4.2 Nonpressure Components: Casing, Breeching, Baffles, Buckstays & Expansion Joints

4.2 Nonpressure Components: Casing, Breeching, Baffles, Buckstays & Expansion Joints

Core Trade Concept: Nonpressure boiler components—including inner casing, outer lagging, flue gas breeching, baffles, buckstay frameworks, and expansion joints—do not contain steam or water under pressure. However, they form the structural skeleton and thermodynamic envelope of the boiler. Boilermakers must install, seal-weld, insulate, and align these components to contain high-velocity toxic flue gases, direct convective heat transfer, prevent casing burn-through, and safely absorb massive thermal movements.

1. Boiler Casing, Skin Casing, Insulation & Lagging

The boiler casing forms the exterior structural envelope that seals the combustion chamber and convective gas passes from the ambient boiler room environment.

                      BOILER CASING & LAGGING CROSS-SECTION
                      
         Hot Furnace Interior (Combustion Gases / 2,000°F+)
       ====================================================
       [ Waterwall Tubes / Membrane Wall or Refractory Tile ]
       ----------------------------------------------------
       [ Inner Seal Casing (10-12 Ga Carbon Steel Sheet)  ] <- Seal-Welded
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       |  Insulation Layer 1: High-Temp Ceramic Blanket   |
       |  Insulation Layer 2: Mineral Wool / Calcium Sil. | <- Stud Pins &
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    Speed Clips
       [ Air Gap / Foil Barrier                           ]
       ====================================================
       [ Outer Metal Lagging (Corrugated Alum. / Galv.)   ] <- Ambient Room

Pressurized Furnace Casing vs. Balanced Draft Casing

Insulation Pins & Lagging Installation

1. Insulation Pin Layout: Threaded or plain steel pins (typically $10\text{ to }12\text{ gauge}$, $3\text{ to }6\text{ inches}$ long) are welded directly to the casing or membrane fins using a portable drawn-arc or capacitor-discharge stud welding gun. Pins are spaced on a grid (typically $12\text{ in.} \times 12\text{ in.}$ centers on vertical walls, or $9\text{ in.} \times 9\text{ in.}$ on overhead surfaces).
2. Insulation Application: Layers of high-temperature refractory insulation are pressed over the pins:
   • Inner Layer: High-temperature ceramic fiber blanket ($1{,}800^\circ\text{F}\text{--}2{,}300^\circ\text{F}$ rating) or refractory blocks directly against the hot casing.
   • Outer Layer: Mineral wool or calcium silicate thermal blocks ($1{,}200^\circ\text{F}$ rating) to reduce the external surface temperature to safe personnel-protection limits (typically $< 130^\circ\text{F}\text{--}140^\circ\text{F}$).
   • Retention: Self-locking stainless steel speed clips (speed washers) are pushed over the pins to compress the insulation tightly, and the projecting pin ends are clipped flush or bent over.
3. Outer Metal Lagging: Heavy-gauge ribbed/corrugated aluminum sheet ($0.032\text{--}0.040\text{ in.}$ thick) or galvanized/aluminized steel sheet is installed over the insulation.
   • Shingle Lap Rule: Vertical sheets must overlap by at least $2\text{ to }3\text{ inches}$ in a "shingle lap" fashion (upper sheets overlapping outside lower sheets) to shed washdown water and rain.
   • Flashing & Drip Edges: Custom bent sheet-metal flashing and drip caps are fitted around all headers, buckstay penetrations, and sootblower wallboxes to ensure an airtight, weatherproof seal.

2. Flue Gas Breeching, Ductwork & Damper Systems

Breeching is the large-cross-section steel ductwork system that transports hot flue gases from the boiler convective exit through the economizer, air preheater, particulate scrubbers/baghouses, and induced draft fan to the exhaust stack.

                      FLUE GAS BREECHING & DAMPER ARRANGEMENT
                      
         From Boiler Convection Pass (Flue Gas ===>)
       +-----------------------------------------------------------+
       |                                      [Turning Vanes]      |
       |   BREECHING DUCT                      \   \   \   \       |
       |   (External Structural                 \   \   \   \      |
       |    Stiffener Grids)                     \   \   \   \     |
       +------------------------------------------+---+---+---+----+
                                                  |   |   |   |    |
                                 [Guillotine]     |   V   V   V    |
                                 [Damper    ] ===>|                |
                                 (Isolation)      |  To Scrubber/  |
       +-------------------------+                |  Baghouse      |
       | \                     /                  |                |
       |  \  ASH HOPPER       /                   +----------------+
       |   \ (55°-60° Valley)/ 
       |    \   Angle       /
       +-----+-------------+-----+
             | Rotary Valve| -> Ash Disposal

