Fiber Reinforced Plastic (FRP) pipes and flanges are increasingly used in the oil and gas industry where metal is simply too heavy and expensive. In addition to cost pressure, the need for lightweight chemically resistant materials are also driving the use of fiberglass pipes and flanges. Innovations in FRP flange design coupled with improvements in manufacturing technology have allowed FRP piping to be used in even more demanding applications. However, these demanding applications have added challenges for sealing the bolted flange connections.
More aggressive media and higher internal pressures have pushed the limits of the commonly used rubber gasketing materials, such as Nitrile Butadiene Rubber (NBR), Ethylene Propylene Diene Monomer (EPDM), or neoprene. Obtaining a reliable seal in FRP flanges using conventional gasketing materials has become more difficult.
Fiberglass pipes are generally known to have strength limits and a lower pressure resistance, making sealing fiberglass flanges difficult. Gore solved this problem with its patented expanded polytetrafluoroethylene (ePTFE) gasket, specifically designed to seal flanges at low stresses. This solution was successfully demonstrated in a multistage testing procedure conducted in cooperation with a globally leading manufacturer of anti-corrosive fiberglass pipe systems. Continue reading GORE Universal Pipe Gaskets Have Great Success in Aggressive Media Applications→
The connections to shell and tube heat exchangers pose immense challenges for the seals that are used because of both the chemically aggressive media and the frequent temperature load changes. Despite its excellent chemical resistance,polytetrafluoroethylene (PTFE) is not typically suitable as a sealing material in this case because the creep tendency of this material jeopardizes a reliable seal.
Shell and tube bundle heat exchangers generally include not only several connection pieces but also a shell cover flange with a significantly larger nominal diameter.The shell cover flange creates the seal to the tube bundle flange, which then seals the shell flange. These connections are subjected to the full operating pres-sure and test pressure of dozens of bar and seal it from the environment. Oftentimes the tube bundles are arranged in several passes that are channeled by the use of partition plates. This requires a seal at the head of the heat exchanger and between the tube passes at the partition plates where the pressure difference is considerably lower. The numerous potential different arrangement of the passes requires specially adapted seal designs. The sealing challenges that pertain to the shell cover flange position can be attributed to a combination of different conditions:
Temperature load changes caused by starting up and shutting down part of the process
Complex sealing geometries including separators, often combined with large nominal diameters
Chemically aggressive media
Damaged sealing surfaces, due to corrosion or warping
The economic necessity of minimizing downtime
The requirement to document legal compliance
“Creeping” gasket tapes made of PTFE
Gasket tapes are occasionally used to seal the static connections to shell and tube heat exchangers — sealing materials with a defined width and thickness, but an undefined length. They offer a way of functionally connecting the junction points and can be shaped as desired at the point of installation. Since the gasket circumference is completed at the time of installation, it is no longer necessary to completely dismantle the heat exchanger, removal of the tube bundle, for example. It is only important that the sealing surfaces be sufficiently accessible. Gasket tapes have been around in many different industrial fields for several decades. By introducing the Gore Joint Sealant, Gore launched the first gasket tape made of expanded PTFE (ePTFE) back in 1971. However, multiaxially expanded PTFE technology finally made it easier to use ePTFE with shell and tube heat exchangers across a broader range of application conditions.
The need for these material performance improvements by Gore is the low creep resistance of PTFE. In molecular terms, PTFE consists of a chain of carbon atoms that is saturated with fluorine atoms. The strong covalent carbon-fluorine bond is responsible for this polymer’s nearly inert chemical reaction behavior. This explains why this material is desired as a sealing material in aggressive media. However, the lack of reactivity also means that PTFE chains cannot be molecularly linked like other materials, elastomers for example, and this results in a profound cold and warm flow behavior, also known as “creeping”.
Risk of deformation of the sealing material
Creeping refers to the mechanical deformation of a component, in this case the seal, when subjected to loads and temperatures. One reason why seals in flange connections function is because the surface pressure caused by the bolts allows for irregularities in the sealing surfaces to be filled up and for blocking of leakage channels inside the seal. This reduces leakage (depending on the behavior of the sealing material) to an acceptable level. In this sense, leakage refers to the admissible substance flow, irrespective of the prevailing mechanism, in other words the sum of all mass flows from Fick’s law of diffusion to gross leakage.
This sealing function is severely impaired by the creep tendency. Inside a flange connection, creep of a sealing material takes place in such a way that the load is reduced at a certain temperature until a balance between the internal strength of the seal and the external load is achieved. This behavior is called creep resistance. Deformation takes place when equilibrium is reached. With a seal, this is a reduction in the thickness that reduces the bolt elongation and thus the preload. This ultimately means a loss of strength or surface pressure that depends on the stiffness of the sealing system with respect to its extent. This surface pressure loss is called “creep relaxation” and can be so great that the bolts are loosened completely. Creep and the respective relaxation mainly occur during the heating up and cooling phases, in other words during thermal cycling. These effects occur to a significantly lower degree with a constant temperature. Creep relaxation is the main reason why simple PTFE (often brought into shape by sintering) can hardly be used in heat exchangers, which by their very nature are often subjected to significant temperature load changes.
