Showing posts with label Heaters. Show all posts
Showing posts with label Heaters. Show all posts
Visual Inspection of Soot Blowers
Soot blowers can be a root cause for
deterioration if they are not operating properly. Therefore, soot-blower parts
should be inspected for proper alignment, position, and operability. If soot
blowers are out of position or misaligned, the blower blast could impinge on
tubes which will eventually cause tube failure due to erosion. Soot blowers can
also be a source of liquid water that can promote dew point corrosion of tubes,
casing and the blowers themselves. The shut-off valve to the blowers should be
checked to ensure it does not leak while in service. Condensate can form in the
system when the blower is out of service and if it leaks into the firebox can
cause dew point corrosion.
The blower, supporting hangers, and brackets
should be examined visually for soundness and for excessive thinning from
oxidation. Soot blowers for the high-temperature part of the boiler are
sometimes composed of high-chromium alloys that embrittle in service and so
they should be handled/inspected appropriately to avoid fracture. Connection
welds of supporting elements should be inspected for cracks. If the welds look
cracked, a magnetic-particle inspection should be made. Packing glands and all
operating parts of the rotating and retracting types of soot blowers should be
examined for good working condition. Because of the potential difficulty of
repacking soot blowers in service, repacking should be done during down periods
if there is any evidence that repacking might be required.
Inspection of Stacks
An external visual inspection should be made
of brick, concrete, and steel stacks for conditions that may weaken these
structures. Field glasses will be helpful in making inspections of high stacks
because they will enable any defects to be observed fairly well from the
ground. Brick stacks should be inspected for cracks and for the condition of
mortar joints to determine the effect of weathering. Concrete stacks should be
inspected for cracks and spalling that may expose the steel reinforcement.
Steel stacks should be inspected externally for the condition of painted
surfaces, signs of oxidation, and thinning or perforation due to corrosion by
acidic flue gases.
In many cases, cracks in brick and concrete
stacks are a result of insufficient thickness of the internal insulation or to
internal secondary combustion. These potential causes of cracks should be kept
in mind when inspecting the interior of stacks.
The linings of all stacks should be inspected
for cracks, wear, and structural soundness.
While stacks are in service, an external,
infrared thermographic examination can be made that will show hot spots, which
indicate failure of the internal liner.
When liquid fuels are burned, soot accumulates
in the base of the stack and must be removed occasionally. During the internal
inspection, the amount of soot and ash should be noted, and whether they need
to be removed should be decided. The inside of steel stacks should be inspected
for corrosion or cracking due to condensation of acidic flue gases. Areas at or
adjacent to welds are most susceptible to stress corrosion cracking.
Steel stacks in heater and boiler services
should be inspected and checked for wall thickness at time intervals that are
warranted by experience. In addition to the thickness determination, a thorough
hammer inspection should be made of the entire stack, with particular attention
paid to the seams, adjacent areas, and areas adjoining any stiffening rings,
lugs, nozzles, and the like, which may act as cooling fins to cause
condensation of gases and localized corrosion. The minimum allowable thickness
at which repairs will be made should be definitely established for such
structures. One practice is to establish these thicknesses on the same basis as
was used in the original design for the structure.
Bolts at the base flange and at elevated
sections should be checked periodically for loosening and breakage. Elevated
flanged connections that are installed for the purposes of field erection
should be seal welded internally to prevent the escape of corrosive flue gases,
which accelerate bolt failure. In the case of derrick-type flare stacks, the
structure itself should be completely inspected. Careful attention should be
given to the foundations and anchor bolts. Most derricks are assembled by
welding or bolting. Bolts should be checked for looseness and corrosion. If
looseness is found, the shank of the bolt should be checked for abrasion from
the movement of structural members. The flare-stack roller guides and guide
arms should be checked for alignment and operability and should be realigned or
freed if necessary. Ladders, platforms, and all structural members should be
checked for atmospheric corrosion to determine whether any section is
approaching the minimum allowable thickness.
The guy lines to guyed steel stacks should be
inspected visually for corrosion. The wire rope should be inspected for:
a. Reduced diameter due to internal or external
corrosion.
b. Corroded or broken wires at end connections,
especially at the deadman and the top of the stack, where moisture can be
retained.
c. Cracked, bent or worn end connections.
d. Worn/broken outside wires.
e. Kinks, cuts or unstranding.
Electromagnetic inspection techniques based
upon flux leakage principles are available for inspecting wire rope as well.
These technologies involve a crawler capable of inspecting the length of wire
for localized strand defects and general thinning of the wire cross-section.
This allows for a quantitative assessment of the wire integrity. The stack
painters’ trolley and cable should be inspected visually for corrosion or
mechanical damage before being used and before being returned to storage. The
condition of the connections at the top of the pulley and of the trolley ring
and its connections to the stack should be determined carefully.
