Double Block and Bleed (DBB) is an isolation method that uses two consecutive blocking points and an intermediate relief or check point. The purpose is to increase safety before maintenance work or intervention on a network section.
How it works
Two independent isolation elements are closed, and the intermediate space is vented or monitored. This allows for detecting if one of the valves is not sealing completely.
Why it is safer
A single closed valve can have internal leakage without being obvious. The second block and the bleed reduce the probability of unwanted gas entering the work section.
Relationship with LOTO
DBB does not replace the lockout/tagout procedure. It is used in conjunction with the facility's isolation procedures and work permits.
Bleed line
The bleed line must lead to a safe location. Venting inside an enclosed space can create a hazardous atmosphere.
Zero energy verification
Before working, it must be confirmed that there is no pressure or gas in the isolated section using appropriate means and in accordance with the approved procedure.
