Sometimes terms can be confusing. Terms safety valve and relief valve are confusing too. So, to make it clear, let see the definition based on API 521.
Pressure relief valve is a generic term applied to relief valves, safety valves, and safety relief valves. It is designed to automatically reclose and prevent the flow of fluid.
Relief valve is a spring-loaded pressure relief valve actuated by the static pressure upstream of the valve. The valve opens normally in proportion to the pressure increase over the opening pressure. A relief valve is used primarily with incompressible fluids.
Safety relief valve is a spring-loaded pressure relief valve that may be used as either a safety or relief valve, depending on the application.
Safety valve is a spring-loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. A safety valve is normally used for compressible fluid.
Can you spot the differences?
Relief valve is used for liquid and open in proportion to the pressure increase, while safety valve is used for gas and open rapidly.