What is a double block and bleed ball valve?

A single ball valve with two seating surfaces and a bleed port between them. In the closed position it blocks flow from both ends and lets you vent the space between the seats, so you can check that both barriers hold.

How does a double block and bleed valve work?

Closing the valve brings the ball between two seats, one facing each end. Opening the bleed valve vents the cavity between the seats; if the pressure stays at zero, both seats are holding.

What is the difference between a DBB valve and two separate valves?

A DBB valve puts two seating surfaces in one body, with one bleed port and two end connections. Two separate valves with a spool and a bleed valve do the same job with independent barriers, but take more space and have more joints.

What is the difference between self-relieving and double piston effect seats?

A self-relieving seat lets excess cavity pressure escape upstream, which protects the cavity from thermal expansion. A double piston effect seat seals against pressure from either direction and does not relieve the cavity by itself, so a separate relief may be needed.

Where are double block and bleed valves used?

Where a line section has to be positively isolated and proved at zero pressure, such as metering runs, pipeline stations, tank farms and chemical injection points. The valve is also used as the first isolation in a maintenance procedure.

Draft page. The factory document pack for this page is still under review. Values marked “To be confirmed” have not been verified yet.

Double block and bleed ball valves

Ball valves · API 6D

A double block and bleed ball valve isolates a line section with two seating surfaces and a bleed port between them, so the space can be vented and checked at zero pressure. API 6D defines double block and bleed and sets requirements for DBB valves. Pipeline and metering stations use it where a positive isolation check is needed.

Who makes this valve

Signed panel factories with API 6D in force4

How licences are checkedExpiry date in the API Composite List, not the “Active” flag

Registry checked8 Nov 2026Next check 8 Feb 2027

Manufacturer and licence no.Named in your quotation, with the registry link

We publish one number per API specification, never a factory-by-factory row: a row with its licence set and expiry year would point at a single registry entry. One RFQ is quoted by several panel factories, and each quotation names the manufacturer, its API licence number and where to verify it, so you can check the source before you order. How we check licences

How it compares

Single double block and bleed valve vs two isolation valves with a bleed (general)
Feature	Single DBB valve	Two valves plus a bleed
Components	One body with two seating surfaces and a bleed port	Two separate valves and a spool with a bleed valve
Space and weight	One body and two end connections	Two valves, a spool and three flanged joints
Isolation	Both seats in one body; the cavity between them is vented	Two independent barriers; the spool between them is vented
Maintenance	Serviced as one valve	Each valve can be maintained or replaced on its own
Typical use	Metering runs, pipeline stations and tank farms where space is tight	Where full independence of the two barriers is required

Specification range

Grades listed are common ASTM designations for this valve type. The size and class range offered by the partner factory is confirmed against its datasheet before this page goes live.

Double block and bleed ball valve specification: size × pressure class × ASTM grade
ASTM grade	Form	Sizes (NPS)	Pressure classes	End connections	Partner factory supplies
ASTM A216 WCB	Cast carbon steel body	To be confirmed	To be confirmed	To be confirmed	To be confirmed
ASTM A105	Forged carbon steel body	To be confirmed	To be confirmed	To be confirmed	To be confirmed
ASTM A350 LF2	Forged low-temperature carbon steel	To be confirmed	To be confirmed	To be confirmed	To be confirmed
ASTM A351 CF8M	Cast 316 stainless steel	To be confirmed	To be confirmed	To be confirmed	To be confirmed
ASTM A182 F316	Forged 316 stainless steel	To be confirmed	To be confirmed	To be confirmed	To be confirmed

Pressure–temperature rating (ASME B16.34)

The pressure class and the body material set the maximum working pressure at each temperature. The table shows ASME B16.34 Standard Class values for material group 1.1 (for example WCB and A105) at −29 °C to 38 °C. Seats and seals may limit the valve below the shell rating.

ASME B16.34 Standard Class, Group 1.1, −29 °C to 38 °C. Shell rating only; the valve rating can be lower because of seat and seal materials.
Pressure class	Max. working pressure (bar)	(psi)
Class 150	19.6	285
Class 300	51.1	740
Class 600	102.1	1,480
Class 900	153.2	2,220
Class 1500	255.3	3,705
Class 2500	425.5	6,170

Partner factory P-T curve for this valve: To be confirmed

How a double block and bleed valve works

In the closed position the ball is sealed by two seats, one facing each end of the valve, and the cavity between the seats is closed off from both sides. Opening the bleed valve vents that cavity, so a gauge or a drain at the bleed port shows whether either seat is passing.

