API 6D calling out the distinction between a double block and bleed valve and a double isolation and bleed valve.
But I do not think this distinction is properly recognized by the whole valve industry.
we still see many technical documents written where it calls out double block and bleed which is what would is really needed is double isolation and bleed.
I will give a brief explanation on it below.
The main difference between the two
A double block and bleed valve is a valve that seals from either direction with a bleed between the two.
A double isolation of bleed is a valve that seals from one direction with two separate seals in a bleed between the two.
And the important distinction between the two is it on a double blocking 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 blocking bleed even though the name implies double security gives security in two different directions each a separate seal.
I hope more and more of my peers will look deeper into this issue, which will enable us to provide with more high-performance valves of safe design.
