Valves Types

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Ball Valves

The ball valve (Fig. A10.20) is a quarter-turn valve suitable for clean gas, compressed air, and liquid service. They also can be used for slurry service, but provisions for prevention of crud buildup must be made.

Ball valve in closed position

The use of soft-seat materials such as nylon, delrin, synthetic rubbers, and fluorinated polymers imparts excellent sealing ability. With fluorinated polymer seats, ball valves can be used for service tempera-tures ranging from —450 to 500°F (—270° to 260°C); with graphite seats, service temperatures to 1000°F (538°C) or even higher are possible. Also, with metal- backing seats, the valves can be used in fire-safe services. Ball valves are similarto plug valves in operation. They are nonbinding and provide leak-tight closure. The valves exhibit negligible resistance to flow because of their smooth body and port.

Construction of a Ball Valve

Major components of the ball valve are the body, spherical plug, and seats. Ball valves are made in three general patterns: venturi port, full port, and reduced port. The full-port valve has an inside diameter equal to the inside diameter of the pipe. In the venturi and reduced-port styles, the port is generally one pipe size smaller than the line size. Stem sealing is accomplished by bolted packing glands and O-ring seals. Valves are also available with a lubricant-seal system that is similar to that available for plug valves. A typical lubrication system is illustrated in Fig. A10.21.

Lubricant-seal system in a ball valve

A ball valve may be unidirectional, bidirectional, or multidirectional, depending on the number of valve ports and the number of valve seats. Therefore, ball valves are referred to as 2-way, 3-way, 4-way, or 5-way multiport valves. A 2-way ball valve with a single seat will be unidirectional with the flow direction indicated. Even a 3-way, 4-way, or 5-way ball valve can be unidirectional when flow must enter through a designated port. A 2-way ball valve provided with two seats, one on the upstream side and the other on the downstream side of the ball, is termed a bidirectional valve. Multiple-port ball valves permit flows in more than one direction, thus eliminating the need for several valves.

Ball valves are manufactured in several different body configurations: top entry, side entry, split body, and three-piece body. The valve ends are available as butt welding, socket welding, flanged, threaded, soldering, or brazing ends.

Ball valves are manufactured in high and low-pressure classifications. The advances in ball valve designs have made it possible to use these valves in high pressure and high-temperature applications.

Types of Ball Valves

The following provides a brief description of types of ball valves:

Ball valves, split body, regular port, flanged ends
Ball valves, top entry, regular port, socket weld ends
  • Split-Body Ball Valve. The split body design consists of a two-part body, a cover, ball, seat rings, stem, and other internals. The two-part body is held together by a flange connection. One body part is smaller than the other. The ball is inserted in the larger body part, and the smaller body part is assembled by a bolted connection. The stuffing box is constructed integral with the larger body part. On smaller size split-body ball valves, the two-part body is joined by threaded connection. The flanged or threaded joint between the two-part body is an added source of potential leakage. Flanged end connections are commonly available on all sizes; however, they are standard for large size valves. NPS 2 (DN 50) and smaller split-body ball valves are furnished with screwed ends. The split-body ball valves are manufactured in sizes ranging from NPS ¹⁄₂ (DN 15) through NPS 36 (DN 900). Refer to Fig. A10.22.

  • Top-Entry Ball Valve. Top-entry ball valves allow access to valve internals for assembly, disassembly, repair, or maintenance by removal of the valve bonnet-cover. The valve is not required to be removed from the pipeline. Refer to Fig. A10.23.

  • End-Entry Ball Valve. End-entry ball valves have a single-piece body. The ball is inserted from one end and is retained by an insert. These valves have flange or screwed-end connections. This design is commonly used for inexpensive small valves. They are also available in larger sizes up to NPS 6 (DN 150).

  • Three-Piece—Body Ball Valve. The middle part of the valve is the major part that holds all valve internals, and the stem passes through a hole in the top. Two end caps are held together with the middle body by bolts or studs and nuts. The end connections are part of the end caps, and they may be butt-welding, socket welding, threaded, or flanged. This design is available in sizes ranging from NPS ¹⁄₂ (DN 15) through NPS 36 (DN 900). The two end cap joints are two additional sources of potential leakage. The trunnion-mounted ball-valve design is used with split-body large size valves, whereas the floating-ball design is the most common design for all other types of ball valves. The torque required to actuate a trunnion-mounted ball valve is substantially smaller than the torque required for a floating-ball design.

