5.2 Gasket Materials, Flange Facings, Bolt Torque Sequences & Joint Assembly

5.2 Gasket Materials, Flange Facings, Bolt Torque Sequences & Joint Assembly

Core Trade Concept: A bolted flanged joint is a complex mechanical system consisting of three interacting components: the flanges, the gasket, and the threaded studs. To maintain leak-tight containment under high hydrostatic pressure, intense thermal cycling, and external bending moments, the bolted joint must maintain a minimum compressive gasket seating stress ($y$) and operating gasket factor ($m$) without exceeding the yield strength of the flange hub or crushing the gasket element.

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1. Industrial Flange Types & Pressure Classes (ASME B16.5 / B16.47)

Flanges provide demountable mechanical joints to connect pipe sections, valves, pumps, vessels, and heat exchangers. Flanges are categorized by attachment geometry and pressure rating classes (Class 150, 300, 400, 600, 900, 1500, 2500 per ASME B16.5 for sizes up to NPS 24, and ASME B16.47 for NPS 26 to NPS 60).

                         INDUSTRIAL FLANGE TYPES
                         
   WELD NECK (WN)           SLIP-ON (SO)            LAP JOINT (LJ)
      +---+---+               +---+---+               +---+---+  +-- Stub End
      |   |   |               |   |   |               |   |   |  |   Face
      |   |   |               +---+---+               |   +---+--+---+
      |   |   |               |       | Fillet        |   |   |      |
     /     \  | Butt         /  Pipe  | Welds         |   |   | Pipe |
    /  Hub  \ | Weld        +---------+              +---+---+      |
   /         \|             |         |              |   |   |      |

Flange Architecture Comparison

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2. Flange Facings & Surface Finish Specifications

The flange facing is the machined contact surface that compresses the gasket to establish a fluid-tight seal.

                         FLANGE FACING PROFILES
                         
   RAISED FACE (RF)           FLAT FACE (FF)          RING TYPE JOINT (RTJ)
      +---+---+                  +-------+                  +---+---+  <- Gasket
      |   |   |                  |       |                  |   |/ /|     Groove
   +--+   +---+--+            +--+       +--+            +--+   +---+--+  (Trapezoidal)
   |  |   |   |  |            |  |       |  |            |  |   |   |  |
   |  +---+---+  |            |  +-------+  |            |  +---+---+  |
   | 1/16" or 1/4"            (Full Contact Plane)       (Solid Metallic Ring)

Facing Profiles

1. Raised Face (RF):
   • A raised annular lip concentrated inside the bolt circle.
   • Height: $\frac{1}{16}\text{ in.}$ ($1.6\text{ mm}$) for Class 150 and 300; $\frac{1}{4}\text{ in.}$ ($6.4\text{ mm}$) for Class 400 through 2500.
   • Surface Finish: Features a phonographic continuous spiral or concentric serrated groove profile with a roughness of $125\text{--}250\text{ }\mu\text{in. AARH}$ (Arithmetic Average Roughness Height). These serrations bite mechanically into the soft gasket filler, preventing radial gasket blowout under pressure.
2. Flat Face (FF):
   • The entire mating face lies in the same plane across the full diameter, with no raised central section.
   • Critical Trade Rule: Used on cast iron, bronze, and fiberglass equipment (pumps, valves, cast headers). Bolting a Raised Face flange to a Flat Face cast iron flange creates a high bending moment that cracks the brittle cast iron flange ear. Flat Face flanges require Full-Face Gaskets extending past the outer bolt holes.
3. Ring Type Joint (RTJ):
   • A precision-machined trapezoidal groove cut directly into the flange face inside the bolt circle.
   • Designed for extreme high-pressure/high-temperature services (Class 600 through 2500, up to $10{,}000\text{ psi}$).
   • Ring Cross-Sections:
     • Oval Rings (Legacy): Circular cross-section curve contacts flat $23^\circ$ groove walls at a single line contact.
     • Octagonal Rings (Modern Standard): Flat $45^\circ$ chamfered facets mate flatly against the $23^\circ$ groove angle, providing superior sealing contact area and carrying higher torque loads without sliding.
   • Hardness Rule: The metallic RTJ ring must be at least 20 to 30 Brinell Hardness points (HB) softer than the flange body to ensure the replaceable ring deforms elastically/plastically into the groove rather than indenting the permanent flange face.

