Introduction to Double Block and Bleed Valve System
A Block and Bleed Valve System is a combination of one or more block/isolation valves, usually ball valves, and one or more bleed/vent valves, usually ball or needle valves. The purpose of the block and bleed valve system is to isolate or block the flow of fluid in the system so the fluid from upstream does not reach other components of the system that are downstream. This enables the engineers to bleed off or vent or drain the remaining fluid from the system on the downstream side in order to execute some kind of work (maintenance/ repair/ replacement), sampling, flow diversion, chemical injections, integrity check for leakage etc.
Double Block and Bleed Valves operate on the principle that isolation can be achieved from both the upstream and downstream fluid flow / pressures. This is achieved by two isolation valves (may be ball, gate, globe, needle, etc.) placed back to back, with a third bleed valve (usually NPS 0.75 or NPS 1 size isolation valve) in the center cavity. Once isolation has been achieved in one or more of the main process isolation valves, the cavity that is created between these isolation valves can be drained through the bleed valve. Bleed valves will either be vented directly to atmosphere locally or via a hose connection / piping to an appropriate closed disposal system.
In simple terms as explained by ValveMagazine.Com, “It’s time to do maintenance on a section of process. You don’t want to shut down the entire facility, so you decide to block off and depressurize just the section you’re working on. Just upstream is a double block and bleed valve—a trunnion-mounted ball valve with self-relieving seals and a bleed valve to vent the cavity. You close the ball valve and open the bleeder. Now you can de-pressurize the line downstream and open it up to work on it.”
Types of Block and Bleed Valve Systems
Single Block and Bleed
Single Block and Bleed Valve System consists of one block valve and one bleed valve. This system is sometimes refereed as only “Isolation Valve System”.
Single block and bleed valves are usually used for non-critical process service, such as low pressure system or non toxic, non hydrocarbon, non-hazardous process fluids. For critical service, double block and bleed valve assemblies must be used.
Double Block and Bleed
Double Block and Bleed Valve System consists of two block valve and one bleed valve. A double block and bleed valve is like having three valves in one.
API 6D defines a Double Block and Bleed Valve System 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 vending/bleeding the cavity between the seating surfaces.”
OSHA defines a Double Block and Bleed Valve System as a “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”.
Single Unit Double Block and Bleed
Single Unit Double Block and Bleed Valve provides double block and bleed in a single valve. This style can isolate piping on both sides of the valve to vent/bleed the valve cavity between the seats.
Using a single unit double block and bleed valve system versus 3 separate valves saves installation time, weight on the piping system, and space. This design also has operational advantages,
- There are significantly fewer potential leak paths within the double block and bleed section of the pipeline.
- The valves are full bore with an uninterrupted flow orifice they have got a negligible pressure drop across the unit.
- The pipelines where these valves are installed can also be pigged without any problems.
- All the valve components are housed in a single unit, the space required for the installation is dramatically reduced thus freeing up room for other pieces of essential equipment.
- Shorter drain times is required.
- One actuator instead of two is required.
Double Block and Bleed Valve Applications
Double block and bleed valves are most commonly used in the oil and gas industry, but can also be helpful in many other industries. It’s typically used where bleeding the valve cavity is required, where piping needs isolation for maintenance, or for any of these other scenarios:
- Prevent product contamination.
- Remove equipment from service for cleaning or repair.
- Meter calibration.
- Liquid service near waterways or municipalities.
- Transmission and storage.
- Chemical injection and sampling.
- Isolate instrumentation such as pressure indicators and lever gauges.
- Primary process steam.
- Shut off and vent pressure measuring instruments.
Double Positive Isolation
Many people take the term “Double Block & Bleed” (DBB) to mean the same thing as “Double Positive Isolation” (DPI). While this may seem like a small matter, but in reality it means that some users may think they’ve achieved positive isolation when they haven’t. The key to understand the difference between Double Block & Bleed (DBB) and Double Positive Isolation (DPI) can be found in API 6D. API 6D wasn’t always as clear as it could have been in spelling out the difference between DBB and DPI, but the addition in 2008 of several notes has clarified it.
API 6D defines a Double Block and Bleed Valve (DBB) 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.” The 2008 note points out that this valve does not provide positive double isolation when only one side is under pressure.
API 6D defines a Double Isolation and Bleed Valve (DIB) as a “single valve with two seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the seating surfaces.” The 2008 note adds that this feature can be provided in one direction or in both directions.
The important distinction between the DBB and DIB is, that on a DBB if there is somebody working downstream on the line and the first seal leaks the second seal will not seal in that same direction.
Under normal conditions there is pressure on the upstream valve seal, which (along with an internal spring) keeps it energized. There’s no pressure on the downstream side, so the only thing energizing the seal on that side is a spring. The bleeder valves are open, and the cavity in the ball is at atmospheric pressure. But it’s not uncommon for a valve that’s been in service for a while to leak a bit. The upstream seal is leaking a little, but this should not be a problem because the leakage will be carried away by the bleeder—except when the bleeder is not working, either because one or both of the bleeder valves is closed, or because there’s a clog in the bleed line. The pressure in the valve cavity can then possibly reach as high as it overcomes the spring on the downstream seal and forces it off its seat, discharging fluid downstream to where people may be working. This is clearly not a double isolation and bleed valve.
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