Rubber expansion joints, also referred to as elastomeric expansion joints, have many advantages compared to metal expansion joints. Below are just a few to highlight:
Rubber expansion joints, also referred to as elastomeric expansion joints, have many advantages compared to metal expansion joints. Below are just a few to highlight:
Minimal face-to-face dimensions
Minimal face-to-face dimensions in rubber expansion joints offer untold economies compared with costly bends or loops. The relative cost of the pipe itself may be less or no more than a rubber expansion joint, however, total costs are higher when considering plant space, installation labor, supports, and pressure drops.
Lightweight
Rubber expansion joints are relatively light in weight, requiring no special handling equipment to position, contributing to lower installation labor costs.
Low movement forces required
The inherent flexibility of rubber expansion joints permits almost unlimited flexing to recover from imposed movements, requiring relatively less force to move, thus preventing damage to motive equipment.
Reduce fatigue factor
Compared to steel, the inherent characteristics of natural and synthetic elastomers are not subject to fatigue breakdown or embrittlement and prevent any electrolytic action because of the steel-rubber interface of joints and mating flanges.
Reduced heat loss
Rubber expansion joints reduce heat loss, giving long maintenance-free service. The added piping required for loops contributes to higher operating costs after going on stream due to increase in heat losses.
Corrosion, erosion resistant
A wide variety of natural, synthetic, and special purpose elastomers and fabrics are available to the industry. Materials are treated and combined to meet a wide range of practical pressure and temperature operating conditions, corrosive attack, abrasion, and erosion. Standard and special sizes of rubber expansion joints are available with PTFE/TFE/FEP liners, fabricated to the configurations of the joint body, as added insurance against corrosive attack. Fluoroplastics possess unusual and unique characteristics of thermal stability, non-sticking surface, extremely low coefficient of friction, and resistance to practically all corrosive fluids and forms of chemical attack.
No gaskets
Elastomeric expansion joints are supplied with flanges of vulcanized rubber and fabric integrated with the tube, making the use of gaskets unnecessary. The sealing surfaces of the expansion joint equalize uneven surfaces of the pipe flange to provide a fluid and gas-tight seal. A ring gasket may be required for raised face flanges.
Acoustical impedance
Elastomeric expansion joints significantly reduce noise transmission in piping systems because the elastomeric composition of the joint acts as a dampener that absorbs the greatest percentage of noise and vibration.
Greater shock resistance
The elastomeric type expansion joints provided good resistance against shock stress from excessive hydraulic surge, water hammer, or pump cavitation.
An expansion joint must perform several functions. The rubber joint is the most flexible part of a piping system, but this flexibility has a tradeoff. An expansion joint is usually the weakest component of a piping system and must be carefully selected for the application. Connected to a pump, it reduces vibration. If located in the connected piping, the expansion joint moves with the thermal pipe movement. It does this while providing a seal under system pressure and at temperature and resisting corrosive media. The keys to success are selecting the proper elastomer for the application, knowing the required movement, pressure and temperature and proper installation.
Simply, rubber. Or more technically, elastomers. Elastomers are made of high-molecular weight polymer chains that, when vulcanized (or cured), can be repeatedly stretched and returned to the original length. The different types of polymer chains, when combined with the curing method, give the rubber properties that make it suitable for various applications. But there is more than elastomers to an expansion joint. A typical expansion joint consists of three components: the tube, carcass and cover.
The tube is a protective, leak-proof lining made of synthetic or natural rubber as the service dictates. This is a seamless tube that covers the bore to the outside edges of the flanges. Its purpose is to stop the materials being handled from penetrating the carcass and weakening the fabric. These tubes can be designed to cover service conditions for chemical, petroleum, sewage, gaseous and abrasive materials.
The carcass or body of the expansion joint consists of fabric and, when necessary, metal reinforcement.
Fabric reinforcement: The carcass fabric reinforcement is the flexible and supporting member between the tube and cover. Standard constructions normally use high-quality synthetic fabric. Natural fabrics can also be used at reduced pressures and temperatures. All fabric plies are impregnated with rubber or synthetic compounds to permit flexibility between the fabric plies.