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3. Gasket Categories & ASME B16.20 Construction

               ASME B16.20 SPIRAL-WOUND GASKET (SWG) ANATOMY
               
        +-------------------------------------------------------+
        | 1. OUTER CENTERING RING (Solid Carbon Steel)          |
        |    - Centers gasket on bolts; prevents over-crushing  |
        +-------------------------------------------------------+
        | 2. SEALING ELEMENT (W-Shaped Metal + Soft Filler)     |
        |    - Flexible Graphite or PTFE with 304/316 SS        |
        +-------------------------------------------------------+
        | 3. INNER RETENTION RING (Solid Stainless/Alloy)       |
        |    - Prevents inward buckling into pipe bore          |
        +-------------------------------------------------------+

Gasket Classifications

1. Non-Metallic (Sheet Gaskets per ASME B16.21):
   • Compressed Non-Asbestos Fiber (CNAF): Aramid/glass fibers bonded with nitrile rubber (NBR); rated up to $400^\circ\text{F}$ and Class 300 utility lines.
   • Expanded PTFE (Teflon): $100%$ virgin PTFE sheet; 100% chemically inert ($ ext{pH } 0\text{--}14$); rated up to $450^\circ\text{F}$; ideal for strong acids and corrosive chemicals.
   • Flexible Expanded Graphite Sheet: Pure exfoliated graphite flake ($99%\text{ C}$); rated up to $900^\circ\text{F}$ in oxidizing environments ($5{,}000^\circ\text{F}$ in inert/reducing atmosphere); excellent recovery and zero creep relaxation in high-temperature steam.
2. Semi-Metallic Gaskets (ASME B16.20):
   • Spiral-Wound Gaskets (SWG): The most common gasket in heavy power generation and refining. Composed of alternating V-shaped chevron metal strip windings (304SS, 316SS, Inconel, Monel) and soft filler (flexible graphite or PTFE).
     • Outer Centering / Gauge Ring: Solid painted carbon steel ring that positions the gasket precisely inside the flange bolts and acts as a positive mechanical travel stop to prevent over-compression of the windings.
     • Inner Ring: Solid stainless steel ring positioned between the inside diameter of the windings and the pipe bore. Inner rings are mandatory on all Class 900+ flanges, all vacuum lines, and all PTFE-filled spiral-wound gaskets to prevent the windings from buckling radially inward into the pipe bore under high bolt loads and turbulent fluid flow.
   • Kammprofile (Grooved Metal Core with Soft Facing): Solid serrated stainless steel core with flexible graphite or PTFE layers bonded to each face. Offers superior blowout resistance and requires lower seating stress than spiral-wound gaskets; standard for heat exchanger heads and large-diameter flanges.
3. Metallic Gaskets: Solid forged soft iron, low-carbon steel, 304SS, or 316SS RTJ rings stamped with ring numbers (e.g., R-24, R-45, RX, BX series for subsea).

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4. Bolting Metallurgy & Fastener Specifications

Flange integrity relies on high-strength continuous threaded stud bolts (all-thread studs) and heavy hex nuts manufactured to precise ASTM metallurgical standards.

                     HIGH-PRESSURE STUD ENGAGEMENT
                     
              +---------+                     +---------+
              | 2H Nut  |                     | 2H Nut  |
         <==  +---------+=====================+---------+  ==>
           2-3 Full Threads                 2-3 Full Threads
           Visible Past Nut                 Visible Past Nut

Fastener Metallurgy Breakdown

Thread Engagement Rule

When fully torqued, the stud must extend through the outer nut face by a minimum of 2 to 3 full threads (or flush plus $\frac{1}{4}\text{ in.}$). If the stud does not extend past the outer face of the nut, the nut does not achieve full tensile load capacity, creating a severe blowout risk under pressure.

