Defining “Double block and bleed”

Defining “Double block and bleed” In very basic terms a Double Block and Bleed Valve consists of two block valves and a bleed valve. The block valves act to shut off pipeline flow and the bleed valve is used to drain that particular section of the pipeline. They are often used in high pressure systems or toxic/hazardous fluid processes.

Applications include chemical injection connection, instrument drain, gauge isolation and chemical seal isolation.

The term “Double Block and Bleed” is most commonly used to define the level of isolation provided by the valves. It is used to describe a method of safely isolating a pressurised or hazardous medium so that maintenance and modifications can be carried out. Additionally, this terminology is used to identify the relative integrity of temporary isolation systems.

Although “Double Block and Bleed” is a widely used term to specify a level of isolation, the specific definition used by organisations and regulating bodies can vary.

In their guidance document, The Safe Isolation of Plant and Equipment, the UK Health and Safety Executive, defines double block and bleed as “an isolation method consisting of an arrangement of two block valves with a bleed valve located in between.”

BP – Guidance on Practice for Safe Isolation and Reinstatement of Plant offer the following definition: “Double Block and Bleed consists of the closure of two block valves in series with an intermediate bleed valve. The integrity of both valves shall be tested separately and the bleed valve will then be left in the closed position between periodic integrity checks.”

In the United States, API 6D defines a double block and bleed valve as 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.” A 2008 note states that this valve does not provide positive double isolation when only one side is under pressure.

Peter Smith Valves offer high quality Double Block and Bleed valves supplied to various ratings. Fig. 650 Parallel slide valves are supplied to PN25/40 rating. Alternatively, valves can be supplied PN16 rated and ANSI 1501300 upon request.

Additional Fig. 640 Bleed Flanges are available for fitting downstream from the Fig. 650DBB Double Block & Bleed unit assembly to assist drainage of the system and to verify the closure of both of the double isolation valves and for end of line service capped with a blind flange c/w a drain valve.

The Fig. 640 bleed flange is an approved seal welded assembly, consisting of the body, extension tube and 015mm PN40 flange (or as specified by Customer) in certified carbon steel, B7 tie-rods with 2H nuts and 2 spiral wound graphite filled gaskets (suitable for use on steam). A fig. 280 cast steel globe stop valve, PN25/40 rated is available as a drain valve upon request.

All of the parallel slide valve details are as the current Peter Smith Valve Co. Ltd catalogue. The assembly can be supplied as a unit or in kit form or as a modification to existing valves on site. When assembled, the double block and bleed unit has a straight through, virtually full bore design, with only nominal pressure drop and internal turbulence.

Find out more at: www.petersmithvalve.co.uk