Ball Valves

1.Basic Design
The ball valve is basically a ported sphere in a housing. A quarter turn of the stem will cycle the valve from open to closed or vice-versa.

2.Size Range
Ball valves with are manufactured in sized ranging from ¼ inch to 48 inch and higher. Sizes up to 8 inch are used quite frequently. Ball valves in small sizes are relatively inexpensive.

3.Directional Seating Characteristics
Ball valves with two seats are normally bi-directional while those with a single seat are normally unidirectional. Multiport ball valves (2-Way, 3-Way, 4-Way) are unidirectional. The multiport ball valve is used frequently for diverting flow to several directions using a single valve.

4.Typical Applications
Ball valves are used as bubble-tight stop valves for relatively clean fluid systems, such as air, water, and gas. The popularity of this valve is enhanced by its very small space envelope for a given line size (although this advantage diminishes somewhat in larger sizes) and its quarter turn operation from the closed to open (or open to close) position. Typical applications include fuel oil and fuel gas service.

5.Design Features
Ball valves have a very low center of gravity and are simple designs. With few moving, yet accessible parts, this valve design rates very high with respect to achieving low maintenance costs.
These valves have low pressure drop and low leakage and they are rapid opening. In full port configuration they have the same flow restriction as a straight piece of pipe and generate little or no additional turbulence.

6.Construction Details
Construction details are as follows :
a. Ball valves are manufactured in three basic designs :
- Floating Ball
- Double Trunnion
b. Port sizes are as follows :
- Full Port
- Reduced Port
- Venturi Pattern
c. The full port ball valve has the lowest pressure drop (Or highest Cv) and the highest price of the three port configurations.

7. Limitations
Limitation of ball valves are as follows :
a. Standard ball valves should not be used for any sustained throttling service as severe erosion of the ball and seat ring my result if the ball and seats are not designed for throttling service. Some ball valves have special design features which makes them suitable for throttling service.
b. Soft-seated ball valves have a generic design problem that exists when suspended solid participles in the fluid system (resins, oxide particles, etc.) settle-out and become trapped in the cavities below the ball and in the vicinity of the stem or trunnion areas when the flow has stopped and this “crud” has had time to become encrusted. One solution to this problem is to position all valves mounted in a horizontal run of pipe so that the stems or trunnions are in the horizontal position. This valve position will promote the “self-scouring” motion when flow returns. Another solution is to drill and tap the body for a threaded plug under the ball or in the area of crud build-up; the tapped hole can be used for drawings and/or flushing.
c. Fluid trapped in the ball valve in the closed position may caused problems if the valve is not vented (e.g., water freezing and subsequent cracking the ball).
d. Because the valve can be quick opening and closing, rapid closure may cause water hammer or pressure surges.
e. Typical materials used for seating and seals on ball valves are listed with approximate (they are a function of mechanical design as well as material properties) recommended maximum service temperatures.

8.Access for Disassembly and maintenance
The internals of most ball valves may be accessed from one of the valve ends; this requires the pipe line to be loosened at the valve removed. More desirable are “top entry” designs. Also swing-away designs are available in small sites; this design allows in-service access, also which does not require dismantling the pipe line.

9.Basic Seat Seal Design
In order to maintain the bubble-tight closure expected of a ball valve, materials used for seating and sealing must be compatible for the system service temperatures. The majority of the ball valve design used a resilient material seat ring in contact with the polished surface of the ball disc. This valve seating design is described as “nonlubricated” and depends on the natural lubricating qualities of the material to provide low friction operation.

10.“Lubricated Seat” Designs
Some manufacturers produce valves with secondary seating as well as primary. This is accomplished by the use of sealant material forces under pressure into machined passages along the periphery of the ball affecting a seal that can be maintained until the primary seal can be replaced. This design is known as a lubricated seat design.

11.Stem Seals
Stem seal design varies with the valve sizes and service temperatures. Simplified stuffing boxes are used with flat packing rings compressed by screwed glands and stuffing boxes that us chevron type packing rings that are pressurized with a sealant. Other designs use a series of O-ring seals backed up by a secondary sealant seal.

12.Metal-To-Metal Seated Ball Valves
Heavy duty, meal-to-metal seated ball valves are available for high temperature, high pressure service.
This type of valve has been seen in increasing usage due to the following advantages :
a. High resistance to erosion, making this design an excellent selection in high solids service;
b. Temperature capability equal to metal seated gate or glove valves in high temperature service;
c. Options for throttling.
The drawbacks of these design are :
a. High initial costs;
b. A high stem torque requirement which means that a gearbox is required in smaller sizes than for a conventional ball valve.