Can anyone explain what is anti static and double block and bleed in a ball valve, please explain how it works.

A trunnion mounted valves can in some designs be supplied with a bleed valve inbetween the seats - this make a _test_ of the tightness for both seats possible. This will on occation by sales reps be solds as "double block and bleed" BUT THIS IS NOT TRUE since the seats will only be tight with line pressure both upstream and downstream. Something that you usually dont have if you need a DBB.

Neither answer is complete.
1.  Because the ball and stem in a ball valve are susspended on non matalic parts , i.e. the seats and stem seals, there is a possibility when cycled a static charge can build on the ball-stem.  Ball valves usually have a method to maintain a metal-to-metal contact between the rotaing ball/stem and the valve body which will ground any charge to the valve body.  The valve body is then grounded to the pipe by the pipe connection or the addition of a ground strap around the flanges.  This ground need is also present in plug valves.

2.  A true double block and bleed is three valves w/very ggod seating connected by a Tee fitting (some times done with two valves and a smaller drain valve installed between).  Purpose is to provide a method to change products flowing in a system without having them mix.  The two products valves are closed then the space between them is drained then the second line is opened.  A trunnion ball valve can function as a block and bleed as today most designs include a soft seat element that provides a positive seal and the seat holders are spring loaded to provide a seal at the ball without line pressure.  In a true block and bleed one still needs a shut off valve to change fluid.  But a trunnion can be used to shut off line and bleed valve opened to confirm shut-off.  This is not a true block and bleed application.

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