What Is DBB Valve? Double Block and Bleed

Quick Answer

A double block and bleed (DBB) valve provides two independent sealing barriers with a bleed (drain/vent) between them. Closing both seals and opening the bleed valve confirms positive isolation by verifying that neither seal is leaking. DBB is the standard isolation method for maintenance, tie-ins, and hot work on pressurized piping and pipelines.

How DBB Works

A DBB arrangement creates two independent seals on either side of a cavity. The bleed valve, connected to this cavity, is opened after both block seals are closed. If no pressure builds up at the bleed, both seals are holding and the downstream section is safely isolated. If pressure appears at the bleed, one of the seals is leaking and isolation is not achieved.

This is fundamentally different from a single block valve, which provides only one sealing barrier with no way to verify its integrity without depressurizing the entire system.

DBB Configurations

Configuration	Description	Sealing
Single valve DBB	One trunnion ball valve with two independent seats and a body cavity bleed	Both seats seal against the ball; cavity is bled through body bleed port
Two-valve + bleed	Two separate block valves in series with a bleed valve between them	Each valve provides one seal; bleed valve on the pipe between them
DIB-1 (single isolation)	Single sealing barrier per API 6D; relies on one seat sealing in one direction	Not true DBB; adequate only for low-risk isolation
DIB-2 (double isolation)	Two sealing barriers per API 6D; both seats independently tested	True DBB; required for positive isolation

API 6D Classification

API 6D defines isolation capability categories for pipeline valves:

Category	Definition	Seats Required	Bleed Required	Application
Category A	Single isolation (one sealing direction)	One seat seals	Not mandatory	Routine block valve
Category B	Single isolation + bleed	One seat seals + cavity bleed	Yes	Basic isolation with verification
Category C	Double isolation	Both seats seal independently	Not mandatory	Positive isolation
Category D (DIB-2)	Double isolation + bleed	Both seats seal independently + cavity bleed	Yes	Positive isolation with verification

Category D (DIB-2) is the only configuration that provides true double block and bleed per the strictest interpretation. It requires a trunnion mounted ball valve with independently sealing upstream and downstream seats and a body cavity bleed connection.

When DBB Is Required

• Pipeline maintenance requiring personnel to enter or work on an isolated section
• Hot work (welding, grinding) on live piping systems
• Equipment tie-ins to operating pipelines
• Instrument calibration requiring verified zero-pressure conditions
• Chemical injection point isolation
• Any situation where single-valve isolation poses an unacceptable safety risk

DBB vs Single Block Isolation

Feature	Single Block Valve	Double Block and Bleed
Sealing barriers	One	Two (independent)
Leak verification	Not possible without depressurization	Bleed valve confirms seal integrity
Safety level	Standard	High (required for hot work, confined space)
Valve type	Any isolation valve	Trunnion ball (single-body DBB) or two valves + bleed
Cost	Lower	Higher

Common Mistake

Assuming a floating ball valve provides DBB capability. In a floating ball valve, only the downstream seat is pressurized against the ball. The upstream seat may not seal independently. True DBB requires a trunnion mounted ball valve where both seats are spring-loaded and seal independently, or two separate block valves in series.

For pipeline valve standards, see the API 6D overview. For valve types and selection, see the complete valve guide.

Read the full guide to valve types