There are many times when maintenance or testing must be done in a chemical processing plant. As it is impossible to shut the plant down when these issues need attention, it is important that positive isolation be achieved so the maintenance can be undertaken while the plant is live. Normally a double block valve has a bleed valve as well; all three valves are assembled into a common manifold. A double block valve is used when the process is critical or the materials that are being handled are under extremely high pressure or the material is toxic. In those areas which are less critical, a single block valve is often acceptable.
The double block valve is employed when it is necessary to repair, maintain or shutdown a piece of equipment or a process. The arrangement is used as follows; both valves are closed as a first step. The side of the valve that is designed for shutdown is then drained or vented through the bleed valve, this valve allows for trapped fluid to be removed. The shutdown side is now isolated from the process side, allowing the work on the downside to commence. The double block assembly eliminates a number of possible leak points compared to the older method where two individual valves were coupled together with a spool, which contained the bleed valve. In this method there were four faces where a leak could occur, in a double block valve, the number of faces have been reduced by 50 percent.
Space is always at a premium in a process facility, this has become even more important with the amount of offshore work that is being done in the oil and gas industry. A rig is designed to be as compact as possible, space on an offshore rig costs a great deal. By eliminating the spool used in a traditional double block valve, the space is greatly reduced as is the weight, in many cases as much as 60 percent.
In less critical applications a double block is not required, a single block and bleed valve is acceptable. The function is the same except there is only one main valve to close and isolate the upstream side of the process from the downstream. A single block valve is used when the pipes are carrying non hazardous or non-hydrocarbon fluids.