Extended inspection intervals for a hydrogen shift reactor


  • Hydrogen shift reactor vessel, operating at 425°C and 20bar with a process hydrogen content of 75%, fabricated from C½Mo steel


  • After around 250,000h service with no untoward events or adverse inspection findings, it is desired to extend the inspection interval from 3 to 5 years


  • To satisfy the Competent Body under the European Pressure Equipment Directive that there is a technical basis for extending the inspection interval


  • Assessment of the kinetics of hydrogen attack, so as to demonstrate that unacceptable levels of damage could not arise during a five-year period of normal operation
  • Assessment of the growth rates of hypothetical cracks, so as to demonstrate that no credible defect could reach a critical size during a five-year period of normal operation


  • Whilst the operating conditions lay above the relevant Nelson Curve in the current edition of API 541, it could be demonstrated that the design was consistent with the version ruling at the time of construction
  • Currently available kinetic models, including those in API 581, indicate that there is little likelihood of general hydrogen attack becoming evident in foreseeable timescales under present operating conditions
  • Predicted rates of creep and fatigue crack growth from detection-limit defects are slow enough to be negligible over the expected future operating period
  • Critical defect sizes are well in excess of current detection limits, and leak-before-break can be reasonably inferred


  • The technical basis for extending the inspection interval was accepted by the Competent Body
  • The extended inspection interval brought this item into conformity with the remainder of the unit, allowing saving of inspection costs
  • Future integrity of the reactor vessel was assured and a basis for risk management was established
Hydrogen solubility in low-alloy steels


Brear, J.M. and Church, J.M.
“Technical basis for API Publication RP941 (Nelson Curves)”
Third Int Conf ‘Engineering Structural Integrity Assessment’, Cambridge, September 1996

Plant Integrity