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5. Controlled Bolting: Torque Patterns, Lubrication & Bolt Stretch

                STANDARD 8-BOLT CRISS-CROSS (STAR) TORQUE PATTERN
                                    (1)
                                   O   O (8)
                            (5) O         O (4)
                            (3) O         O (6)
                                   O   O (7)
                                    (2)
                Sequence: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8

The Physics of Torquing: $T = \frac{k \cdot D \cdot F}{12}$

Applied torque ($T$, in $\text{ft-lbs}$) is calculated using the standard short-form formula:

$T = \frac{k \cdot D \cdot F}{12}$

Where:

• $T =$ Required torque wrench setting ($\text{ft-lbs}$)
• $k =$ Nut factor / Friction factor (dimensionless)
• $D =$ Nominal stud bolt diameter (inches)
• $F =$ Target bolt preload / clamping tension (lbs, typically $50%\text{--}70%$ of stud yield strength)

The Impact of the Nut Factor ($k$)

$\text{Friction Distribution: } \approx 50\% \text{ Nut Face Friction} + \approx 40\% \text{ Thread Friction} + \approx 10\% \text{ Useful Bolt Stretch}$

Over $85\text{--}90%$ of applied wrench torque is consumed solely in overcoming friction! Only $10\text{--}15%$ produces actual bolt stretch (preload).

• Nickel / Moly Anti-Seize Paste: $k = 0.11\text{--}0.15$ (consistent, low friction).
• Light Machine Oil: $k = 0.18\text{--}0.22$.
• Dry / Rusty Steel Threads: $k = 0.30\text{--}0.45$.

Critical Trade Scenario: Applying $300\text{ ft-lbs}$ of torque to a dry, unlubricated stud ($k=0.35$) produces less than half the clamping force of the same torque applied to a lubricated stud ($k=0.14$). An unlubricated flange will leak during hydrostatic testing even though the torque wrench clicked at full value!

ASME PCC-1 Multi-Pass Crisscross Bolting Procedure

1. Pre-Assembly Verification: Verify flange alignment. Maximum gap variation across the face must not exceed $\frac{1}{16}\text{ in.}$ ($1.6\text{ mm}$), and bolt holes must align within $\frac{1}{8}\text{ in.}$ without forced drifting.
2. Clean & Lubricate: Clean stud and nut threads with a wire brush. Apply anti-seize lubricant liberally to all stud threads and the bearing face of the heavy hex nuts.
3. Pass 1 (Crisscross / Star Pattern): Tighten all studs in the numbered star pattern to $20%\text{--}30%$ of target torque to seat the gasket evenly.
4. Pass 2 (Crisscross / Star Pattern): Tighten all studs in the star pattern to $50%\text{--}60%$ of target torque.
5. Pass 3 (Crisscross / Star Pattern): Tighten all studs in the star pattern to $100%$ of target torque.
6. Pass 4 (Final Circular / Rotational Pass): Move sequentially clockwise around the flange, bolt-by-bolt, at $100%$ of target torque. Continue circular passes until no nut rotates, recovering all gasket creep relaxation.
7. Pass 5 (Optional Re-Torque): On high-temperature lines ($> 500^\circ\text{F}$), perform a final torque check at $100%$ torque after 4 to 24 hours of ambient relaxation or following an unpressurized thermal cycle per plant engineering specifications.

Direct Bolt Stretch Measurement (Ultrasonic Verification)

Torque wrenches measure rotational resistance, which is heavily distorted by thread friction. In critical high-pressure joints (reactor heads, turbine casings, main steam headers), boilermakers measure direct bolt elongation (stretch) using an ultrasonic bolt extensometer or precision micrometer:

$\Delta L = \frac{F \cdot L_{eff}}{A \cdot E}$

Where $\Delta L$ is the measured physical stretch (typically $0.008\text{--}0.015\text{ inches}$ ), $L_{eff}$ is effective clamp length, $A$ is root tensile area, and $E$ is Young's Modulus ($29.5 \times 10^6\text{ psi}$ for steel). Direct stretch measurement completely eliminates friction errors.