HRSGs and power boilers are built by assembling large numbers of components, mostly by welding. Design choices in optimizing heat transfer against overall cost lead to configurations that aim to minimize the number of welds per square meter of heat transfer surface. In addition, fabrication of the modules is frequently subcontracted by the OEM for manufacture in regions where manpower is cheaper. The OEM becomes in effect an integrator of components built by others. How can Product Quality for pressure parts be assured in that context?
Under the ASME Power Boiler Construction Code, the approach is to initially to certify a manufacturer by approving a suitable QC program, including the qualification of welding procedures and welders themselves. Going forward, Quality would then be assured by the manufacturer’s adherence to their approved program and by the presence of the ASME Authorized Inspector on site.
The Authorized Inspector has a number of compulsory checks that must be made in person but, in practice, a large portion of the welding continues without any further follow-up, except where the Code requires NDE. If the approved QC program becomes a box-ticking exercise – for example when schedule constraints take priority – weld quality can decline resulting in unrepaired defects. The problem for the Purchaser is to determine whether the manufacturer’s QC program can be trusted to guarantee a quality product.
Tetra’s experience is that product delivered by a number of OEM’s has on frequent occasions displayed unacceptable weld quality, despite the OEM and/or subcontracting fabricator having a valid quality certification in place. The cause is systemic management focus on schedule and cost, with quality given a lower priority. Many rejectable weld defects are not visible from the outside and only infrequently will the module fail the field hydro tests (although this has occurred). The main concern is that a large population of weld defects present in a set of tube modules will affect pressure part longevity, particularly if the plant is in cyclic operation.
To assist clients in their pressure part procurement process, Tetra Engineering performs:
During a recent Tetra project thousands of still images of tube to header welds were taken using a borescope in several HRSGs at a plant site just after commissioning. The images were thereafter reviewed independently by at least two engineers. The tube to header welds were found to be characterised by a general lack of good workmanship with numerous defects or flaws apparent. The initial inspection of one set of upper SH headers yielded 20 tubes that had weld defects serious enough to require repair. It should be noted that repair can be difficult because of poor access. Similar results were found in SH modules from the other units.
Tetra Engineering held its first steam Boiler & Steam Cycle Knowledge Workshop at the Media One Hotel in Dubai. Five of the seven workshop topics were presented by a team of four instructors from our company, the other two were presented by external collaborating partners: Mr. Jamie Pollock of Mettek Ltd. and Messrs. Tord Forslund & Leif Andersson from CCI Valves. Attendees could sign up for topics “à la carte”, these ranged from how to meet B.31 requirements for piping inspections to particular problem issues such as waterside deposition or creep.
One of the first applications of the borescope was for inspecting steam turbine rotor discontinuities in the 1920s. Today a wide variety of commercial borescopes and videoprobes can be used for access within HRSG components. These include both flexible and rigid borescopes and a variety of videoprobes and end effectors. Mirror or prism attachments can be used to get right angle views of tubes or pipe connections. Access can be via hand-holes or other penetrations.
Flexible video inspection probes employ high-resolution camera at the probe tip to capture detailed color inspection footage. Interchangeable lenses bring subjects both near and far into sharp focus. Variable-intensity fiber optic illumination floods hidden targets with cool, white light. Knobs on the handset control the tip’s up/down and left/right articulation.
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