There are 2 types of safety valve, standard valve and double safety valve.
The major different between these 2 valves is the junction of sealing and the shutter design. The standard one is metal to metal sealing while the double safety valve is metal to silicone sealing. Hence, the double safety valve have a better sealing.
Further to this, there is a shutter rod on a double safety valve in which consumers can move the rod during cleaning. It can prevent the malfunction of valve as the mineral scales (especially in hard water condition) will built up in the valve.
Moka Pot operate at pressure around 1 bar. But in some cases, such as too fine coffee grounds is being used or coffee grounds is being tamped, the boiling water may not able to pass through the coffee grounds. Hence, pressure inside the boiler built up.
In order to prevent pressure continue to build up, the safety valve will be operated at pressure around 2.5 bar. Pressure inside boiler will be released.
High quality safety valve is important. It can help to prevent pressure inside boiler build up.
Cross-section of Double Safety Valve
Cross-section of Standard Valve