Bezzera Boiler Valve 1/2"M 1.65 Bar CE
Its job is to keep the pressure from exceeding a predetermined level. Most espresso machines control that pressure by using this simple boiler valve. Also referred to as a pressure relief valve or over-pressure valve.
The scaleup build are normally the cause for the damage to the safety valve. The spring will become weak with years of pressure pushing against it and eventually it will start to leak.
If you have gentle hissing and possibly some dripping from the pressure relief valve, it is time to change the boiler valve.