Key Components of Boilers

In general, the critical components are those whose failure will directly affect the reliability of the boiler. The critical components can be prioritized by the impact they have on safety, reliability, and performance.

Critical Components

Component	Description
Drums	The steam drum is the single most expensive component in the boiler. Consequently, any maintenance program must address the steam drum, as well as any other drums, in the convection passes of the boiler. In general, problems in the drums are associated with corrosion. In some instances, where drums have rolled tubes, rolling may produce excessive stresses that can lead to damage in the ligament areas. Problems in the drums normally lead to indications that are seen on the surfaces—either inside diameter (ID) or outside diameter (OD). Assessment:Inspection and testing focuses on detecting surface indications. The preferred nondestructive examination (NDE) method is wet fluorescent magnetic particle testing (WFMT). Because WFMT uses fluorescent particles that are examined under ultraviolet light, it is more sensitive than dry powder type magnetic particle testing (MT) and it is faster than liquid dye penetrant testing (PT) methods. WFMT should include the major welds, selected attachment welds, and at least some of the ligaments. If locations of corrosion are found, then ultrasonic thickness testing (UTT) may be performed to assess thinning due to metal loss. In rare instances, metallographic replication may be performed.
Headers	Boilers designed for temperatures above 900°F (482°C) can have superheater outlet headers that are subject to creep — the plastic deformation (strain) of the header from long-term exposure to temperature and stress. For high-temperature headers, tests can include metallographic replication and ultrasonic angle beam shear wave inspections of higher stress weld locations. However, industrial boilers are more typically designed for temperatures less that 900°F (482°C) such that failure is not normally related to creep. Lower temperature headers are subject to corrosion or possible erosion. Additionally, cycles of thermal expansion and mechanical loading may lead to fatigue damage. Assessment: The nondestructive examination (NDE) method should include testing of the welds by magnetic particle testing (MT) or by wet fluorescent magnetic particle testing (WFMT). In addition, it is advisable to perform internal inspection with a video probe to assess waterside cleanliness, to note any buildup of deposits or maintenance debris that could obstruct flow, and to determine if corrosion is a problem. Inspected headers should include some of the water circuit headers as well as superheater headers. If a location of corrosion is seen, then ultrasonic thickness testing (UTT) to quantify remaining wall thickness is advisable.
Tubing	By far, the greatest number of forced outages in all types of boilers are caused by tube failures. Failure mechanisms vary greatly from long term to short term. Superheater tubes operating at sufficient temperature can fail long term (over many years) due to normal life expenditure. For these tubes with predicted finite life, Babcock & Wilcox (B&W) offers the NOTIS® test and remaining life analysis. However, most tubes in the industrial boiler do not have a finite life due to their temperature of operation under normal conditions. Tubes are more likely to fail because of abnormal deterioration such as water/steam-side deposition retarding heat transfer, flow obstructions, tube corrosion [inside diameter (ID) and/or outside diameter (OD)], fatigue, and tube erosion. Assessment: Tubing is one of the components where visual examination is of great importance because many tube damage mechanisms lead to visual signs such as distortion, discoloration, swelling, or surface damage. The primary nondestructive examination (NDE) method for obtaining data used in tube assessment is contact ultrasonic thickness testing (UTT) for tube thickness measurements. Contact UTT is done on accessible tube surfaces by placing the ultrasonic transducer onto the tube using a couplant, a gel or fluid that transmits from the ultrasonic transducer sound into the tube. Variations on standard contact UTT have been developed due to access limitations. Examples are internal rotating inspection system (IRIS)-based techniques in which the signal from the ultrasonic transducer is reflected from a high rpm rotating mirror to scan tubes from the ID—especially in the area adjacent to drums. A second system is B&W's immersion ultrasonic transducer, where a multiple transducer probe is inserted into boiler bank tubes from the steam drum to provide measurements at four orthogonal points. These systems can be advantageous in the assessment of pitting.
