01 Oct CASE STUDY: HP SUPERHEATER P91 HEADER
Identification and repair of P91 material soft spots
PEI were engaged to assist in the retrofit installation of a set of 4 off HP Superheater headers fabricated from Grade 91 material in a HRSG. The client had experienced a tube to header weld failure – T91 tube to P22 header – which was found to be influenced by the differing material properties at the dissimilar weld. Their solution to minimising future failures was to change the HP Superheater header material to A335-P91.
The client had received the replacement headers from an overseas fabricator. A review of the MDR / QA Pack identified that the PWHT set up was inadequate for P91, not following best practice guidelines. Considering the susceptibility of P91 to overheating and consequential influence of degraded material properties, baseline hardness testing was carried out on the new P91 headers.
The hardness testing identified that the two ends of 1 of the headers were “soft” and non-compliant. As a result of this non-conformance a number of repair options were considered:
- Leave as is
- The HP Superheater Headers were barely accessible for monitoring of the material condition in-service.
- Normalise and temper to recover material properties
- difficult to ensure available furnaces had adequate temperature control and size for even temperature distribution on the component
- Concerns about management of temperature of the component with differing thicknesses of components, i.e. there were tube stubs welded to the headers
- Concerns about whether there would be dimensional impact on the header assembly as a result of the heat treatment cycle
- Part replacement and repair – remove soft ends of the headers and replace with new sections
- Complete replacement of header
- would materials be available at short notice?
- timeline required is greater than option 2
Option 2 – Removal of soft ends, replacement with new P91 material – was selected following consideration of the risks, the timeframe available, and consequences of the selected repair methodology on the long term performance of the equipment.
PEI was able to assist our client in the following ways:
- identification of poor methodology being applied to the fabricator’s post weld heat treatment plan that led to the suspicion that the purchase equipment may not be in the as-new condition as expected by the client
- identification of the appropriate method of inspection – hardness traverse – in order to identify if an overheating event had occurred on the replacement HP Superheater headers.
- Identification of the options available to the client in the event of the findings of low hardness material
- Evaluation of the pros and cons of the options
- Detailing the appropriate repair methodology for the rectification works
- Management of the repair works, including
- Quality Assurance
- Coordination with 3rd party inspectors
- Development of the PWHT plan to ensure no repeat events
- Progress reporting
The repair works were carried out in parallel with the installation of the 3 remaining HP Superheater headers, which allowed PEI to ensure that the client’s original schedule was not impacted by the rectification works which needed to be carried out.