The difference is generally in capacity and setpoint. A relief valve is ment 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 ment 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 valveis 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 safey 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 adaquatley sizing safety valves.