Duct Stiffening & Ash Hopper Geometries

• External Structural Stiffeners: Breeching duct walls are fabricated from $3/16\text{ in. to }1/4\text{ in.}$ carbon steel plate (ASTM A36). Because large rectangular ducts have huge flat surface areas, they will flex and collapse under internal draft pressures. Heavy structural angles, channels, or fabricated tee-sections are welded externally around the perimeter in structural grids. Under NFPA 85 (Boiler and Combustion Systems Hazards Code), ductwork must be stiffened to withstand transient pressure excursions of at least $\pm 35\text{ in. w.g.}$ without permanent plastic deformation.
• Aerodynamic Turning Vanes: At sharp $90^\circ$ duct turns, flue gas tends to swirl, creating severe pressure drops (draft loss) and causing suspended fly ash to drop out of the gas stream. Curved aerodynamic turning vanes (fabricated from heavy abrasion-resistant steel plate) are welded diagonally across the elbow to guide gas smoothly around the corner with uniform velocity profile.
• Ash Hopper Valleys: Where breeching routes underneath convective tube banks or air heaters, pyramidal ash hoppers collect dropping fly ash.
  • Valley Angle Rule: To guarantee continuous gravitational sliding of abrasive fly ash into bottom evacuation valves without bridging or clogging, the hopper valley angle (the intersection where two sloping hopper plates meet) must be at least $55^\circ\text{ to }60^\circ$ from horizontal.

Damper Configurations & Operational Mechanics

Boiler duct systems utilize several distinct damper types to isolate equipment, regulate gas flow, and control draft distribution:

                            TYPES OF FLUE GAS DAMPERS
                            
    GUILLOTINE DAMPER         OPPOSED-BLADE LOUVER       PARALLEL-BLADE LOUVER
      +-------------+            +---------------+          +---------------+
      | [Seal Air]  |            |  /     \     /|          |  /    /    /  |
      |  +-------+  |            | /       \   / |          | /    /    /   |
      |  | Blade |  |            |/         \ /  |          |/    /    /    |
      |  | (Down)|  |            +---------------+          +---------------+
      +--+-------+--+            (Precise Linear            (Rapid On/Off
      (100% Zero Leakage)          Flow Modulation)            Flow Diverting)

1. Guillotine Dampers:
   • Consist of a single, heavy solid steel blade that drives completely across the gas duct via rack-and-pinion, chain-drive, or screw-jack actuators.
   • Zero-Leakage Seal Air System: The perimeter blade track is equipped with flexible metallic seal strips and an internal chamber pressurized with ambient air from a seal-air blower (at a pressure $+3\text{ to }+5\text{ in. w.g.}$ higher than duct pressure). Any leakage across the seals is clean ambient air blowing into the duct, ensuring $100%$ zero flue gas leakage to protect boilermakers working downstream during online maintenance.
2. Multi-Louver Dampers:
   • Consist of multiple parallel aerodynamic blades linked together by an external mechanical drive rod and crank arms.
   • Opposed-Blade Configuration: Adjacent blades rotate in opposite directions (clockwise/counter-clockwise). This creates a centered, symmetrical throttling orifice, providing precise linear flow and draft modulation.
   • Parallel-Blade Configuration: All blades rotate in the same direction simultaneously. This imparts a swirling directional flow, making it ideal for rapid open/close isolation or flow diverting, but poor for fine throttling.
3. Butterfly Dampers: A single circular or rectangular disc pivoting on a central shaft; used for tight shutoff in small-to-medium circular ducts.
4. Bypass Dampers: Heavy-duty diverter dampers used to route flue gas around economizers, air preheaters, or SCR reactors during startup to prevent cold-end acid condensation.

3. Baffles, Refractory Walls & Erosion Shields

Convective Flow Directing & Baffle Construction

In watertube boilers with convective generating banks, flue gas naturally attempts to take the shortest, straightest path from furnace exit to stack. If unguided, gases bypass the tube bundles, resulting in high stack temperatures and drastic loss of boiler efficiency.

                    CONVECTIVE BAFFLE FLOW ARRANGEMENT
                    
        +-------------------------------------------------------+
        |  Steam Drum                                           |
        |  +=================================================+  |
        |  | |||   |||   |||   |||   |||   |||   |||   |||   ||| |  |
        |  | |||   |||   |||   |||   |||   |||   |||   |||   ||| |  |
   Gas  |  | |||   |||   |||   |||   |||   |||   |||   |||   ||| |  |
   In   |  | |||   |||   +-[ Baffle 1 ]-+  |||   |||   |||   ||| |  |
  ====> |  | |||   |||                  |  |||   |||   |||   ||| |  |
        |  | |||   |||                  |  |||   +-[ Baffle 2 ]+ | ====>
        |  | |||   |||                  |  |||   |               |   Gas
        |  | |||   |||                  |  |||   |               |   Out
        |  +=================================================+  |
        |  Mud Drum                                             |
        +-------------------------------------------------------+

• Baffles: Structural walls constructed of interlocking refractory tiles, heat-resistant chrome alloy steel plates, or castable refractory poured between tubes. Baffles force the flue gas to make multiple cross-flow passes (sweeping across the tubes at $90^\circ$ angles). Cross-flow provides significantly higher convective heat transfer coefficients than parallel longitudinal flow.
• Gas Bypass Prevention: Baffle tiles are notched to fit precisely around tube profiles. If a baffle develops cracks, tiles fall out, or joints open due to thermal vibration, flue gas short-circuits directly to the outlet. This causes localized tube overheating downstream, cold-end fouling, and a drop in total steaming capacity.

