Single block and bleed valves are used to achieve positive isolation when performing maintenance activities in a live process plant. Typically one block valve and a bleed valve are manufactured as a single assembly and this single block and bleed valve manifold can be readily installed for isolation purpose. Single block and bleed valves are usually used for non-critical process service, such as low pressure system or non toxic, non hydrocarbon, non-hazardous process fluids. For critical service, double block and bleed valve assemblies must be used.
To perform positive isolation while performing maintenance, repair or shutdown activities, single block and bleed can be used in the following way. The block valve is closed first. The shutdown side of the block and bleed assembly is then drained/vented/purged to make ready for maintenance. Block and bleed valves are also often used with spectacle blind or blind spacers to provide positive isolation. Block and bleed valve assemblies allow fewer potential leakage paths compared to traditional separate installations of individual valves, thus increasing safety of the design.
Usually, ball valves are used as block valves or isolation valves and ball or needle valves are used as bleed valves or drain valves in the block and bleed assembly.