An expansion joint can relieve stress in piping systems and prevent flange gaskets from being crushed. But which expansion joint is best for your specific application? Let us first describe the two types of expansion joints:
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Rubber &#; a flexible connector fabricated of natural and/or synthetic elastomers and fabric and, if necessary, internal metallic reinforcements designed to provide stress relief in piping systems due to thermal movements and mechanical vibration.
Metal &#; a flexible element (bellows) constructed of relatively thin gauge material (generally stainless steel) designed to absorb mechanical and thermal movements expected in service.
Typical Metal Expansion Joint
Temperature
Rubber joints with standard construction and materials have an upper range to 230°F. Most manufacturers, however, can offer special constructions up to 400°F. Metal expansion joints do offer a far greater range, from -420°F to +°F. However, working pressures are reduced at elevated temperatures.
Pressure
Rubber joints typically, depending on diameter, can have pressure capabilities up to 250-psi with a full vacuum rating. Metal joints can be designed for pressures up to -psi. The strength of metal is definitely an advantage in high pressure applications; however, the relative stiffness or spring rates coupled with thrust forces should be carefully examined. Piping systems/anchors must be designed to handle the combined load.
Typical Rubber Expansion Joint
Movements
Rubber and metal expansion joints have similar movement capabilities in the axial plane (compression and extension). However, rubber joints are certainly able to absorb far greater lateral movements when compared to metal joints that have similar face to face dimensions. Constructions (dual or universal) are available for metal joints where large movements in the lateral plane are required but these are considered special design and can be costly.
Spring Rates
Defined as the total force required to move an expansion joint 1&#; in any direction. Rubber and metal joints do have similar characteristics in the axial plane for the standard face to face dimensions. Metal joints are much stiffer when subjected to lateral motion and, therefore, typically have a much lower lateral movement capability. Note that all spring rate values are at 0-psig. Both rubber and metal joints produce thrust forces when pressurized that must be considered for proper system design.
Acoustical Impedance
Although well designed (multi-ply) metal joints can lower the transmission of visible vibration, they will continue to transmit distracting and/or damaging noise. Rubber joints significantly reduce the undesirable transmission in piping systems. The elastomeric composition of the joint acts as a dampener that absorbs the greatest percentage of perceptible noise and vibration.
Abrasion/Erosion Resistance
Metal joints typically have a wall thickness anywhere between .012&#; to .080&#;. Rubber joints on the other hand are much thicker, 0.5" to over 1". The thin gauge construction of metal joints makes them susceptible to erosive chemicals and abrasive liquids and slurries. Rubber joints are highly resistant to abrasion and erosion of all types and do outperform metal joints in the applications where these conditions prevail. Drop-in or fixed liners can be provided to enhance the life of metal joints in many of these applications but at best can only prolong the time to eventual failure.
Fatigue/Cycle Life
The fatigue life of a metal joint is affected by many factors such as temperature, pressure, movement, vibration and, of course, how the joint was initially designed. Typically, metal joints have a defined cycle or fatigue life that can be calculated through various formulas. Metal joints frequently succumb to fatigue failure from excessive cycling/movement. Rubber joints on the other hand are constructed of resilient elastomers and the joint itself acts as a vibration dampener, not susceptible to fatigue/cycle failure.
Installation/Maintenance
As a rule of thumb, rubber joints are 25% to 50% than metal joints. Rubber joints do not require additional gasketing and, in many cases, are installed easily by one or two men without the use of special handling equipment. Metal joints must be serviced occasionally to insure that the flange gasket is still intact and not deteriorated. For both rubber and metal expansion joints, control units are recommended to minimize possible damage to the expansion joint caused by excessive motion of the pipeline and in some applications, to absorb thrust forces. When control units are set to eliminate axial extension and compression, the only movement the joint can take is in the lateral plane.
Metal expansion joints are applied more frequently than rubber primarily because application conditions, e.g. temperatures and pressures, favor their use. Some experts in this industry estimate the metal expansion joint market to be 4X larger than the rubber expansion joint market. The writer of this article has been in the manufacturing business of both rubber and metal joints for over 35 years and would venture to say that is a good estimate.
It may appear that the above comparison is slanted toward the rubber expansion joints. The fact is, rubber can&#;t replace metal in all applications, but it is a better choice for many applications involving high vibration and sound dampening within the guidelines mentioned above. Consultation with a reputable manufacturer for your specific application is always recommended.
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This article is a reprint of an article that appeared in Pumps & Systems in September of , and is included in the Fluid Sealing Association's Expansion Joints - Piping Technical Handbook.
Although sometimes overlooked or treated like an afterthought in process design, expansion joints are critical elements in chemical process industries (CPI) operations for providing flexibility and stress relief to piping systems. Failures in expansion joints can result in leaks of process fluids, with implications for personnel safety, environmental protection and operational uptime. Selecting the correct type of expansion joint for the process conditions will maximize the longevity of the joint and reduce downtime for replacing it, while also maintaining the joint&#;s integrity. By considering a full set of process characteristics when selecting expansion joints, plants can maximize production and operational uptime while still maintaining safety and environmental stewardship.
For the selection process, plant engineers should consider taking advantage of expansion-joint vendors, who can offer specialized expertise and on-the-ground experience to guide users to the optimal type of expansion joint for certain process conditions. Vendor companies serve as a valuable resource for arriving at the best expansion-joint choice.