Tube Erosion Shields

In coal-fired, biomass, and recovery boilers, flue gas carries abrasive fly ash particulates traveling at velocities of $40\text{ to }80\text{ ft/sec}$.

• Erosion Vulnerability: Tubes located in the first $1\text{--}2$ rows of convective banks, near baffle turnaround turns, and adjacent to sootblower discharge lanes suffer aggressive mechanical thinning from ash impingement.
• Tube Shields (Wear Plates): Curved half-round protective shields fabricated from AISI Type 304, 309, or 310 stainless steel or high-chrome casting alloys ($12\text{ to }36\text{ inches}$ long) are fitted directly over the leading exterior face of the tubes. They are secured using stainless steel band clamps or small stitch tack-welds on the trailing edge. These sacrificial shields absorb abrasive impact, preventing tube wall loss and catastrophic pressure blowouts.

4. Buckstay Systems & Transient Pressure Restraint

                    BUCKSTAY CORNER CONNECTIONS & TIE-BACKS
                    
                     North Waterwall Tube Panel
         ====================================================
         [ Buckstay Beam - North Wall                       ]
         +--------------------------------------------+-----+ 
                                                      |     | 
                                                      | PIN | <- Hinged Link Pin
                                                      |     |    (Allows 2-Way
         +--------------------------------------------+-----+    Thermal Growth)
         | Buckstay Beam - East Wall                        |
         ====================================================
                     East Waterwall Tube Panel

Furnace Pressure Excursions & Structural Integrity

Under transient operating conditions, industrial and utility furnaces experience sudden, violent pressure fluctuations:

1. Furnace Explosion (Puff): Delayed ignition of accumulated unburned fuel vapor during burner light-off causes an instantaneous internal positive pressure spike.
2. Furnace Implosion: A sudden Main Fuel Trip (MFT) under full firing causes the hot gas volume to cool rapidly, dropping internal furnace pressure to severe negative levels (down to $-20\text{ to }-35\text{ in. w.g.}$), threatening to suck the waterwall panels inward.

Mechanical Components of the Buckstay Framework

• Buckstay Beams: Heavy structural wide-flange H-beams or double-channel sections banded horizontally around the boiler exterior at vertical intervals of $8\text{ to }15\text{ feet}$.
• Corner Tie Connectors: Where north-south buckstays meet east-west buckstays at the furnace corners, they cannot be rigidly welded together because the walls expand outward in two orthogonal directions. Instead, heavy forged steel corner tie links with structural hinge pins and slotted brackets connect the beams. The link pins transfer tension loads across corners to restrain wall blowout while allowing the walls to expand freely outward.
• Tie-Back Scissor Guides: Welded to the membrane fin tubes, these sliding guides hold the buckstay beam web flush against the wall while allowing unlimited vertical expansion of the waterwalls.

5. Fabric & Metallic Expansion Joints

Because boiler pressure vessels and flue gas ductwork operate at temperatures up to $1{,}000^\circ\text{F}$, thermal expansion can produce several inches of differential movement between connected equipment. Flexible expansion joints are installed in ductwork and piping runs to isolate thermal stress, absorb vibration, and maintain an airtight seal.

                    EXPANSION JOINT TYPES & MOVEMENTS
                    
        METALLIC BELLOWS JOINT                 NON-METALLIC FABRIC JOINT
        +---------------------+                 +-----------------------+
        |  /|  /|  /|  /|  /| |                 |  === FABRIC BELT ===  |
  ====> | | | | | | | | | | | | ====>     ====> |  [Ceramic Pillow]     | ====>
        |  \|  \|  \|  \|  \| |                 |  === FABRIC BELT ===  |
        +---------------------+                 +-----------------------+
        (Stainless / Inconel)                   (Multi-Layer Fluoroelastomer)

     AXIAL COMPRESSION             LATERAL SHEAR             ANGULAR ROTATION
        +->     <-+                     ^                           ^
        |  | | |  |                    / /                         / /
        +->     <-+                   v v                         v /

Metallic Bellows vs. Fabric Expansion Joints

Critical Installation Rules: Shipping Bars & Cold-Springing

1. Shipping Restraints (Shipping Bars): All expansion joints are manufactured with rigid steel angles or painted bars bolted or tack-welded across the joint flanges to maintain the exact neutral face-to-face dimension during shipping, rigging, and duct alignment.

   CRITICAL BOILERMAKER DIRECTIVE: All expansion joint shipping bars, locking pins, and transit restraints MUST be removed after duct installation is complete and prior to boiler startup or hydrostatic testing. Failure to remove shipping bars locks the joint in a rigid state, causing severe duct rupture, tearing of the bellows, or buckling of structural anchors as soon as thermal expansion begins.

2. Cold-Spring Preset: To maximize the cyclic fatigue life of an expansion joint, boilermakers often install the unit with a cold-spring preset (deliberately pre-compressing or pre-extending the joint during cold erection). When the duct heats up and expands during full-load operation, the joint moves from its pre-stressed cold position into a relaxed, stress-free neutral midpoint, preventing over-extension at peak operating temperatures.