Difference between safety valve and relief valve
Relief Valve: The  relief valve is used 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. 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. The capacity of PRV are generally small.
Safety Valve: This device is used to  relief 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. 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.     _{Your Comment}