Boiler Outage Inspection - Piping, Tubes

Piping:

A visual inspection should be made for evidence of leakage in pipe and threaded or flanged pipe joints. Water leaks may be detected by the presence of moisture or deposits at the point of leakage and steam leaks by the appearance of the adjacent metal.

Leaks may sometimes be a result of strains caused by deformation or misalignment of the piping system. Deformations may be caused by lack of provision for expansion or by improper supports. If not eliminated, pronounced deformation may place strains of sufficient magnitude to cause failure in small connections. A careful inspection should determine if such defects are present.

When flanged connections are opened, gaskets and gasket seats should be inspected carefully. Gaskets may be damaged by leakage or by improper centering of the gasket when the joint is made up. Gasket seats may be scored by a steam leak at the joint, improper handling, or careless use of tools. Seating surfaces should be inspected for tool marks, other mechanical abuses, and evidence of the type of erosion commonly called steam cutting or wire drawing. Mechanical damage may lead to erosion if not corrected. Either a scored seat must be machined to provide a proper gasket face or the flange must be replaced; otherwise, leaks will recur. Before joints are re-made, ring gaskets should be examined to determine their fitness for reuse. Other types of gaskets should be replaced with new ones.

Tubes: 

All tubes should be inspected for signs of overheating, corrosion, and erosion. Waterwall tubes and generating tubes nearest the burner are particularly susceptible to overheating and should be closely examined for bulging, blistering, quench cracking, sagging, and bowing. Inspection for blisters and local bulging is easily accomplished by shining a flashlight parallel to the length of the tube so that bulges, blisters, and other deformities cast shadows. Cleaning of a slagged tube may be necessary to find minor blisters. The tube’s circumference should be measured at the blister or bulge.

Boiler tubes should be inspected at the steam-drum connection for gouging and caustic corrosion due to steam blanketing. Roof tubes are generally designed for heat pickup on one side only. Therefore, a sagging roof tube due to burned out hangers is especially susceptible to overheating. These tubes should be straightened, and the hangers should be replaced.

Waterside corrosion, generally caused by faulty water treatment, can usually be detected by ultrasonic thickness measurements or laser profilometry of representative tubes. Measurements can also be made from inside the steam drum for a distance of 8 in. – 10 in. (20 cm – 25 cm) into the tubes. The locations measured and thicknesses found should be recorded to establish a tube corrosion rate.

Fireside corrosion is generally caused by moisture that accumulates in fly-ash deposits. Although fireside corrosion may occur anywhere in the tube nest, it usually occurs where the tubes enter the lower drums or headers. Moisture-causing fireside corrosion can come from leaks in tubes, drums, headers, and faulty steam soot-blower shutoff valves. Other sources can come from rain water through stacks and roofs, and from condensation from the atmosphere during downtime.

Specific attention should be given to tubes near any openings, like viewports, as air in-leakage can cause increased external corrosion.

Steam tubes should be examined for the type and thickness of internal scale. Ultrasonic techniques exist to nondestructively measure steam-side scale thickness from the outside surface. An assessment of tube remaining life can be made from the measured scale thickness of the tube if operating in the creep range.

When a tube rupture occurs, the tube should be visually inspected. Its appearance may indicate the cause of failure. If the cause is not evident, samples of the tube in the original condition, with deposits and scale intact, should be taken and analyzed chemically and microscopically. The tube sample should be cut at least 1 ft (30 cm) on either side of the failure.

The inside of bent tubes and of straight tubes, as far as it is accessible, should be examined with strong illumination. Straight tubes should be examined by illuminating the end away from the observer.  Ultrasonic and laser profilometry test methods can provide inspection over the tube’s full length.

Internal cleanliness is required to conduct a satisfactory tube inspection. When tube cleanliness is in doubt, a turbinetype cleaner should be used to remove internal deposits. The loosened deposits should be trapped at the discharge ends. The weight of trapped deposit and the internal surface area will indicate the average thickness of the deposit removed.

Fiber optics or borescopes are of limited use on bent tubes but are satisfactory for viewing straight tubes and may also be used to inspect tube internals. Tube ends should be checked for proper projection and flaring. Calipers, micrometers, laser profilometry, and ultrasonic instruments can be used to measure tube diameters, dimensions of bulges on tubes, depth of corrosion pits, and tube-wall thickness. These measurements are of great value in determining the effects of corrosion and erosion and in estimating the future lives of the parts measured. Tubes should also be checked for any cutting due to cleaning.

Erosion of exterior surfaces is caused by the impingement of fly ash or raw fuel solids at excessive velocity or by soot blowers. Fly-ash tube erosion can be arrested by installing shields or by reducing the gas velocity. If erosion is due to soot-blower medium impingement, the soot blowers should be checked for alignment, warpage, and operating wear. Wastage of exterior tube surfaces can be caused by flame impingement, which should be corrected by adjustments to the firing equipment.

Some types of waterwalls have tubes widely spaced and the area between the tubes covered by steel fins attached to the tubes. The fins may become overheated and burn or crack. The fins should be inspected for cracks that may extend into the tubes. The tubes should be inspected for signs of leakage that may result from the cracks.

Waterwall tubes should also be checked for alignment. All gas passages should be inspected for slagging or bridging from fly ash or slag buildup. The first gas pass is particularly susceptible to this condition.