In the pulp and paper industry, there are numerous components that make the operation work. Understandably, the larger, more expensive pieces of equipment tend to get the most attention. However, when considering the potential challenges and issues, users can succeed by paying attention to more minor, but still crucial, components such as gaskets and seals.
When considering the systems and functions of a pulp and paper operation, it is easy to overlook low-cost, high-consequence sealing products. The failure of a gasket that only costs a few dollars can trigger serious issues, like loss of process efficiency, product loss, injury to workers or unplanned outages and downtime. This article will take a closer look at how gasket selection for any flanged connection—whether a small pipe flange, a heat exchanger, or even the critical digester—can positively or negatively impact how well the various systems and the mill perform as a cohesive unit.
Often, the gasket is considered a lower priority. The function of a gasket is to seal two surfaces and prevent the release of process fluids or the intrusion of atmospheric contaminants. In a sense, it is the softest, weakest part of the assembly or system, yet no less critical than the flanges or bolts themselves.
The first step to ensuring that the gasket selected is the right choice for a service is to remember an acronym: STAMP (size, temperature, application, media and pressure).
Water is a critical ingredient for any pulp and paper mill, and in recent years water conservation has become a key metric for mill success. Commonplace drips that used to be acceptable are now considered an opportunity for cost savings. Also, with the amount of water moving through these mills and being returned to water sources like rivers and lakes, users need to ensure the water is not contaminated by process chemicals, equipment lubricants or any other non-eco-friendly manufacturing byproducts.
Leaks are often attributed to flanges that are damaged or worn from years of use. The best way to address this issue is to consider a gasket that offers improved compressibility or conformability. This can be accomplished by using softer materials like elastomers or, if chemicals are involved, highly compressible polytetrafluoroethylene (PTFE) products. Another way to address uneven or worn surfaces is by using products that offer more conformability through engineered surface features that reduce the contact area on the gaskets and optimize the force being applied from the bolts in the assembly.
Lower load sealing products can be more forgiving from an installation perspective and can help to compensate for less than desirable bolt loads.
Many nuisance leaks can also be attributed to vegetable or cork fiber gaskets commonly found in gearboxes, inspection covers and pump casings. Vegetable fiber gasketing’s flaw is that it will eventually leak no matter what users do. Since these products are made from plant fiber, consider how a plant takes water from the ground to the top of the plant through tiny “tubes” called the xylem and phloem. When these “tubes” are chopped up to create the sheet material, gaskets end up with millions of tiny leak paths that will eventually allow water, oil or other media to pass through.
As with most things, gasketing technologies continue to evolve. New sealing solutions are designed to eliminate these annoying and costly leaks for minimal additional cost. These products are often designed to be more compressible, which is necessary for gearboxes, covers and casings that tend to be lighter weight castings and/or sheet metal. Improved compressibility, in combination with engineered controlled swell properties, has resulted in gasketing materials that are designed to absorb a small amount of water or oil from the process fluids, expand slightly and create a tighter seal. While the traditional vegetable fiber material may come at a lower cost, it is important to look beyond and consider the cost of the fluids being lost, the slip and fall safety concerns as well as the associated cost of those leaking fluids that find their way into drains where remediation becomes necessary.
Liquors & Other Process Chemicals
Compatibility on the pulping side of the mill is essential as well. Having products that are chemically resistant to process fluids that could potentially be caustic or acidic is critical based on the variety of liquors used in the processing of wood fiber into pulp used to make paper products. When selecting a product for liquors and caustic solutions, it is important to consider the resistance of all components in the gasket material.
In the late 1980s and early ’90s, many mills attempted to use compressed fiber gasket products with ethylene propylene diene monomer (EPDM) binders for strong caustic and liquor service. Unfortunately, many reported premature failures such as leaks and blowouts because the gasket was not as appropriate as originally thought. While the EPDM portion of the gasket was suitable for the caustic and liquor, that only represented around 10% to 20% of the product composition. The remaining fiber and clay fillers were not resistant to the caustic. In addition, these EPDM bonded sheets were used in other areas, such as the turpentine/tall oil recovery systems. In this case, the opposite was true. The fiber and fillers were fine, but the EPDM binder was degraded by the hydrocarbons.
PTFE-based gasket materials have shown to be a good solution for the chemical and liquor-containing processes found in pulp mills. The inherent creep characteristics of PTFE had to be addressed with inorganic fillers such as barium sulfate. Furthermore, specialized processing techniques create PTFE products that are not only chemically resistant but also able to perform long term under less-than-ideal installation conditions and severe service conditions. Using the previously mentioned surface profiling optimizes the gasket stress through a reduced contact area. This is another way to create a more reliable, chemically resistant sealing solution with improved load retention (reduced creep) and compressibility and conformability for older, worn flange surfaces.
Flange Design Challenges
Today it is more common for older metal piping systems and tanks to be replaced with nonmetallic components made of polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) and fiber reinforced plastics (FRP). Not subject to corrosion like traditional steel components, these products typically offer improved service life. Lightweight by design, such products tend to be easier to handle and install while offering cost savings over more exotic alloys. However, it is important to recognize that a fiber or traditional PTFE gasket that was used in the previous steel/metallic systems is not likely to be a good fit. Most nonmetallic flanges and vessels are designed for elastomers (typically 50 to 70 durometer Shore A). This can be a learning curve, as selecting an elastomer is more than just picking the right polymer. The phrase “you get what you pay for” is true with elastomeric sheet and gasketing products. Many of the products found in the market today are lower-grade materials that are blended with cheaper polymers and fillers to reduce the cost. Many elastomeric products can be manufactured with a cheapened process, having a major impact on the material properties that are needed to handle the service conditions.
In cases where the service conditions and/or chemicals exclude the more readily available polymers (e.g., EPDM, neoprene, fluorocarbon-based fluoroelastomer [FKM], etc.), engineered PTFE solutions with reduced contact area are often an option. This turns relatively low allowable torques found in nonmetallic flanges into higher applied stresses by reducing the contact area while still properly supporting the flanges and preventing excess bending or rotation.
Regardless of industry, every flanged connection, whether in piping, a tank, a heat exchanger or other equipment, is a small system. The success of that system is based on all components working together correctly. While a gasket, bolt or similar component may seem like a small detail, ask the question: “If one of these items was to malfunction, what impact would that have on the entire process?” This thought process will help users take a comprehensive approach in selecting the right products for any operation.