Hot Hydrogen Attack (HTHA)
The presence of Hydrogen in industrial plants is a source of damage. HTHA is one such form of degradation which is due to the formation of Methane (CH4) by reaction with carbon in steel. HTHA can occur either in the parent material or in the weld itself and manifests itself in several types of alloys. The probability of HTHA damage in plant is dependent on the partial hydrogen pressure, operating temperature, and alloy. Methane formed by the reaction accumulates at internal voids on the grain boundaries, where under certain conditions the build-up can produce micro-cracks. In the weld material this leads mostly to a leak before break situation but when the damage is in the parent material, the result is mostly catastrophic for people and the environment.
The inspection methodology developed by Mistras gives an absolute indication of the presence of HTHA in parent material, as long it is above the (low and safe) detection threshold. Decisions can be made immediately, there is no risk of an ambiguous result. If necessary in order to measure the progress of attack, the inspection methodology generates data that can be monitored very accurately, on-line, and at temperatures of up to 450 degree Celsius.
The inspection methodology uses advanced techniques including Time Of Flight Diffraction (TOFD), ultrasonic backscatter, and velocity ratio measurements. This inspection methodology meets the HTHA inspection approach proposed in API 941.