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Double block and bleed isolation valve

What is the difference between a regular Double Block and Bleed valve and a Double Isolation and Bleed valve? Fred Turco of Omni Valve offers clarifications in this interview at Valve World StockExpo 2013 (coming soon: transcript of the interview below the video):

David: Fred, I’m hoping you can give me an explanation on a hot topic in the industry. What’s the essential difference between a double block and bleed valve and a double isolation and bleed valve?


Fred: David, I’m glad you asked that question. This has been a hot topic for many years. API-6D, Edition 23 that just came out recently, has finally recognized the difference between those two items. A double block and bleed valve is a valve that seals from either direction with a bleed between the two. A double isolation and bleed is a valve that seals from one direction with two separate seals and a bleed between the two. And the important distinction between the two is that on a double block and bleed, if there is somebody working downstream on the line and the first seal leaks, the second seal will not seal in that same direction. A double isolation and bleed has two seals, each of which seal in one direction. Meaning, somebody working downstream has the reliance that they can rely on the fact that the first seal is sealing. If it’s not, it’s detected by the bleed, and the second seal has to be overcome before that person would be in danger. So a double isolation bleed gives double security. A double block and bleed, even though the name implies security, gives security in two different directions, each a separate seal.


David: So do you think this distinction is properly recognized by the valve industry?


Fred: Yes, I do more and more so, but not as much as it needs to be. We still see many specifications written where it calls out double block and bleed which what is really needed is double isolation bleed. So even a lot of the engineering houses, I’m not sure everyone is fully aware or abreast of the recent developments of API-6D calling out the distinction between the two types. So I’d like to see more of it, but it is becoming more and more accepted.


David: Now Fred, you’ve been working in the industry for about 35 years. What are you seeing as key drivers in valve selection nowadays?


Fred: Yes, that’s a tough one David. Price is always an issue. But what I think is happening now is the security in the valve design, the integrity of the valve is more and more a requirement in the industry today. We’re seeing the requirement for really high integrity sealing, material test reports that go back to the actual foundries that show the different chemical and physical properties in the materials used. We almost never get an order today that doesn’t require an MTR from the foundry. Twenty years ago that was the exception.


David: So you’re saying that safety is paramount?


Fred: Definitely. Absolutely. Definitely. Price is still an important factor but safety is the driving force today.