Double Block & Bleed
A Double Block & Bleed
installation shall be implemented by means of two block valves in series with
an interconnecting piping spool fitted with a bleed connection. Where
practical, Double Block & Bleed sets shall be installed fitting to fitting
to minimize the trapped volume between the two block valves. The bleed valves
should be arranged to ensure that any discharge cannot harm personnel and are
easily accessible for checking.
Bleed valves will either be
vented directly to atmosphere locally or via a hose connection/hard piped to an
appropriate disposal system. The decision as to whether an inventory is hosed
to flare or vented locally will be addressed as part of the Task Risk
Assessment carried out prior to the issue of the permit to work. However, where
the volume between the block valves is significant, i.e. the DBB set is not
fitting to fitting and the fluid is hazardous, bleeds will be hard piped to the
LP flare or closed drain system. The requirement to hard pipe bleeds may be
established case by case during detailed design as the piping design of the
systems is developed. Bleed valves not connected to the flare system must be
blinded off with a fully rated blind flange or plugged when not in use.
Emergency Shutdown (ESD)
valves and Shutdown valves (SDV) may be used as block valves in Double Block & Bleed installation provided the valve itself is not in need of service and can be
reliably immobilized and the actuator is in its de-energized fail closed (but
not fail open) position. The valve shall be rendered “passive” whilst closed
for isolation purposes to ensure it cannot be inadvertently opened. When used
in a Double Block & Bleed DBB arrangement, the actuated valve must be the secondary isolation valve.