Double Block & Bleed

[muidrees]

I live in Canada and my query is with regards to Double block and bleed valve. My understanding is that a professional engineer can sign off on a smaller bleed line size for Double Block & Bleed applications or requirements, however this has been challenged that this only relates to confined space entry applications.
I want to know if there is a limitation to the bleed line size based on the purpose of Double Block & Bleed valves/devices? What I would like to identify is this, are there limitations to a professional engineer on signing off a variance for a Double Block & Bleed devices.

 

[LittleInch]

I'm really following this.

a smaller bleed line size than what?

The purpose of a double block and bleed assembly is to provide positive isolation which a means of determining if that isolation is holding. The purpose of the bleed is to test that isolation. It is not there to vent gas or liquid which is passing one part of the block therefore its size is relatively irrelevant IMO. If you have a bleed passing gas or liquid then you don't have a block and you should fix the block before continuing.

A DBB valve on the other hand is a different animal and a DBB valve does not provide the same level of isolation and ability to isolate items as it only blocks from one side or the other.

Apologies if you already know this, but don't mix terms like DBB assembly / valve.

Can you "sign it off" - I don't know - depends on what the design code and isolation philosophy say and what your authority level is for making and approving decisions in your company or authorizing deviations.

Remember - More details = better answers
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