Types And Designs of Ball Valves

 

Ball Valve

Ball Valve

 

Ball Valve Types

Ball valves may be classified according to their housing assembly, ball design, and bore profile.

 

Housing Assembly

• One-Piece Ball Valve

A one-piece ball valve has a single-piece cast body that houses the internal components of the ball valve. This eliminates the risk of leakage of the fluid from the valve. One-piece ball valves are the cheapest ball valves and always have a reduced bore. A welded one-piece ball valve is more common but cannot be dismantled for cleaning and repaired once damaged; therefore, it is only used for applications with a low possibility of particle build-up, and where sanitation is not a major concern. On the other hand, screwed one-piece ball valves can be cleaned, serviced, and repaired, but dismantling requires special tools.

 

Split Body Ball Valves

Split Body Ball Valves are valves that are assembled on the sides of their balls. A split body ball valve may be a two-piece or a three-piece ball valve:

• Two-Piece Ball Valve

A two-piece ball valve consists of housing divided into two pieces that are fitted together. The main piece contains the ball and a connection to one end, and the other piece holds the internal components together and has a connection to the other end. Two-piece housing is the most common type among ball valves. The two parts can be dismantled for cleaning, maintenance, and inspection but it requires removal of the valve from the pipe.

• Three-Piece Ball Valve

A three-piece ball valve consists of the housing for the internal components of the valve which are fitted and held together by bolt connections to its two ends. The ends are threaded or welded to the main pipe.

Three-piece ball valves are used for applications that rely heavily on valves, that their maintenance activities must frequently be done. They can be cleaned and serviced easily and their seats and seals can be routinely replaced by just taking the valve body out without disturbing the two ends. Three-piece ball valves are commonly used in the food and beverage and pharmaceutical industries where sanitation is crucial to safety and product quality.

 

Top Entry Ball Valve

A top entry ball valve allows access to the internals of the valve by simply removing the bonnet on top of the valve. This allows in-line maintenance activities (i.e., dismantling, cleaning, inspection, and repairing the valve) without removing the ball valve from the main pipe.

 

Ball Valve Designs

Floating Ball Valves

 

ANSI Floating Ball Valve

ANSI Floating Ball Valve

 

The floating ball is the most common ball design in ball valves. The ball is suspended inside the valve and free to move in a lateral direction when the valve is in a closed position. It is sandwiched between two seats that support the valve and hold it in place. The ball is connected to the shaft in a slot on one end while the other end is free. When the valve is in an open position, the shaft connection to the slot at the top of the ball prevents the ball from moving laterally.

The sealing action is only dependent on fluid pressure. During an operation of a floating ball valve, the inlet pressure of the fluid forces the ball to the outlet seat which prevents the fluid from escaping from the valve body. The fluid pressure on the ball and the seats are higher when the ball valve is in the closed position.

Floating ball valves have the simplest design. They come in smaller diameters and are suitable for liquids and gases operating under low to moderate pressures. The application of floating ball valves is limited by the amount of pressure the seats can handle. At high fluid pressure, the seats can be deformed from the pressure exerted by the ball which can affect the sealing characteristics of the valve under low pressure. Furthermore, the torque to rotate the stem depends on the force required to counteract the same fluid force acting on the ball and seals.

 

Trunnion Ball Valves

In a trunnion ball valve, the ball is supported by an additional shaft at its bottom which is called the trunnion. This holds the ball in its place and limits the movement of the ball to its axis. The ball can only move if the valve shaft rotates. Trunnion ball valves also feature spring-loaded seats. The inlet fluid pressure activates the springs towards the ball held by the trunnion, which creates a tight sealing.

Trunnion ball valves are available in small to large diameters, but it is more expensive than floating ball designs. They can operate efficiently in a wide range of pressures and they are ideal for high-pressure applications since the fluid pressure is also dissipated to the trunnion and the springs of the seats. Hence, they are easier to operate with a lower operating torque or a small actuator.

 

Vented Ball Valve

A vented ball valve is constructed and operates in the same way as a standard ball valve, except that the vented ball has small orifices drilled into its side. When the valve is closed, the orifice is directed to the outlet side of the valve. The drilled hole is used to vent trapped gases which causes a build-up of internal pressure inside the valve, to prevent leaking, valve failure, and explosion.

Vented ball valves are used in compressed air systems, cryogenic processing and conveying volatile liquids are also referred to as the "cryogenic valve" because of their usefulness in cryogenic processing.

 

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