Enhancing the surface profile can improve sealing capabilities, extending the functionality of aging piping systems in chemical plants.
There are many aged and aging process plants in operation today. In fact, many of the processing plants for power, chemicals, oil, etc., have been in service for more than 50 years. And while the piping itself may remain intact, their bolted flange gasket joints and connections are becoming misaligned, corroded and damaged due to repeated handling, chemical exposure and thermal cycling. This can lead to costly ruptures that may result in millions of dollars in damages, downtime, noncompliance penalties, irreparable environmental impact and litigation.
There is a solution that can extend the life of aging piping systems, preserving their functionality: raising the surface profile on polytetrafluoroethylene (PTFE) gaskets. This design modification can prevent leaks, spills and other releases in chemical processing plants by reducing and managing the contacted area of the gasket, thus achieving and maintaining a strong seal.
A Brief History of Gasket Technology
Traditionally, gasket thickness and sealability always involved a performance tradeoff. One could use 1/16-inch-thick (1.6 millimeter) gaskets when flanges were in good condition, achieving a tight seal with reduced creep.
However, when the flanges had bad or misaligned surfaces, the seal integrity was degraded.
In those instances when the flanges are in poor condition (or if the shape of the flange condition is unknown), one would choose a 1/8-inch-thick (3.2 mm) gasket. The reason? A user does not want to risk installing a thinner gasket and discover that it does not seal properly, which then requires a timely and costly uninstall and reinstall. However, the thicker gaskets do not seal as well as their 1/16-inch counterparts when placed under comparable load. Additionally, with the thicker gaskets, creep is higher, requiring re-torque.
To address the limitations of both gasket options, the ideal gasket should combine the creep resistance of a 1/16-inch gasket with the compressibility and conformability of a 1/8-inch gasket—easier said than done.
Historically, gaskets have not always been forgiving, easy to use or simple to remove. Yet technology has evolved, allowing sealing products to be engineered and designed to optimize the work that is put into them, delivering a tighter, more durable seal.
The approach is one that does not focus on the gasket thickness but rather its surface profile. The results produce gaskets that reduce leaks, spills and other releases from piping systems, including those of aging chemical plants.
Raising the Gasket Profile
The concept of using surface profiling to reduce area and increase stress is found in many products, such as running shoes and car tires. Reducing the contact area while maintaining a given amount of compressive force results in increased stress. In the case of shoes or tires, this stress provides traction. In the case of gaskets, traction or friction between a gasket and a flange face is critical to holding internal pressure. If the downward force created by the fasteners in a flange is evenly spread over a larger area, the created stress contributes to making a seal more effective. This approach enables the aging piping system to maximize its sealing potential.
Impact on Raising Gasket Profile
Surface profiling positively impacts gasket technology in five key areas: compressibility, pressure resistance, scalability, load retention and dimensional flexibility.
Compressibility is a critical functionality of gaskets, as it represents the ability of the gasket to conform to the surfaces that it seals. Adding raised features to the surface of a gasket directly impacts compressibility by reducing the contact area and increasing the resulting stress.
When flange surfaces are worn, pitted or scratched—such as those in aging piping systems in chemical plants—it can be cost prohibitive and nearly impossible to repair/replace the flange to a “good as new” condition. The more compressible the gasket, the better chance of producing an effective seal with the flanges.
To create an effective seal, a gasket must perform two functions:
- Conform to the flange face. This will help prevent the media from passing between itself and the flange faces. This is where compressibility becomes important.
- Resist or prevent permeation. On a microscopic level, most nonmetallic gasket materials have small voids or spaces. The key to preventing media from permeating through these voids or spaces is to close them off through the use of compressive force. This can sometimes be difficult if a gasket is extremely hard, or the surface area of the flanges is relatively large. The use of surface profiling or raised features can be beneficial as the force is concentrated, generating higher densification (improved gasket consolidation), which results in improved permeation resistance. This concentration of the compressive force is exerted at the point where the raised features contact the sealing surfaces/flanges. This results in higher surface stresses and, ultimately, more efficient sealing.
Load Retention & Blowout Resistance
The performance of a gasket is directly related to stress. The more stress that a gasket can retain, the better it will function in the long term. By contrast, the inability to retain stress results in detrimental creep relaxation.
All gaskets experience load loss over time, which underscores the need to use materials with properties of compressibility/conformability, thus allowing the gasket to seal with a given amount of force.
When a flange assembly is pressurized, the internal media pushes in all directions, axially to spread the flanges apart and radially to push the gasket out of the joint. The axial forces of the internal pressure try to spread the flanges apart and push the gasket out of the joint. If the load retention is low due to high/detrimental gasket creep, the axial forces (due to internal pressure) can overcome the forces left in the bolts and the joint will fail. Thinner gaskets creep less, maintaining a higher bolt load retention and better blowout resistance. Thicker gaskets creep more, resulting in lower bolt load retention and making them more susceptible to failure by blowout.
The high-performing gasket should be able to execute a seal regardless of the sealing surface shape or size. Most gaskets with a profiled or raised surface are molded in a specific shape or size that requires tooling or has a size limitation. Since there are countless shapes, sizes and configurations of sealing surfaces in pumps, valves, mixers, reactors, etc., the best solution is a sheet gasket that can easily be cut into various shapes.
Raising the surface profile of a PTFE gasket eliminates the shortcomings inherent in both 1/16-inch- and 1/8-inch-thick gaskets, improving pressure resistance, resisting permeation, optimizing load retention and enhancing dimensional flexibility. The result? A better performing gasket that allows chemical plants, with their aging piping systems, to reduce the risk of leaks, spills and other releases.
Can you afford anything less?
The original article can be found on the Pumps & Systems website and was written by Jim Drago, Principal Applications Engineer at Garlock.