Double block valves have been used in process industries for years, wherever a pipe line had to be isolated, you would find a double block valve.
The idea of the valve is rather simple, you have two valves in line and between the valves there is a cavity. When the valves are closed, one after another, and the cavity is bled of any remain fluid, it is possible to do any repair or maintenance downstream as the flow has been interrupted.
Many years ago the traditional manner in which this isolation was done was to use two separate gate valves. The valves were connected to one another by a spool, a length of pipe with flanges on both ends. Located in the spool was a third valve, this valve was used to bleed the cavity so that the downstream side was absolutely safe to work on. Although this system worked well, it was heavy and bulky.
Double block valves with a built in bleed have now been perfected where the entire system is contained in a single housing. The double valves; one for upstream and one for downstream are located on either end of the valve body, the bleed valve is integral with the housing and bleeds a chamber between the main valves. This improved design eliminated a great deal of the weight and bulk from the original multi-valve arrangements.
Although the double block valve is used in the process flow it is also an important feature in instrumentation. The valve allows for the introduction of chemicals into the line.
The primary benefits of the integrated valve is a dramatic reduction in weight, the valve is considerably smaller than the built up unit using separate components and perhaps most importantly, it reduces the number of places where there could be a leak of hazardous material. In a conventional system, each independent valve has two flanges, the spool meets on either side with a flanged connection; the process pipeline meets on the other. This combination means there are four potential surfaces where leaks can occur. In the case of the integral double valves, the valve and the spool are all in one machined component, there are flanges on either end which are connected into the process pipe. This simple design change eliminates 50 percent of the potential leak points.