This is a follow on to my previous question about blanking spades. There's a gas pipeline with a butterfly valve mounted between to flanges. I want to work on one side (call it the inert side) and need to close his side with a blanking spade to the other side (gas side). To enter the spade I have to close the valve and open a few bolts so I can push the spade in. But the moment I remove bolts, the force (What's Kraftschluss in english) between the valve and the flange on the gas side is weaker and I get a leak. At least that's what I think happens.
Depending on pressure on the gas side, I either have a gas leak and a potential hazardous atmosphere in the working area, or air infiltration into the pipe and a potential explosion hazard there.
So far all no butterfly valve I've seen could be mounted as a headpiece.
So either there's a trick to it or a short time hazardous atmosphere is accepted (and can be worked around with appropriate safety measures).
So what is the actual procedure to enter a spade into a pipeline in light of this?
P.S. I don't do or supervise these works. I'm just curios how the folks doing it (should) go about it.