Metal seal
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Using Metal Seals Correctly

Using metal seals can be a viable alternative to polymer and rubber seals. They are used extensively in extreme applications, such as rocket engine nozzles and missile guidance housings, as well as in ultra-high-vacuum equipment and cryogenic transfer systems. This article focuses on where metal seals are used and the pros and cons of their use.

Where Using Metal Seals is a Good Approach

Metal seals do their best work in extreme environments where polymer and elastic seals typically fail, including extremely high temperatures (above 200°C) and pressures. 

For example, in the oil and gas industry, they are used with wellhead equipment, subsea Christmas trees, valve bonnets, and high-pressure pipework, which require metal ring gaskets (such as API ring joint gaskets) because they can withstand extreme downhole pressures and resist corrosive fluids.

In aerospace and defense, metal seals are found in rocket engine nozzles, spacecraft propulsion systems, and missile guidance housings. Such applications rely heavily on metal seals to maintain integrity through thermal cycling, vibration, and vacuum conditions.

The nuclear industry uses metal seals in reactor pressure vessels, containment flanges, and coolant piping. Metal seals (often manufactured from Inconel or stainless steel) perform extremely well because they resist radiation embrittlement and provide reliable long-term sealing without the problems associated with outgassing.

Semiconductor and vacuum systems employ metal seals in ultra-high-vacuum (UHV) equipment such as particle accelerators, electron microscopes, and chip fabrication chambers. These applications often use knife-edge metal seals (CF flanges) to achieve near-zero leak rates without risking contamination in clean environments.

Metal seals are also used in cryogenic applications, including liquid nitrogen or liquid hydrogen storage and transfer systems. Polymers become brittle at cryogenic temperatures, while soft metals like aluminum or copper remain ductile, making them an ideal solution for use at cryogenic temperatures.

Engineers also use metal seals extensively in chemical processing applications, such as reactors and heat exchangers, where aggressive acids, solvents, or oxidizers (which use metal spiral wound or ring gaskets)   are used. Metal seals work well where the chemical compatibility is a significant concern.

Benefits of Using Metal Seals

Metal seals used for several reasons. First, metal seals do not experience creep or relaxation when exposed to heavy, sustained loads, which can be a major issue with some elastomers and polymers. Depending on the material chosen, metal seals can also be engineered to be chemically inert to most process fluids, including strong acids, solvents,  alkalis, and oxidizers. In addition, they are not subject to permeation or outgassing issues that can occur with some elastomers. They are also inherently fire safe because of their high melting point, and can handle some of the most aggressive cleaning and sterilization processes.

Metal seals can also survive extreme thermal cycling without experiencing permanent set. In fact, metal seals exhibit excellent temperature resistance, with some metals capable of withstanding temperatures from -270°C to over 1000°C, which cannot be achieved with a polymer or elastomer. They can also handle extreme pressures, from ultra-high vacuum pressures to tens of thousands of psi. Finally, they experience a long service life in static applications and do not have a limited shelf life or aging concerns.

Things to Keep in Mind when Using Metal Seals

Metal seals do require stricter surface finish requirements than polymer and elastomeric solutions, and a high seating stress is required to achieve a reliable seal. Most metal seals are single-use, and the cost of their replacement can add up quickly over the equipment’s life cycle. Metal seals are also more sensitive to misalignment and vibration, which makes them unsuitable for applications involving thermal distortion across joints of movement. 

The raw material costs of metal seals (e.g., Inconel, stainless steel, titanium) and the precision machining required to manufacture them make them more expensive than their polymer and elastomeric options.  In addition, soft metals (e.g., copper, aluminum, soft iron) can gall or seize against the flange during installation, which can lead to expensive rework. Metal seals can also be difficult to disassemble and are not suitable for low-pressure applications. Finally, metal seals have limited self-healing properties.

Conclusion

Metal seals have applications where they significantly outperform their elastomer and polymer counterparts, including those involving temperature and pressure extremes. If you are in the process of selecting a metal seal for your design, contact the seal experts at Advanced EMC today.

An example of a subsea AUV
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Polymer Bearings for Subsea Robotics: Surviving Pressure, Saltwater, and Zero-Maintenance Windows

The ocean floor remains one of the most unforgiving environments on Earth. Remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) descend thousands of meters to inspect pipelines, support offshore drilling, and conduct scientific surveys. And these ROVS do all of this in conditions that would destroy conventional mechanical components within hours, including bearings. For engineers designing subsea robotic systems, the question is never whether the environment will push the hardware to its limits. The question is whether the hardware is ready for it.

