Calculations of Hydrodynamic Cavitation Devices

6 Selection of Structural Materials

Structural material selection for a cavitation device cannot be reduced to a single alloy ranking. The pressure boundary, cavitation generating insert, rotor, shaft, wear plate, fastener, seal face, gasket, and product contact surface perform different functions. Pressure retaining walls require weldability and code allowable stress; removable inserts require erosion resistance and dimensional stability; rotors require fatigue strength and balance; and sanitary surfaces require corrosion resistance, cleanability, low metal release, and controlled finish. Assigning all functions to one material can increase cost while reducing lifecycle performance [22–27].

The preferred design strategy first controls the hydraulic collapse location, then assigns replaceable wear components, and only afterward considers alloy or coating upgrades. No practical material is immune to sufficiently aggressive cavitation. A hard surface may resist plastic indentation but fail by brittle cracking or coating delamination. A corrosion resistant alloy can repeatedly lose its passive film under impact, while a tough alloy may deform and work harden but release metal. Material selection must therefore reflect the actual chemistry, temperature, solids, gas content, pressure history, geometry, permitted contamination, and inspection interval [23–25].

Figure 6.1. Structural material selection workflow for hydrodynamic cavitation equipment [32].

6.1 Functional decomposition of the device

The calculation package should identify the material function of every component:

Pressure boundary: contains maximum allowable working pressure, relief pressure, vacuum, thermal load, and piping reaction.

Cavitation insert or throat: defines the hydraulic geometry and may be intentionally replaceable.

Collapse chamber or wear plate: receives the highest expected microjet and cloud collapse loading.

Rotor and shaft: resist centrifugal stress, torque, fatigue, corrosion, and erosion while maintaining balance.

Stator and clearance surfaces: maintain geometry under pressure, temperature, and possible rub.

Seals, gaskets, and bearings: remain compatible with chemistry and temperature while preventing product contamination.

Welds and heat affected zones: meet pressure code, corrosion, fatigue, and surface finish requirements.

Instrumentation connections: avoid crevices, dead legs, thin diaphragms exposed to collapse, and plugged impulse lines.

A replaceable insert allows the pressure shell to be selected primarily for code, corrosion, and fabrication, while the wear component is selected for cavitation exposure. The insert should have positive location, repeatable dimensions, no bypass leakage, and an inspection datum. Its failure must not release a loose part into downstream equipment.

6.2 Pressure boundary screening

For a cylindrical wall under internal pressure, a simplified thin wall screen can be written:

where $p$ is design pressure, $D$ is the selected diameter basis, $S$ is allowable stress, and $E$ is weld joint efficiency. Equation (6.1) is only a preliminary screen. Final thickness, reinforcement, flange rating, fatigue, external pressure, nozzle loads, weld details, examination, testing, and material allowable must comply with the applicable pressure vessel or piping code, such as ASME BPVC Section VIII or ASME B31.3 [26, 27].

A practical nominal thickness expression is:

where $t_{code}$ is code required thickness, $c_{corr}$ is corrosion allowance, $c_{eros}$ is a justified erosion allowance, $c_{mach}$ is machining or resurfacing allowance, and $m_t$ is the negative mill tolerance fraction. An erosion allowance should not replace inspection or a wear insert when cavitation attack is localized.

Worked structural screen

For $p=1.6$ MPa, $D=0.100$ m, $S=115$ MPa, and $E=0.85$, Equation (6.1) gives $t_s=0.825$ mm. This small value illustrates only membrane pressure demand. It is not a final wall thickness. A real device may require substantially more thickness for code minimums, threading, nozzles, flange loads, cyclic pressure, vibration, manufacturing tolerance, corrosion, erosion, and rigidity needed to preserve a small internal clearance.

6.3 Cavitation erosion and erosion corrosion interaction

Cavitation erosion is produced by repeated pressure pulses, microjets, cloud collapse, and associated cyclic deformation. The incubation period may show little measurable mass loss while subsurface fatigue accumulates. It can be followed by an accelerating stage, a maximum rate, and a later rate modified by roughness and the evolving flow. A single test duration can therefore misrepresent long term behavior [23, 24].

