3.3 Erosion/Erosion-Corrosion & Cavitation

Erosion and Erosion-Corrosion — API RP 571 Section 3.27

1. Synergistic Mechanism Description

Erosion is the purely mechanical removal of surface material resulting from the repetitive impingement of solid particles, liquid droplets, or gas bubbles suspended in a flowing fluid. Erosion-Corrosion is the synergistic acceleration of metal loss resulting from the combined, simultaneous interaction of mechanical wear and chemical or electrochemical corrosion.

In pure chemical corrosion, metal dissolution frequently slows over time because the metal forms a protective surface scale or passive film—such as iron sulfide ($FeS$) in sour services, iron carbonate ($FeCO_3$) in sweet services, or chromium oxide ($Cr_2O_3$) on stainless steels. In erosion-corrosion, turbulent fluid flow and impacting particles mechanically strip, abrade, or spall this protective film away. Bare metal is repeatedly exposed directly to the corrosive electrolyte, driving accelerated electrochemical dissolution. The resulting metal loss rate is vastly greater than the simple arithmetic sum of erosion alone plus corrosion alone.

Fluid Flow + Entrained Solids / Droplets
   │
   ▼ Mechanical Shear & Impingement
[ Protective Scale / Passive Film Removed ]
   │
   ▼ Bare Metal Substrate Exposed to Electrolyte
[ Rapid Electrochemical Dissolution (Fe → Fe2+ + 2e-) ]
   │
   ▼ Film Re-nucleates (Repassivation attempt)
[ Immediate Mechanical Stripping by Next Impact ]
   │
   ▼ Continuous High-Velocity Metal Loss (Grooving, Gullies, Horseshoe Pits)

2. Critical Factors Governing Erosion and Erosion-Corrosion

• Fluid Velocity and the Power Law: Fluid velocity is the single most dominant operating variable. Metal loss rate generally increases exponentially according to an empirical power law: $\text{Wear Rate} \propto V^n$ Where velocity exponent $n$ typically ranges from 2.0 to 5.0, depending on metallurgy, entrained particle hardness, and impingement geometry. Every material-fluid combination possesses a "critical threshold velocity" above which protective films cannot adhere.
• Angle of Impingement and Material Ductility:
  • Ductile Materials (Carbon Steel, Austenitic Stainless Steel, Copper Alloys): Exhibit maximum erosion rates under shallow, oblique impingement angles between 20° and 40°. At these low angles, impacting abrasive particles shear, gouge, and slice micro-chips from the metal surface.
  • Brittle Materials (Refractory Linings, Ceramics, Hardfacings, Cast Irons): Exhibit maximum erosion rates under perpendicular / normal 90° impingement. Direct normal impacts induce high subsurface Hertzian stresses, causing fatigue micro-fracturing and chipping of brittle grains.
• Entrained Solids Characteristics: Solid particulate concentration, size, shape, and hardness directly govern wear. Angular catalyst fines in Fluid Catalytic Cracking (FCC) units, quartz sand in crude oil gathering, and coke fines in delayed coking units represent severe erosive agents.
• Multiphase Flow Regimes: Two-phase flow (such as high-velocity steam containing condensed water droplets, flashing hydrocarbons downstream of letdown valves, or gas-entrained slurries) produces violent droplet impingement and turbulent shear stresses.

3. Affected Refinery Equipment and Vulnerable Locations

• Piping Geometries: Short-radius elbows, pipe tees, return bends, concentric/eccentric reducers, orifice plates, and piping downstream of flow control valves.
• FCC Units: Reactor and regenerator cyclone bodies and diplegs, catalyst transfer slide valves, catalyst withdrawal lines, and slurry oil pumparound circuits.
• Distillation & Hydroprocessing: Crude unit vacuum ejector systems, overhead condenser tube inlets, hydrocracker letdown valves, and delayed coker hydraulic decoking lines.

4. Morphological Hallmarks of Erosion-Corrosion

• Directional Patterns: Smooth-bottomed grooves, gullies, waves, ripples, and scalloped troughs oriented strictly parallel to the direction of local fluid flow.
• Horseshoe-Shaped Pits: Classic morphology consisting of crescent or horseshoe-shaped pits whose open ends point downstream (in the direction of flow).
• Scale-Free Luster: Corroded surfaces exhibit a bright, clean, bare metallic luster completely free of corrosion product scales, magnetite, or iron sulfide films due to continuous mechanical polishing.

Cavitation — API RP 571 Section 3.16

1. Physical and Thermodynamic Fundamentals

Cavitation is a purely mechanical localized degradation mechanism caused by the rapid formation, growth, and violent collapse (implosion) of vapor bubbles within a flowing liquid. Cavitation occurs without requiring solid abrasive particles or corrosive chemicals.

The thermodynamic cycle unfolds in two distinct stages:

1. Bubble Nucleation: When local static fluid pressure drops below the fluid's equilibrium vapor pressure ($P_{\text{static}} < P_v$) at the local operating temperature, the liquid instantaneously boils, creating millions of microscopic vapor cavities.
2. Violent Implosion: As these vapor bubbles are swept downstream into regions of recovered static pressure ($P_{\text{static}} > P_v$), the vapor inside condenses almost instantaneously. The surrounding liquid rushes inward to fill the void.

