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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Double Block and Bleed versus Trunnion
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(Petroleum)
(OP)
29 Nov 04 18:39Hi all,
I'm sorry for posting, but unfortunately the search isn't working.
Could someone please explain the difference between a floating ball valve and a trunnion mounted ball valve? And what is double block and bleed on a trunnion mounted ball valve?
Thanks in advance.
2
zdas04(Mechanical)
29 Nov 04 23:38A floating ball valve relies on a differential pressure to shift the ball a few microns into the downstream seat. This takes the ball off of the upstream seat and allows upstream pressure to fill the ball cavity.A trunnion ball valve uses springs to push both of the seats against the ball (which is held rigidly against lateral movement by the trunion bearings). A trunnion ball valve has two seating surfaces against the process fluids and has a body bleed to evacuate the cavity in the ball when the valve is shut. This is similar to having two floating ball valves with a vent valve in between.In cases where safety (or the government) dictates that you have "double block and bleed" (e.g., energy isolation for hot work in a situation where no one is working in a confined space), a trunnion ball valve meets the definitions (it is up to each company to satisfy themselves if it is truly a safe practice, but it does meet the definitions).On the other hand, some manufacturers sell "double-block-and-bleed" valves which are not trunnion ball valves. These valves typically have two complete valves and a vent in one casting. They use various technologies and probably work as advertized. I'm not terribly familiar with them, but one that I looked at was not piggable and I went back to Trunnion Ball Valves.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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(Petroleum)
(OP)
30 Nov 04 11:12Thanks Dave. It finally makes sense.
(Industrial)
10 Dec 04 07:09Zeroman & David,Dave has just about covered the "how they work issue" although there are some variants. Piggable double block and bleed valves, or DBB's, are available so are floating ball versions, I've acquired them for projects before. Try a google/yahoo search for "alco dbb valves". or try www.mono-flange.com this has more details on the actual types and variations you can also refer to the EEMUA 182 standard for a deeper understanding.
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