Ball valves are also classified based on the body construction :
• Single piece design
• Two piece design
• Three piece design
• The short pattern
• The long pattern
• Sandwich design.
• Flush bottom design
In the single piece design valve, the body will be cast/forged as one piece. The insertion of the ball will be through the end of the body and is held in position by body insert. This design restricts the valve to be of regular port only.
In two piece design, the body is constructed in two pieces and the ball is held in position by body stud. There can be full port or regular port design possible in this construction. In case of three piece construction, the body has two end pieces and one centre piece. These are held
by body studs.
The three piece construction permits in line servicing without disturbing the existing pipe work. If the valves have socket weld, screwed or butt welding ends, this design totally dispenses with the necessity of companion flanges.
The short pattern and the long pattern of the body is on the basis of the end dimensions Normally short pattern body is adopted by the manufacturer upto 300 NB valves for 150 LB class. In case of 300 NB to 400 NB class 150 short pattern valves, the ball may protrude
beyond the body and flange faces when the valve is in closed position.
The sandwich design is the flangeless design adopted by some manufacturers. This is to confine the use of the high cost exotic materials like Alloy-20. Hastelloy-B. Hastelloy-C etc. To the wetted areas only. The valve is designed to fit between the flanges. The body cover gets
bolted to body with studs or hexagonal head screws.
The seat rings are renewable in the ball valves except for those having one-piece sealed body construction. The two different types of seat construction are possible, viz. The fire safe design and the non-fire safe design.
In the fire safe design, a secondary metal seat will be provided so that when the soft seat is fully burnt, the ball will shift its position and seat against secondary metal seat and arrest full leakage.
In an actual fire, the heat intensity of the fire could be so different that it is impossible to ensure that elastomer seats are fully damaged during fire. If the seats are only partially damaged, the ball cannot take seating against the secondary metal seat and hence the valve
would leak. Hence, in my opinion, none of the soft seated ball valves can be declared fire safe since the valves are bound to leak in case of partially damaged seats.
The manufacturers have come up with metal seated ball valves which are fully fire safe. Here the elastomer seats are replaced by metal seats which could even be deposited with high temperature resistant materials.
The fire safe design should also ensure that any development of static electricity should be fully discharged by proper design and manufacture of valve. Such an arrangement is called the ‘Anti-static’ design. This ensures to have a discharge path from ball to the spindle and from spindle to the valve body with an electrical resistance of not greater then 10 ohms when the valve is new. A typical method of achieving earthed continuity is to provide stainless steel spring loaded plungers, one fitted between the stem tongue and ball and second fitted
between stem and body.
The ball could be of full bore or a reduced bore. The design aspect of the same has been explained earlier. The ball at the bottom end of the body could be supported fully by the seat or it could be trunnion supported. The ball can be solid ball or of hollow construction with
cavity. The cavity is to be sealed when the valves are used in volatile liquid. This design of the ball is called sealed cavity design.
The gland shall be bolted type or screwed. Internally screwed stuffing box is not allowed by code. Bellow sealed bonnet is also provided in case of valves used in lethal services. There are two basic bellow seal designs available. The same is explained under plug valves.
The valves shall be operated by wrench or by hand wheel with gear arrangement. The wrench shall be designed so that it is parallel to the flow passage of the ball. The valve shall be closed, by turning the wrench or the hand wheel in a clockwise direction. The length of the
wrench or the diameter of the hand wheel shall be such that the minimum force is required to operate the valve under the maximum differential pressure.
When added emission control is required additional packing and leak off port are options that can be added.
The valve having pressure balanced is called dynamically balanced plug. M/s Audco called these types of valves as ‘Super H’ pressure balanced valves. The break away torque required to operate these valves are lower than (almost half) that for the reduced port ball valves.