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Expansion Joint Types
Expansion joints can be split into three general categories: rubber expansion joints, stainless steel expansion joints and hose-and-braid flex connectors. Each type has its own advantages and limitations, and each category can be further broken down into subcategories, because there are several different types of elastomers used for expansion joints, as well as multiple types of stainless steel. In addition, there are multiple design variations with different joint geometries that help their performance in certain specific applications.
Figure 1. Expansion joints can be divided into three general categories, as shown here: Rubber, hose and braid, and stainless steel
For rubber expansion joints, the most common elastomers are ethylene propylene diene monomer (EPDM) rubber, polytetrafluoroethylene (PTFE), polychloroprene (Neoprene), nitrile rubber, butyl rubber, natural rubber, & synthetic elastomers. Rubber expansion joints are generally more versatile than other types of joints, although issues of chemical compatibility, pressure and temperature may preclude them from being used under some process conditions. Rubber expansion joints effectively handle compression (movement inward), and extension (movement outward), as well as lateral offset motion, vibration and sound dampening in a short overall length.
Stainless steel expansion joints are also available in several variants, such as 304, 316, and 321 stainless steel grades. Expansion joints made from other metals, such as titanium, or high-temperature alloys, such as Hastelloy and Inconel, are also available. Stainless steel expansion joints are good for handling compression, extension, lateral motion and vibration, but they will exert a larger overall force on the piping system and are longer in overall length
Stainless steel expansion joints should not be confused with stainless steel flex connectors, which are hose-and-braid-style connectors with a stainless steel braided material on the outside. They can address lateral movement and vibration in pipe and equipment joints.
Figure 2. Rubber expansion joints shown on a process pump and piping setup
Factors For Selecting Expansion Joints
When selecting expansion joints for process pipes and equipment, engineers should begin by following a set of criteria that has been generally agreed upon by the industry as a method of taking a comprehensive look at the requirements of the joint. The set of criteria is known by the acronym &#;STAMPED.&#; By gathering information according to this set of factors and questions will provide an excellent basis for which expansion joints should be used. The components of the acronym are defined here:
The STAMPED criteria can go a long way toward narrowing down the choices in expansion joints to steel or rubber, but there are still a wide variety of choices within each of those categories, and different designs available for particular conditions and applications. For example, if an application requires a great deal of movement, there are joints with more convolutions or arches built into the bellows of the joint. Again, expansion joint vendors can be a resource here.
Selection Recommendations
Once you have established the information for the questions associated with STAMPED, there a several additional considerations to help arrive at the final selection of expansion joints. They are discussed here:
Rule of 250. The &#;Rule of 250&#; concerns pressure and temperature of the media. If the pressure is lower than 250 psi and temperature lower than 250°F, then rubber expansion joints should be used, but for pressures above 250 psi and temperatures over 250°F amount probably requires stainless steel.
Figure 3. Rubber expansion joints can be used in applications where temperatures are lower than 250°F
Specific temperature requirements. Beyond the so-called Rule of 250, the selection of expansion joint material should get more specific. EPDM and butyl rubber expansion joints are suitable up to 250°F, but Neoprene is only good to 220°F and nitrile rubber up to 212°F.
MSDS. Material safety data sheets (MSDS) provide details on various compounds that might be present in process media. The information can include details about the potential health and environmental hazards of the components, as well as chemical compatibility. It is highly recommended that users consult MSDS before selecting an expansion joint. Two similar compounds used in a process, such as ethylene glycol and propylene glycol, could require different expansion joints. For example, ethylene glycol is suitable for EPDM expansion joints, while propylene glycol works with nitrile rubber. The degree of specificity that users can have for the type of media that the expansion joint will encounter can have huge impact on the type of elastomer selected.
Concentrations. Beyond the identity of the process media that is present, concentrations of various components within the media are also important. For example, a 5% solution of a caustic substance might be suitable for a particular type of elastomer, but if the concentration of the same caustic material is increased to 20%, for instance, the rubber material might break down. In these cases, stainless steel expansion joints might be required.
Gas handling. For processes involving gases, the material should be considered carefully also, since rubber expansion joints are somewhat permeable to gases. Cases where gas escape would be a problem might also require the use of stainless-steel expansion joints.
Life expectancy. It can be useful to identify factors that might reduce or extend the lifetime of an expansion joint. The average lifespan of a rubber expansion joint is roughly 7-10 years. Depending on the particulars of an application, however, expansion joints might only last a matter of 2-3 months, while other joints might function well for 25 years or more. Some of the factors that can contribute to a shorter life expectancy are high pressure, high temperature, aggressive or abrasive media, exposure to outdoor weather and extensive levels of movement.
Expansion joint surveys. Although there are lengths of time that expansion joints can be expected to last, a definitive method for determining exactly when to replace expansion joints or to identify the exact moment when they will fail does not exist. The best way to prepare for an expansion joint failure, and to limit downtime in the event of a failure, is an expansion joint survey. Member companies of the Fluid Sealing Association (FSA; Wayne, Pa.; www.fluidsealing.com) offer expansion joint surveys. These surveys can be critical for plant maintenance, especially with plants that are trying to maximize productivity and profits while also limiting downtime. Surveys have the potential to help a plant avoid environmental concerns associated with expansion joint failures.
No matter what process details are found in a process, expansion-joint manufacturers will be able to guide users toward the correct type of expansion joint for given operational conditions.
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