THE TRUE MEANING OF
DOUBLE BLOCK
AND BLEED
PHOTO COURTESY CAMERON
Many people refer to
these valves when
what they are really
referencing are
double positive
isolation valves. But
there’s a difference,
says Exxon Mobil
Development’s Rudy
Garza, and it’s an
important one.
BY PETER CLEAVELAND
It’s time to do maintenance on a section of a process. You don’t want to shut down the entire facility, so you decide to block off and depressurize just the section on which you’re working. Just upstream is a double block and bleed valve—a
trunnion-mounted ball valve with self-relieving seals and a bleed valve to vent the
cavity. You close the ball valve and open the bleeder. Now you can de-pressurize the
line downstream and open it up to work on it.
Not so fast, says Rudy Garza. You may think that valve gives you double isolation, but it doesn’t—and that could be dangerous.
On March 4 Garza, Mechanical Lead—Static Equipment Engineering Group at
ExxonMobil Development Company, spoke at the VMA Technical Seminar in San
Antonio on “Isolation Philosophies” in which he asserted that many people take
the term “Double Block & Bleed” (DBB) to mean the same thing as Double Positive Isolation (DPI). While this may seem like a small matter, it means that some
users may think they’ve achieved positive isolation when they haven’t, Garza says.
Part of the problem is that designers and users don’t always understand the capabilities of the valves in question. And the design of a particular valve can vary from
one manufacturer to another, he adds.
