13.2 Thermal Fatigue & Thermal Shock

13.2 Thermal Fatigue & Thermal Shock — API RP 571 Sections 3.64 & 3.65

Thermal stress cracking in refining and petrochemical operations encompasses two related yet fundamentally distinct damage mechanisms cataloged in API Recommended Practice 571:

1. Thermal Fatigue (Section 3.64): Subcritical, progressive cracking driven by repeated, cyclic temperature fluctuations that induce alternating thermal expansion and contraction strains under mechanical restraint.
2. Thermal Shock (Section 3.65): Non-cyclic or low-cycle catastrophic structural failure resulting from a sudden, massive temperature gradient where localized thermal strain instantaneously exceeds the yield or ultimate tensile strength of the material.

Both mechanisms stem from the physics of thermal expansion, but their operational timescales, morphology, and engineering mitigations differ markedly.

---

Thermomechanical Stress Principles & Mechanics

When a metallic component experiences a temperature change ($\Delta T$), it undergoes a dimensional change proportional to its mean coefficient of thermal expansion ($\alpha$):

$\frac{\Delta L}{L} = \epsilon_{th} = \alpha \cdot \Delta T$

If the component is free to expand and contract without external or internal restraint, zero mechanical stress is induced. However, in pressure vessels, heat exchangers, and piping systems, thermal movement is inevitably restrained by:

• Rigid piping anchors, vessel nozzles, and structural guide supports.
• Heavy cross-sectional wall thickness differences (e.g., a thin nozzle neck welded to a thick shell course).
• Steep internal-to-external temperature gradients across the wall thickness.
• Bimetallic joints (dissimilar metal welds) joining alloys with differing thermal expansion coefficients.

When thermal expansion or contraction is fully restrained in one dimension, the resulting thermal stress ($\sigma_{th}$) is expressed as:

$\sigma_{th} = -E \cdot \alpha \cdot \Delta T$

For equibiaxial restraint (such as the surface of a thick pressure vessel wall), Poisson's ratio ($\nu$) amplifies the induced stress:

$\sigma_{th} = -\frac{E \cdot \alpha \cdot \Delta T}{1 - \nu}$

where $E$ is Young's modulus of elasticity. For carbon steel, with $E \approx 30 \times 10^6\text{ psi}$ and $\alpha \approx 6.5 \times 10^{-6}\text{ in/in/°F}$, a sudden unconstrained surface temperature drop of only 200 °F (111 °C) generates a theoretical localized surface tensile stress exceeding 55,000 psi (380 MPa)—well above the room-temperature yield strength of typical ASTM A106 Grade B pipe or A516 Grade 70 plate.

                     THE MECHANICS OF THERMAL FATIGUE CYCLING

      Hot Operating State                     Cold Quench / Temperature Swing
   ┌────────────────────────┐                ┌────────────────────────┐
   │ Metal attempts to      │                │ Metal attempts to      │
   │ expand against rigid   │──────CYCLE────>│ contract; exterior     │
   │ restraint.             │     SWING      │ surface develops high  │
   │ Compressive plastic    │                │ cyclic TENSILE stress. │
   │ yielding occurs.       │                │ Cracks initiate at ID. │
   └────────────────────────┘                └────────────────────────┘

---

Critical Factors Driving Thermal Fatigue (API RP 571 Section 3.64)

1. Temperature Swing Magnitude ($\Delta T$)

API RP 571 gives a rule of thumb that thermal fatigue cracking may be suspected where temperature swings exceed about 200 °F (93 °C).

Thermal fatigue is primarily strain-controlled (low-cycle fatigue). While high-frequency thermal stripping with temperature fluctuations of 50 °F to 100 °F (28 °C to 56 °C) can cause cracking over millions of cycles, larger swings exceeding 200 °F (111 °C) produce rapid failure within hundreds to thousands of cycles. The rate of damage accumulation accelerates exponentially with the magnitude of $\Delta T$.

2. Cycling Frequency and Thermal Conductivity

Materials with low thermal conductivity (such as austenitic stainless steels, $k \approx 15\text{ W/(m K)}$) establish much steeper through-wall thermal gradients than carbon steels ($k \approx 50\text{ W/(m K)}$) or copper alloys. Consequently, austenitic stainless steels generate higher localized surface thermal strains under rapid thermal cycling.

3. Dissimilar Metal Welds (DMWs)

When ferritic steels (such as 2.25Cr-1Mo or carbon steel) are welded directly to austenitic stainless steels (such as 304H or 347) in high-temperature hydrocrackers, cat crackers, or furnace headers, their differing thermal expansion coefficients create severe cyclic stresses:

• Ferritic steel thermal expansion: $\alpha \approx 12 \times 10^{-6}/\text{K}$ ($6.7 \times 10^{-6}/\text{°F}$)
• Austenitic stainless steel thermal expansion: $\alpha \approx 18 \times 10^{-6}/\text{K}$ ($10.0 \times 10^{-6}/\text{°F}$)

Under thermal cycling, the austenitic side expands ~30% more than the ferritic side. High cyclic shear stresses concentrate at the fusion line, causing thermal fatigue cracking along the ferritic heat-affected zone (HAZ).

