It is theoretically possible to build a standard ball valve in double block and bleed configuration, but it is not practical because of the poor seating ability of the upstream seat. Only a trunnion-mounted sprungseat design is practical to build as a double block and bleed. Many valves are built with bleeds, but generally, this is done when it is desirable to purge the body cavity of some fluid right after the valve is closed.
Even though it is true that the seats do not hold pressure that well, it is also true that the leakage rate can be very slight. A valve that has been closed under pressure, then removed from the line, may retain a surprising amount of pressure that can cause serious injury if not properly relieved prior to maintenance activities.
The body cavity may also retain quantities of line fluid, which could require thorough purging if the fluid is hazardous. This can be avoided somewhat by means of a ball vent, which is drilled through the ball to equalize pressure in the body cavity with the inside of the flow area.
Sometimes this hole is drilled through the top of the ball, under the stem slot, which is the thinnest point in a solid ball. Occasionally it is drilled through the side of the ball. There are services where a vented ball is required, such as in cold services where a liquid trapped behind the ball could vaporize and build up high pressure so quickly as to explode.
There are other cases where vents are prohibited, such as in food service where it represents a pathway that would allow buildup of uncleanable contaminants.
There are very small barstock ball valves, but the barstock valve is just as likely to be built as a plug type. Quarter-turn valves, in general, are unusual in barstock or tubing-type valve usage. The needle or globe type is more common. Ball valves are chosen in these sizes for specific abilities, such as having full-port straight-through flow passages, which provides the capability of being rodable.
This feature is being used more commonly now on gage valves and other small instrument takeoffs, or root valves. Externally, these valves do not really look any different from the corresponding plug or globe type. Another specialty adaptation of the ball valve is the hermetically sealed design with a stem containing magnetic inserts, inside the sealed bonnet, with a handle containing magnets outside the stem area. This permits the ball to be operated with no physical contact between the handle and the stem.
Very large ball valves, above 24" (DN 600) are primarily found in cross-country pipeline applications because they will shut off quickly without prelubrication. Their primary competitor is the throughconduit gate valve. The ball valve is at a disadvantage, though, because of its metal mass, which means increased cost. It also means serious dynamic problems, since the ball valve can be made to operate rather rapidly.
There are actuators capable of opening and closing quarterturn valves very quickly. However, the valve must be capable of withstanding the high forces required to get the ball in motion and those that result when the ball stops at the end of its travel.
The advantage of a ball valve over the through-conduit valve is that the overall envelope of the ball valve is much smaller. Even including the actuator, the ball valve is not much larger than the pipeline, while the through-conduit gate can be four to six times the pipeline size in overall length or height.
