In thread408-48267: Double-Block Valves and Double Block and Bleed Valve on 10Mar03, davefitz (Mechanical) posted a comment stating,"double block and bleed is required by code in section I (boiler) service if there are 2 or more boilers interconnected on a common steam line and the boilers have a manway in the steam drum. This is to avoid cooking the maintenance person that enters the steam drum on the idle boiler. A similar philosphy would be prudent for any pressure vessel that a person can enter , if the vessel is interconnected to another active system which has dangerous fluids."

What code is davefitz refering to?  I ask this because at our facility we are currently installing a second boiler that will be connected to our existing boiler. I'm not the engineer over this project, but I would like to ensure that no one could potentially be hurt.

It is important to not forget why the regs are in place. In this case it is to ensure the area (tank, boiler or process equipment) can be positively isolated. The definition of positive isolation is stopped flow and a method of proving there is no flow past the isolation point. Hence the primary, secondary and bleed valves in the double block and bleed system. In bygone days a spade or blind was inserted into the line downstream of the primary isolation valve. Bleed past the primary isolation valve(Forged Steel Valves) could pressurise behind the spade and create a hazardous situation for the person removing the spade. Hence the progression to double block and bleed. Having stated that rather than referring to legislation, apply safe operation guides and put the block and bleed valves in. A HAZOP would also identify this requirement.

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