Gaskets in your piping and process-equipment systems are key factors in your plant’s ability to meet production goals, ensure process safety and comply with environmental regs.
How easily and quickly you install and maintain these vital components is critical both to their success and yours.
It has been one of industry’s stickiest problems: Flange adhesion by many types of gaskets has posed a continual challenge for operations and maintenance personnel. The often-difficult task of separating flanges is just one part of the problem; removing their adhering gasket material without damaging the flanges is another, possibly tougher one.
While solvent-based gasket removers can be effective, they also can present health and safety issues. Thus, many plants turn to wire brushes or wheels, which, if improperly applied, can damage flanges on pipes and process equipment. Just try to imagine what could happen to your operations if particles loosened by a wire brush were to contaminate a mission-critical piping system at your facility…
The mechanism of adhesion
Many theories have been put forth to explain the mechanism of adhesion. To date, though, no single theory has been able to address the issue in a comprehensive way. What we do know for sure is that the bonding of an adhesive material to another surface is the result of a sum of mechanical, physical and chemical forces that overlap, interact and influence one another.
As detailed here, the prevailing theories include adsorption, mechanical interlocking, chemisorption and electrostatic attraction.
- The adsorption theory assumes that when an adhesive material is applied, it spreads spontaneously, “wetting” the adherent surface. For this to occur, the surface tension of the adhering material must be lower than the surface free energy of the adherent. Adhesive strength results from intimate contact between the adhesive and adherent through secondary intermolecular forces at the interface, collectively known as van der Waals forces. These include dipole-dipole forces, dispersion forces and hydrogen bonding.
- The mechanical interlocking theory is based on the fact that at the microscopic level all surfaces are rough, consisting of crevices, cracks and pores. Bonding occurs when the adhesive penetrates or surrounds these irregularities and hardens.
- The theory of chemisorption—like that of adsorption—is based on adhesive “wetting” of an adherent, resulting in the formation of ionic, covalent or metallic chemical bonds. These are much stronger bonds than those produced by van der Waals forces.
- The electrostatic theory posits the formation of a double electrical zone at the adhesive-adherent interface, where the transfer of electrons creates positive and negative charges that attract one another.
It has been determined empirically that minimal flange adhesion occurs with flexible graphite gaskets, since they consist of interlocked “worms” of exfoliated flakes with no organic binders. In fact, flake graphite is often used as an anti-stick coating for other types of gaskets. It has also been established that PTFE-based gasketing is subject to only minimal flange adhesion.
The well-known non-stick properties of PTFE are the result of its low surface energy. Like flexible graphite, it too is often used as an anti-stick coating for other gasketing compositions.
The problem of flange adhesion is largely associated with the use of compressed fiber gasketing. While there are many available compositions, they all contain organic rubber binders. The extent of curing and degree of cross-linking between these binders are typically lower than that of homogeneous rubber gaskets. The result is a softer, less cross-linked rubber that allows the gasket to conform to the flange and create a seal. Under heat and pressure, however, the binder flows out and wets the flange, allowing adsorption, chemisorption and mechanical interlocking to occur. These forces can be quite high, resulting in flange adhesion.
Preventing the problem
Flange adhesion can be mitigated by coating gaskets with a low-surface-tension semisolid or liquid, e.g. PTFE, silicone, a platy solid or an anti-seize compound, to prevent the binder from wetting out on the flange. Anti-seize compounds vary in composition but typically consist of metal particles in a petroleum-based carrier with other additives. These compounds are not recommended for three reasons: Under heat and pressure, the metal particles can adhere to the flange, distorting the facing, filling in the serrations and eventually rendering the gasket incapable of sealing.
Coating gaskets with anti-seize compounds can cause problems as the gaskets are compressed. A lubricated gasket not only has a tendency to extrude and split, but also can be forced out of the flange by internal pressures and lack of friction. In addition, the petroleum in anti-seize compounds can attack and soften some gasketing.
Although the use of silicone anti-stick agents can be effective, they can contaminate system media. If they come into contact with paint or photographic chemicals, for example, these agents can cause a lack of adhesion. For this reason, the use of silicone is excluded from many gasketing applications.
PTFE-based anti-stick agents can be effective, but lack thermal stability. PTFE begins to decompose above 500 F—well below the maximum service temperatures of most compressed fiber gasketing. In addition, hazardous and corrosive halogenated byproducts can be formed during decomposition.
A better solution is to use an inorganic material, such as talc, mica, vermiculite or graphite, to prevent the binder from wetting out on the flange. Due to their layered crystal structure, these materials are effective as anti-stick agents, cleaving to form thin sheets which, when milled, result in flat, plate-like structures. These structures allow the particles to form a laminar barrier on the surface of the gasket.
However, these naturally occurring substances vary in their morphology, degrees of purity and levels of undesirables. Graphite is also an electrical conductor that can result in severe galvanic corrosion in wet or humid environments. Wetted by seawater, graphite gaskets can cause rapid localized attack of most stainless-steel alloys, and at elevated temperatures they can carburize some stainless and nickel alloys, making them more susceptible to intergranular corrosion.
Developing an advanced solution
For end users seeking a solution to flange adhesion problems around their operations, an advanced anti-stick agent is now available (see Sidebar, pg. 19). It has the desirable flake morphology—but without some of the undesirable characteristics of mined materials. The particles used in the new gasket coating are synthesized from refined materials under highly controlled conditions, resulting in a uniform, high-purity material. Because these particles are extremely compliant, they stack well to create a continuous barrier and enhance gasket sealability. Unlike graphite, the particles are non-conductive and contain extremely low levels of potentially corrosive halogens and sulfur compounds.
