I'm looking for which spec or code states that when work is carried out on equipment on an operational plant that the piece of equipment is isolated by two valves. Any one remember where this is written down.

I have seen and heard this question asked many times in other forums. The code and most standards only provide minimum requirements, and are not intended to specify or regulate company safety programs. Safety programs require common sense and must be controlled by the organization that owns/operates or repairs hazardous equipment.

With that said, in certain applications - for example separate boilers (not twin furnace boilers) that feed into a common steam outlet header the ASME B&PV code requires a stop and check valve as a form of double isolation to assure that steam from one boiler does not back feed into the other boiler. This is part safety, and to protect isolated equipment from damage.

Regarding non-boiler applications, double isolation is a good thing, regardless. Safety is most important, and if you need to provide a zone of isolation using two valves to assure personal safety so be it. You might want to check OSHA requirements for more information.

MORE NEWS