Metal reinforcement: Wire or solid steel rings embedded in the carcass are frequently used as strengthening members of the joint. The use of metal sometimes raises the rated working pressure and can supply rigidity to the joint for vacuum service.
The exterior surface of the joint is formed from natural or synthetic rubber, depending on the service requirements. The prime function of the cover is to protect the carcass from outside damage or abuse. Special polymers can be supplied to resist chemicals, oils, sunlight, acid fumes and ozone. Also, a protective coating may be applied to the exterior of the joint for additional protection.
A rubber expansion joint may be compared to a tire in several ways. Both are made of reinforced rubber, customized for various conditions as necessary and contain pressure.
There are two main styles of rubber joints: spool and spherical. Spool-type joints have cylindrical bodies with steel reinforcing rings. This style of joint will typically also have arches to accommodate additional movement. Spool-type joints may be custom-manufactured for specific installation requirements. The sealing surface is a full-face flange.
Spherical expansion joints use the body shape to provide a strong and flexible connector. The spherical body shape results in a uniform pressure distribution throughout the body, precluding the requirements for reinforcing steel and only using fabric reinforcement. Spherical joints typically have floating flanges and a bead seal instead of a full-face flange.
Although a rubber joint appears flexible and may be easily bent, twisted and stretched, there are plenty of considerations when these joints are incorporated into a piping layout.
Pressure thrust sometimes surprises the installers, and it could be a rude surprise. Since a rubber joint has a flexible wall, when put under pressure it will inflate like a balloon. The fabric and reinforcing steel will restrain stretching in the radial direction, but there is no real restraint in the axial direction. The only axial restraints will be external in the form of either control rods or pipe anchors.
As a result, the axial restraints will be subject to a pressure thrust force in addition to the spring force of the rubber body. The total force on a pipe anchor will be the pressure thrust added to the spring force of the rubber body. For an expansion joint with control rods, the pressure thrust will be restrained by the control rods unless the joint is compressed.
In Equation 1, say the pipe and expansion joint are 6 inches in diameter, the operating pressure is 100 pounds per square inch (psi) and the thermal movement of the pipe is calculated at 1 inch. The pressure thrust from the expansion joint will be the pressure multiplied by the joint effective area. This example will use an effective area of 52 square inches (this is different for each manufacturer and can be found in the product literature).
The spring force may be obvious but the pressure thrust may not be. If the expansion joint is installed without restraints, the pressure thrust can easily move the connected pipe.
Another common installation has the expansion joint as a pump connector. For this installation, it is also common to include control rods to restrain the joint.
For this installation, no anchors would be required because the pressure thrust is restrained by the control rods. If the same parameters are being used as the anchored pipe example, the control rods would be restraining the 5,200 lbf pressure thrust with no thermal movement.
The FSA is the main standards organization for rubber pipe expansion joints. There are three standards and one handbook to reference for more detailed information:
In addition to these standards, elastomeric expansion joints can be fabricated to additional standards as required. Examples are American Society of Testing Materials (ASTM) F, National Sanitation Foundation (NSF)-61 and various military specifications. Also, the FSA website contains information on the Knowledgebase at fsaknowledgebase.org
Like car tires, rubber expansion joints have a service life and will be a maintenance item for the system. Unlike car tires, the service life is not measured in miles. The service life of an expansion joint depends on several factors including operating temperature and pressure, ambient conditions, movement and proper installation.
The best expansion joint for an application can be rendered useless if it is not installed according to the manufacturer&#;s instructions and following FSA recommendations.
The general rule for replacement of an expansion joint is five years for a joint in a critical service application. For joints not in a critical service, observe the joint at regular intervals and plan to replace after 10 years. Some joints can last as long as 30 years depending on the operating conditions.
Expansion joint failures are routinely traced back to installation problems. Here are some of the more common installation issues:
We&#;ve only scratched the surface of the subject of rubber joints. To learn more about the subject, read FSA&#;s &#;Piping Expansion Joints Handbook.&#; The same authors of the handbook have also written several in-depth articles on the subjects mentioned above, and they are also free downloads from the FSA.