With an oxidation threshold of approximately 800 C, the particles are thermally stable and have unusually high chemical stability. This thermal and chemical stability contributes to their non-toxicity, making them more environmentally friendly than other anti-stick compounds. Relative to exposure, there are no specific limits for these particles; they are classified simply as “nuisance dust.” The particles are not classified as being carcinogenic to humans by any of the relevant agencies, and there are no regulations regarding their use, transport or disposal.
A stable dispersion of the optimized coating formulation was applied uniformly to sample gaskets for testing against both uncoated gaskets and gaskets coated with other anti-stick compounds. Among the factors evaluated were crush and blowout resistance, sealability and adhesion.
Testing the new coating
ASTM Test Method F607 provides a means of determining the degree to which gasket materials under heat and compressive load adhere to metal surfaces. The two-part test not only measures the force required to separate the flanges, but also evaluates the amount of residual gasket material left on them after the test. Compressed fiber gaskets with Neoprene binders were tested at 400 F (204 C) for 22 hours. Predictably, the rubber binder and fragments of synthetic fibers from the uncoated gasket adhered to the flange (Fig. 1), compared with only traces of the coating and ink from the printed side of the gasket coated with the new anti-stick agent (Fig. 1a).
In addition to preventing flange adhesion, it was imperative that the new coating not adversely affect gasket functionality and performance.
Crush and blowout resistance…
Crush resistance is particularly vulnerable to the use of the wrong type of anti-stick coating. Crushed gaskets deform laterally toward their IDs and/or ODs when they cannot accommodate more compression.
The friction between gaskets and flanges has a major impact on the amount of stress that can be applied before the gaskets begin to split apart. To the extent that a coating affects that friction, it can reduce the crush resistance of a gasket. Unfortunately, bolt lubricants are commonly applied to gaskets, causing them to crush and split or blowout.
In compression testing, a gasket material treated with a copper anti-seize coating underwent a sudden reduction in thickness at a stress of approximately 17,000 psi, while the same material with the new anti-stick coating withstood 30,000 psi without crushing.
Surface coatings also can affect blowout resistance, or the maximum internal pressure that a flanged joint can hold before gross leakage and/or gasket rupture. As with crush resistance, the friction between a non-metallic gasket and flange is the determining factor in a joint’s pressure capability.
Blowout tests were conducted on a gasket treated with the new anti-stick agent, an uncoated gasket and an uncoated gasket on a flange with talc. The tests were run in two-inch 2500# raised-face flanges, heated to 1000 F (538 C) and pressurized until the joint leaked or the gasket ruptured. Test results (see Fig. 2) showed that the new anti-stick coating did not adversely affect the gasket’s pressure resistance.
Sealability also was studied, with the uncoated values serving as the baseline for comparison. Gasket sealability was determined using ASTM F 37 for fuel A and nitrogen tests. Fuel A tests were done at a compressive load of 500 psi and an internal pressure of 9.8 psig, while nitrogen tests were conducted at 3000 psi and 30 psig respectively. Test results indicate the new anti-stick coating had little to no effect on how well the gaskets sealed.
In evaluating the adhesion properties of the gaskets, the same array of coated and uncoated materials was tested using ASTM F 607 methods. Test platens were assembled with a two-square-inch gasket at a stress of 3000 psi and heated in an oven at 212 F and 400 F. The force required to separate the platens was measured, quantifying the degree to which the gaskets adhered to them. The sample gasket with the new anti-stick coating exhibited dramatically lower separation stresses than the other treated and untreated test materials.
As noted, the residue left on a flange face is just as important as separation stress. In some cases, entire gaskets can be stuck. In others, a thin film is left in the serrations. The new anti-stick coating minimized both separation stress and residue left on the flange. The carbon fiber gasket treated with the new anti-stick coating (Fig. 3) was easily removed by hand; the untreated carbon fiber gasket (Fig. 4) and the vermiculite gasket (Fig. 5) were completely adhered to the flange.
The extensive research and testing that has gone into developing this new anti-stick coating confirms that the binders in compressed fiber gaskets act as visco-elastic materials that tend to flow at elevated temperatures and pressures. As these binders “wet out” and make contact with the face of a metallic flange, chemical adhesion, mechanical interlocking and other modes of adhesion occur. This fact has enormous implications for plant maintenance. That’s because removing such gaskets can be a tedious, labor-intensive and costly task that can damage equipment.
The coating referenced in this article acts as a barrier that prevents binders from wetting out, thereby making gaskets easier to remove. Moreover, gaskets treated with the new anti-stick coating reduce the potential for residual particles to adversely affect the performance of replacement gaskets or to break loose, contaminate piping systems and impair the operation of downstream equipment such as pumps and valves. Because they can be removed intact, the gaskets are also easier to dispose of properly. MT
Mike McNally is a senior chemist, Howard Lockhart is an applications engineer and Dave Burgess is a senior applications engineer with Garlock, in Palmyra, NY. For more information, e-mail email@example.com.
Garlock Sealing Technologies has recently moved to establish a trust to pay current and future asbestos claims under Section 524(g) of the U.S. Bankruptcy Code. The company ceased production of all compressed sheet gaskets containing asbestos in December 2000. For more details, visit www.enproindustries.com.