Wednesday, October 30, 2013

Ball Valves Types, Construction, Applications and Advantages

The ball valve (Fig. 1) 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. The use of soft-seat materials such as nylon, delrin, synthetic rubbers, and fluorinated polymers imparts excellent sealing ability. 
Ball valve in closed position
Fig. 1: Ball valve in closed position
With fluorinated polymer seats, ball valves can be used for service temperatures 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 similar to 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. 2.
Lubricant-seal system in a ball valve
Fig. 2: Lubricant-seal system in a ball valve
A ball valve may be unidirectional, bidirectional, or multi-directional, 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 multi-port 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:

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.
Ball valves, split body, regular port, flanged ends
Fig. 3: Ball valves, split body, regular port, flanged ends
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. 3.

Top-Entry Ball Valve

Ball valves, top entry, regular port, socket weld ends
Fig. 4: Ball valves, top entry, regular port, socket weld ends
Top-entry ball valves allow access to valve internals for assembly, dis-assembly, repair, or maintenance by removal of the valve bonnet-cover. The valve is not required to be removed from the pipeline. Refer to Fig. 4.

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.

Double Trunnion Ball Valves

In this ball-valve design, the ball is provided with two integral short-shaft extensions called the top and the bottom trunnions. These trunnions are fitted in bearings and rotate freely when the shaft installed in the top trunnion is turned to open or close the valve. The ball is held firmly in place, unlike the ball in other designs where the ball is supported by the two seat rings and is allowed to float in the direction of the slot on top of the ball. A shaft installed in the top slot is turned to open or close the valve.

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 Non-lubricated 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. 2. The valves with lubrication seal systems are termed lubricated ball valves, while others are called non-lubricated.

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. Multi-port 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 feed water systems
5. Steam service

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