What is the difference between a pipeline relief valve and a safety valve?

You mean that thermal expansion valve installed on pipes, right? It’s a type of safety valve

Pipe safety valves are suitable for liquid media, with a slight opening mechanism

In a broad sense, so-called safety valves include relief valves. From the perspective of regulatory requirements, they are valves that must be installed directly on steam boilers or pressure vessels of category 1, and it is necessary for them to be approved by the technical supervision authorities; in a narrow sense, these are referred to as safety valves, while the others are generally called relief valves. Safety valves and relief valves are very similar in structure and performance; both discharge the fluid inside automatically when the set pressure is exceeded, in order to ensure the safety of the production equipment. Due to this fundamental similarity, people often confuse the two when using them; furthermore, some manufacturing equipment also allows either one to be used as per the regulations. Therefore, the differences between the two are often overlooked. As a result, many problems have arisen. If a clearer definition for both is to be provided, it can be understood in accordance with the definition outlined in Part 1 of the ASME Boiler and Pressure Vessel Code: (1) Safety Valve, an automatic pressure relief device driven by the static pressure of the medium in front of the valve. It is characterized by a sudden, full-open starting action. Used for gas or steam applications. (2) Relief Valve, also known as a overflow valve, is an automatic pressure-relief device driven by the static pressure of the medium in front of the valve. It opens proportionally as the pressure exceeds the opening force. It is mainly used in fluid applications.