Stack / breeching

Stack breeching is the large flue-gas ductwork that connects the boiler outlet, air pollution control equipment, induced-draught fan and chimney or stack. In older plant language, "breeching" often means the horizontal or sloped duct just before the stack, but the term is also used for major flue-gas headers between process units.

This region is mechanically simple but operationally important. It carries treated or partly treated flue gas at lower temperature, often near the conditions where acid condensation, ash settling and plume issues become visible.

Operating issues

Common stack and breeching problems include:

• Ash settlement in low-velocity zones, corners and expansion sections
• Acid dew point corrosion where metal temperature falls below the sulphuric acid dew point
• Chloride corrosion on waste and biomass lines
• Water ingress from stack drains, rain caps or wash systems
• Lining failure, refractory cracking and expansion-joint leakage
• Flow maldistribution into the induced-draught fan or stack liner

Because breeching is often large and difficult to enter safely, small deposit problems can remain hidden until pressure drop, vibration, corrosion or stack emissions change.

Design and inspection implications

Good breeching design avoids dead legs, sharp expansions, poor drain falls and unnecessary cold surfaces. Access doors, lighting, wash drains and safe platforms matter because inspection is usually done during short outages. Material selection depends on gas temperature, moisture, acid gases and whether the duct is upstream or downstream of wet scrubbing.

Corrosion and deposition often coincide. A damp ash layer traps acid, holds chlorides against the steel and hides metal loss from external inspection. Ultrasonic thickness checks should be targeted to cold corners, underside plates and downstream of flow disturbances.

Sonic horns on breeching

Sonic horns can keep dry ash mobile in wide duct sections, turning boxes and low-velocity headers, especially upstream of dry collection equipment. They are less useful where the deposit is wet sludge, hardened scale or acid mud. In Sylio-style assessments, breeching horns are considered when deposits are dry enough to move and the duct has enough acoustic volume for the selected wavelength.

Operating and inspection context

Stack breeching is a common location for problems that are created upstream. Low gas temperature, poor mixing, acid dew point approach, wet scrubber carry over, ammonia salt formation, expansion-joint leakage and ash settlement can all show up in the breeching before they are obvious at the stack outlet. The geometry often includes elbows, dampers, fan outlets, silencers and sampling ports, so local velocities and deposit patterns can be very uneven.

Design variables include gas flow, temperature, corrosive chemistry, pressure class, internal lining, insulation, drainage, access doors, support loads and allowance for thermal expansion. If the breeching carries saturated or acid-bearing gas, low points need drains and materials must suit condensate quality. Poor insulation or air in-leakage can create local cold spots that trigger corrosion even when the average gas temperature looks acceptable.

For acoustic-cleaning work, the breeching is sometimes a useful target when dry ash settles in large horizontal runs or around turning vanes. It is usually a poor target for wet sludge, acid condensate or compacted deposits downstream of a wet scrubber. Maintenance inspections should look for lining damage, loose refractory, ash shelves, corrosion at welds and fasteners, cracked expansion joints, blocked drains and safe access to sampling points. Any cleaning device mounted here also needs noise review because breeching is often close to grade, control rooms or plant boundaries.

Related terms

• Duct
• ID fan
• Acid dew point
• Cold-end corrosion / dew-point corrosion