9.2 Tube Preparation, Bending Radii, Ovality & Alignment Tolerances

8.2 Tube Preparation, Bending Radii, Ovality & Alignment Tolerances

Core Trade Concept: Fabricating and bending replacement boiler tubes requires strict compliance with ASME Section I material specifications, geometric bending limits, and surface preparation standards. Bending alters tube wall geometry—stretching and thinning the outer radius (extrados) while compressing and thickening the inner radius (intrados). Boilermakers must calculate wall thinning, measure ovality, enforce post-bend heat treatment rules, and prepare tube ends to pristine bare metal.

1. Boiler Tube Material Specifications & Metallurgy

Boiler tubes are classified by manufacturing method (seamless vs. welded), carbon content, and alloying elements. Selecting the correct ASME Section II / Section I material specification is vital for pressure integrity, corrosion resistance, and high-temperature creep strength.

                  BOILER TUBE METALLURGY SPECTRUM

   LOW TEMPERATURE / LOW-MEDIUM PRESSURE =====> HIGH TEMPERATURE / HIGH PRESSURE
   
   +---------------+  +---------------+  +---------------+  +---------------+ 
   |    SA-178     |  |    SA-192     |  |    SA-210     |  |    SA-213     | 
   |  (ERW Carbon) |  | (Seamless CS) |  | (Med-Carbon)  |  | (Chrome-Moly) | 
   +---------------+  +---------------+  +---------------+  +---------------+ 
   - Grade A: Low C   - Seamless drawn   - Grade A-1 / C    - T11 (1.25Cr-0.5Mo)
   - Grade C: Med C   - High-pressure    - 60-70 ksi TS     - T22 (2.25Cr-1Mo) 
   - Firetubes &      - Generating banks - High-heat-flux   - T91 (9Cr-1Mo-V)  
     Waterwalls       - Superheaters       Waterwalls       - TP304H/347H SS   

Material Classifications

1. ASME SA-178 (Electric Resistance Welded Carbon Steel):
   • Description: Manufactured from flat steel strip formed into a cylinder and joined by high-frequency resistance welding without filler metal. Grade A is low-carbon ($0.06\text{--}0.18%\text{ C}$); Grade C is medium-carbon ($0.35%\text{ C}$ max, tensile strength $\ge 60\text{ ksi}$).
   • Applications: Firetube boilers, low-to-medium pressure watertube generating banks, and waterwall panels operating below $800^\circ\text{F}$.
2. ASME SA-192 (Seamless Carbon Steel for High-Pressure Service):
   • Description: Hot-finished or cold-drawn seamless carbon steel ($0.06\text{--}0.18%\text{ C}$) engineered for severe high-pressure service.
   • Applications: Utility and industrial steam drums, economizers, and generating banks operating up to $850^\circ\text{F}$.
3. ASME SA-210 (Seamless Medium-Carbon Steel):
   • Description: Higher-strength carbon steel tubing (Grade A-1: $60\text{ ksi}$ tensile; Grade C: $70\text{ ksi}$ tensile). Higher carbon content provides higher allowable stress values, allowing thinner tube walls.
   • Applications: High-heat-flux furnace waterwall panels and riser circuits.
4. ASME SA-213 (Seamless Ferritic and Austenitic Alloy Steel):
   • Description: High-temperature, creep-resistant alloy tubing formulated with chromium (for oxidation/corrosion resistance), molybdenum (for creep strength), and vanadium/nitrogen (for carbide stabilization).
     • Grade T11 ($1.25%\text{ Cr -- }0.5%\text{ Mo}$): Economizer and low-temperature superheater passes up to $950^\circ\text{F}$.
     • Grade T22 ($2.25%\text{ Cr -- }1.0%\text{ Mo}$): High-temperature superheaters and reheaters up to $1{,}050^\circ\text{F}$.
     • Grade T91 ($9%\text{ Cr -- }1%\text{ Mo -- V}$): Advanced martensitic alloy engineered for supercritical and ultra-supercritical superheater/reheater circuits up to $1{,}150^\circ\text{F}$. Requires strict preheat, interpass, and controlled post-weld/post-bend heat treatments.
     • Grades TP304H / TP347H (Austenitic Stainless Steels): High-temperature superheater/reheater finishing loops operating above $1{,}100^\circ\text{F}$ under severe coal/biomass ash corrosion conditions.

2. Tube Bending Mechanics & Tooling: Rotary Draw vs. Ram Bending

Boiler tubes must be bent to precise angles and radii to route around burner throats, sootblower wallboxes, inspection doors, and drum penetrations.

