How Does Double Block And Bleed Ball Valve Work?

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Any time you&#;re going to work on a valve, separator, or other piece of equipment on a well site, we highly recommend you use the practice &#;Double Block and Bleed.&#;

The Hiss

Maybe you&#;ve had this experience.

You&#;re going to repair a valve that&#;s in line. You know this means you need to block off and depressurize the section you&#;re working on. So you close off the ball valve upstream, open the bleed valve to vent pressure, and get to work.

You square up to the valve in need of repair and start unthreading bolts. Then you hear it: a hiss. You quickly snug the bolts back down.

I could have sworn I just isolated this valve, you think. What did I miss?

One possible cause of the hissing in this scenario is that the valve is getting pressure from downstream.

What is Double Block and Bleed?

Double block and bleed (DBB) is the practice of shutting in a section of pipe on both sides of the valve rather than just one.

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It means you close the ball valves to block both the upstream and downstream sides of your working area, and then bleed any pressure that remains in the piping and valve.

The practice of double block and bleed reduces the risks of harm to people and the environment, which is why many oil and gas companies have adopted the practice as mandatory when working on field equipment.

Stay safe out there!

(Note: This article refers to the safety practice; double block and bleed may also refer to a specific kind of valve.)

DBB definitions

There are two entities in the United States that define DBB&#;API and OSHA. According to API, a DBB valve 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." API also notes in this definition that this valve does not provide positive double isolation when only one side is under pressure.

In contrast, OSHA describes DBB as "the closure of a line, duct, or pipe by closing and locking or tagging two inline valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves."

API's DBB definition does not achieve the same level of isolation as OSHA's. API allows DBB valves to be one single valve with two unidirectional seats, while the OSHA standard can only be achieved with two separate valves with a method to bleed pressure in between. There are some valves that utilize a twin-valve design. By combining two valves into one body, a twin-valve design reduces weight and potential leaks paths while meeting the OSHA requirements for double block and bleed.

Valve associations usually choose to follow either API's or OSHA's definition, but some have created their own handbook with their own definitions for industry terms. For example, the British Valve and Actuator Association (BVAA) defines DBB 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)."

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