Published on 02 October 2013
TWI was asked to carry out an independent quality assessment of the fabricated components of a power plant on delivery to site. An expert visual examination of the shop welded headers and steam pipework sub-assemblies for a combined cycle gas turbine (CCGT) power plant revealed manufacturing defects that could have exposed the structure, and subsequently the personnel and the environment, to severe risk. The defects found included lack of root fusion, lack of root penetration and excessive bore protrusion.
The plant consisted of gas turbines, four heat recovery steam generators (HRSGs) with supplementary firing and steam turbines. The HRSGs were designed to the requirements of ASME Section I, but without code stamping. The high pressure steam pipes and the main superheater outlet headers were manufactured from P91 steel (9Cr 1Mo modified); header stubs from P22 steel (2Cr 1Mo) and the low pressure components from low carbon steel.
An experienced TWI welding engineer was deployed to site to carry out a visual examination of the accessible pipe and header welds to form an opinion as to the general level of workmanship and weld quality and to comment on whether the welds were acceptable to the ASME Section I code.
The visual inspection of those welds that were readily accessible was carried out using both available light and hand held torches. Measurements of defect dimensions such as undercut, root concavity, misalignment etc was carried out using a hand held TWI welding gauge. Time constraints and the accessibility of the inside diameter (ID) of headers and pipework, particularly on two heat recovery steam generators (HRSGs), meant that the survey was somewhat restricted in scope.
However, the ID of sufficient welds on the remaining two HRSGs were accessible such that a good general picture, representative of the overall quality of shop fabricated welds, could be established. It was observed that the majority of the observed welds in the P91 HRSGs did not comply with the ASME Section I quality requirements. The C-Mn steel headers appeared to be of a generally better quality than the P91 headers, but still had defects outside the ASME Section I acceptance criteria.
In many cases dimensional tolerances were such that accurate joint fit-up could not be achieved without plastically deforming the pipework. Hardness test results raised serious concerns about the heat treatment or chemical composition of some of the P91 pipework.
Following this review, it was recommended that a full visual and NDE inspection be performed on all four HRSGs to identify those welds requiring remedial work.
Subsequently, TWI was requested to provide a fully qualified expert welding engineer to provide four weeks of site support for:
- Full visual and NDE inspection of all four HRSGs to identify those welds requiring remedial work.
- Establishing the effects of multiple PWHT (time at temperature) on P91 weldments.
- Monitoring and advising on weld repair procedure qualifications.
- Supervising removal of defects by machining/grinding to eliminate the need for PWHT.
- Investigating the low hardness values found on the P91 pipework.
TWI on-site work including recommendations for and supervision of the remedial work reduced the adverse impact on the plant project schedule. TWI technical support also enabled the client to take appropriate decisions, with respect to further destructive testing of suspect welds, replacement of critical components and repairs to components at site.
For more information on TWI's P91 fabrication quality auditing service or how TWI can help with regards to welding engineering support, training for fabricating with P91 or fabrication and quality management with these materials, please contact us