A Double Block and Bleed (DBB) valve is a type of valve assembly that provides two block valves and a bleed (vent) valve within a single compact unit.

This design is used to achieve positive isolation in critical applications where the prevention of fluid leakage is crucial. The primary purpose of a Double Block and Bleed valve is to ensure that both upstream and downstream flow paths are blocked off before any maintenance or repair work is performed on the system.

Here's how a Double Block and Bleed valve typically works:

Blocking Function

1. First Block Valve (Isolation Valve 1): The upstream flow is blocked by closing the first block valve. This prevents the fluid from flowing into the downstream section of the pipeline.

2. Bleed (Vent) Valve: Once the first block valve is closed, the bleed valve is opened. This allows any trapped fluid between the first and second block valves to be safely vented or bled off, reducing the pressure in the isolated section.

3. Second Block Valve (Isolation Valve 2): With the bleed valve open, the second block valve can be closed to block the downstream flow. Now, both the upstream and downstream flow paths are isolated from each other.

Verification and Maintenance

1. The operator can verify the isolation by checking pressure gauges or indicators to ensure that both block valves are closed and the bleed valve is open.

2. Maintenance or repair work can be performed on the equipment or pipeline between the block valves without the risk of fluid leakage.

Opening the System

1. When the maintenance work is completed and it's safe to resume operation, the second block valve is opened.

2. The bleed valve is then closed to allow normal flow to resume.

The Double Block and Bleed valve design provides an extra layer of safety by offering redundancy in blocking fluid flow and providing a means to safely vent any residual fluid. It is commonly used in industries such as oil and gas, petrochemicals, and other applications where fluid containment and safety are critical considerations.