What is the main operational characteristic that differentiates a relief valve from a safety valve?

The main operational characteristic that differentiates a relief valve from a safety valve is that a relief valve opens partially under pressure. This partial opening allows for the gradual release of excess pressure, which is essential in systems where pressure build-up needs to be controlled without a complete discharge of flow. Relief valves are typically designed to manage varying amounts of pressure, and they can modulate their opening to release pressure continuously rather than operate only in a binary (fully open or fully closed) manner.

In contrast, safety valves are designed to open fully at a predetermined set pressure to quickly relieve pressure in critical situations, ensuring safety by preventing catastrophic failures. They are generally used in applications where a rapid release of pressure is necessary, such as with gases or steam, rather than for continuous control of pressure like relief valves.