Seals in the oil and gas industry face some of the most intense challenges, HPHT (High Pressure, High Temperature) and corrosive media. Trying to find the right rotary shaft seal for such an application can be challenging but is far from impossible.
Introduction to Oil and Gas Seals
Seals are needed in many different areas of the petrochemical industry. They can be found in compressors, motors, gearboxes, pumps, top drives, and steam turbines. Seals are required for mixers, extruders, fans, cooling towers, and blowers. Swivel stacks, wellhead connectors, subsea connectors, and rotary drill bits are also in need of reliable seals to do their job safely and effectively.
And whether these seals are for upstream, midstream, or downstream petrochemical processing, they often serve in a critical application: when a seal in the oil and gas industry fails, it is more than an inconvenience. Seals failures can quickly prove devastating to personnel, the equipment, and the environment. And the costs of seal failure can easily extend beyond equipment downtime and repair costs, as there may be fines related to environmental damage as well as costly lawsuits and out-of-court settlements involved.
With all that said, it can be agreed that seals for rotary shaft applications must be especially reliable, extremely rugged, and able to provide excellent performance in some of the most hostile operating environments in existence.
Spring-Energized Seals
Spring-energized seals include a spring energizer that keeps the seal lip in contact with the sealing surface when other seals would fail. They can maintain a seal even in the presence of wear, out of roundness, eccentricity, and runout. In addition, spring-energized seals outperform traditional lip seals in extreme pressures (including vacuum pressures) and extreme temperatures (including cryogenic temperatures).
Most oil and gas applications using spring-energized seals take advantage of canted coil springs and their ability to provide a consistent load on the seal lip over a wide deflection range. In addition, canted coil springs are also highly resistant to damage unless they are stretched upon installation.
Labyrinth Seals
Labyrinth seals do an excellent job of sealing and are very effective at contaminant exclusion. They accomplish this through a barrier with a complicated set of maze-like paths (hence the term “labyrinth”) that (1) restricting the clearance through which leaks can occur and (2) creates areas of turbulent flow and high flow friction to further prevent leaks and contamination. They are considered a dynamic mechanical clearance seal.
One of the benefits of labyrinth seals is their ability to provide sealing without making contact with the sealing surface. This means they experience far less wear than traditional lip seals and generate less frictional losses, as well. They are highly efficient, extremely reliable, and are even easy to install.
Seal Materials
Rotary shaft seals can be exposed to highly corrosive chemicals including methanol, H2S, aromatic hydrocarbons, oil, and supercritical CO2. Many of the chemicals that seals must interact with are also highly flammable gases and liquids. Operating temperatures can range from cryogenic below zero temperatures all the way up to 800°F, depending on the application and media involved. To complicate matters further, some liquid products are handled near their vapor temperature where they can unexpectedly flash into vapor. In addition, the materials used need to be resistant to chemical permeation while also being extremely wear-resistant and flameproof.
Two options typically lie at the top of the list for seal jacket materials: PEEK and PTFE. Both of these high-performance polymer materials are extremely compatible with a wide range of chemicals. They offer excellent performance in the HPHT environment found in most oil and gas applications: PEEK has a maximum operating temperature of 500°F and PTFE can handle temperatures up to 575°F. They also perform well in below-zero cryogenic temperatures, such as those used with LNG.
These engineering polymers are also wear-resistant and flame resistant with low coefficients of friction and thermal expansion. In addition, they are dry-running so no additional lubricants are required. And PEEK in particular offers excellent performance even in sour gas environments, which often destroys other seal materials.
For spring-energized seals, both PEEK and PTFE are excellent choices. For labyrinth seals, however, PEEK or Flourosint (enhanced PTFE) usually performs more effectively. And keep in mind that these materials can be enhanced with fillers to increase their mechanical properties, including stiffness, strength, and wear resistance.
Conclusion
When traditional rotary shaft lip seals fail in oil and gas operations, consideration should be given to spring-energized seals and labyrinth seals combined with PTFE, PEEK, or enhanced PTFE. This combination of sealing technology and engineering polymers can provide the reliable performance needed in critical applications where seal failure is simply not an acceptable option. Both types of seals and the materials described here have proven performance records for their ability to handle the rugged, corrosive, HPHT environments of the petrochemical industry.