We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to .
Minimal face-to-face dimensions
Minimal face-to-face dimensions in rubber expansion joints offer untold economies compared with costly bends or loops. The relative cost of the pipe itself may be less or no more than a rubber expansion joint, however, total costs are higher when considering plant space, installation labor, supports, and pressure drops.
Lightweight
Rubber expansion joints are relatively light in weight, requiring no special handling equipment to position, contributing to lower installation labor costs.
Low movement forces required
The inherent flexibility of rubber expansion joints permits almost unlimited flexing to recover from imposed movements, requiring relatively less force to move, thus preventing damage to motive equipment.
Reduce fatigue factor
Compared to steel, the inherent characteristics of natural and synthetic elastomers are not subject to fatigue breakdown or embrittlement and prevent any electrolytic action because of the steel-rubber interface of joints and mating flanges.
Reduced heat loss
Rubber expansion joints reduce heat loss, giving long maintenance-free service. The added piping required for loops contributes to higher operating costs after going on stream due to increase in heat losses.
Corrosion, erosion resistant
A wide variety of natural, synthetic, and special purpose elastomers and fabrics are available to the industry. Materials are treated and combined to meet a wide range of practical pressure and temperature operating conditions, corrosive attack, abrasion, and erosion. Standard and special sizes of rubber expansion joints are available with PTFE/TFE/FEP liners, fabricated to the configurations of the joint body, as added insurance against corrosive attack. Fluoroplastics possess unusual and unique characteristics of thermal stability, non-sticking surface, extremely low coefficient of friction, and resistance to practically all corrosive fluids and forms of chemical attack.
No gaskets
Elastomeric expansion joints are supplied with flanges of vulcanized rubber and fabric integrated with the tube, making the use of gaskets unnecessary. The sealing surfaces of the expansion joint equalize uneven surfaces of the pipe flange to provide a fluid and gas-tight seal. A ring gasket may be required for raised face flanges.
Acoustical impedance
Elastomeric expansion joints significantly reduce noise transmission in piping systems because the elastomeric composition of the joint acts as a dampener that absorbs the greatest percentage of noise and vibration.
Greater shock resistance
The elastomeric type expansion joints provided good resistance against shock stress from excessive hydraulic surge, water hammer, or pump cavitation.
An expansion joint must perform several functions. The rubber joint is the most flexible part of a piping system, but this flexibility has a tradeoff. An expansion joint is usually the weakest component of a piping system and must be carefully selected for the application. Connected to a pump, it reduces vibration. If located in the connected piping, the expansion joint moves with the thermal pipe movement. It does this while providing a seal under system pressure and at temperature and resisting corrosive media. The keys to success are selecting the proper elastomer for the application, knowing the required movement, pressure and temperature and proper installation.
Simply, rubber. Or more technically, elastomers. Elastomers are made of high-molecular weight polymer chains that, when vulcanized (or cured), can be repeatedly stretched and returned to the original length. The different types of polymer chains, when combined with the curing method, give the rubber properties that make it suitable for various applications. But there is more than elastomers to an expansion joint. A typical expansion joint consists of three components: the tube, carcass and cover.
The tube is a protective, leak-proof lining made of synthetic or natural rubber as the service dictates. This is a seamless tube that covers the bore to the outside edges of the flanges. Its purpose is to stop the materials being handled from penetrating the carcass and weakening the fabric. These tubes can be designed to cover service conditions for chemical, petroleum, sewage, gaseous and abrasive materials.
The carcass or body of the expansion joint consists of fabric and, when necessary, metal reinforcement.
Fabric reinforcement: The carcass fabric reinforcement is the flexible and supporting member between the tube and cover. Standard constructions normally use high-quality synthetic fabric. Natural fabrics can also be used at reduced pressures and temperatures. All fabric plies are impregnated with rubber or synthetic compounds to permit flexibility between the fabric plies.
