THE TEMPERATURE MYTH

Many LDS clients own and use temperature measurements for various purposes. But temperature is not a reliable indication of valve leakage. Temperature often produces false positives as well as false negatives.

Hot does not necessarily indicate the presence of a leak, and cool does not necessarily indicate tightness.
Leaks of high-temp water and steam under pressure begin as a refrigeration process. The expansion causes cooling (not heating). The same process can be seen in compressed air discharges and also the discharge from unlit butane lighter.

A leak is usually going to saturation conditions. There can not be a temperature rise unless the pressure rises also. The pressure downstream of a leak will not rise until the flow becomes so strong as to cause a restriction in the pipe. Often the downstream pipes are sized to cause little or no restriction to flow with the valves wide open. This thermodynamic process is known as the Joule-Thomson Effect. It describes the temperature change of a gas or liquid when it is forced through a valve while kept insulated so that no heat is exchanged with the environment.