ASTM G32 provides a vibratory apparatus method for comparative cavitation erosion testing [23]. Its output is valuable for screening materials, heat treatments, and coatings under controlled conditions, but it should not be converted directly into plant service life. A hydrodynamic device has different impact frequency, bubble cloud scale, pressure recovery, chemistry, temperature, solids, and wall geometry. The strongest program combines standardized comparison, a representative flow test, and pilot inserts in the actual process.

Mean depth of erosion calculated from mass loss is:

where $\mathrm{\Delta }m$ is mass loss, ${\rho }_s$ is solid density, and $A_e$ is exposed area. The mean depth penetration rate is:

A combined erosion corrosion loss may be expressed conceptually as:

where $W_{cav}$ is purely mechanical cavitation loss, $W_{corr}$ is corrosion loss without cavitation, and $W_{syn}$ is the interaction term. The interaction can be positive because impact removes passive films and corrosion weakens the surface, but the sign and magnitude must be measured for the actual environment [24].

For an observed approximately linear penetration rate after incubation, a remaining life screen is:

where $h_{allow}$ is the maximum permitted local penetration and $h_{meas}$ is measured penetration. This calculation is valid only when the rate is stable and the measured location corresponds to the most severe attack. Local pits or crack like defects may govern before average depth.

6.4 Material property requirements

Cavitation resistance is influenced by hardness, yield strength, strain hardening, toughness, fatigue behavior, elastic modulus, microstructure, residual stress, phase stability, corrosion resistance, surface finish, and defect population. No single property predicts performance across all regimes. The desired combination is a surface that resists repeated indentation and cracking, a substrate that supports the surface without excessive deformation, and a chemistry that maintains passivity or avoids rapid dissolution [24, 25].

Material screening should consider:

Strength and fatigue: pressure cycles, rotor stress, passage frequency loading, and vibration.

Toughness: resistance to crack initiation and propagation under repeated impact.

Hardness and work hardening: resistance to plastic indentation, with attention to brittle failure.

Corrosion: uniform corrosion, pitting, crevice corrosion, stress corrosion cracking, galvanic coupling, and corrosion fatigue.

Fabrication: weldability, heat treatment, distortion, machinability, repair, and nondestructive examination.

Surface condition: roughness, inclusions, machining marks, weld profile, coating defects, and residual stress.

Process compatibility: extractables, toxic metal limits, food or pharmaceutical conformity, product discoloration, and catalyst poisoning.

Lifecycle: cost, lead time, replaceability, inspection method, and disposal or repair.

6.5 Candidate metallic materials

Table 6.1. Qualitative material selection matrix for cavitation devices.

Material family	Potential role	Principal advantages	Principal cautions
Carbon and low alloy steel	External pressure shell or nonhygienic benign service	Low cost, strength, weldability, code familiarity	Corrosion products, product contamination, localized erosion corrosion, coating dependence
304L austenitic stainless steel	Mild sanitary or water service	Fabricability, cleanability, availability	Lower chloride pitting margin than Mo bearing grades; not immune to cavitation
316L austenitic stainless steel	Common hygienic pressure boundary and wetted components	Weldability, cleanability, improved pitting resistance over 304L	Can pit or crevice corrode in hot chlorides; moderate hardness; erosion can remove passive film
Duplex stainless steel 2205	Higher strength chloride service, pressure parts, selected rotors	Higher strength and chloride resistance than common austenitic grades	Phase balance and welding control; surface finishing and hygienic qualification require care
Super duplex or high alloy austenitic steel	Severe chloride or oxidizing aqueous service	High pitting and crevice corrosion resistance	Cost, fabrication control, availability, possible over specification if erosion dominates
Martensitic or precipitation hardening stainless steel	Shafts, rotors, replaceable wear parts	Higher strength and hardness after heat treatment	Lower corrosion resistance in some chemistries; heat treatment control; not automatically suitable for sanitary contact
Nickel base corrosion resistant alloy	Strong acids, mixed chemicals, severe corrosion	Broad corrosion resistance in selected environments	High cost, galling, density, weld procedure, chemistry specific limitations
Titanium alloy	Chloride rich service, low density rotor or wetted component	Low density and strong passive behavior in many chloride environments	Galling, galvanic coupling, hydrogen effects, fabrication cost; cavitation resistance remains condition dependent
Cobalt base or nickel base hard facing	Local replaceable wear surface	Hardness, work hardening or high temperature wear capability	Cracking, dilution, repair, cost, possible metal release restrictions
Copper alloy	Specialized marine or antimicrobial service	Thermal conductivity and selected seawater applications	Metal release, galvanic effects, chemistry limitations, generally unsuitable where copper contamination is unacceptable

The table is a screening matrix, not a substitution rule. The specific grade, product form, heat treatment, weld, surface finish, and service chemistry determine performance. Material certificates and positive material identification should be tied to the component drawing.

