College of Product Knowledge: GENERAL APPLICATION VALVES PART III
PLUG VALVESThis type uses a rotating cylinder with a port (hole) through it. When the port is aligned with the passageway, fluid is permitted to flow. When the cylinder is rotated 90? (commonly referred to as a "quarter turn" activation), a solid portion of the cylinder blocks passage of the fluid through the valve (the "off" position). Plug valves have some of the same operating characteristics as the gate variety, including a straight-through flow. And like gate valves, this type is not usually recommended for throttling applications, and is intended primarily for "on-off' service.
Construction: The two basic approaches to plug valve design are called the "lubricated" and "non-lubricated" types. It is important to understand that the term "lubricated" in this case refers to a means of sealing the plug against leaking. Filling grooves on the plug with a viscous lubricant (sometimes called "sealant") provides a leakproof seal in the area between the plug and valve body, also assuring smooth and easy operation. Non-lubricated plug valves, on the other hand, do not use such a means of sealing, typically using "O" rings instead. There are two common ways to achieve sealing of the passage in a plug valve using a non-lubricated approach. The first involves a design in which the plug is allowed to wedge tightly into its mating cavity when rotated into the "off" position (thus providing a tight seal), and is made to lift slightly when turned on (to relieve binding). The other approach uses an elastomeric (resilient) sleeve around the plug, or the surface of the plug itself in a resilient material.
As far as the design of the port through the plug is concerned, there are several common options: standard, venturi, full-port and throttling port.
In addition to the standard "single bore" plug valve designs with a single inlet and single outlet, there are also special "multi-port" types that can be used for switching flow. These include "three-way" and "four-way" designs, providing a number of switching and diverting combinations.
BALL VALVESThere are similarities between ball valves and plug valves in both mechanical concept as well as application. Instead of a cylinder with a port through it, however, ball valves employ a simple spherical or "ball" component with such a channel. Also like the plug valve, the ball variety is characterized by a straight-through flow with quarter-turn activation.
One of the key differences between the two types, however, is the means of sealing. Rather than relying on a generalized wedging to accomplish the seal, as in the case of most plug valves, these types achieve a more specific sealing by means of resilient seat rings on either side of the ball. And rather than relying solely on the tightness of the mechanical assembly to assure the leak-proof seal, ball valves take advantage of the fluid pressure to enhance the seal. In other words, pressure pushing against the ball from the upstream side (when the valve is in the "off' position) tends to push it even more firmly against the seat on the downstream side.
Construction: There are several approaches to the construction of a ball valve, reflected in catalog terms such as "one-piece," "two-piece," and "three-piece." The number of "pieces" in each case refers to the component makeup of the basic valve body. These terms are all related to the means by which the ball and seats are secured in place. In terms of servicing, the top entry design offers the advantage of the ball and seats being accessible without removing the valve from the connecting piping. Top entry valves usually employ a one-piece body. End entry designs are more common, however. As the name implies, the ball and seats are inserted from one end of the valve. The stem is not an integral part of the ball, but rather, is inserted into a mating socket once the ball has been positioned in place inside the body.
As far as the porting (size of the opening through the ball) is concerned, there are three common types used: standard or regular port, full port, and reduced port. Of the three, only the full port provides a passage capacity the same as the connecting piping. The standard or regular types have port sizing that is smaller, and reduced port models smaller yet. Like plug valves, there are multi-port ball valves for switching applications. Seats for ball valves are made of a variety of resilient materials, including TFE (Teflon), nylon, and numerous rubber compounds.
BUTTERFLY VALVESThis next category of valves is based on what we have termed the "rotating disc" concept of closure. The disc is a wafer-like section of material that blocks a general passageway through a valve when set in the "off" position. Rotating the disc 90? opens the valve to its full flow capacity, and unlike most other types of valving, this is accomplished by leaving the complete control member in the passageway. Because the disc has such a thin profile, it causes little restriction or pressure drop as it "stands in the stream." This design has a versatile combination of operating applications, including "on-off' service, throttling, and installations requiring frequent activation. Most commonly, butterfly valves are specified for service involving high volume fluid with relatively low pressure. They are especially well suited for the handling of fluids that contain suspended solids (often called "slurries").
Construction: The key mechanical components of a butterfly valve are the disc and seat. Discs are specified according to the material required, including types coated with corrosion-resistant and wear-resistant plastic or rubber. The seat of a butterfly valve is a ring-shaped component that keys into place in a corresponding groove or shoulder in the body passageway. Options in seat material include metal (usually used when complete shutoff is not required), and various plastic and rubber specifications. Like ball valves, the stem is not an integral part of the disc itself. It is keyed into the disc once the disc has been set in position within the valve body. In addition to these "traditional" butterfly valve designs, there is another variety that reverses the typical sealing arrangement. By this, we mean that the resilient sealing member is located around the rim of the disc (like an "0" ring in a groove around the edge), and seals on a smooth metal surface inside the valve body.
DIAPHRAGM VALVESThis final valving concept is the diaphragm type which involves the compressing of a resilient membrane across a passageway in order to control flow. Diaphragm valves are usually specified for primary on-off service, though there are special designs suitable for throttling applications. They are especially well suited for service involving slurries, corrosives and for applications in which sanitation of the media is important (such as food and beverage processing).
Construction: Bodies for diaphragm valves are generally one piece, but there are also two-piece models in which the top half takes the form of a separate, bolted-on component. This makes possible the assembly and retaining of the diaphragm inside. Diaphragms themselves are available in a variety of resilient materials, ranging from natural rubber to a number of synthetic compounds. These are specified according to the temperature requirements, as well as the chemical compatibility with the media involved.
In terms of activation means, the one part unique to diaphragm valves is a component called the compressor. As the linkage between the stem and the diaphragm, this is the part that actually acts upon the membrane to accomplish the sealing (pushes it down onto the seat).