Increasing the creep resistance through expansion
There are different ways to reduce the creep resistance of PTFE. One way is to add filler materials. This technique is only appropriate for use with gasket tapes to a limited extent due to the fact that the flexibility of filled PTFE is extremely limited. Strips of simple PTFE are also not very useful for the same reason because it is difficult to form them into the required shape at the time of installation. Expanding the PTFE material offers an effective solution. This creates a microporous structure through mechanical stretching. This structure causes a significant change in properties. On the one hand, it improves the conformability due to the higher compressibility as well as creep resistance on the other hand. Here, it is important for the expansion to take place as evenly as possible. This is crucial to ensuring that the sealing properties are and remain the same at all positions of the seal. It can still have a texture, however, although it should not extend throughout the entire seal (in other words for the entire length of a band). In fact, a controlled transverse texture (with a circular transfer radially) can have a positive effect on deployability and bending stiffness. While such a structure changes the sealing properties (the transverse strength, for example), this can be taken into consideration structurally. Expansion allows for tapes to be produced that are flexible enough to be laid in even narrow radii.
The following properties are characteristic of gasket tapes made of ePTFE:
Easy and fast adaptation to suit even complex flange geometries
Design flexibility because of the various widths, which then allow for higher surface pressures (higher tightness classes)
High creep resistance compared to other PTFE-based materials
Excellent adaptability to flange irregularities
Not reusable (due to the high compression and low spring back
Non-biodegradable, do not age, UV-resistant
Resistant to all types of media (pH 0 to 14)
Long shelf life, limited only by the declining bonding strength of the mounting aid adhesive tape.
Mounting of ePTFE gasket tapes
ePTFE gasket tapes are mounted on one of the flat sides of the tape using adhesive tape. The adhesive only serves as a mounting aid and has little or no impact on the sealing effect. The adhesive is also considerably less temperature- and chemical-resistant than ePTFE. This means it usually decomposes during operation. This makes it easier to remove later on. When closing the seal made of an ePTFE gasket tape, one must make sure that the ends overlap and the right method is used. Basically, there are two different types of ePTFE gasket tape materials that require different overlapping methods: monoaxially and multiaxially ePTFE.
Monoaxial expansion is the older technique used to produce ePTFE gasket tapes. With this method, the PTFE is stretched to the tape only longitudinally. The texture that this creates is responsible for the weaker transverse strength. This results in greater broadening during initial compression, an aspect that must be taken into consideration in the design. Broadening of 20 % and more is quite common. The advantage that monoaxially expanded gasket tapes offer is that they can be compressed to be very thin during installation. This means they are able to adjust to micro unevenness very easily. Macro unevenness on the other hand is quite difficult to compensate for with this type of gasket tapes. Furthermore, unlike with multiaxial expansion, the ends can be overlapped. Nevertheless, monoaxially expanded gasket tapes are used only very rarely in heat exchanger applications because the lack of transverse strength results in a higher creep tendency. Although the small installation thickness compensates for this to some degree, the performance gap to multiaxially expanded PTFE gasket tapes increases as temperatures rise.
The multiaxial expansion of the material takes place in the direction of two vectors that stand at right angles to each other. This results in a multiaxial structure, in other words an orientation of the emerging fibrils in all directions. The transverse texture can be produced by varying the intensity of the expansion. Among other things, it helps to keep the nodes small because this non-oriented material contributes to creeping. Multiaxial expansion is of particular advantage in that it offers high transverse strength and allows for gasket tapes that are particularly resistant to creep.
Multiaxially expanded PTFE gasket tapes seal reliably
The process of expansion compensates for the disadvantage of the PTFE material, its high creep tendency, and results in a chemically stable, creep-resistant sealing material. If expansion takes place multiaxially, the creep tendency of the material will be reduced to such an extent that it can then be used as a sealing material under extreme conditions of both chemically aggressive media and frequent major temperature load
The original article was written by Christian Wimmer, Product Specialist at W. L. Gore & Associates and was featured in PROCESS Worldwide.
A newly developed gasket tape made by Gore – of expanded polytetrafluorethylene (ePTFE) is specifically designed to address the challenges of creating reliable seals in large glass-lined steel equipment.
Equipment made of glass-lined steel is used when manufacturing or processing aggressive chemicals such as aniline derivatives and sulphuric or hydrochloric acid. The Achilles heel of such systems is the gaskets needed to seal the joints between components. Exposure to aggressive media causes the seals to degrade overtime, resulting in damage to equipment and posing a health risk to operators. Replacing the seals costs a great deal of time and effort, with a corresponding drop in production output.
