Double Block and Bleed Valves

Double Block and Bleed Valves:

Double Block-and-Bleed Valve – API Definition

According to API 6D “Specification for Pipeline Valves”

• Double block and bleed valves is a “single valve with two seating surfaces that, in the closed position, provides a seal against pressure from both ends of the valve with a means of venting/bleeding the cavity between the seating surfaces.
• This valve does not provide positive double isolation when only one side is under pressure.
• Valve seals against pressures from both sides of the valve
• In a Double Block and Bleed valve, there are two unidirectional self-relieving seats.

Double Block-and-Bleed Valve – OSHA Definition

• DBB – “the closure of a line, duct or pipe by closing and locking, or tagging, two in-line valves and by opening and locking, or tagging, a drain or vent valve in the line between the two closed valves.”
• Can only be achieved with two separate valves with a method to bleed pressure in between

Double Block and Bleed Valves – British Valve & Actuator Association (BVAA) Definition

British Valve & Actuator Association (BVAA) defines

• Double block-and-bleed as “a manifold that combines one or more isolation valves, usually ball valves, and one or more bleed/vent, usually a needle-style global valve, into one assembly for interface with other components (e.g., pressure measurement transmitters, pressure gauges, and switches).
• Also, says that for DBB capabilities, only one valve is required, not a system.
• “DBB valves replace the previous traditional technique employed by pipeline engineers to create a double block-and-bleed configuration in the pipeline, usually by fabricating three valves using flanges, ‘Tee’ pieces, and associated bolting.”

Double block and bleed valves

Double block-and-bleed valve is a compact valve or valve system that provides more reliable isolation in critical areas than a standard, single valve would.

• This smaller system or single valve unit serves to
• reduce the installation footprint
• saves on extra piping requirements
• reduces weight in critical areas
• save space, time, and cost
• reduce the need for costly multivalve systems
• provide zero leakage capabilities from upstream to downstream
• allows for an integrity check of seat seals
• used in a variety of applications and markets,
  • LNG, natural gas industrial processes
  • petrochemical, transmission and storage
  • liquid pipelines, and refined products transmission lines

Double isolation-and-bleed (DIB) – API Definition

• Important to use a DIB valve instead of a DBB valve in applications that require an additional pressure barrier that seals separately of the main pressure barrier.
• This is important to fulfil
  • certain operational safety requirements
  • according to the nature of the service, such as ▪ low tolerance for leakage ▪ cleanliness of the fluid.
• Valve cannot relieve body cavity pressure past the seats – meaning its seats are not self-relieving.
• When using a DIB valve, an external relief system is necessary to relieve pressure build-up. 

Single Block and Bleed Valves

One block valve and a bleed valve are manufactured as a single assembly

• Used to achieve positive isolation in live process plant usually used for non-critical process service, such as low pressure system or nontoxic, non-hydrocarbon, non-hazardous process fluids
• The block valve is closed first. The shutdown side of the block and bleed assembly is then drained/vented/purged to make ready for maintenance.
• Often used with spectacle blind or blind spacers to provide positive isolation
• Usually, ball valves are used as block valves or isolation valves and ball or needle valves are used as bleed valves or drain valves in the block and bleed assembly.

Slimline Primary Isolate Valves

Designed to replace the traditional primary isolate valve by combining a primary isolate piping valve with instrument double block and bleed functions

• Can be installed as the primary isolate valve, in either single block, block and bleed or double block and bleed versions.
• Is of outside screw and yoke construction and is designed to ASME VIII specifications
• Dual instrument connections enable instrument to be mounted vertically on either horizontal or vertical line mounting application.
• Use standard or heavy duty needle valves, for different pressures.
• Leak paths greatly reduced
• Process connections from ASME 1/2″ 150 class up to ASME 2″ 2500 class
• Instrument port of 3/4″ NPT and a vent port of 1/2″ NPT.

Gauge Block Monoflange Valves

Gauge Block Monoflange valves work in conjunction with a pre-installed primary isolate valve

• They provide a very compact instrument Double Block and Bleed valve
• This range is also available in a Single Block and Double Block and Bleed configurations
• Capable of horizontal or vertical orientation
• Use standard or heavy duty needle valves, for different pressures

Installation – Typical

Process to Instrument Valves

The units that utilize ball valves are referred to as a Monoflange

• Monoflange describes how the unit has a single flange for connection to the process flange
• Outlet (instrument) and bleed connections are normally screwed.
• Ball valves are not torque seated valves.

• The term Slimline describes how the unit has a single flange with multiple drillings;
• It has tapped instrument and bleed connections.
• Needle type globe valve units are used for clean service, and are extremely compact.
• Application temperature is limited to 150 °C (300 °F) for all classes due to the thermoplastic seals that are used to achieve emission class A or B.