Rubber vs. Metal Expansion Joints

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.

Advantages: Metal Expansion Joints

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.

Advantages: Rubber Expansion Joints

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.

Summary

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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If you have questions about what joint is right for your application, please contact Gallagher's Engineering Department.

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.

Rubber Expansion Joints are defined as connectors between vessels or used in all systems transporting fluids, slurries or gases operating under pressure at widely different temperatures. These rubber expansion joints are installed in air and gas duct systems in all power plants, gas turbine systems, diesel engine installations, petrochemical and chemical plants, pump systems, cement works, steel works, incinerators, on- and offshore systems, ships and cruise vessels, etc.

Rubber expansion joints are used widely in most energy and industrial plants today. Made from natural or synthetic elastomers, rubber expansion joints are vital components used as flexible connectors in which special fabrics are embedded to provide physical reinforcement. They are used in a variety of applications. Rubber expansion joints are available in standard sizes or customized to the customer's individual needs from a variety of high quality synthetic rubber materials.

Uses of Rubber Expansion Joints

• Compensate for pipe misalignment
• Absorb compression and extension
• Accommodate pressure loads
• Reduce noise
• Isolate vibration
• Neutralize axial, lateral, angular stresses
• Prolong life of motive equipment
• Prevent damage to pipe-work or ducting system
• Relieve stress of on equipment flanges
• Adjust for seismic movement

Applications of rubber expansion joints

In a relatively short space, rubber expansion joints provide the ultimate solution to:

HVAC Applications

Rubber expansion joints are used in a variety of heating, ventilation, and air conditioning systems, which includes commercial and industrial buildings, hotels, apartments, schools, shopping plazas and aboard ships to relieve stress in ductwork. Rubber expansion joints are installed to isolate equipment like pumps, compressors and chillers from the piping system. A variety of configurations of joints are available accommodating different shapes, service and connection requirements.

Process Piping
Protection of a process piping system is an important use for rubber expansion joints. The joints reduce noise and vibration caused by forces in pumps. They also centrifuges by behaving as a shock absorber in systems handling abrasive slurries, corrosive chemicals, or very reactive liquids or gases. Thermal movement in a piping system can also be avoided by using rubber joints. The excellent chemical resistance of rubber and synthetic elastomers make these joints ideal for process piping systems.

Power Plant Applications
Nuclear and fossil fuel plants use Rubber expansion joints so as to reduce thermal expansion and contraction on condenser lines, condensate lines, steam turbine exhaust lines, aeration systems and cooling water lines. Reliability, large size capability, long life, and the ease with which they accommodate themselves to movements make these expansion joints ideal for power plant applications.

Industrial Ducting Systems
Rubber expansion joints have been installed in demanding industrial ducting systems where there are problems like heavy vibration, dry or wet corrosive materials, flutter etc.

Marine Systems
Eliminating the destructiveness of electrolysis and at the same time insulating against the transmission of sound, rubber expansion joints allow the necessary motion and flexibility in a 'moving' ship's piping system. The compactness, resilience and low stress features of rubber make these joints ideally suited for shipboard piping systems. The rubber joints are popularly used in areas like auxiliary condenser, air intake on diesel engines, overboard discharge line between condenser and hull, ventilation lines, flush and sanitary systems, fire and bilge pumps, to name a few.

Pollution and Wastewater Treatment
Water treatment plants, waster water treatment plants, Sewage treatment plants, and air scrubber systems all employ the use of rubber expansion joints. Due to their resistance to abrasion and corrosion as well as their flexibility feature, rubber expansion joints are ideal to be used in raw and secondary sewage lines, sludge pumps, centrifugal air blowers, scrub stacks.

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