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I often see specifications for refurbishing works on gas pipelines calling for blanking spades, to separate the area worked on from the rest of the gas system. This is in addition to closing valves.
Googling showed me some pictures of how blanking spades look like, but I don't yet understand the process of installing them. The most straightforward way would be to mount them between flanges, after a section of pipe has been valved off and flushed. How are blanking spades actually used?
The most straightforward way would be to mount them between flanges, after a section of pipe has been valved off and flushed.
This is correct. Blanking spades, also known as slip plates, slip blinds or spectacle blinds, are usually slipped in between two flanges after a process line has been double blocked and bled. They are usually secured in place by the flange bolts and sealed by a gasket or O-ring, similar to how wafer pattern valves are installed.
Sometimes dedicated 'blind flanges' are provided in the pipeline for this purpose, but often the inlet or outlet flanges on valves or equipment are used.
Blanking plates are used to provide an additional layer of protection during maintenance work, guarding against leaking valves or negligent opening of an isolation valve. Other important considerations when using them are:
You are correct, blanks are installed at a pipe flange where two runs connect. Typically you break the joint and pry the flange apart to install the blank, but this can cause problems if/when the prying action torques the next joint in the piping system and damages the gasket. I've heard of a person replacing every gasket in a large system because every time a joint was pried apart it ruined the adjacent gasket.
