications. However, flow is limited because of the globe-style flow path. Needle valves also suit lighter, less viscous fluids.
For the most precise flow control, consider fine metering valves. Typically found in laboratory settings, these are a type of needle valve with a long, fine stem that lowers through a long, narrow channel. This makes for a pronounced globe pattern, ideal for marking fine gradations of flow. Some fine metering valves aren't designed for shutoff.
Quarter-turn plug valves are economically priced utility valves. Quarter-turn actuation rotates a cylindrical plug with an orifice in a straight-through flow path. Plug valves commonly are used for low-pressure throttling applications in addition to shutoff.
Rising plug valves, like needle valves, lower a tapered element into an orifice to reduce flow. They differ from needle valves in their flow path, which is straight-through rather than globe patterned. Because of the straight path, the valve isn't as effective at providing fine gradations of flow. The rising plug is roddable -- and so it's a good choice if there's a risk of clogging with system media.