Bearings are among the most critical and vulnerable components in a subsea robot’s drivetrain, joint assemblies, and thruster systems. Make the wrong design choice here, and the entire mission fails. Make the right design choice, and you have a system that can run reliably at depth for extended deployments without ever surfacing for a service interval. Polymer bearings for subsea robotics are emerging as the answer for engineers who are finding that conventional metal bearings simply cannot provide the performance needed in the harsh environment beneath the ocean.

Pressure, Saltwater, and Maintenance-Free Operation

Subsea robotics engineers, whether they are designing robots for inspecting oil and gas pipelines or monitoring fish farms, face three specific challenges that conventional bearing materials are not designed to handle simultaneously: hydrostatic pressure, saltwater corrosion, and lubrication.

Hydrostatic pressure increases by roughly one atmosphere for every 10 meters (~32.8 ft) of depth. At 3,000 meters (~9840 ft), which is a routine working depth for deepwater inspection ROVs, the pressure exceeds 300 atmospheres. Metal bearings rely heavily on lubrication films and tight tolerances that can easily be compromised under such loads. Pressure-induced deformation, lubricant displacement, and housing distortion can all cause premature failure.

Saltwater corrosion is another relentless issue that must be addressed for successful bearing performance. Corrosion degrades surface finish, tightens clearances, and eventually seizes rotating assemblies entirely. Marine environments expose bearing surfaces to chloride ions, dissolved oxygen, and biological fouling. Steel bearings corrode; bronze bearings can undergo dezincification; and even stainless alloys remain vulnerable to crevice corrosion in low-oxygen zones.

Zero-maintenance windows may be the most operationally demanding constraint of all. A bearing on an offshore ROV working a deepwater site cannot be pulled, inspected, relubricated, or replaced on a daily basis. Many deployments run for weeks or months between topside recovery. Polymer bearings, such as those made from PEEK or PTFE composites, eliminate the need for periodic lubrication or adjustments, reducing operational costs and downtime, and ensuring continuous mission success because they are self-lubricating.

Polymer Bearings: Material Solutions for Subsea Demands

Bearing-grade polymer materials address all three of these challenges — pressure, saltwater, and maintenance-free operation — often simultaneously. Several proven engineering polymers stand out as solutions that can inspire confidence in subsea robotic applications.

PEEK (Polyether ether ketone) is the workhorse of high-performance polymer bearings in subsea use. It maintains exceptional dimensional stability even under hydrostatic loading, exhibits near-zero water absorption, and resists saltwater, hydraulic fluids, and most chemicals commonly encountered in offshore environments. Furthermore, PEEK bearings operate without external lubrication, drawing instead on the material’s inherently low friction coefficient. For ROV thruster assemblies and joint pivots, PEEK offers a compressive strength that approaches or exceeds many metal alloys while eliminating corrosion.

Filled PTFE successfully extends the lubrication-free advantage further. Unfilled PTFE is too soft for structural bearing applications, but glass-filled, carbon-filled, or bronze-filled PTFE delivers self-lubricating performance with meaningful load capacity and low friction. In slow-rotation or oscillating applications, such as the articulating arms of an inspection ROV, field PTfE bearings provide a smooth, stick-slip-free motion with no maintenance requirement and complete resistance to saltwater attack.

UHMWPE (Ultra-High Molecular Weight Polyethylene) brings outstanding impact resistance and surface toughness to applications where debris ingestion is a risk. Its extremely low coefficient of friction, combined with high chemical resistance and excellent fatigue life in wet environments, makes it well-suited for guide bearings and bushing applications in subsea manipulators and cable management systems.

Finally, Torlon (PAI, Polyamide-imide) is ideal for harsh, high-temperature, and high-load bearing applications where even PEEK approaches its limits. With excellent creep resistance and impressive compressive strength, Torlon-based bearings perform well in compact, high-load joint designs where space constraints are tight, and operating cycles are demanding.

Building Reliability Into Every Depth

The common thread across all of these materials is simple: each one removes a failure mode that metal bearings experience in the subsea environment. No corrosion, no lubrication intervals, dimensional stability under pressure, and compatibility with the full range of subsea fluids and cleaning agents are critical properties that high-performance polymers such as PEEK, filled PTFE, UHMWPE, and Torlon possess. 

For operators running extended deepwater missions, polymer bearings are not merely an alternative to metal bearings; they are the more reliable choice. Lower system weight, reduced drag on thruster assemblies, and the elimination of corrosion-driven maintenance costs all contribute to a lower total cost of ownership over the working life of AUVs and ROVs.

Ready to specify the right bearing material for your subsea robotic application? The bearing experts at Advanced EMC have strong expertise in bearing-grade polymers for even the most demanding marine and offshore environments. Contact Advanced EMC today to discuss your project requirements and get material recommendations backed by real engineering experience.