Glowworm Pressure Relief Safety Valves (Prv)
The pressure relief valve helps to control pressure build up within a heating system. It is set to release water if the pressure is too high to prevent catastrophic boiler malfunction.

Glow-worm 0020118190/SD Pressure Relief Valve - Betacom/Easicom/Ultimate
Part No: 1340566 / MPN: 0020118190/SD