It is in fact an aerjec or airsep these are made by several companies Myson being one. They do in fact allow air to exit the system via the expansion pipe, but their main function is to provide a nuetral pressure point within the system so that the pump always has the same pressure in front of...
Dont even attempt to weld it, I'm a coded welder and it won't work. cast iron will crack if heated to welding or brazing temperatures in one spot.
drill out, tap, and put in screw with sealant on it, as suggested previously.
As this is a nursing home, I take it that the pipe-work would be regularly chlorinated to prevent legionella etc, as is the case in the hospitals I work in. If so, plastic pipe is suscepitble to high doses of chlorine, maybe that is the problem.