  • Lubricated or Nonlubricated Ball Valves. Like other valves, the stem sealing is usually accomplished by bolted packing glands and O-ring seals. Some valve designs are available with a lubricant-seal system similar to the one used in plug valves. Such a design is shown in Fig. A10.21. The valves with lubrication seal systems are termed lubricated ball valves, while others are called nonlubricated.

Advantages of Ball Valves

The following are the advantages of ball valves:

  1. Provides bubble-tight service.

  2. Quick to open and close.

  3. Smaller in size than a gate valve.

  4. Lighter in weight than a gate valve.

  5. Multiport design offers versatility not available with gate or globe valves. It reduces the number of valves required.

  6. Several designs of ball valves offer flexibility of selection.

  7. Can be used in clean and slurry applications.

  8. High-quality ball valves provide reliable service in high-pressure and high-temperature applications.

  9. Force required to actuate the valve is smaller than that required for a gate or a globe valve.

Disadvantages of Ball Valves

  1. They are not suitable for sustained throttling applications.

  2. In slurry or other applications, the suspended particles can settle and become trapped in body cavities causing wear, leakage, or valve failure.

Typical Applications of Ball Valves

The following are some typical applications of ball valves:

  1. Air, gaseous, and liquid applications requiring bubble-tight service

  2. Low-point drains and high-point vents in liquid, gaseous, and other fluid services

  3. Instrument root valves

  4. Cooling water and feedwater systems

  5. Steam service

Plug Valves

Plug valves, also called cocks, generally are used for the same full-flow service as gate valves, where quick shutoff is required. They are used for steam, water, oil, gas, and chemical liquid service. Plug valves are not generally designed for the regulation of flow. Nevertheless, in some applications, specially designed plugs are used for this purpose, particularly for gas-flow throttling.

Plug valves generally can be readily repaired or cleaned without necessitating removal of the body from the piping system. They are available for pressure service

from vacuum to 10000 psi (69000 kPa) and temperatures from —50 to 1500°F (—46 to 816°C). Also, plug valves are available with a wide variety of linings suitable for many chemical service applications.

Construction of a Plug Valve

The basic design of plug valves is illustrated in Fig. A10.24.

Plug valve with lubricant system, as specified in API Standard 600

Full flow is obtained when the opening in the tapered plug is aligned in the direction of flow. When the plug is rotated a quarter turn, flow is terminated. The body and tapered plug represent the essential features in plug valves. Careful design of the internal contours of the valve produces maximum flow efficiency. The port in the tapered plug is generally rectangular. However, valves are also available with round ports. Major valve patterns or types are identified as regular, venturi, short, round-port, and multiport.

Plugs are usually tapered downward, while in some cases they are tapered upward. Most of the plug valves are top entry. In top-entry plug valves, the tapered plug is installed from the top of the valve. In some cases the plug is tapered upwards and is installed from the bottom opening in the valve body. Such a plug is called bottom entry or inverted plug valve.

Plug valves are also available with cylindrical plugs. The cylindrical plugs provide for larger port openings equal to or greater than the pipe flow area.

The regular pattern employs the tapered form of port openings, the area of which is from 70 to 100 percent of the internal pipe area. In some cases, the face-to-face lengths are greater than those of standard gate valves. The venturi pattern provides streamlined flow and thus permits reduction in the port size. The port opening area is approximately 40 to 50 percent of the internal pipe area. In most of the plug valves, the port opening varies from 60 to 70 percent of the pipe area. The round-port full-bore pattern has a circular port through the plug and body equal to or greater than the inside diameter of the pipe or fitting. Operating efficiency is equal to or greater than that of gate valves of the same size.Use of multiport valves is advantageous in many installations because it provides simplification of piping and convenience in operation. One three-way or four-way multiport valve may be used in place of two, three, or four straightway valves. Refer to Fig. A10.25.

