TPS 2025: Centrifugal Pumps for Compressible Supercritical Applications

The 2025 Turbomachinery & Pump Symposia featured a heightened focus on supercritical fluids, equipment, and applications compared with previous years, with presenters and exhibitors outlining new technologies and demonstrating projects that validate centrifugal pumps for supercritical operations. Jake Wharton, Lead Application & Hydraulics Engineer for Sulzer, led a session titled Application and Design Considerations of Centrifugal Pumps in Compressible Supercritical Services, which discussed this topic in-depth.

Supercritical fluids—existing at temperatures and pressures above their critical points—behave unlike traditional liquids or gases. They combine liquid-like density with gas-like compressibility, challenging conventional turbomachinery design. Among them, CO₂ stands out as the most common, with growing applications across enhanced oil recovery, injection, transportation, and the broader carbon capture and storage value chain.

Pumps vs. Compressors in Supercritical Service

Although compressors are traditionally associated with compressible fluids, Wharton explained that centrifugal pumps can outperform them under certain supercritical conditions. Pumps typically operate with three to seven blades, a configuration that balances efficiency and reliability. Their lower inlet angle, slower operating speeds, and ability to handle higher mass flow rates make them well-suited for dense, compressible fluids.

Radial pump designs, in particular, have shown success in managing low-density supercritical flows. Both horizontal and vertical configurations are viable, depending on footprint, application, and site requirements. Performance calculations, however, remain a demanding exercise, as engineers must consider inlet temperature, pressure, flow rate, fluid composition, discharge pressure, and non-dimensional pump curves. Temperature rise, which limits compression ratio, can be mitigated through the use of multiple pumps in series with interstage cooling.

Cavitation, Rotordynamic, and Mechanical Design

Cavitation takes on a different character with supercritical fluids. While possible, the phenomenon is significantly less severe than in liquid-only service. Centrifugal pumps are capable of tolerating small amounts of cavitation, especially when dealing with fluids with densities similar to water or water vapor. Operators are advised to maintain a clear margin between inlet conditions and the saturation line to prevent cavitation-related instabilities. In practice, this requires careful monitoring and process control, but the results can provide stable, long-term operation even under challenging service conditions.

Supercritical applications also pose unique rotordynamic hurdles. The low viscosity of supercritical fluids provides poor lubrication, and wear parts offer minimal damping. For this reason, lateral analysis is considered mandatory in pump design for such services. Operators must also account for minimum operating speeds, particularly for low-density fluids, to prevent instability. Lightweight profile couplings are increasingly favored to reduce overhung modes and extend reliable operation.

HPcpV vertical pumps for HISEP system | Image Credit: Sulzer

Sealing the Deal: Mechanical Seal Options

One of the most critical elements of pump reliability is seal selection. Supercritical conditions demand solutions that can withstand rapid phase changes, chemical exposure, and high pressures. Options presented included dry gas seals, liquid double seals, and elastomeric seals. Double seals, which isolate process fluid from the seal face, are particularly effective for minimizing contamination and ensuring stable performance across state changes. Elastomeric seals add a layer of chemical resistance.

According to Wharton, material selection is equally important. Seals, piping, and drain valves must be chemically compatible and corrosion resistant, while low-temperature alloys are recommended for drain valves and piping. Non-metallic wear parts can further reduce galling and extend service life.

Testing, Validation, and Controls

Because full-scale testing with supercritical CO₂ is not always practical, water is frequently used as a surrogate fluid. After testing, pumps must be thoroughly dried before deployment. Water-based validation of seals also helps prove designs under more accessible conditions. Pumps for supercritical service are typically driven by variable frequency drives, which provide the flexibility needed to respond to process variation. Custom controls add further safeguards, preventing overload, overspeed, and instability at low flow rates. Minimum flow protection remains a critical feature, ensuring that pumps stay within their designated operating range.

Case Studies: From Lab to Field

Real-world examples continue to demonstrate the promise of centrifugal pumps in supercritical applications.

• A 4-stage diffuser pump successfully handled water, CO₂, and nitrogen, delivering stable hydraulic and rotordynamic performance even at densities as low as 340 kg/m³.
• 6-MW full-scale pumps are currently in production, underscoring the technology’s readiness for large-scale deployment.
• Qatar Gas implemented a 9-stage diffuser pump with a large tandem dry gas seal.
• The LaBarge Expansion project utilized a 12-stage volute inner casing pump for high-demand supercritical operations.

Offshore, Petrobras’ operators are turning to pumps for CO₂ reinjection strategies. The HISEP system, for instance, separates and reinjects CO₂-rich dense gas directly on the seabed, reducing topside processing demands. On the company’s pre-salt floating production storage and offloading units, as much as 65% of the topside footprint is dedicated to pumping and reinjection systems.

Looking Ahead

The message from Sulzer's Wharton was clear: Centrifugal pumps and compressors are not competitors in the supercritical space but partners. Each technology offers unique strengths, and together they provide a robust toolkit for industries managing CO₂ and other supercritical fluids. Conventional centrifugal pumps, with the right mechanical and hydraulic modifications, are not just viable—they’re often the better choice.

But success requires a close partnership between customer and supplier to ensure each installation meets the challenges of supercritical operation. As global industries look to scale carbon management, enhanced oil recovery, and alternative energy pathways, the centrifugal pump is poised to play an outsized role in the supercritical era.