A newly developed gasket tape made of ePTFE (expanded polytetrafluorethylene) is specifically designed to address the challenges of creating reliable seals in large glass-lined steel equipment.
Operators of chemical plants choose sealing materials according to a wide range of criteria such as process medium, flange type, sealing performance, pressure and heat resistance, cost and longevity. Other important selection criteria include time required for installation and inventory management. And, of course, a plant operations prior gasket experience weighs in as well. Gaskets for glass-lined-steel equipment are safety-relevant parts because their failure can endanger human lives and/or harm the environment, but they are often treated for administrative purposes as C-class items, that is, parts of minor significance.
This classification doesn’t reflect the true importance of these sealants. There is a need for more explicit regulations to supplement the general legislation pertaining to occupational health and safety and the handling of hazardous substances. The introduction of a European-wide regulatory basis for establishing detailed, standard processes would be welcome, for instance with respect to approval procedures and safety. As things stand today, companies are obliged to find their own compromise between varying sets of requirements. These include compliance with EU-wide and national directives concerning environmental protection and occupational health and safety. At the same time, companies are making efforts to augment the reliability of their products, simplify inventory management and installation processes, and reduce downtime and overall costs. An added factor in both cases is specific process requirements with respect to temperature, pressure and media.
One particular challenge is that of choosing the right sealant for glass-lined steel systems, because these involve the use of aggressive media such as aniline derivatives and sulphuric or hydrochloric acid under demanding conditions. Glass-lined steel presents the advantage of being highly resistant to corrosive and/or abrasive media. Other characteristic features of this material are its smooth surface, which is easy to clean due to its low adhesion properties, and its biologic and catalytic inert behaviour. Nonetheless, it can be difficult to achieve reliable seals in glass-lined steel equipment. This is because the glass lining is more brittle than the metal, and can therefore split or splinter if handled incorrectly. As a result, the gasket load that can be applied to the seal is lower than that for an all-steel flange. Consequently, care must be taken to limit the pressure applied when installing gaskets between interconnecting parts of the system.
Another problem is that of achieving a reliable seal if the flange surface is uneven or has surface deviations. Once the glass lining has fused, its surface cannot be reworked. The challenges posed by these characteristics of glass-lined steel, combined with the exposure to aggressive chemicals and high temperatures, must be met by the chosen sealant. In practice, these difficult conditions often lead to premature sealing failure and a greater risk of corrosion. The further consequences of sealing failure include leaks and uncontrolled emissions, damage to equipment, high replacement and repair costs, production losses, unplanned maintenance and downtime, and potential risks to employees’ health and safety.
Since 1958, Gore has developed products that improve lives. At the center of these solutions is polytetrafluoroethylene (PTFE), a polymer with exceptional properties like high tensile strength, a low dielectric constant, UV resistance and many more. In 1969, the possibilities for PTFE expanded with Bob Gore’s discovery of expanded PTFE, or ePTFE.
In the years since, Gore has developed unparalleled expertise in manipulating ePTFE and other fluoropolymers. Gore’s engineers can change a material’s structure, shape, thickness and surface geometry, then pair it with complementary materials to provide the performance qualities required by the application and the customer. The resulting product can be strong or permeable, rigid or flexible, thin or thick — with many additional combinations of properties that can be applied to meet the end use requirements.
Since its very founding, Gore has been passionate about solving the complex challenges of their global customers. From the first suggestion of a product need, to its delivery to market, this passion is apparent in everything Gore does.
Curiosity: From keeping water off a person’s skin to preventing leaks from happening in chemical containers, Gore listens to their customers and analyze the challenges to determine the underlying problem.
Competency: Gore determines how they can apply their expertise in fluoropolymer science to deliver solutions that are valued and differentiated from the competition.
Commitment: Gore rigorously tests their products to ensure they deliver failure-free performance and suit their customers’ needs and applications, the first time and every time.
Looking to consolidate your inventory into as few materials or parts as possible? Would eliminating the installation of incorrect gasketing material interest you? GORE® Universal Pipe Gaskets may be the answer.
GORE® Universal Pipe Gaskets (Style 800) provide a reliable seal for steel, glass-lined steel and fiber reinforced plastic (FRP) flanges, in the full spectrum of strong acid, alkali, and solvent process media, including the most challenging thermal cycling and elevated temperature applications. This single gasket solution can reduce the process safety and production downtime risks caused by the use of an incorrect gasket material. The highly conformable 100% ePTFE also reliably seals irregular surfaces.
GORE® Gasket Tape is a 100% ePTFE form-in-place gasket that delivers worry-free sealing performance for large steel flanges (Series 500) and large glass-lined steel equipment (Series 1000). While this product has a number of advantages, GORE® has now made their gasket tape line even easier to use.
Improved installation-aid adhesive.
GORE® has upgraded the removable adhesive backer on GORE® Gasket Tape Series 500 and Series 1000 to a new higher strength material. This ensures that when the backer is removed at installation, it peels away in one easy-to-remove piece.