Material validation is a critical step when designing a high-performance expansion joint. Therefore, it is important to know what is on the inside of the expansion joint and not just the outside. There are numerous issues that could arise from improper material selection and layering construction that will ultimately affect the life and performance of the expansion joint. Some of the key issues users will see are delamination between expansion joint layers, leaks at the flange, low pressure and vacuum ratings.
Delamination
Expansion joints are built by layering varied materials on top of each other. Each material selected, and its placement in the expansion joint’s build-up, is critical. Testing and understanding the individual material properties is important for both media selection and overall expansion joint performance.
Even with the best materials selected, the performance of the expansion joint could be poor if material compatibility and testing are not performed between each layer. Oftentimes, this leads to delamination and weak spots. Delamination is the separation of layers within the expansion joint. This often occurs due to failure of an adhesive or other bonding agent.
For practical application in the field, the expansion joint would experience the conditions of the operating system, including line pressure and temperature. Delamination can occur if the bonding between the two dissimilar layers in the expansion joint is not properly and carefully tuned. The joint illustrated in Image 1 demonstrates layer delamination after exposure to water at 225 pounds per square inch (psi) line pressure and 225 F.
Leaks at Flange
When selecting an expansion joint, one should consider certain design factors to reduce potential installation issues and maximize anticipated equipment up time. The first consideration is the amount of rubber and fabric a manufacturer places into the flange. Image 2a depicts the flange of a performance expansion joint, while Image 2b shows a low-cost one. As seen in the picture, the performance expansion joint is manufactured with multiple flange plies. This increases sealability and reduces bolt load creep. Conversely, low-cost joints will use less fabric and more rubber to save money. While less expensive to manufacture, adding less fabric within an expansion joint flange decreases the expansion joint’s ability to create and hold an effective seal and allows the rubber to creep and loosen the bolts. Over time, a flange with minimal fabric will lose its ability to hold a seal and the joint will begin to leak.
Cycle Life Ratings
One of the most important ratings for an expansion joint is its cycle life rating. An expansion joint can provide excellent movement, burst and vacuum ratings, but if it cannot withstand time and regular use, the end user is left dealing with frequent replacements, safety concerns and plant shutdowns. Having a high-performance expansion joint with a rigorously tested and validated cycle life allows the user to have confidence in the expansion joint’s actual performance, as opposed to only having theoretical values.
Despite efforts to enhance the material bonding between the layers to address system conditions, the joint may still fail over time due to the continuous expansion and contraction of the system during operation. In Image 3, the joint failed after 569 compression-elongation cycles at 225 psi water pressure and 225 F with the rated compression and elongation distances.
Material Validation Testing
One of the ways to improve the expansion joint is to better understand and enhance the individual layers within the joint. Evaluating the individual material layers allows for proper material selection. This will ensure the performance expansion joint is built to withstand high-temperature and pressure conditions, while also providing desirable movement ratings.
Textile/Fabric Selection
Textile and fabric selection is critical to maximize the best performing expansion joint. Tire cord (textile) selection is important for the strength and movement of the expansion joint. When looking at the proper fabric selection, tensile strength is important to give the expansion joint added strength to perform in high pressure situations.
The tensile strength of the cord will determine the durability of the expansion joint. Likewise, a joint built with a tire cord that provides favorable elongation ratings will help increase the expansion joint’s overall movement ratings. Finding a tire cord that can balance both these properties will result in a highly rated performance expansion joint.
To evaluate tire cord, it is best to use a testing apparatus designed to test materials in accordance with the American Society for Testing and Materials (ASTM), International Organization for Standardization (ISO) or other industry standards. This will provide reliable data during both the design and quality control testing.
To maximize the textile or fabric properties, rubber compound must be applied either as a topper to the textile or incorporated into the fabric. This will give the proper adhesion to create the layers for the expansion joint. It will also give the performance properties the expansion joint is rated for.
Compound Selection
For optimal durability, it is critical for the skim compound, the rubber compound adhering to the textile, to have high bonding strength to several different elastomer layers. Improving rubber-textile adhesion will reduce variable results of the joint. A better bonding between layers of fabric and rubber will also reduce delamination, which can cause a major failure of an expansion joint.
Creating a high-performance compound involves precise science and engineering of rubber additives, including process aids, fillers and curing agents. This results in a desired blend with optimal physical and mechanical properties. Adhesion bonding tests can be used to verify and validate rubber bonding in the buildup of rubber layers and fabric.
More Than Meets the Eye
The development of a high-performance expansion joint starts from the inside out. Research and development of each individual layer is crucial in material selection processes. A performance expansion joint should have best-in-class performance ratings that are backed by validation testing. The buyers, engineering, procurement and construction (EPC) engineers, pipeline designers and facility maintenance management teams need to have the complete picture of their expansion joints to ensure they are making a fully informed buying decision. Having a full understanding of research and design methodologies in material selection, and layering construction, will allow the end users to ask the right questions and have the confidence they need to choose the right product for their application.