With both seats holding, the bleed port stays at zero pressure. If the pressure rises, the upstream or downstream seat is leaking, and the valve needs attention before the line section is opened.

Self-relieving and double piston effect seats

API 6D defines both seat types. A self-relieving seat lets excess cavity pressure escape to the upstream side, which protects the cavity from thermal expansion of trapped liquid. A double piston effect seat holds pressure from either direction, so it isolates more strictly but does not relieve the cavity by itself.

The choice changes the isolation direction and the need for a separate thermal relief. State the seat type and the relief arrangement in the purchase order, and check that the flow arrow matches the intended upstream side.

Testing a DBB valve

Each seat is pressure tested from its own side, and the cavity is then vented and checked. API 6D and API 598 give the shell and seat test procedures; the cavity test sequence and the acceptance criteria for the bleed port are agreed in the purchase order.

Ask for the seat test record, the cavity test record and EN 10204 3.1 material certificates. On a DBB valve the seat test record is the evidence that both barriers hold, so it should identify the serial number and the test pressures.

Fire-safe design

A DBB valve is often an isolation point in hydrocarbon service, so a fire test is commonly specified. API 607 and ISO 10497 test the assembled valve with its seats and seals and then check that it still isolates; the certificate must cover the valve type and size range.

No fire-test certificate for a partner factory valve is published on this page.

Sour service: NACE MR0175 / ISO 15156

For wet H2S service the body, trim, bolting and welds have to meet NACE MR0175 / ISO 15156. Whether the standard applies depends on the H2S partial pressure, the in-situ pH and the temperature, so the RFQ should state those values instead of only the word “sour”.

A valve that is acceptable in sweet service may need a different trim or heat treatment on a sour line, and the bleed port and its fittings are part of the pressure boundary that has to meet the same requirements.

Datasheet and drawings

Datasheet: To be confirmed

A datasheet, dimension drawing and STEP file will be published here, without factory branding, after the panel factory authorises them. Until then, ask for them in your RFQ.

Prices, lead times and minimum order quantities are quoted per enquiry against the datasheet you send; they are not listed on this page.

Frequently asked questions

What is a double block and bleed ball valve?: A single ball valve with two seating surfaces and a bleed port between them. In the closed position it blocks flow from both ends and lets you vent the space between the seats, so you can check that both barriers hold.
How does a double block and bleed valve work?: Closing the valve brings the ball between two seats, one facing each end. Opening the bleed valve vents the cavity between the seats; if the pressure stays at zero, both seats are holding.
What is the difference between a DBB valve and two separate valves?: A DBB valve puts two seating surfaces in one body, with one bleed port and two end connections. Two separate valves with a spool and a bleed valve do the same job with independent barriers, but take more space and have more joints.
What is the difference between self-relieving and double piston effect seats?: A self-relieving seat lets excess cavity pressure escape upstream, which protects the cavity from thermal expansion. A double piston effect seat seals against pressure from either direction and does not relieve the cavity by itself, so a separate relief may be needed.
Where are double block and bleed valves used?: Where a line section has to be positively isolated and proved at zero pressure, such as metering runs, pipeline stations, tank farms and chemical injection points. The valve is also used as the first isolation in a maintenance procedure.

Standards cited

API Spec 6D: Pipeline and piping valves: design, manufacture, testing and documentation of ball, gate, plug and check valves.
API Std 598: Valve inspection and pressure testing (shell, backseat and seat).
EN 10204: Types of inspection documents, including the 3.1 and 3.2 certificates.
API Std 607: Fire test for quarter-turn valves with non-metallic seats: a type test of the design.
ISO 10497: Fire type-testing requirements for valves: test procedure and acceptance criteria.
NACE MR0175 / ISO 15156: Metallic materials for use in H2S-containing environments in oil and gas production: material selection, heat treatment and hardness limits for sour service.
ASME B16.34: Valves with flanged, threaded and welding ends: pressure–temperature ratings by material group and pressure class. Edition quoted to: ASME B16.34-2025.