Lightning rods on stacks and their grounding cables
should be inspected visually to see that they are secured and unbroken. The
ground rod should be inspected visually to see that it is firmly attached to
the cable and that it extends to a ground depth sufficient to provide an
electrical resistance of not more than 25 ohms. This should be checked
periodically, particularly in dry weather.
The ladders on steel, concrete, and brick stacks should
be inspected visually for corrosion and should be tested physically by applying
test weights in excess of those that may be imposed by the personnel using
them.
The caps on radial brick and concrete stacks sometimes
become damaged, causing loose brick to fall or the reinforcing steel to be
exposed. Stack caps should be inspected visually so that any necessary repairs
can be made, thereby eliminating a hazard from falling bricks and preventing
damage to steel reinforcement.
Tube Supports - Steam/Methane-Reforming Heaters
Tube support methods vary in
steam/methane-reforming heaters. Some designs require full support from the
top. In these designs the pigtail may be below the tube and unable to take any
load from the catalyst-filled tube. Counterweights are often used and may
support two or more tubes. The lever or pulley system must work as designed.
Interference from tube flange bolts, slipping of supports off tube flanges, and
other similar problems have led to pigtail failures.
Inadequate support also allows tube bending,
which puts a bending moment on a pigtail that exits the tube from the side,
thus causing localized high stress at the fitting on the tube or the outlet
headers.
Outlet headers grow, usually from a center anchor
point. Bottom tube supports on short pigtailed tubes must allow movement of the
tube bottom to minimize stress on the pigtail. If the tube is designed for
bottom movement, the upper tube supports must allow the tube to move at the
bottom end. To prevent a pigtail bending moment, the heater lining must not
press on the tube. Loose bricks are often used to help close openings. The
bricks must move freely if the tube presses on them.
If support springs are used, those that have
been stretched should be replaced. A stretched spring cannot support a tube.
When the tube is heated up after shutdown, the spring will no longer support it
as designed.
Visual Inspection of Burners
Burners should be visually inspected to ensure
proper operation once per shift. Conditions that need to be corrected include
flame impingement on tubes and supports, abnormal flame dimensions and pattern,
oil drippage, and smokey combustion. In addition, the burners should provide an
even heat distribution. Poor firing from unbalanced burners can cause serious
deterioration of the heating elements and setting. Defective burners that can
not be repaired in service should be replaced so that they do not lead to
premature failure of other components. Obtain the burner drawings from the
burner vendor. The drawings will include the installation tolerances, tile
diameter and tip drilling information.
Burner plugging problems sometimes can be
solved by proper sizing of the fuel drum and demister pad, heat tracing the
fuel gas delivery lines, and providing filters or coalescing systems.
Regardless, deposits should be analyzed to determine if the source of the
plugging can be identified and eliminated.
The following guidelines are general
recommendations for maintenance outage inspection. Always consult the
guidelines from the burner vendor.
a. The burner tile is an air orifice. It controls
the amount of air flow. Since it is extremely desirable to have even air flow
to each burner, the tile dimensions are critical. Typical installation
tolerance is ± 1/8 in. (0.32 cm) on the diameter as shown on the burner
drawing. Measure the diameter in three to four locations. Most round tiles are
installed as a slight oval shape. This will result in poor fuel-air mixing and
a bad flame shape. Burner tiles should not be cracked or spalled. The use of a plywood
template helps set the tiles in the proper diameter and concentricity. The
burner tile must be centered on the gas tip to obtain uniform fuel air mixing.
Typical installation tolerance is ± 1/8 in. (0.32 cm). Poor installations
result in bad flames.
b. Check the burner drawing for the number of tip
drillings and the drill bit size of the port. Use drill bits to check the
proper hole size and the proper included angle of the drillings. Do not mix
parts from other burners. Be sure to install tips with high-temperature
anti-seize compound.
c. The installation tolerance on gas risers is
typically ± 1/8 in. (0.32 cm) on horizontal spacing and ± 1/8 in. (0.32 cm) on
vertical spacing. Bent gas risers can cause these dimensions to be wrong. A
common problem is different lengths on the gas risers. Many burners are
designed such that the gas jets intersect in the center of the burner. The tips
usually have arrows or cutouts to aid in tip orientation. Welding rods can be a
valuable check for alignment.
d. Air dampers and registers should be checked
for operability. Sticking air registers and dampers are a problem. Sometimes,
dry graphite lubricant can improve air register operability. Penetrating oil,
grease fittings and the addition of bearings to damper shafts can improve
operability.
