Stano,
We are moving away from the original subject, but OK.
DB&B is normally not considered safe (positive) isolation in case you have to enter a vessel or a tower. Some operating companies accept it only if other ways of isolating (e.g. spectacle blind, spade, removable spool) is not practical and if the DB&B is clearly tagged and locked. Other operating companies do not accept it. Reasoning is that chances are too high that someone may open the valves without knowing that someone is inside the vessel.
In your case (fuel gas lines to heater), the reason for the DB&B is to prevent leakage of gas into the heater box that could result in an explosive atmosphere inside the heater box. Here I have always seen the DB&B as described in the article from Driedger. In case the heater is stopped, the two main valves close and the valve in the vent line is automatically opened. Only in case of small pilot gas lines I have sometimes seen that only the double block valves are used, without the vent.
Now there is a more general discussion about how to use the bleed in the DB&B: Is the bleed used to remove pressure on the second block valve (in which case it should be open when the block valves are open and hooked up to safe location), or is the bleed used to frequently test whether one or both block valves are leaking (in which case the bleed is normally closed and only opened to do the leak testing of the block valves). Reasoning of the second possibility is that if you frequently test the valves, the likelihood that the two valves in series will fail is very small, in other words: a tolerable risk.
It seems like NFPA 86 now recommends to use the bleed is a leak testing device instead of a way to remove pressure from the second block valve.
A few weeks ago we did some research to see what the generally accepted method is to use of DB&B and found that there is no such general acceptance (e.g. between operating companies or international standards).
