CO2 Immersion Gear Pump Strength Testing and Determination

CO2 immersion gear pump strength determination verifies that gear pumps used in carbon dioxide service, such as R744 refrigeration and heat pump circuits, carbon capture systems, and supercritical CO2 processes, retain full mechanical integrity after prolonged exposure to CO2. This article explains the test samples, evaluated parameters, test methods, applicable international standards, and typical applications of this specialized assessment. All procedures described can be performed by accredited third-party laboratories in accordance with ISO, ASTM, EN, GB, UL, and JIS standards.

Technical Overview

Carbon dioxide is now one of the most important working fluids in modern industry. It serves as the R744 refrigerant in transcritical heat pumps and commercial refrigeration, as a transport and injection medium in carbon capture, utilization and storage (CCUS), and as a supercritical solvent in extraction and processing plants. Gear pumps are widely used in these systems to circulate lubricant, transfer liquid CO2, and dose process fluids at elevated pressure. Because transcritical CO2 circuits commonly operate between 9 MPa and 15 MPa, the pressure-containing strength of the pump must be demonstrated under realistic conditions, not merely in the as-manufactured state.

CO2 creates several degradation mechanisms that justify immersion conditioning prior to strength testing. Dense or supercritical CO2 diffuses into elastomeric seals and polymer bearing materials, causing swelling and, upon rapid depressurization, explosive decompression damage known as rapid gas decompression (RGD). In the presence of moisture, CO2 forms carbonic acid that can attack carbon steel components. Flashing liquid CO2 can also produce low temperatures that embrittle certain materials. Consequently, a complete strength determination program consists of baseline characterization, CO2 immersion conditioning, and post-exposure mechanical strength and integrity verification.

Testing Samples

Complete Gear Pump Assemblies

Typical test articles include external gear pumps, internal gear pumps, and magnetically coupled or canned-motor gear pumps intended for CO2 service. Pumps should be supplied in full production configuration, including factory-installed seals, gaskets, bearings, shafts, and fasteners, with documented rated pressure, displacement, and materials of construction.

Pressure-Containing Components

Casings, end covers, flanges, mounting feet, shafts, and drive components may be tested individually when design verification of a specific part is required. These components are assessed for proof pressure, burst pressure, and fatigue strength after the same CO2 conditioning applied to the complete assembly.

Non-Metallic Functional Parts

O-rings, shaft seals, gaskets, and gear bearings made of PEEK, PTFE composites, carbon, and elastomeric compounds are the parts most affected by CO2 absorption. They are evaluated for swelling, softening, blistering, and loss of mechanical strength following immersion and decompression.

Material Coupons and Reference Specimens

Flat coupons of the casing alloy and molded specimens of the seal compounds are conditioned in parallel with the pumps. These coupons allow quantitative measurement of mass change, volume change, and mechanical property retention without sacrificing complete assemblies.

Testing Items

The strength determination program typically covers the following test items:

• Hydrostatic proof pressure strength at 1.0 to 1.5 times the maximum allowable pressure
• Ultimate burst pressure strength of the casing and pressure boundaries
• Cyclic pressure fatigue (impulse) strength over 10,000 to 1,000,000 pressure cycles
• Static and running torque strength of shafts, keys, and gear trains
• External leakage rate under pressurized CO2 or equivalent test media
• Elastomer swelling evaluated as mass change, volume change, and dimensional change
• Hardness change and tensile property retention of seal materials after CO2 exposure
• Rapid gas decompression (RGD) damage rating of seals and non-metallic parts
• Corrosion, carburization, and embrittlement assessment of metallic components
• Permanent deformation and dimensional stability of critical fits and clearances
• Performance retention, including displacement, flow rate, volumetric efficiency, and input torque
• Post-test non-destructive examination by dye penetrant and radiographic inspection

Testing Methods

CO2 Immersion Conditioning

Samples are placed in a pressure autoclave and purged with CO2 to displace air. The vessel is then pressurized to service-simulating conditions: liquid CO2 conditioning is typically performed between 2 MPa and 8 MPa at sub-ambient to ambient temperatures, while supercritical conditioning is performed above the critical point of 7.38 MPa and 31.1 degrees Celsius, commonly at 8 MPa to 15 MPa and 35 to 60 degrees Celsius. Exposure durations range from 72 hours to 500 hours depending on the qualification target. Depressurization is controlled at a defined rate, or deliberately rapid when RGD resistance is being challenged, following the elastomer qualification philosophy of ISO 23936-2 and immersion practices adapted from ASTM D471.

