Cartridges on coffee drier anti-explosion systems


Today on a tour of the Nestle UK coffee roasting plant in Hayes, just outside London, they showed me the anti-explosion system on the coffee syrup drier. The drier is a massive machine five stories high where coffee syrup is sprayed into the top of the machine, and dried to a powder using superheated air that is injected in. The anti-explosion system uses heated water that is pressurised by the steam above it that heating produces. When a fire is detected inside the drier machine, the anti-explosion system causes a cartridge to be detonated (not sure whether percussion/electric) which allows the pressurised water to flow in and prevent an expolosion due to fine powder in air. Does anyone have any idea what calibre this (presumably blank) round would be? I couldn’t se it as it was inside the valve.