In a method and system for testing the integrity of pressure vessels, or welds on flanges or the like, at elevated temperatures, a pressurized mixture of a gaseous environmentally safe composition is formed within an indicator chamber in a housing receiving a pressurized gas and containing a surface in the chamber upon which is placed an indicator composition including a marker component, whereby the marker component is carried by the carrier gas from within the chamber and through a conduit to the pressure vessel or weld area. The carrier gas and the indicator or marker component are injected in an area between the inner and outer weld of a terminal flange welded to a tubular pipe section or casing. The pressurized composition includes a marker component or sub-composition which is detected by a detection apparatus which scans the pressure vessel area to be tested, or weld area. The pressure of the gas may also be monitored to observe losses in pressure indicative of flaws in the pressure vessel area or welds. The integrity of the pressure vessel or welds is tested at elevated temperature permitting remedial repairs to be made without reheating the work area of the pressure vessel or on a tubular pipe or casing.