Showing posts with label Furnace. Show all posts
Showing posts with label Furnace. Show all posts
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.
DETAILED EXAMINATION AND DESTRUCTIVE TESTING OF TUBE SAMPLES
When deterioration cannot be effectively
identified or monitored in service, obtaining tube samples for destructive
examination may be appropriate. Metallographic examination can be performed for
deterioration mechanisms, such as decarburization, carburization, hydrogen
attack, and stress cracking. Physical testing of creep life can be appropriate
for severe services and for affirming any calculated remaining life.
Oftentimes, calculated remaining tube life includes several assumptions of tube
operating history that can lead to inaccurate results. The density of scale
samples can be measured and may provide information on the tube operating
history. Furthermore, physical properties of the tubes can be measured that can
assist in damage assessments.
Prior to removal of a tube section,
consideration should be given to the ability to make the repair weld between
the new tube section and the existing tube sections.
In many heaters, tubes are not accessible for
an internal visual inspection. Some companies make a practice of thoroughly
inspecting all tubes that are condemned and removed from a heater, regardless
of the reason for the tubes’ removal. This inspection is made by cutting a tube
into short sections of 2 ft – 3 ft (60 cm – 90 cm) so that the inside surface
can be examined. Measurements for metal-wall thicknesses are made at the ends
of each section. In some cases, the sections are split longitudinally, thus
exposing the entire inside surface for examination. The ends of the tube rolled
into the fitting should be removed for examination. They may then be inspected
to determine the general condition and effectiveness of the rolled joint.
When external deterioration, including that
due to oxidation, scaling, cracking, and external corrosion is suspected,
especially in the case of convection tubes, the use of ultrasonic (UT) base
intelligent pigging should be considered to carry out full inspection of the
serpentine coil. Intelligent pigging will
provide complete condition assessment of regions of the coil (convection,
cross-over, radiant, etc.) which it passes through. As a less comprehensive alternative, representative tubes may be removed
from the heater and then cleaned and examined thoroughly. The selection of the
tubes to be removed may be guided by the tube locations in the heater, the
length of time the tubes have been in service, and the general appearance of
the tubes in the area. If the tubes chosen for inspection are found to be defective
or unfit for further service, other tubes in the same area and of the same or
similar age and general appearance should also be inspected.
Tube diameter/circumference/sag/bow measurements
Tubes sustaining creep/stress rupture damage
will exhibit diametral
growth or sagging. Shining a flashlight along the tube length can be a quick
way to identify major bulges, sags and bows.
Use of ultrasonic based intelligent pigs can also provide diametral
growth measurements throughout the full coil length. Frequently, the diametral
growth will be localized to a small section of the tube and appear as a bulge.
The bulge may be limited to the hot face of the tube and, therefore, will not
be a uniform circumferential growth. Diametral growth occurs from the operating
hoop stresses. Localized bulging can result from internal coke deposits causing
the wall temperature to become excessive. Another potential cause is flame
impingement, which could elevate temperatures. It is not uncommon to find
evidence of both since the elevated temperatures can promote coking in some
hydrocarbon services. The amount of bulging provides an indication of the
extent of damage up to the point of failure.
Tube sagging is another indication of creep
damage or short-term overload. Except in
this case, the stresses are longitudinal and usually result from inadequate
support or excessive temperature. Excessive sagging is often caused by short
term overheating because of runaway decoking or loss of flow. In such cases,
the tubes should be hardness checked to insure adequate strength remains, and
the thickness should be measured. If sagging is caused by long-term creep, a criterion
such as 1% – 5% elongation should be used depending on the material.
Tube diameter or circumferential measurements
provide an indication of the amount of damage. These measurements can be
compared to original measurements to determine the amount of bulging and is
often presented as a percentage growth/bulge. Measurements of the circumference
can be made with a narrow tape measure often referred to as external
“strapping”. This method is limited only
to sections of coil which are readily accessible. Other techniques include tube gauges
calibrated to specified diameters to determine growth or are set to specific
percentage growth and used as “pass-fail” gauges. Tube gauges can be fabricated
from thin SS plate (0.10 in. – 0.125 in. [0.25 cm – 0.32 cm]). The cutout
dimension is the sum of tube OD, mill out of round tolerance per ASTM-530 (approximately
0.0625 in. [0.16 cm]), and creep/stress rupture growth. Typical gauges are made
for 2%, 5%, and 10% creep growth. These measurement techniques may not be
precise if the tube is heavily scaled. In these instances, the growth may
appear more severe if the scale is not removed. One could increase the
tolerance of the tube gauge to account for some scale thickness.
The circumference and diameter can also be
accurately determined using crawler devices equipped with special sensors. The
sensors transducers sensors can be either laser, ultrasonic or eddy current
liftoff types. These specialized external tube crawlers are commonly used when
inspecting steam methane/naphtha reformer tubes.
Tube sags are estimated as the distance offset
from straight. In reality, the tube is flexible and will have some natural
amount of sag and so the measurement will not be precise. Sag measurements can
be used similar to diameter measurements to estimate the extent of creep. A
significant amount of sag can be tolerated before rupture is a concern and so
this condition is not considered serious unless it prevents cleaning or causes
headers to jam and wedge against other headers or against the sides of the
header compartment. In convection sections, sagging of the tubes in upper rows
to a point between those in lower rows can prevent the free passage of flue gas
around the tubes. This condition, called nesting, will cause overheating of
adjacent tubes and draft loss. If this condition is found, the offending tubes
should be replaced.
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