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Metal reinforcement: Wire or solid steel rings embedded in the carcass are frequently used as strengthening members of the joint. The use of metal sometimes raises the rated working pressure and can supply rigidity to the joint for vacuum service.
The exterior surface of the joint is formed from natural or synthetic rubber, depending on the service requirements. The prime function of the cover is to protect the carcass from outside damage or abuse. Special polymers can be supplied to resist chemicals, oils, sunlight, acid fumes and ozone. Also, a protective coating may be applied to the exterior of the joint for additional protection.
A rubber expansion joint may be compared to a tire in several ways. Both are made of reinforced rubber, customized for various conditions as necessary and contain pressure.
There are two main styles of rubber joints: spool and spherical. Spool-type joints have cylindrical bodies with steel reinforcing rings. This style of joint will typically also have arches to accommodate additional movement. Spool-type joints may be custom-manufactured for specific installation requirements. The sealing surface is a full-face flange.
Spherical expansion joints use the body shape to provide a strong and flexible connector. The spherical body shape results in a uniform pressure distribution throughout the body, precluding the requirements for reinforcing steel and only using fabric reinforcement. Spherical joints typically have floating flanges and a bead seal instead of a full-face flange.
Although a rubber joint appears flexible and may be easily bent, twisted and stretched, there are plenty of considerations when these joints are incorporated into a piping layout.
Pressure thrust sometimes surprises the installers, and it could be a rude surprise. Since a rubber joint has a flexible wall, when put under pressure it will inflate like a balloon. The fabric and reinforcing steel will restrain stretching in the radial direction, but there is no real restraint in the axial direction. The only axial restraints will be external in the form of either control rods or pipe anchors.
As a result, the axial restraints will be subject to a pressure thrust force in addition to the spring force of the rubber body. The total force on a pipe anchor will be the pressure thrust added to the spring force of the rubber body. For an expansion joint with control rods, the pressure thrust will be restrained by the control rods unless the joint is compressed.
In Equation 1, say the pipe and expansion joint are 6 inches in diameter, the operating pressure is 100 pounds per square inch (psi) and the thermal movement of the pipe is calculated at 1 inch. The pressure thrust from the expansion joint will be the pressure multiplied by the joint effective area. This example will use an effective area of 52 square inches (this is different for each manufacturer and can be found in the product literature).
The spring force may be obvious but the pressure thrust may not be. If the expansion joint is installed without restraints, the pressure thrust can easily move the connected pipe.
Another common installation has the expansion joint as a pump connector. For this installation, it is also common to include control rods to restrain the joint.
For this installation, no anchors would be required because the pressure thrust is restrained by the control rods. If the same parameters are being used as the anchored pipe example, the control rods would be restraining the 5,200 lbf pressure thrust with no thermal movement.
The FSA is the main standards organization for rubber pipe expansion joints. There are three standards and one handbook to reference for more detailed information:
In addition to these standards, elastomeric expansion joints can be fabricated to additional standards as required. Examples are American Society of Testing Materials (ASTM) F, National Sanitation Foundation (NSF)-61 and various military specifications. Also, the FSA website contains information on the Knowledgebase at fsaknowledgebase.org
Like car tires, rubber expansion joints have a service life and will be a maintenance item for the system. Unlike car tires, the service life is not measured in miles. The service life of an expansion joint depends on several factors including operating temperature and pressure, ambient conditions, movement and proper installation.
The best expansion joint for an application can be rendered useless if it is not installed according to the manufacturer&#;s instructions and following FSA recommendations.
The general rule for replacement of an expansion joint is five years for a joint in a critical service application. For joints not in a critical service, observe the joint at regular intervals and plan to replace after 10 years. Some joints can last as long as 30 years depending on the operating conditions.
Expansion joint failures are routinely traced back to installation problems. Here are some of the more common installation issues:
We&#;ve only scratched the surface of the subject of rubber joints. To learn more about the subject, read FSA&#;s &#;Piping Expansion Joints Handbook.&#; The same authors of the handbook have also written several in-depth articles on the subjects mentioned above, and they are also free downloads from the FSA.
We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to .
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