I’ll start this post with a confession: I’m a bit new to the valve game. But I’m learning quickly. There lies, on the desk next to me, an array of valves I poke and prod and ask questions about. Colleagues drop by and I ask questions. The fascinating world of valves.
No wonder, then, that I soon bumped into an old industry debate: what exactly, is the difference between a “double block and bleed” (DBB) and “double isolation and bleed” (DIB) valve?
Apologies if you’re an industry stalwart, but to the enthusiastic newcomer such as myself, and anybody else out there, this topic seemed to deserve further exploration.
To start off, Fred Turco does a great job explaining in one of our interviews. Fred refers to the API definition, so here it is:
Article 4.7 defines a double-block-and-bleed valve as “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” with an additional note that “this valve does not provide positive double isolation when only one side is under pressure”
Article 4.8 defines a double-isolation-and-bleed valve 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” with an additional note that “this feature can be provided in one direction or in both directions”.
- a DBB is a valve that seals from either direction with a bleed between the two, if the first seal leaks, the second will not seal in same direction.
- a DIB is a valve that seals from one direction with two separate seals and a bleed between the two. Here we have two seals from the same direction.
A DBB valve implies double security, but the difference is that it actually gives security in two different directions, each with a separate seal. The important distinction is when a DBB valve is in use and the first seal leaks the second seal will not seal in that same direction.
Good, I’m making progress.
At this point the topic can be discussed further, depending on the type of valve used and it’s intended application. Ron Manson, Cameron’s Director of Applications Engineering says on their website.
“When determining whether to use a DBB or DIB valve and which definition to follow, API or OSHA, it is important to have a clear understanding of the similarities and differences of the types of block-and-bleed valves and the specific application the valve will play in. Additionally, it is important that the features required for isolation are fully tested during factory acceptance testing of the valve.”
Ingolf Fra Holmslet from Klyde Consultants also contributed a series of articles to Valve World Magazine last year on the topic (see 2015 Issues 1, 3, 5, 7 & 9).
However the different definitions to me are interesting. We have an API defintion, an OSHA definition, and the British Valve & Actuator Association (BVAA) apply their own definition yet again. In their efforts to simplify the situation I’ve read that suppliers publish their own internal guildelines as well.
Is it just me or is this confusing? Feel free to let us know your thoughts on our LinkedIn forum:
Is it an industry issue?
Do many in the industry still refer to DBB when whey really mean DIB?
Is there (despite the API modifiations in 2008) a confusion in the industry on the different types of of block-and-bleed valves and their intended application?
Your input will be enthusiastically received.