What are Double Block Bleed Valves. Where are the valves used. Some more constructional features are welcome. What is the basic configuration of the Valve (Gate/Globe/Ball), is it a substitute to any of the above valves.

In this standard, OSHA provides a couple of relevant definitions:
 
"Isolation" means the process by which a permit space is removed from service and completely protected against the release of energy and material into the space by such means as: blanking or blinding; misaligning or removing sections of lines, pipes, or ducts; a double block and bleed system; lockout or tagout of all sources of energy; or blocking or disconnecting all mechanical linkages.

"Double block and bleed" means the closure of a line, duct, or pipe by closing and locking or tagging two in-line valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves.

The simplest form of a double block and bleed setup is two gate valves with a short pipe between them with a small, typically 3/4" "bleeder" gate valve in between them. The two primary gate valves can be closed and the bleeder opened to bleed off any fluid which is trapped or is entering through a leaking upstream valve. The likelyhood of the fluid continuing through the downstream valve with no pressure to drive it is minimal, thus protecting the downstream piping (or equipment) from the fluid.

Other options include two back to back gate valves. One of the valves has a tap in the body to provide the bleed function. Other valves exist which can perform the entire double block and bleed function with one valve (eg: a double disk gate valve with the cavity in between discs equipped with a bleeder).

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