Boiler Outage Inspection - Drums

All internal surfaces and the connections to all outside attachments, including water-column connections and safety valve nozzles, should be examined for deformation, corrosion, pitting, grooving, cracking, scale deposits, and sludge accumulation. Special attention should be paid to all seams, whether welded or riveted, and to the areas adjacent to them. If seams are heavily coated, they may have to be grit blasted or wire-brushed before a visual examination is possible.
Welded seams and connections should be examined for cracks. Riveted joints should be checked for loose or broken rivets, cracking, or other evidence of distress. Rivets should be hammer-tested for soundness. If there is any evidence of leakage or other distress in lap joints, it should be investigated thoroughly, and if necessary, rivets should be removed or the plate should be slotted to determine whether cracks exist in the seam.

Corrosion along or immediately adjacent to a seam may be more critical than a similar amount of corrosion away from the seams. Such points should receive a close visual examination and ultrasonic wall thickness measurements. Grooving and cracks along longitudinal seams are especially significant, as they are likely to occur when the material is highly stressed. Severe corrosion is likely to occur where the water circulation is poor. Both the internal and external surfaces of the drum need examination. The top external surface of drums should be cleaned of all deposits, and the surface should be examined for corrosion.

When a more thorough examination for cracks and other defects in plate and weld metal is desired than can be obtained by a visual inspection, a radiographaphic, magnetic-particle,
ultrasonic, or dye-penetrant tests may be used as follows:

a.  Radiography can identify cracks below, at, or near the metal surface if they are of sufficient size and oriented properly to make a discernable change in the film density.
b.  Dry powder magnetic-particle test can determine cracks at or near the surface.
c.  Wet fluorescent magnetic-particle test uses either a black or blue light for finding discontinuities and is more sensitive to tight cracks than dry powder.
d. Ultrasonic straight beam and shearwave tests can indicate discontinuities in the metal at any depth.
e.  Dye-penetrant test is used to locate surface cracks in large or small areas.
f.  Electromagnetic inspection techniques may be used for surface and sub-surface crack detection instead of magnetic particle and dye penetrant testing.

Inspection of the steam drum should also include observations of the normal water level. Any bulges or uneven areas that would indicate excessive heat input from leaking fireside baffles should be noted. Evidence of poor circulation may be indicated by waterline gouging along the top half of the top one or two rows of downcomers. This is sometimes accompanied by flash marks on the drum surface at the tube openings. If a sample of the boiler drum is needed for chemical analysis or microscopic examination, a section may be trepanned from the wall. The resulting cavity would need to be evaluated for repair by a suitable method such as welding. Normally, the wall thickness is measured ultrasonically and recorded to establish corrosion rates and remaining life estimates.

Safety-valve nozzles and gauge-glass connections, especially the lower connections, should be examined for accumulations of sludge or debris. A flashlight should be used to visually inspect the nozzle or connection. If the inside cannot be observed directly, a small hand mirror may be used for indirect observation. Special forms of illuminating equipment, mirrors, and magnifying devices are very useful for this type of inspection. When the boiler contains more than one drum, usually only one of the drums will have safety valves on it.

Any manhole davits should be tested for freedom of movement and for excessive deformation. Manhole and handhole cover plates and nozzle seats should be examined for scoring in the manner described in preceding text for pipe flanges. Cover plates should be inspected for cracks.

Drum internals and connections to the drum should be inspected when the drum is inspected. Drum internals, including internal feed header, distribution piping, steam separators, dry pipes, blowdown piping, deflector plates, and baffle plates, should be inspected for tightness, soundness, and structural stability. The vigorous turbulence of the steam and water mixture present in the drum may vibrate such parts loose from their fasteners, attachments, or settings. When these parts are welded in place, it is not uncommon for the welds to crack from vibration. Welds or rivets attaching internals or connections to the drums should be inspected in the same manner as welds or rivets in the drum proper. Steam separators and baffles should be carefully inspected for tightness, corrosion, and deterioration, and associated welds should be checked for cracks. Any bypassing of the steam separator will permit carryover into the superheater, causing salt deposition, resultant overheating, and possible tube failure. Steam separators should be free from deposits that might impair their operation. Some boilers do not have steam separators and depend entirely on dry pipes for water separation.

The holes in dry pipes should be free from any deposits that might restrict flow. Since dry pipe holes are in the top of the pipe near the top of the drum, it may be necessary to inspect the holes indirectly with a hand mirror. Any drain holes in the pipe should also be inspected for freedom from deposits and scale. Not all drums contain dry pipes.

Ultrasonic testing, laser profilometry testing, and hammer sounding are good methods of checking for tube wall loss caused by corrosion.  Tube ligaments should be examined for cracks. If tubes are covered by baffle or deflector plates, a few of these plates should be removed to permit a spot check of the condition of the tubes behind them.

The methods described in API RP 572 are applicable to all drums forming any part of a steam boiler.

No comments: