Over the past 20 years, there have been a number of significant revisions to Section 1 of the ASME Boiler and Pressure Vessel Code relating to water level instrumentation. Understanding the timelines associated with these revisions provides a partial explanation for the instrumentation that was originally installed on a particular vintage of boiler by the OEM. Other factors may include instruments that are in addition to those required by the Code and others that have been replaced by the user, which may or may not be Code compliant.
Prior to 1991, for operating pressures up to 400 psi, the Code required two water gage glasses or one water gage glass combined with a set of three gage cocks (also known as try cocks, and they are typically installed on a water column). Gage cocks are valves that vent to the atmosphere and when actuated (by the operator) provide a visual confirmation of steam or water that emits from the discharge nozzle. In 1991, the use of gage cocks or a second water gage glass was eliminated from the Code for operating pressures up to 400 psi (Ref: Subsection PG60.4). This was based on the high reliability of water gage glasses on these applications. However, some operators still continue to use gage cocks for confirmation.