DIN Standard Safety Valve Features: 

Safety valves were first used on steam boilers during the industrial revolution. Early boilers without them were prone to accidental Explosion.
Safety valves also evolved to protect equipment such as pressure vessel (fired or not) and heat exchangers. Safety valve term should be limited to compressible fluid application (gas, vapor, steam).

The two general types of protection encountered in industry are thermal protection and flow protection.

For liquid-packed vessels, thermal relief valves are generally characterized by the relatively small size of the safety valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly incompressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure.

DIN Standard Safety Valve Specification

Safety Valve (Pressure Relief Valve)  Dimension