High Temperature Hydrogen Attack (HTHA)
- 18 Sep 2024
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High Temperature Hydrogen Attack (HTHA)
- Updated on 18 Sep 2024
- 2 Minutes to read
- Contributors
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- PDF
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Take Note
There are some specific cases where the S-RBI assessment approach in IMS PEI changes, depending on the Degradation Mechanism.
This section gives guidance for High Temperature Hydrogen Attack (HTHA).
StF – Step 7
See Step 7.
There are three different StF questionnaires:
StF Non PWHT Carbon Steels
StF 0.5Mo STEELS
Stf ALL STEELS − Excluding 0.5Mo Steels and non-PWHT Carbon Steel
An appropriate StF questionnaire will be selected based on the user-provided input, depending on the entered BASE MATL and PWHT Specs on the Component level.
For a more detailed overview of the requirements for each StF questionnaire, as well as the details for each questionnaire, consult the spreadsheet attached below.
Take Note
In principle the API RP 941 should be leading. The S-RBI assessment has been adapted to include existing situations where 0.5-Mo Steels and as welded Carbon Steels (non-PWHT) are still in use for historical reasons.
Confidence – Step 10
See Step 10.
For HTHA, the Confidence assessment has been extended from 3 to 4 questions and the scoring has been tightened to make it more difficult to extend inspection intervals unless process stability, process monitoring, and inspection are clearly understood. All questions need to be answered positive to gain Very High confidence. Consult Table 64 for guidance.
Confidence Assessment for HTHA guidance:
Key Questions | Main Input | Guidance | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Degradation mechanism can be properly controlled. | Corrosion Engineer | Process Stability: Over the last run, has the unit has been operating
| ||||||||||||||||||
Relevant process parameters are reliably monitored. | Operator / Process Engineer and the Corrosion Engineer | Have the operating conditions been verified to be within the IOW?
Process Monitoring Validation Schedule for HTHA:
| ||||||||||||||||||
Reliable inspections have been carried out. | Inspection Engineer |
| ||||||||||||||||||
Total percentage of susceptible areas inspected at the last inspection. | Inspection Engineer |
This question allows the inspection interval to be increased to the maximum, based on the ratio of the amount of the susceptible areas inspected. A 50% inspection sample size is used as it allows the interval to be increased by a factor of two, but this can only be achieved if all the Confidence questions are answered positively. |
IMS sums the total points from the four questions. Half scores, i.e., a total of 0.15 are not rounded up to 0.20 but is left at 0.10.
MII – Step 11
See Step 11.
IMS implements the following table to determine the MII.
HTHA MII Lookup table:
Inspection Intervals (yrs)
| Confidence Rating | |||||
VL | L | M | H | VH | ||
Criticality | E | 0 | 0 | 0 | 0 | 0 |
H | 0 | 1.5 | 3 | 6 | 9 | |
MH | 1.5 | 3 | 6 | 9 | 12 | |
M | 3 | 6 | 9 | 12 | 15 | |
L | 6 | 9 | 12 | 15 | 18 | |
N | No inspection | No inspection | No inspection | No inspection | No inspection | |
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