What type of line contains only vapor that will not condense?

The type of line that contains only vapor that will not condense is a Dry Leg Compensating Line. This line is designed specifically to maintain a pressure reference without allowing any liquid to accumulate. In the context of instrumentation, especially in level measurement applications, a dry leg is advantageous because it prevents the effects of liquid column height and associated density changes, which could affect pressure readings or cause erroneous measurements.

By ensuring that only vapor occupies the line, it provides a stable reference point that can reliably indicate measured levels or pressures. The dry leg essentially functions to transmit pressure from a vapor space directly, with the understanding that any changes in vapor pressure are directly related to the level of the liquid being measured, without the interference of condensation or liquid presence that could compromise accuracy.

Understanding these functionalities is crucial in process automation and control, where accurate measurement and control of levels or pressures can influence safety and efficiency in operations.