On-stream Inspection through Infrared Scanning
Infrared scanning provides a means to check
the accuracy of tube skin thermocouples.
Furthermore, infrared thermal scanning of tubes helps fill the gaps
created with the tubeskin thermocouples. Infrared scanning inspection can
determine local tube metal temperatures in the areas not covered by the tube
skin thermocouples. Generally, tube ruptures occur in very local areas of
overheat. Infrared scanning has proven effective in identifying localized “hot
spots” before they cause a failure. A periodic scan of heaters is common practice. The inspection intervals should be shorter
for furnaces with coking tendencies (such as crude, vacuum, heavy oil
hydroprocessing and coker units), furnaces susceptible to fouling (such as dry
point fouling in naptha hydrotreating unit), and steam methane reforming
furnaces. Longer intervals may be used
for furnaces in non-coking and non-fouling susceptible services. For furnaces that have frequent decoking
activities (such as ethylene/olefin cracking units), a case-by-case evaluation
is needed to determine how often infrared scanning should be performed to
complement routine monitoring that typically uses a handheld pyrometer.
Personnel performing Infrared (IR) surveillance of
heaters should be knowledgeable and appropriately trained and qualified per the
owner users requirements for IR scanning (e.g. ASNT SNT-TC-1A, PCN Condition
Monitoring or owner user standard/practice).
In addition, scanning personnel should be aware of the factors that may
impact IR survey results such as flame environment, emissivity, infrared
radiation and characteristics of different materials, infrared camera
functions, and the limitations and accuracy of the method. When performing IR
scanning, inspection personnel should review prior IR survey results, current
heater operating status, Integrity Operating Window temperature limits, and any
new operations or maintenance issues to ensure that all areas of concern are
inspected.
An external IR scan of the heater should include an
assessment of heater casing and stack for refractory damage. Internal IR scans of the firebox through each
sight port should include an assessment of:
a) viewable (i.e. radiant or convection) heater
tubes for overall temperatures and hot spots,
b) tube skin thermocouples,
c) tube supports and refractory for spalled
refractory or broken tube supports, and
d) burner tiles and fuel gas tips for damage or
plugging.
IR surveys should be conducted on a scheduled
interval based on Integrity Operating Windows, API 530/Omega design metal
temperature(DMT) limits, unusual/poor operation or control, steam air decoking
or when deemed necessary. External and
internal IR scan results that indicate significant temperature differences from
previous or anticipate IR survey results should be evaluated. The owner user should specify guidelines for
acceptable temperature limits for each heater.
Results outside of owner user guidelines should be reported immediately. Inspection reports should include
documentation of IR camera settings used for the survey, drawings of heater
tube locations and results, and images of all significant findings.
Accuracy of infrared scanning can be
influenced by the skill of the operator, the angle of incidence, the nature of
the combustion products, flame patterns, and scale on the tubes. The infrared operator ideally would be
certified in infrared technology and have experience scanning heaters and
boilers. The presence of flames can mask the tubes if the operator must scan
through the flames. Another significant limitation is scale on the tubes. The
temperature of scale on the tubes tends to be hotter than the tube since it may
not be tightly bonded to the tube. During outages it can be beneficial to
remove scale in areas to allow the operator to scan a “scale-free” area and
compare to other scaled areas. This could allow better interpretation of
results. Grit blasting stainless steel tubes has also been shown to improve the
accuracy of infrared by mottling the surface enough to reduce reflectivity
which artificially causes a higher temperature measurement.
External casing can be inspected on-stream
both visually and using infrared. Visual examination can identify areas of distortion
and holes. These can indicate hot areas of lost refractory and promote
continued deterioration. A periodic infrared scan of the case is more effective
than visual examination in identifying “hot spots”, holes, and cracks. Regular
inspection of the firebox is critical for reliability. Inspection can identify
poor flame pattern of improperly firing burners, fuel rich operation as
evidenced by afterburning, and changes in appearance of tubes, supports,
refractory, etc. These inspections help identify changes early. Any changes or
problems can be addressed or analyzed to prevent further damage and
deterioration from occurring.
Header boxes should be visually examined for
evidence of process leaks. These could indicate a leaking plug header for those
heaters with fittings or a leaking instrument connection like a a thermowell.
If there is evidence of process leakage, understanding the cause should be
investigated.
Tube skin thermocouples and infrared scanning
have some limitations in reading temperatures accurately. The particular type
of thermocouple should have a mid-range rating for the expected tube metal
temperature for improved accuracy. Thermocouple wires can have the potential to
drift with time at temperature and so they require recalibration or periodic
replacement. Another significant problem is the attachment of the thermocouple
to the tube. If it is poorly attached, the thermocouple can separate from the
tube and begin reading firebox temperatures.
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