Which term describes the first escape of steam before a safety valve actually lifts?

The term that accurately describes the first escape of steam before a safety valve actually lifts is "pre-lift." This term refers to the initial release of steam that occurs when the pressure in the boiler approaches the set point of the safety valve but is not yet high enough to lift the valve fully. This phenomenon can indicate the system is nearing its safety limits and serves as a proactive measure to ensure the valve operates effectively.

In this context, terms like "simmer," "fizzle," and "leak" do not specifically address the point of steam escaping at a pressure below the safety valve's lift point. "Simmer" generally pertains to a low boil, "fizzle" implies a slight or ineffective escape of gas or steam, and "leak" refers to any unintended release of steam or fluid from a system. Therefore, "pre-lift" is the most accurate term to describe the initial escape of steam prior to the safety valve lifting entirely, relating closely to the operational mechanics of steam systems and safety measures.