The most effective method is to remove the insulation, check the surface condition of the pipe, and replace the insulation. This is also the most expensive method in terms of cost and time lost. The logistics of insulation removal will probably involve asbestos and its attendant complications. Process related problems may occur, if the insulation is removed while the piping is in service. Infrared In the right conditions, infrared can be used to detect damp spots in the insulation, because there is usually a detectable temperature difference between the dry insulation and the wet insulation.
Corrosion is a distinct possibility in the areas beneath the wet insulation.
This system is designed to detect wet insulation on pipes and vessels. A radioactive source emits high energy neutrons into the insulation. If there is moisture in the insulation the hydrogen nuclei attenuate the energy of the neutrons. The instrument's gauge detector is only sensitive to low energy neutrons. The count displayed to the inspector is proportional to the amount of water in the insulation.