---

High-Risk Refinery Units & Equipment Assets

                     HIGH-RISK REFINERY THERMAL FATIGUE SERVICES

       Mixing Tees & Quench Points                    Delayed Coker Drums
  ┌─────────────────────────────────────┐    ┌─────────────────────────────────────┐
  │ - Hydroprocessing reactor quench    │    │ - 24 to 48-hour batch cycle         │
  │ - Desuperheater steam/water inject  │    │ - 900°F (482°C) oil filling         │
  │ - FCC fractionator reflux tees      │    │ - Rapid cold water quench           │
  │ - Turbulent thermal stripping at ID │    │ - Skirt attachment cracking         │
  └─────────────────────────────────────┘    └─────────────────────────────────────┘

       Boiler Tubes & Soot Blowers               Dissimilar Metal Welds (DMW)
  ┌─────────────────────────────────────┐    ┌─────────────────────────────────────┐
  │ - Cold steam/air soot blower blast  │    │ - Low alloy to austenitic SS welds  │
  │ - Rapid local tube chilling         │    │ - 30% thermal expansion mismatch    │
  │ - Transverse craze cracking         │    │ - Fusion-line cyclic shear tearing  │
  │ - Thick oxide scale packing         │    │ - Catalytic reforming / hydrocracking│
  └─────────────────────────────────────┘    └─────────────────────────────────────┘

1. Mixing Tees and Quench Injection Stations

Mixing tees represent the single most common location for thermal fatigue failures. When two fluid streams at differing temperatures merge (such as cold hydrogen quench injected into hot hydrocracker reactor effluent lines, cold condensate into steam desuperheaters, or cold wash oil into FCC overhead vapors), turbulent mixing does not occur instantaneously. Unstable eddies of hot and cold fluid alternate against the inner pipe wall at frequencies of 1 Hz to 50 Hz (a phenomenon known as thermal stripping). This causes rapid cyclic thermal strains in the inner wetted skin, initiating through-wall cracking within months.

2. Delayed Coker Drums

Delayed coking is a severe batch process operating on a 24- to 48-hour cycle. Empty coke drums are preheated with steam, filled with heavy residuum at 900 °F (482 °C), and subsequently quenched by pumping cold water into the bottom of the drum to cool the solid coke bed. Water quench introduces rapid localized thermal gradients that contract the vessel shell unevenly. This severe thermal cycling induces:

• Circumferential weld cracking and shell bulging ("elephant footing" or drum ovalization).
• Catastrophic fatigue cracking at the skirt-to-cone attachment weld, where thermal expansion of the hot cone is mechanically restrained by the cooler cylindrical skirt.

3. Boiler Waterwall and Superheater Tubes

Boiler tubes in utility and refinery power plants suffer thermal fatigue in soot-blower lanes. When soot blowers inject cold steam or compressed air against hot operating tubes (700 °F to 1,000 °F / 371 °C to 538 °C) to dislodge slag, localized surface chilling induces cyclic tensile stress, forming transverse craze cracks.

---

Morphology and Physical Appearance of Thermal Fatigue

Thermal fatigue generates a unique, unmistakable morphological signature under visual and metallographic inspection:

                   THERMAL FATIGUE MORPHOLOGY: CRAZE CRACKING

       Pipe / Vessel Inner Wetted Surface (Plan View)
       ┌──────────────────────────────────────────────────────────┐
       │   ┌──────┬───────┐  ┌───────┬─────────┐   ┌─────────┐    │
       │   │      │       └──┘       │         └───┘         │    │
       │   │      │   "CRAZE CRACKING" / "ALLIGATOR HIDE"    │    │
       │   │      │   Multidirectional mosaic network        │    │
       │   └──┬───┴───────┐   ┌──────┴─────────┐   ┌─────┬───┘    │
       │      │           └───┘                └───┘     │        │
       └──────┴──────────────────────────────────────────┴────────┘

       Cross-Sectional Microscopic View
       ═════════════════════════════╤════════════════════════════════
                                    │ ◄── Broad, surface-breaking mouth
                                   / \
                                  /   \  Dense, dark iron oxide
                                 │  █  │ scale filling the fissure
                                 │  █  │ (Magnetite Fe3O4 / Spinels)
                                  \ █ /
                                   \ / ◄── Wide, dagger-shaped,
                                    ▼      blunt-tipped transgranular crack

1. Macroscopic Appearance: "Craze Cracking" or "Alligator Hide"

• Because thermal strains in a pipe or vessel wall are typically equibiaxial (acting equally in circumferential and longitudinal directions), thermal fatigue cracking manifests as an interconnected, multidirectional network of surface fissures. This pattern is universally described in API RP 571 as craze cracking, alligator hide, or elephant hide.
• If thermal restraint is directional (such as longitudinal bending restraint in a heat exchanger tube or circumferential hoop restraint in a thick-wall mixing tee), cracks orient perpendicular to the direction of maximum tensile strain.