Multiport valves

Types of Plug Valves

The following describes salient features of different types of plug valves:

  • Lubricated Plug Valves. The plug in a lubricated plug valve is provided with a cavity in the middle along its axis. This cavity is closed at the bottom and fitted with a sealant-injection fitting at the top. The sealant is injected into the cavity, and a check valve below the injection fitting prevents the sealant from flowing in the reverse direction. The sealant oozes out from the center cavity through radial holes into lubricant grooves that extend along the length of the seating surface of the plug. The sealant or the lubricant performs the following functions:

  • Establishes a renewable seal between the plug and the body. As a result, internal leakage is prevented or minimized.

  • Protects the seating surfaces against corrosion.

  • Acts as a lubricant, thus reducing the force required to open or close the valve.

The lubricant pressure developed by a turn of the lubricant screw or injection of lubricant with a pressure gun exerts a powerful hydraulic jacking action on the plug, momentarily lifting it from the seat and making it easy to turn. Since the lubricant pressure is greater than the line pressure, it is virtually impossible for solids to lodge between the valve body and plug.

The type of sealant to be used must be compatible with the flow medium in the pipe. The sealant must not dissolve or be washed away by the flow medium. The washed-away or dissolved sealant could contaminate the fluid, and the seal between the plug and the body would be destroyed, resulting in leakage. In addition, the sealant used must be able to withstand the flow-medium temperature.

Lubricated plug valves are normally manufactured in sizes ranging from NPS

¹⁄₂ through 36 (DN 15 through 900). They can be used in applications with pressures over 2500 psi (17250 kPa). They have been used in services involving air, gases, acids, alkalis, water, steam, oils, fuels, among others. Lubricated plug valves are less subject to seizing or wear and may exhibit somewhat greater resistance to corrosion in some service environments.

  • Nonlubricated Plug Valves. Nonlubricated plug valves contain an elastomeric body liner or a sleeve, which is installed in the body cavity. The tapered and polished plug acts like a wedge and presses the sleeve against the body. Thus, the nonmetallic sleeve reduces the friction between the plug and the body. Nonlubricating plug valves are used where maintenance must be kept to a minimum. Like lubricating plug valves, these valves also provide a bubble-tight closure and are of compact size.

Lubricants

The word lubricant does not precisely define the part this material plays in the efficient functioning of lubricated plug valves. More properly such valves might be called plastic sealed valves, and the lubricant could better be designated plastic sealant. The use of an effective lubricant is important, as, in operation, the valve structure and plastic scaling film are an integral unit, and each component is dependent on the other for ultimate performance. The lubricant in effect becomes a structural part of the valve, as it provides a flexible and renewable seat. This eliminates the necessity of force fits and metal-to-metal distortable-seat contacts to effect a seal. For this purpose, the lubricant must exhibit proper elasticity as well as resistance to solvents and chemicals to avoid the destructive action of the line fluid and to form an impervious seal around each body port, even under pressure. The film of lubricant also protects the metal surfaces between the plug and body from corrosion. The seal formed by the lubricant transmitted in a system of lubricant grooves circuiting each port aids in maintaining the essential film on the metal, closure surfaces.

Advantages of Plug Valves

The following summarizes the advantages of plug valves:

  1. Simple design with few parts.

  2. Quick to open or close.

  3. Can be serviced in place.

  4. Offers minimal resistance to flow.

  5. Provides reliable leaktight service. Seal can be maintained by injection of sealant or by replacement of sleeve, in addition to utilizing the wedging action of a tapered plug.

  6. Multiple port design helps reduce number of valves needed and permits change in flow direction.

Disadvantages of Plug Valves

The disadvantages include:

  1. Requires greater force to actuate, due to high friction.

  2. NPS 4 (DN 100) and larger valves require use of actuators.

  3. Reduced port, due to tapered plug.

  4. Typically, plug valves may cost more than ball valves.

Typical Applications of Plug Valves

As indicated earlier, the plug valves can be and have been used in many different fluid services. They perform well in slurry applications. They are primarily used in bubble-tight services as on-off stop valves. The wiping-off action of a plug does not permit suspended particles to accumulate and form crud. The following are some typical applications of plug valves:

  1. Air, gaseous, and vapor services

  2. Natural gas piping systems

  3. Coal slurries, mineral ores, mud, and sewage applications

  4. Oil piping systems

  5. Vacuum to high-pressure applications

Diaphragm Valves

All diaphragm valves are bidirectional. They can be used as on-off and throttling valves. Diaphragm valves offer advantages in certain low-pressure applications not possible with other types of valves. Their fluid passages are smooth and streamline