High temperature hydrogen attack (HTHA) is a phenomenon which occurs within some industrial processes when the presence of hydrogen, subjected to high temperature and pressure, makes plants and systems susceptible to damage. This reaction can cause an industrial plant to suffer serious failure, which may lead to both expensive repairs and loss of production.
HTHA can be detected in a number of ways, but the reliability of some of the techniques used in its detection is questionable. We have developed a series of procedures featuring the latest advances in ultrasonic imaging and data processing techniques in order to achieve a greater level of confi dence in both HTHA inspection results and inspection repeatability.
Available techniques in accordance with American Petroleum Institute (API 941)
The backscatter technique is used to detect suspicious areas affected by hydrogen attack. The front of the attack in the material can be measured and imaged. The remaining wall thickness (un-attacked) is assigned a color.
A typical display of the backscatter technique
Time of fl ight diffraction (TOFD)
With time of fl ight diffraction (TOFD) testing it is possible to inspect welds for defects resulting from HTHA. It can be used for the detection of both macro and micro cracks. Macro cracks can easily be identifi ed with TOFD. Micro cracks, however, can be challenging to discriminate from other types of weld defects. Also here a verifi cation is found by cross-referencing the results to a Pulse-Echo or Phased Array UT examination.
Velocity ratio measurement
Velocity ratio measurement is based on the principle that the longitudinal and transverse sound velocities are affected by HTHA, by means of which the percentage of attack can be measured. By cross-referencing the results with the backscatter measurement, a confi rmation of the amount of attack is established. In addition, the methodology used allows for a differentiation between small inclusions and HTHA.
Ts = Unattacked wall thickness
Ta = Attacked wall thickness
Cl = Longitudinal sound velocity in unattacked material
Ct = Transverse sound velocity in unattacked material
Cla = Longitudinal sound velocity in attacked material
Cta = Transverse sound velocity in attacked material
Areas for application
Backscatter and velocity ratio measurement are used for the
inspection of the parent material. Time of flight diffraction
is used for the inspection of the weld material.
Benefits at a glance
- Digital techniques
- High reliability and reproducibility
- Clear presentation of results
- Numerous different geometries can be examined
- Damage progress can be accurately monitored.
At or above Nelson Curves
High inspection coverage: backscatter mapping, velocity ratio measurement, ToFD, 45° shear wave inspection
Up to 25 °F below Nelson Curve
Medium inspection coverage: backscatter mapping, manual, velocity ratio measurement, ToFD, 45° manual. (no requirement to open vessel)
Between 50 °F below Nelson Curve
Low inspection coverage: manual backscatter, ToFD
More than 50 °F below Nelson Curve