6.6 Coatings, surface treatments, and claddings

Coatings can increase surface hardness or corrosion resistance while retaining a tough structural substrate. Candidate technologies include physical vapor deposited nitrides, high velocity oxy fuel coatings, weld overlays, laser cladding, nitriding, shot peening, and engineered surface remelting. A recent study of CrN coated 316L illustrates that coating architecture can improve comparative cavitation behavior, but the result depends on adhesion, residual stress, porosity, substrate support, and the test environment [25].

A coating qualification should include:

• substrate grade, heat treatment, and surface preparation;

• coating composition, thickness, hardness, porosity, and residual stress;

• adhesion and edge condition;

• cavitation test before and after chemical exposure;

• thermal cycle and clean in place compatibility;

• inspection method for pinholes, cracks, and delamination;

• permitted particle or metal release;

• repair procedure and dimensional effect.

A very hard thin coating on a compliant substrate can crack when repeated pressure pulses deform the substrate. A thick sprayed coating can contain pores and reduce a small hydraulic clearance. Coating a sharp orifice edge can change its radius and discharge coefficient. The final hydraulic geometry should therefore be measured after coating.

6.7 Ceramics and cermets

Technical ceramics and cermets offer high hardness and corrosion resistance but are sensitive to tensile stress, impact defects, thermal shock, and attachment details. They are better suited to compressively supported inserts, sleeves, nozzles, or tiles than to complex unsupported pressure boundaries. A ceramic insert should have generous radii, controlled interference or compliant support, and protection against assembly impact. The consequences of fragment release must be considered for food, pharmaceutical, turbine, membrane, and precision downstream equipment.

Tungsten carbide and chromium carbide systems may be used as bulk inserts or sprayed coatings, but binder chemistry and galvanic behavior must match the process. Hardness alone does not guarantee cavitation life because binder removal, pore attack, and brittle grain pullout can control failure.

6.8 Elastomers, polymers, seals, and bearings

Elastomer selection is governed by the complete fluid mixture, temperature, pressure, cleaning chemicals, oxidants, solvent swelling, gas decompression, and regulatory status. EPDM, FKM, FFKM, nitrile, silicone, and PTFE based materials have distinct compatibility envelopes; no generic ranking is valid for all cavitation applications. Compatibility should be confirmed using supplier data and immersion or compression testing in the actual process and cleaning fluids.

Seals should be located away from the strongest collapse zone and pressure pulsation where possible. A dynamic seal calculation should include chamber pressure, shaft speed, heat generation, face loading, flush plan, solids, gas, and dry running risk. Double mechanical seals or barrier systems may be required when product containment, air exclusion, or hazardous chemistry governs. Bearings should be isolated from process liquid unless they are specifically designed as product lubricated components.

Thermoplastics can be useful for transparent development chambers, low pressure inserts, or corrosion isolation, but pressure rating, creep, solvent compatibility, temperature, ultraviolet exposure, and brittle fracture must be checked. A transparent chamber used for visualization should have a protective enclosure and a defined service life.

6.9 Hygienic and cleanable construction

Food and pharmaceutical devices should use cleanable geometry, compatible product contact materials, controlled surface finish, drainability, and validated cleaning. Inline rotor stator equipment may be evaluated against applicable 3-A requirements, but the owner remains responsible for the complete installation, including piping, seals, instrumentation, and cleaning cycle [22].

Important design details include:

• continuous welds with controlled penetration and ground or polished product surfaces where required;

• no gasket intrusion into the flow path;

• elimination of dead legs and unvented high points;

• drainable housings and sample ports;

• accessible or validated internal cavities;

• surface finish measurement after fabrication and repair;

• avoidance of threaded product contact joints unless specifically accepted;

• material and elastomer traceability;

• verification that wear debris cannot exceed product contamination limits.

