Ball valves are cheaper and easier to make than other valve types. They are quick opening, needing only a quarter-turn from full open to full close. They are non-sticking, give tight closure, and have a negligible pressure drop characteristics because of their smooth, full-opening port. These valves are easy to repair and maintenance costs are low. They are not suitable for throttling because in the partially open position, sealing surfaces on the exterior of the ball are exposed to abrasion of the process fluids.
The ball valve is not new, the design has been around for over 75 years. In the original design form the ball valve could not provide bubble-tight seal because it utilized a metal-to-metal seat. The introduction and use of elastomers (rubber, nylon, Teflon, etc) for seating, and improvement in machining capability, has overcome these disadvantages. These new materials not only provide a tight seal, but also contributes to easy operation.
Leakage passed the stem is prevented by Teflon or plastic seal rings which are firmly held in position by a seal retainer but still allows movement of the valve stem or spindle. These seals are not replaced on line such as the packing of a gate valve can be. The system must be depressurized. A handle or some other means of actuation is attached to the stem. The low friction material used helps prevent excessive wear and there is little problem from outside leakage, that is, fluid passing through the stem seal to the outside atmosphere. Ball valves are divided into two categories, top entry or end entry valves. The advantage of the top entry ball valve is that the ball can be removed without removing the valve or opening the pipe work.
FIGURE 3 BALL VALVE
1.3 Plug Valves
Plug valves are suitable for the same application as ball and gate valves. The advantages of plug valves are that they generally require a minimum amount of installation space, they are simple in operation, having a minimum of moving parts, and they are quick acting, requiring only a quarter-turn to open or close. The tapered plug insures a close fit and, with lubrication, pressure tightness is increased.
Although a close fit is required between the plug and the valve body, the plug has to be loose enough to be able to turn. Friction can wear out the plug and body. If the plug is worn out, it will not seat properly in the body. With a worn plug, it is not possible to get complete shut-off and the valve passes.
When a suitable lubricant such as grease is forced into the plug through the fitting, it passes through an axial hole in the centre of the plug.
This axial hole feeds the lubricant through radial holes into grooves in the surface of the plug where the plug makes contact with the inside of the valve body. Provided adequate lubricant exists between plug and body, friction is kept to a minimum, reducing wear. Besides helping to reduce friction, the lubricant acts as a sealant to prevent leakage. If the lubricant is lost, excessive wear occurs due to friction and generally results in excessive leakage. Plug valves need to be frequently lubricated. This is one reason why plug valves should not be used in throttling service. The fluid flow will leach away the lubricant.