The crack started on the inside of the thermowell. The chloride came from the air outside.
What happened
At a refinery, several austenitic stainless steel thermowells on a reactor vessel were found cracked. The thermowells were several metres long, carried thermocouples at different depths, and had screwed or flanged connections to instrument boxes. The UK Health and Safety Executive (HSE) published a safety alert on the finding, aimed at operators of major accident hazard plant.
What the investigators found
The cracking was chloride stress corrosion cracking. It started on the inside surface of the thermowells and progressed outwards. Most was in the extension above the top of the reactor vessel; some had initiated just below the vessel flange.
The HSE describes the mechanism as breathing. As the vessel heats and cools, so does the air inside the thermowell. Heating the tube from 20 to 420 °C would raise its internal pressure by about 2.4 bar at constant volume. On cooling, the pressure falls below atmospheric and outside air is drawn back in through connections that are not gas tight. Halides carried in that air concentrate in crevices, and with the stress in the tube, cracking can start.
The HSE notes that susceptibility is greatest where the outside air contains halides, such as at coastal locations or near cooling towers. Had the cracks gone through the wall, hazardous substances could have escaped from the reactor to the atmosphere.
What the MTIS Failure Investigation Assistant made of it
This test ran on V0, the pre-release version of the MTIS Failure Investigation Assistant's failure mode diagnosis engine. The current version is limited to helping an engineer identify the failure mode of a metallurgical failure. V1 is planned for commercial release in November 2026.
Given only the published facts, the Assistant ranked chloride stress corrosion cracking first at all three stages of the test. Its final score, after its own follow-up questions, was 26.1 out of 100 (23.5 before them), in the low band, with the runner-up at 13.3. It found the right failure mode but stated it with low confidence, a calibration point we are working on before V1. This is a test on one published case, not an accuracy claim.
What it means for asset owners
A thermowell is small, often outside the pressure-equipment inspection scheme, and its failure path runs straight from process to atmosphere. Many plants in Malaysia are coastal, humid, and have cooling towers close to the process units, which are the conditions the HSE highlights.
What an engineer should check
- Do you have a register of stainless steel thermowells whose failure could release a hazardous substance?
- Are their instrument-box connections gas tight, or can they breathe?
- Does the inspection programme look for cracking from the inside, where this mechanism starts?
Source. HSE Safety Alert HID 2-2013, Chloride induced stress corrosion cracking of stainless steel thermowells: potential for ingress of atmospheric moisture, 8 October 2013. Read the alert. The HSE refers to HSL Research Report RR902 for background on chloride stress corrosion cracking in austenitic stainless steel. This article summarises the source in our own words; see the original for full details. It is general technical information, not advice on a specific asset.
Related reading
- Stress corrosion cracking explained
- Materials selection for corrosive environments
- MTIS failure investigation services
- The MTIS Failure Investigation Assistant
- Follow MTIS on LinkedIn for new failure reviews and industry briefings.
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