Piping	Main Steam—For lower temperature systems, the piping is subject to the same damage as noted for the boiler headers. In addition, the piping supports may experience deterioration and become damaged from excessive or cyclical system loads. Assessment: The nondestructive examination (NDE) method of choice for testing of external weld surfaces is wet flourescent magnetic particle testing (WFMT). Magnetic particle testing (MT) and penetrant testing (PT) methods are sometimes used if lighting or pipe geometry make WFMT impractical. Non-drainable sections, such as sagging horizontal runs, are subject to internal corrosion and pitting. These areas should be examined by internal video probe and/or ultrasonic thickness testing (UTT) measurements. Volumetric inspection (i.e., ultrasonic shear wave) of selected piping welds may be included in the NDE. However, concerns for weld integrity related to the growth of subsurface cracks is a problem associated with creep of high temperature piping and is not a concern on most industrial installations. Feedwater—A piping system often overlooked is feedwater piping. Depending upon the operating parameters of the feedwater system, the flow rates, and the piping geometry, the pipe may be prone to corrosion or flow assisted corrosion (FAC). This is also referred to as erosion-corrosion. If susceptible, the pipe may experience material loss from internal surfaces near bends, pumps, injection points, and flow transitions. Ingress of air into the system can lead to corrosion and pitting. Out-of-service corrosion can occur if the boiler is idle for long periods. Assessment: Internal visual inspection with a video probe is recommended if access allows. NDE can include MT, PT, or WFMT at selected welds. UTT should be done in any location where FAC is suspected to ensure there is not significant piping wall loss.
Deaerators	Overlooked for many years in condition assessment and maintenance inspection programs, deaerators have been known to fail catastrophically in both industrial and utility plants. The damage mechanism is corrosion of shell welds, which occurs on the inside diameter (ID) surfaces. Assessment: Deaerators' welds should have a thorough visual inspection. All internal welds and selected external attachment welds should be tested by wet fluorescent magnetic particle testing (WFMT).
Air Openings	Assessment: Verify that combustion and ventilation air openings to the boiler room and/or building are open and unobstructed. Check operation and wiring of automatic combustion air dampers, if used. Verify that boiler vent discharge and air intake are clean and free of obstructions.
Flu gas vent system	Assessment: Visually inspect entire flue gas venting system for blockage, deterioration, or leakage. Repair any joints that show signs of leakage in accordance with vent manufacturer's instructions. Verify that masonry chimneys are lined, and ensure that the lining is in good condition, and that there are no openings into the chimney.
Pilot and main burner flames	Assessment: Visually inspect pilot burner and main burner flames. Pilot Proper pilot flame Blue flame. Inner cone engulfing thermocouple. Thermocouple glowing cherry red. Improper pilot flame Overfired – Large flame lifting or blowing past thermocouple. Underfired – Small flame. Inner cone not engulfing thermocouple. Lack of primary air – Yellow flame tip. Incorrectly heated thermocouple. Main Burner Check burner flames—Main burner Improper main burner flame Overfired – Large flames. Underfired – Small flames. Lack of primary air – Yellow tipping on flames (sooting will occur). Yellow-orange streaks may appear (caused by dust).
Boiler Heating Surfaces	Assessment: Use a bright light to inspect the boiler flue collector and heating surfaces. If the vent pipe or boiler interior surfaces show evidence of soot, clean boiler heating surfaces. Remove the flue collector and clean the boiler, if necessary, after closer inspection of boiler heating surfaces. If there is evidence of rusty scale deposits on boiler surfaces, check the water piping and control system to make sure the boiler return water temperature is properly maintained. Reconnect vent and draft diverter. Check inside and around boiler for evidence of any leaks from the boiler. If found, locate source of leaks and repair.
Burner and base	Assessment: Inspect burners and all other components in the boiler base. If burners must be cleaned, raise rear of each burner to release from support slot, slide forward, and remove. Then brush and vacuum the burners thoroughly, making sure all ports are free of debris. Carefully replace all burners, making sure burner with pilot bracket is replaced in its original position and all burners are upright (ports up). Inspect the base insulation.