'Pressure Safety Valve' and 'Pressure Relief Valve' are commonly used terms to identify pressure relief devices on a vessel. Frequently these terms are used interchangeably and it may entirely depend on a particular project or company standards to identify all the pressure relief devices either as 'safety valves' or as 'relief valves' or sometimes even as 'safety relief valves'.
Although freely used interchangeably, these terms differ in the following aspect,
Pressure Relief Valve - is the term used to describe relief device on a liquid filled vessel. For such a valve the opening is proportional to increase in the vessel pressure. Hence the opening of valve is not sudden, but gradual if the pressure is increased gradually.
Pressure Safety Valve - is the term used to describe relief device on a compressible fluid or gas filled vessel. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.
