Gathering all the information available regarding a system using an expansion joint is of utmost importance. It is the only way to reach the design life expectancy of any expansion joint. There is no such thing as too much information when it comes to the selection process.
The acronym “STAMP” can be beneficial in remembering all the essential parameters. STAMP stands for:
- Application (movements—axial, lateral and angular)
These parameters should be considered of equal importance. However, size has a major impact on the ability to handle the other parameters. Then, knowing the pressure is essential—not only the operating pressure, but any excursion, if there are pulsations, or even shock.
Media and temperature will determine the expansion joint tube material, as it must be chemically compatible and able to handle the process temperature. Finally, the movement needed will determine the expansion joint design.
Larger displacements may require more than one convolution. Angle and lateral offsets can also be built into the expansion joint, thus minimizing stress on the joint and piping.
Expansion joint failure is rare when conditions are known and verified. Failures typically occur when known conditions are given incorrectly, or the system exceeds one or more factors of the information known. In both cases, failure would be the result of misapplication.
The nonmetallic expansion joint industry, following the Fluid Sealing Association (FSA) best practices, provides conservative designs and expansion joints with capabilities that can handle adverse conditions that may not have been considered during the initial selection process. One factor that leads to this is that the construction involves several materials—unlike metallic expansion joints—and performance is almost entirely determined by the bellows single composition material. With nonmetallic expansion joints, specifically elastomeric expansion joints, there are several factors:
- The elastomer itself
- The reinforcement material
- The design and interaction of the different material
- The interaction with the flange attachment
It is the manufacturer’s responsibility to provide options of product design for use in a specific application. The following should help the specifying engineer or end user make an intelligent selection based on design conditions.
All elastomers have temperature limits. There is a range of temperatures in which elastomers will remain pliable and elastic. If the temperature is below the glass temperature, the elastomer will lose elasticity and will not be able to respond to the required motion.
When temperatures get near or exceed the higher limit, the elastomer can get hard and brittle, leading to cracking, and expansion joint failure is bound to occur. The most sophisticated and strongest reinforcement material could be used, but it will be ineffective if the elastomer is not maintained within its temperature range.
Temperature & Pressure
Maximum pressure and temperature limits cannot be considered separately. As temperatures increase, the pressure capabilities of an expansion joint are reduced. Image 1 shows a general guideline of derating the pressure capability based on temperature level for ethylene propylene diene monomer (EPDM) or butyl elastomer. Thus, at 300 F the catalog-rated pressure limit must be reduced in half to ensure reliability and operating life. If the temperature exceeds the capability of an elastomer, a different compound with a higher temperature limit can be selected. Image 2 lists temperature limits for common elastomers.
There are four basic fabrics used as reinforcements in elastomeric piping expansion joints: polyester tire cord, aramid fiber (Kevlar), fiberglass and nylon. Nylon is rated at a max temp of 230 F, polyester tire cord 300 F and Kevlar/fiberglass up to 400 F, which is higher than any other elastomer except fluoroelastomer. It is important to match the max elastomer capability with the maximum fabric reinforcement capability to achieve the highest expansion joint capability. Due to differences in the coefficient of thermal expansion for the various material, conservative evaluation should prevail.
PTFE Molded Expansion Joints
Polytetrafluoroethylene (PTFE) is generally rated for over 400 F, but this limit is not for expansion joints when used for pressure retention. It is not recommended to use molded PTFE joints at over 300 F.
As the temperature rises, PTFE gets softer and begins to creep. This creep reduces the hoop strength of the PTFE bellows and, therefore, lowers the pressure rating. Image 3 lists pressure ratings according to temperature and the number of convolutions.