2. Microscopic Characteristics

• Crack Path: Predominantly transgranular, propagating directly across the crystalline grains.
• Crack Profile: Cracks are characteristically wide, dagger-shaped, and blunt-tipped. Unlike the ultra-fine, sharp fissures of mechanical fatigue or stress corrosion cracking, thermal fatigue cracks exhibit significant crack-mouth opening displacement.
• Oxide Wedging: Thermal fatigue fissures are heavily packed with dense, dark oxide scale (magnetite $Fe_3O_4$ in carbon steels or complex chrome-iron spinels in alloy steels). The thick oxide scale wedged within the crack opening acts as a mechanical wedge during the compressive portion of thermal cycles, concentrating high mechanical stress at the crack tip and accelerating propagation.

---

Thermal Shock (API RP 571 Section 3.65)

Thermal Shock is defined as structural fracturing or through-wall rupture resulting from sudden, extreme thermal gradients experienced in a single cycle or very few cycles. While thermal fatigue is a progressive endurance failure, thermal shock is an acute overload failure.

Operational Causes of Thermal Shock

1. Rainstorms on Uninsulated Hot Equipment: Vessels, heavy piping flanges, or exchanger shells operating above 400 °F (204 °C) that lack thermal insulation (or where insulation jacketing has blown off) suffer massive localized chilling during sudden rainstorms, generating instantaneous surface tensile strains that shatter brittle materials.
2. Emergency Cold Water Injection: Uncontrolled injection of cold emergency cooling water into dry, overheated steam drums or fired heater tubes.
3. Cryogenic Spill / Inadvertent Auto-Refrigeration: Flashing light hydrocarbons (LPG, propane, butane) venting across carbon steel components, dropping metal temperatures below the ductile-to-brittle transition temperature (DBTT) while thermal contraction generates massive tensile stresses.

---

Prevention and Engineering Mitigations

                 ENGINEERING MITIGATION: THERMAL MIXING SLEEVE

     Hot Main Process Stream
    ───────────────────────┐                                 ┌──────────────
                           │   Annular Stagnant Gap          │
                           │   (Protects Pressure Boundary)  │
    ═══════════════════════╪═════════════════════════════════╪══════════════
    ═══════════════════════╡ ┌─────────────────────────────┐ ╞══════════════
                           │ │ Thermal Mixing Quill/Sleeve │ │
                           │ │ (High-Nickel Alloy / 316SS) │ │
     Cold Quench Stream    │ │                             │ │
     ──────────────────────┘ │                             │ └──────────────
                             └─────────────┐ ┌─────────────┘
                                           │ │ Cold Quench
                                           │ │ Centerline Discharge
                                           ▼ ▼ (Mixing occurs in bulk flow)

1. Thermal Sleeves & Mixing Quills: In all quench mixing tees where $\Delta T > 100\text{ °F}$ ($56\text{ °C}$), install an internal thermal mixing sleeve or quill. The quill discharges the cold quench stream directly into the centerline of the hot flow, allowing turbulent mixing to complete within the fluid core before contacting any metal surface. An annular stagnant gap between the sleeve and the pressure-retaining pipe wall insulates the pressure boundary from temperature fluctuations.
2. Controlled Ramp Rates: Enforce strict computerized ramp rates for heat-up and cool-down cycles. In delayed coke drums and heavy hydrocracker reactors, cool-down rates are typically limited to 50 °F to 100 °F per hour (28 °C to 56 °C/hr) per owner and equipment-manufacturer procedures.
3. Dissimilar Metal Weld Design: When joining ferritic and austenitic steels, use nickel-base weld consumables (such as Alloy 82, Alloy 182, Alloy 52, or Alloy 152). Nickel alloys have an intermediate thermal expansion coefficient ($\alpha \approx 14 \times 10^{-6}/\text{K}$) that bridges the gap between ferritic and austenitic steels, cutting interfacial cyclic shear stresses by more than 50%.
4. Weatherproofing and Insulation: Hot operating vessels ($> 200\text{ °F} / 93\text{ °C}$) should be protected with continuous, well-maintained thermal insulation and weather-sealed aluminum or stainless steel jacketing to prevent thermal shock from cold rain impingement.

---

Inspection, Non-Destructive Examination (NDE) & Detection