Inspection Technology for Heat Recovery Steam generators (HRSG)

In addition to selecting inspection technology, the group responsible for inspection must have a clear understanding of what they are looking for and what actions will be required depending on inspection outcomes.  For example, if video inspection of connecting piping welds such as downcomers or related piping is planned, there must be a clear plan as to how to treat indications of “potentially bad welds”.  That is, should a review of plant radiograph records be triggered by such a situation?  Will it be necessary to mobilize mechanical contractors and personnel to erect scaffolding to provide access to the area?  What additional inspection techniques can be performed to determine the significance of this indication? 

Inspection of the interior of the HRSG gas path also has access issues.   In horizontal gas path HRSG’s, easy access is often only possible on the internal casing floor.   Spacing between tube bundles is often tight.   Good lighting is important for both near and remote areas.   Flashlights, halogen lights, drop lights and high-powered searchlight beams are helpful.  The high-powered beams help get visibility to the top of the tube bundles, which can be 60 feet (20 meters) higher than the floor.   Access to upper areas of tube bundles is often possible through hatches in the top of the boiler casing into upper “Penthouse” areas.  The extent of access varies by HRSG OEM and by design.  While providing access to upper headers, vents and riser piping, these areas are typically low, often 3 ft (1 m) or less above the upper headers, with limited access and difficult mobility for inspectors.  Access to the upper tube-to-header weld joints is usually very limited although the 180° return bends typical of some designs can often be accessed. 

While access is difficult, these spaces provide an important opportunity for critical inspections such as of the upper bends in the IP and LP Evaporators or 180° return bends in the HP Economizer.  These areas are often at high risk for erosive wear and FAC damage.  The upper bends in the evaporators are subject to a two-phase form of FAC that results in more rapid wear rates and therefore loss of metal wall thickness than occurs at the bottom of the unit where conditions are single phase in all HRSG modules.  Access to lower headers and drains is often via the removal of baffle plates that allow entry into lower basement areas.  In vertical gas path HRSGs the tube faces between modules are usually accessible, allowing the whole face to be inspected. Depending on the OEM the headers and antlers may be directly accessible from the gas path, or may require the removal of internal baffles or doors. Some OEMs have separate header compartments with their own access doors, allowing detailed inspection of the headers, antlers and 180° returns for all the tubes.  The interior of steam drums is usually via direct entry to the drum.  The drum internals including steam separators, baffle plates, feed piping, and nozzles can be inspected.  Removal of baffle plates may be required to gain access to risers and tubes for internal inspection.  As noted previously, baffle plate wear damage in the drum is often a good indicator for FAC damage in inaccessible components in the IP and LP Evaporator.  Generally, the HP Evaporator/drum operate at a temperature that is not favorable for FAC damage. In all cases good naked eye visual inspection should try to be within 24 inches (60 cm) of the surface, with good lighting.

The table below shows the principal applicable HRSG Inspection methods.