Double Block and Bleed Valve: Safety and Efficiency Redefined

A double block and bleed valve, often shortened to DBB, is an assembly designed to provide two isolation barriers with a bleed point between them.

Close both isolation points, open the bleed, and you can verify that upstream pressure is not sneaking past. If it is, the bleed tells you. If not, you can work downstream with confidence.

You will see DBB valves as single integrated bodies or as small manifolds that pipe together two isolation valves and a bleed valve.

Either way, the principle is the same:

Two blocks, one bleed, clear verification.

How It Works

A DBB assembly creates a small volume between two closed sealing surfaces.

When you open the bleed valve on that volume, any fluid – or even any residual liquid that might affect drainage in the cavity – drains or vents.

If pressure continues to appear at the bleed port, you know one of the isolation points is not sealing.

If the bleed shows no pressure or flow, the isolation test passes.

There are two common ways to build a DBB:

• Integrated DBB valve: A single body with two seating surfaces and a bleed port in the center cavity. Often a trunnion-mounted ball or a plug valve with two seats.
• Piped assembly: Two separate block valves and a small bleed valve between them. Common in legacy plants and in instrument service where modular manifolds are common.

Typical operation looks like this:

1. 1 Close the upstream block.
2. 2 Close the downstream block, if present in your layout.
3. 3 Crack open the bleed valve to depressurize the center cavity.
4. 4 Monitor the bleed for flow or pressure. A pressure gauge or a bubble bottle works well.
5. 5 Once verified, lock or tag the handles if your procedure requires it.
6. 6 Perform the work. Keep the bleed open to prevent pressure build-up in the cavity.

One small, important detail: cavity pressure behavior.

In a DBB ball or plug valve, the seats can be built to self-relieve in one direction or to seal in both directions. That choice affects safety and testing.

More on that in the DBB vs DIB section.

Key components you will see:

• Two sealing elements: these can be seats in a single valve body or the closures of two individual shut-off valves.
• A bleed valve: often a needle or small ball valve, sized to vent and to connect a gauge.
• Body, bonnet, and end connections: flanged, threaded, or welded ends selected to match pipeline standards.

Materials vary by service.

Carbon steel for refined products, austenitic stainless for corrosive media, duplex grades for sour service, and alloys for high temperature or CO2-rich streams.

Seat materials range from PTFE-based compounds to PEEK and metal-seated designs.

Good practice for operation and maintenance:

• Keep the bleed normally closed except when testing or draining.
• Vent to a safe system where needed, like a flare, a closed drain for proper drainage, or a dedicated drainage system, not to open atmosphere in hazardous service.
• If the center cavity could thermally expand a trapped liquid, provide a relief path or a thermal relief valve.

Ordering a DBB valve is mostly about the piping spec, seat design, emissions target, pipeline safety, and applicable regional regulations. The standards below shape those choices.