Hydrostatic Proof and Burst Strength Testing

After conditioning, the pump is subjected to hydrostatic pressure testing using water or hydraulic oil as the pressurizing medium, with methodology aligned to UL 207 for refrigerant-containing components and to the pressure testing provisions of EN 378-2 and ISO 5149-2. Proof pressure is held for a defined period, commonly 1 to 10 minutes, during which the pump must show no leakage, no rupture, and no permanent deformation. Burst pressure is then applied at a controlled ramp rate until failure or until the specified multiple of the maximum allowable pressure, typically at least three times, is achieved. The complete pressure-time curve is recorded for reporting.

Cyclic Pressure Fatigue Testing

To demonstrate fatigue strength, conditioned pumps are cycled repeatedly between a specified minimum and maximum pressure at a frequency of 0.5 to 5 Hz. Intermediate leak and deformation inspections are performed at defined intervals, and a final burst test is often appended to confirm residual strength after cycling.

Post-Immersion Material Assessment

Elastomeric and polymer specimens are measured for mass and volume change, durometer hardness per ASTM D2240, and tensile properties per ASTM D412. Metallic coupons undergo hardness testing, tensile testing, and metallographic examination to detect corrosion or embrittlement. Microscopic examination of seals identifies blisters and internal cracks characteristic of RGD damage.

Performance and Torque Verification

Pumps are run on a calibrated hydraulic test bench using procedures adapted from JIS B 8351 for gear pumps and from the acceptance test methodology of ISO 9906 and GB/T 3216. Displacement, flow, volumetric efficiency, input torque, and noise are compared with baseline values to confirm that CO2 exposure has not degraded operating performance. Design margins of pressure-containing casings may additionally be benchmarked against GB/T 150 pressure equipment criteria.

Safety Considerations

High-pressure CO2 stores significant energy and poses an asphyxiation hazard in confined spaces. All pressurization and burst testing is conducted behind protective barricades with remote data acquisition by trained personnel in accredited third-party laboratories.

Applications

CO2 immersion gear pump strength determination supports qualification and quality release in the following fields:

• Transcritical CO2 (R744) refrigeration systems and heat pump water heaters, including oil circulation and liquid injection pumps
• Carbon capture, utilization and storage plants requiring CO2 transfer and injection pumps
• Enhanced oil recovery skids that inject CO2 into reservoirs at high pressure
• Supercritical CO2 extraction equipment for food, pharmaceutical, and nutraceutical processing
• Beverage carbonation and CO2 dosing systems
• CO2 fire suppression and inerting installations
• Research programs on supercritical CO2 power cycles and advanced cooling technologies

FAQ

Why must the pump be immersed in CO2 before strength testing?

CO2 penetrates elastomers and polymers under pressure. When the pump is depressurized, the dissolved gas expands and can blister, crack, or soften seals and bearings, while moist CO2 can corrode metallic parts. Strength testing without prior immersion would not reveal these condition-dependent failure modes, so immersion conditioning ensures the results reflect real service exposure.

What immersion conditions are typically applied?

Liquid CO2 conditioning typically uses 2 MPa to 8 MPa at low to ambient temperatures, while supercritical conditioning uses at least 7.38 MPa and 31.1 degrees Celsius, commonly 8 MPa to 15 MPa at 35 to 60 degrees Celsius. Exposure duration and decompression rate are selected to match the intended qualification level.

What acceptance criteria are generally applied?

Common criteria include no leakage and no permanent deformation at proof pressure, a burst pressure of at least three times the maximum allowable pressure, successful completion of the required pressure cycles, and seal volume change and hardness change within agreed limits, with no visible RGD blistering or cracking.

Which standards govern the testing?

Pressure test methodology is aligned with UL 207, EN 378-2, and ISO 5149-2. Elastomer exposure and qualification follow the framework of ISO 23936-2 with immersion and property measurement methods from ASTM D471, ASTM D412, and ASTM D2240. Performance verification references JIS B 8351, ISO 9906, and GB/T 3216, with pressure part margins benchmarked to GB/T 150.

How long does a complete test program take?

Depending on the immersion duration, cycle count, and number of samples, a full program typically requires two to eight weeks. Long-duration supercritical conditioning and high-cycle fatigue programs extend the schedule accordingly.

What samples should be submitted?

A typical submission includes three to six complete pumps in production configuration, plus spare seal kits and material coupons. Documentation of rated pressure, materials, and prior type tests allows the laboratory to define precise test parameters before conditioning begins.