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.