               MANDREL ROTARY DRAW BENDER TOOLING

                          [ Clamp Die ]
                                |
                                v
    +-----------------------+=======+ (Tube Clamped Rigidly to Bend Die)
    | Wiper Die             |  TUBE |
    +-----------------------+=======+--------+
    | Pressure Die ===>     |       | Bend   |
    +-----------------------+       | Die    |
            ^                       | (Rotates
            |                       |  with  |
     [ Internal Mandrel ]           |  Tube) |
     (Bullet or Ball Mandrel        +--------+
      positioned inside tube at tangency point)

Tooling Components of Rotary Draw Benders

Mandrel rotary draw bending is the premier industrial process for fabricating high-precision, wrinkle-free boiler tube bends:

• Bend Die (Radius Die): A circular die machined with a matching tube contour groove that rotates to pull the tube around the specified centerline bend radius ($CLR$).
• Clamp Die: Clamps the leading straight section of the tube securely against the bend die, rotating synchronously with it.
• Pressure Die: Supports the outer tube wall along the straight trailing section, feeding forward to prevent the tube from kicking outward while absorbing reaction forces.
• Wiper Die: Positioned immediately behind the bend die tangency point on the inner radius (intrados). It prevents the compressive forces on the inside curve from buckling into structural ripples or wrinkles.
• Internal Mandrel: A precision tool steel or aluminum-bronze mandrel (bullet plug or multi-ball flexible linkage) inserted inside the tube bore directly at the tangency point. The mandrel supports the tube from within, preventing the tensile outer wall from collapsing inward or flattening.

Rotary Draw Bending vs. Ram (Press) Bending

Cold Bending vs. Hot Bending

• Cold Bending: Performed at ambient temperatures below the lower transformation temperature ($< 1{,}300^\circ\text{F}$). Highly efficient, accurate, and preserves clean surface finish. Produces strain hardening (cold work) in the extrados, which may require post-bend heat treatment depending on material alloy and bend severity.
• Hot Bending: Performed by heating the tube bend zone to forging temperatures ($1{,}600^\circ\text{F}\text{--}1{,}950^\circ\text{F}$) using induction coils or gas furnaces. Used for heavy-wall tubing or ultra-tight bends ($< 1.5 \times OD$).
  • Precautions: Hot bending must be performed under strict pyrometric temperature monitoring. Overheating ($> 2{,}000^\circ\text{F}$) causes rapid grain growth, severe decarburization, and heavy internal/external scaling.

3. Minimum Bend Radii, Wall Thinning & Thickening Calculations

During bending, the metal along the outer radius (extrados) experiences severe tensile stretching, causing wall thinning. Simultaneously, metal along the inner radius (intrados) experiences compressive forces, causing wall thickening and risk of wrinkling.

                  BEND GEOMETRY & WALL THICKNESS DYNAMICS

                                  EXTRADOS (Outer Radius)
                             Tensile Stretch ===> Wall Thinning
                                  /-------------------\
                                 /     t_extrados      \
                                /     (Thinned Wall)    \
                               |                         |
   Straight Leg                |       Centerline        |               Straight Leg
   ============================|      Radius (CLR)       |============================
   (Nominal Wall t_orig)       |                         |      (Nominal Wall t_orig)
                                \                       /
                                 \     t_intrados      /
                                  \-------------------/
                                  INTRADOS (Inner Radius)
                          Compressive Strain ===> Wall Thickening

Minimum Bend Radius ($R_{min}$)

Under standard boiler manufacturing practices and ASME Section I (PG-19):

• The recommended minimum centerline bend radius ($CLR$ or $R$) for cold-formed boiler tubes is $R \ge 2.5 \times OD$ (or $3.0 \times OD$ for seamless alloy tubes without special mandrel tooling).
• Ultra-tight bends ($R < 2.0 \times OD$) require heavy-wall starting stock or hot induction forming to ensure the thinned extrados maintains structural integrity.

Wall Thinning Calculation Formulas

The theoretical wall thickness at the extrados ($t_{extrados}$) after bending is approximated by:

$t_{extrados} \approx t_{orig} \cdot \left( \frac{R}{R + 0.5 \cdot OD} \right)$

Where:

• $t_{orig} =$ Original nominal tube wall thickness before bending (inches)
• $R =$ Centerline bend radius ($CLR$, inches)
• $OD =$ Nominal outside diameter of the tube (inches)

The Percentage Wall Thinning is calculated using the measured thinned wall ($t_{min}$):

$\%\text{ Wall Thinning} = \left( \frac{t_{orig} - t_{min}}{t_{orig}} \right) \times 100\%$

$\text{Worked Example:}$ A boilermaker cold-bends a $2.500\text{-inch OD}$ SA-210 Grade A1 boiler tube with original wall thickness $t_{orig} = 0.220\text{ inches}$ to a centerline radius $R = 6.250\text{ inches}$ ($2.5 \times OD$):

