Difference between safety valve and relief valve

Difference bw safety valve and relief valve

Relief Valve – is the term used to describe relief device on a 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.

Safety Valve – is the term used to describe relief device on a compressible filled vessel. For such a valve the opening is sudden. When the set pressure of the valve is reached, the valve opens almost fully.

The difference is generally in capacity and set point.  A relief valve is meant to relieve pressure to prevent an over pressure condition.  A relief valve may have an operator on it to assist in opening the valve in response to a control signal.  A safety valve is meant to relieve pressure without operator assistance and a safety valve, or combination of safety valves, must be have a capacity to relieve more than the energy input to the volume being protected.  

For example, a thermal relief valve is used to bleed off pressure in a heat exchanger if the heat exchanger is isolated but the possibility of thermal expansion of the fluid could cause over pressure conditions.  The capacity of thermal reliefs are generally small.

A safety valve on a boiler or other types of fired pressure vessels must be capable of removing more energy that is possible to be put into the vessel.  110 percent of boiler rated capacity may be acceptable.  The ASME Boiler & Pressure Vessel Code would be the source to check for adequate sizing safety valves.