Because the collapse occurs adjacent to a solid metal boundary, the liquid collapses asymmetrically. The bubble deforms into a toroid, focusing a piercing, ultra-high-velocity liquid micro-jet directed straight at the metal surface. The micro-jet strikes the metal with localized impact velocities of several hundred meters per second, generating transient impact shock pressures exceeding 100,000 psi (700 MPa).

2. Microscopic Metallurgy of Cavitation Damage

The repeated impact of localized micro-jets delivers millions of micro-water-hammer blows per second onto microscopic surface areas:

• The localized impact stress exceeds the dynamic yield strength and fatigue endurance limit of the metal.
• The surface undergoes severe localized plastic deformation and rapid work-hardening.
• Micro-fatigue cracks initiate and propagate along crystal grain boundaries.
• Metal grains fracture and tear away, leaving behind deep micro-voids and micro-spalling.

3. Vulnerable Refining Equipment

• Centrifugal Pumps: Suction eye of impellers, leading tips and low-pressure suction sides of impeller vanes, casing wear rings, and pump cutwaters.
• Piping Systems: Immediately downstream of pressure-reducing control valves, restrictive orifice plates, butterfly valves, and sudden pipe expansions.
• Acoustic and Operational Signatures: Cavitating pumps and valves emit an unmistakable, harsh, deafening rattle and heavy vibration, universally described in refinery operations as sounding like "pumping rocks, marbles, or gravel."

4. Morphological Comparison: Erosion-Corrosion vs. Cavitation

Prevention, Mitigation & Inspection Strategies

1. Engineering Prevention for Erosion-Corrosion

• Hydraulic Velocity Management: Sizing process piping and heat exchanger tubes to ensure operating velocities remain well below critical erosive thresholds (e.g., limiting amine solutions, sour water, and slurries to prescribed maximum velocities).
• Piping Geometry Optimization: Replace short-radius elbows with long-radius bends ($R \ge 3D\text{ to }5D$) to minimize impingement angles. Install target tees (blind tees with fluid/solid buffer pockets) at changes of direction to absorb particle impacts into stagnant liquid cushions.
• Metallurgy & Hardfacing Upgrades: Upgrade carbon steel to erosion-resistant alloys with rapid repassivation kinetics and superior hardness, such as 2205 Duplex Stainless Steel or nickel-base alloys. Apply cobalt-base Stellite weld overlays, tungsten carbide in nickel matrices, or thermal spray cermets to valve trim and thermowells.
• Refractory Linings in FCC Units: Install hexmetal-reinforced, dense abrasion-resistant refractory linings (such as Resco AA-22S) in catalyst transfer lines, cyclone diplegs, and slide valve throats.

2. Engineering Prevention for Cavitation

• Net Positive Suction Head Management: The foundational engineering defense against pump cavitation is guaranteeing that Net Positive Suction Head Available ($NPSH_A$) exceeds Net Positive Suction Head Required ($NPSH_R$) by a robust safety margin across all operating envelopes: $NPSH_A = P_{\text{surface}} + H_{\text{static}} - H_{\text{friction}} - P_{\text{vapor}} > NPSH_R$ A common design practice is a margin of at least 3 to 5 ft (1.0 to 1.5 m) or an $NPSH_A / NPSH_R$ ratio $\ge 1.2\text{ to }1.3$, with pump-specific margins set by the manufacturer and project standards.
• Anti-Cavitation Valve Trim: In high-pressure letdown service, install tortuous-path, multi-stage pressure reduction valve trims. These trims divide a massive pressure drop into multiple small incremental stages, ensuring local static pressure never drops below fluid vapor pressure ($P_{\text{static}} > P_v$).
• Aeration Cushions: Injecting a small, controlled volume of non-condensable gas (air or nitrogen) into the low-pressure zone provides a compressible cushion that dampens bubble collapse shockwaves.
• Cavitation-Resistant Metallurgy: Upgrade from cast iron or carbon steel to alloys with high work-hardening capabilities and superior fatigue toughness: Cobalt alloys (Stellite), Austenitic Stainless Steels (304, 316), Duplex Stainless Steels (2205), or Nickel-Aluminum Bronze (NAB).

3. Inspection & NDE Methodologies

• Profile Radiography (PRT): The premier, most reliable NDE technique for evaluating internal erosion-corrosion in small-bore piping, control valve bodies, thermowell stems, and piping elbows without removing external insulation.
• Grid Ultrasonic Thickness Scanning (UT / PAUT): Structured, high-density straight-beam and phased array ultrasonic grids mapped along the extrados (outer radius) of elbows, downstream of reducers, and around tees to quantify localized wall thinning.
• Vibration & Acoustic Emission Monitoring: Online accelerometer and high-frequency acoustic emission surveillance on pump bearing housings and valve bodies to detect cavitation collapse shock frequencies in real time.
• Internal Visual Examination (VT) & Liquid Penetrant Testing (PT): Direct visual and dye penetrant inspection of pump impellers, cutwaters, and valve plugs during planned turnaround maintenance to detect micro-fatigue cracking and sponge-like cavitation craters.