High Temperature Hydrogen Attack (HTHA) Detection at up to 370C (698F)

Accounts with shop validation on carbon steel samples prior to field trials, on an in-service C 1/2 Mo vessel, were reported at a recent industry conference. The studies were successful in the laboratory and appear to make sense in field trials on a C 1/2 Mo, in-service vessel.

Shop validation tests were performed on carbon steel samples with varying degrees of HTHA damage. UT readings were performed on the shop samples from ambient up to 370C (698F). Results of laboratory trials were:

• Ultrasonic backscatter spectrum analysis (BSA) was used as the basic technique for the base metal exam and TOFD (time of flight diffraction) UT for weld inspection. The BSA approach was demonstrated on a series of carbon steel samples, with varying confirmed levels of HTHA damage, using a 10MHz, broadband, contact sensor at temperatures up to 370C, using a standoff element. The paper suggests good differentiation between no attack, minor damage/moderate damage and severe damage.
• TOFD was the method of choice for welds and heat affected zones. Testing was performed up to 343C (650F). All artificial defects were found, including a 3mm deep simulated lack of fusing defect, which it was considered as representative of advanced stages of HTHA.

Following the HTHA method procedure development and shop demonstration, a risk based inspection plan was developed for a reactor in hydrogen related service which operates at conditions between the API 941 carbon steel and C 1/2 Mo curves and external surface temperatures in the 340C to 365C (644F to 689F) range. These methodologies were considered in the risk based inspection plan.

The based metal exam was performed using the spectrum analysis method and welds and HAZs were inspected using the TOFD method. A relatively cool, low stress area was chosen on the reactor as the “Clean” reference calibration sample. BSA characteristics of this area were similar to the clean qualification sample.

Of the various areas, base metal and welds and HAZs, examined on this reactor, no evidence of damage was found. More sensitive testing will be performed on this vessel during the next scheduled turnaround.