What is Double Block and Bleed?
API 6D defines a double-block-and-bleed valve (DBB) as a "single valve with
two seating surfaces that, in the closed position, provides a seal against
pressure from both ends of the valve with a means of venting/bleeding the cavity
between the seating surfaces." The 2008 note points out that this valve does not
provide positive double isolation when only one side is under pressure.
By contrast, API 6D defines a double-isolation-and-bleed valve (DIB) as a
"single valve with two seating surfaces, each of which, in the closed position,
provides a seal against pressure from a single source, with a means of
venting/bleeding the cavity between the seating surfaces." The note adds that
this feature can be provided in one direction or in both directions. -Valve
Magazine
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Meaning of Double Block and Bleed" at www.ValveMagazine.com