In many ways, they are similar. Thermal relief valves will handle liquids and usually have very small capacities. Safety relief valves can be much larger and can be sized for either vapor or liquid, can be spring actuated, pilot actuated, etc.
Thermal Relief Valves are typically liquid service and have no blowdown adjustment. Many are "cage" design where the guide is threaded onto the nozzle to align the disc. These are low capacity valves, most often threaded inlet. SRVs are designed for Compressible and Incompressible fluids. Many are adjustable blowdown designs. Typical SRVs range in size from 1" to 8" flanged inlets. Larger designs are availble. There are also threaded SRVs very similar to and in some cases used as thermal relief. In short, thermal relief is one application of SRVs. Thermal relief may not be a code application, so most thermal reliefs are non-code designs. HOwever, SRVs are used in ASME COde applications extensively for both compressible and incompressible fluids.