Cavitation can increase cleaning action locally but does not prove that every surface receives chemical concentration, temperature, velocity, and time sufficient for cleaning. A clean in place study should use riboflavin, conductivity, temperature mapping, swabs, microbiology, or another validated method appropriate to the process.

6.10 Welds, residual stress, and fabrication quality

Weld toes, undercut, incomplete penetration, porosity, inclusions, abrupt section changes, and rough repairs can become preferred sites for cavitation attack or corrosion. Welding also changes microstructure and residual stress. Pressure boundary welds require qualified procedures, welder qualification, examination, and code acceptance. Wetted welds may require grinding, polishing, passivation, pickling, or electropolishing depending on alloy and service.

For duplex stainless steels, heat input and interpass temperature affect phase balance and corrosion performance. For precipitation hardening or martensitic grades, welding and postweld heat treatment can alter strength and corrosion resistance. For hardfacing and cladding, dilution and cracking must be controlled. Fabrication records should therefore be part of the material selection calculation package, not a separate purchasing detail.

6.11 Inspection, monitoring, and lifecycle acceptance

Inspection should concentrate on predicted collapse zones and high stress regions. A post fabrication baseline should include dimensional measurement, surface photography, mass of removable inserts, ultrasonic thickness where applicable, rotor balance, vibration, and surface finish. Initial follow up intervals should be conservative and may be extended only after a stable wear trend has been demonstrated.

Table 6.2. Inspection methods by component and damage mechanism.

Component or concern	Baseline measurement	In service indicator	Outage inspection
Orifice or throat	Diameter, edge radius, $C_d$ curve	Drift in flow pressure relation	Optical metrology, replica, mass or profile loss
Collapse chamber	Thickness map, surface photograph	Metal concentration, acoustic change	UT grid, 3D scan, dye penetrant where appropriate
Rotor	Dimensions, balance, runout, vibration	Vibration and power trend	NDE, mass, balance, crack and erosion inspection
Stator and pins	Clearance and feature geometry	Torque or pressure drift	Coordinate measurement, surface inspection
Coating	Thickness, adhesion, defect map	Metal or particle release	Microscopy, holiday or adhesion test as applicable
Pressure shell	Code examination and pressure test	Leak, strain, pressure cycle count	UT, surface NDE, weld examination
Seal and bearings	Leakage, temperature, vibration	Trend alarms	Face, sleeve, bearing and lubricant inspection

An acceptance criterion should specify maximum dimensional change, maximum local penetration, maximum vibration, allowable metal concentration, minimum remaining pressure thickness, rotor balance tolerance, and product quality limits. Replacement should be based on the governing criterion rather than waiting for leakage or catastrophic loss of geometry.

6.12 Material selection decision record

The material decision should be recorded in a table that connects service environment, failure mode, selected material, protective feature, inspection, and spare strategy.

Table 6.3. Example material decision record.

Component	Service and dominant risk	Selected construction	Design control	Verification
Pressure housing	Internal pressure, chloride bearing aqueous liquid	Code listed corrosion resistant alloy	Code thickness, corrosion allowance, qualified welds	Material certificates, NDE, pressure test, UT baseline
Orifice insert	High local velocity and edge erosion	Hardened or coated replaceable insert compatible with product	Positive location, minimum ligament, measured final edge	Flow calibration, dimensional inspection, spare insert
Collapse liner	Cloud collapse and metal release limit	Replaceable corrosion resistant wear sleeve	Move collapse away from pressure shell	Mass and 3D profile at defined intervals
Rotor	Centrifugal stress, fatigue, cavitation, balance	High strength corrosion resistant alloy with qualified heat treatment	Overspeed and fatigue analysis, generous radii	Material traceability, NDE, balance, vibration trend
Stator	Gap integrity and erosion	Tough corrosion resistant alloy or replaceable ring	Clearance budget and anti-galling pair	Gap measurement and contact inspection
Seal	Chemistry, speed, gas, cleaning	Supplier qualified face and elastomer combination	Flush or barrier plan, temperature monitoring	Leak test, compatibility record, planned replacement

The decision record should be revised when process chemistry, cleaning agent, operating temperature, pressure, speed, solids, or allowable contamination changes. A successful material in a water trial cannot be assumed suitable for a chloride, oxidant, acid, caustic, solvent, or abrasive slurry without additional evidence.