Figure 8 Blind

[miker01]

Can anyone please advise on the theory behind the use of a "Figure 8 blind" in front of a 10"-300# ball valve. This is a natural gas filtration application

 

[hacksaw]

The use of orifice blinds is self explanitory: positive definite shut-off.

Use of figure-8 blinds are common in petro-chemical ind. visual id of blind status is required.

That said, does your question refer to their specific use with a ball valve?

 

[miker01]

Thanks Hacksaw,
I picked it up out of a P & ID. It is being shown in between a gas filter/coalescer vessel and a ball valve, at the inlet , outlet and vent connections. I have never seen one called out before on a gas filtration vessel. How does it work? how is this more efficient than a ball valve, what seals it? do you know of a web site or manufacturer that has specific details?
Thanks

 

[saxon]

miker01, Efficiency's got nothing to do with it, SAFETY does! The figure eight blind has the blind on one side and and an orifice ring on the other. When equipment repair or cleanout is needed the blind portion is rotated into position and the orifice ring rotates to the outside, hence providing positive closure/blockage to the vessel.

Simply because the valve is closed, does not mean that it will stay that way. Furthermore, all valves fall into some leakage category, ANSI class I-VI and as valves get older, leakage paths can and will develop.

As for manu'f's check with the Mac Iron Works in Sandusky Ohio.

Hope this helps.
saxon

 

[zdas04]

Miker,
Keep in mind that you have to blow the line down to separate the flange and rotate the blind (many people call them spectacle blinds because they look more like old-time eye glasses than a figure 8). When rotated to no-flow, you have gaskets on both sides of a blind flange and as close to perfect seal as man can devise.

David