$t_{extrados} \approx 0.220 \cdot \left( \frac{6.250}{6.250 + (0.5 \cdot 2.500)} \right) = 0.220 \cdot \left( \frac{6.250}{7.500} \right) = 0.220 \cdot 0.8333 = 0.1833\text{ inches}$

$\%\text{ Wall Thinning} = \left( \frac{0.220 - 0.1833}{0.220} \right) \times 100\% = \left( \frac{0.0367}{0.220} \right) \times 100\% = 16.68\%$

Critical ASME Code Rule: The resulting thinned wall thickness at the extrados ($t_{extrados}$) must never fall below the ASME Section I PG-27 minimum design wall thickness ($t_{min}$) for the boiler's design pressure and temperature. If calculations show $t_{extrados} < t_{min}$, the fabricator must select heavier-wall starting stock.

4. Ovality (Out-of-Roundness) Standards & Post-Bend Heat Treatment

Ovality Calculation & Tolerances

When a tube is bent, the tensile and compressive forces naturally tend to collapse the circular cross-section into an ellipse. Ovality (out-of-roundness) restricts internal steam/water flow and increases pressure drop.

                       TUBE OVALITY CROSS-SECTION

                       D_max (Major Axis / Vertical)
                                    ^
                                    |
                              +-----+-----+
                           /                 \
              D_min <==== |                   | ====> D_min
              (Minor Axis) \                 /       (Minor Axis)
                              +-----+-----+
                                    |
                                    v
                       D_max (Major Axis / Vertical)

Under ASME Section I (PG-19.1) and industry standards, ovality is calculated as:

$\%\text{ Ovality} = \left( \frac{D_{max} - D_{min}}{D_{nom}} \right) \times 100\%$

Where:

• $D_{max} =$ Maximum measured outside diameter across the flattened bend (inches)

• $D_{min} =$ Minimum measured outside diameter across the flattened bend (inches)

• $D_{nom} =$ Original nominal tube outside diameter (inches)

• ASME / NBIC Acceptance Limit: Ovality must not exceed $8%\text{ to }10%$ for general boiler tubes. In high-pressure superheaters or tight-pitch waterwalls, specifications frequently tighten this limit to $\le 5%$.

Post-Bend Heat Treatment (PBHT) Requirements

Cold bending induces severe plastic deformation, generating high residual tensile stresses and strain hardening that reduce material toughness and accelerate stress-corrosion cracking or creep embrittlement.

Under ASME Section I (Table PW-10 / PG-19), post-forming heat treatment is mandatory for cold-formed alloy tubes when the calculated outer fiber elongation (strain) exceeds specific thresholds:

$\text{Outer Fiber Strain } (\%) = \left( \frac{OD}{2R} \right) \times 100\%$

5. Tube End Preparation & Fit-up Standards

Before replacement tubes are installed into drums, headers, or waterwall panels, the tube ends must undergo meticulous mechanical preparation.

                 TUBE END PREPARATION SPECIFICATION

       Bare Bright Metal Polished Zone (Internal & External)
                      |<====== 2" to 3" ======>|
       +==============+------------------------+
       | Tube Wall    | Ground 37.5° Bevel     |
       |              | Chamfered ID / Deburred| ===> 1/16" Root Face (Land)
       +==============+------------------------+
       |                      Tube Bore        |
       +==============+------------------------+
       | Tube Wall    |                        |
       +==============+------------------------+

1. Sizing & Gauge Ball Check: Tube ends must be sized to ensure circularity within $\pm 0.010\text{ in.}$ of nominal ID/OD. Fabricators pass a hardened steel gauge ball (sized to $95%$ of nominal ID) through the entire length of bent tubes to verify there are no internal obstructions, excessive ovality, or unobserved kinks.
2. De-Burring & Chamfering: All sharp internal and external edges resulting from saw or abrasive wheel cutting must be fully deburred using rotary chamfering tools or fine files. Removing internal sharp burrs prevents damage to roller expanders and eliminates stress concentration risers.
3. Bare Bright Metal Cleaning:
   • The internal and external surfaces of each tube end must be cleaned to bare bright metal for a minimum distance of $2\text{ to }3\text{ inches}$ ($50\text{ to }75\text{ mm}$) from the end.
   • Contaminant Removal: Boilermakers use pneumatic flap wheels, non-chlorinated solvent wipes, or silicon-carbide emery cloth to remove all mill scale, rust, varnish, paint, grease, and anti-spatter oils. Any residual varnish or oil will cause porosity in seal welds or compromise friction holding power in rolled joints.