Rubber expansion joint

A rubber expansion joint is a flexible connector manufactured from natural or synthetic elastomers, technical textiles and metal reinforcements. Expansion joints relieve stresses in piping systems, caused by temperature fluctuations, mechanical vibrations and movements. A rubber expansion joint is an arch-type flexible non-metallic connector fabricated from natural or synthetic rubber and fabrics, usually with metal reinforcements, to isolate vibration and noise and provide stress relief in piping systems caused by thermal changes, mechanical vibration and other system movements. Generally, it shall be designed & fabricated in accordance with ASTM F1123 and “Technical Handbook on Rubber Expansion Joints and Flexible Pipe Connectors”, or in accordance with special customer requirements. Besides, the Chinese standard HG/T 2289 Flexible Rubber Joints sometimes may apply.

Rubber expansion joint is also called rubber tube soft joint, flexible rubber joints, rubber soft joints, flexible rubber joints, high pressure rubber joints, rubber shock absorbers, compensators, etc.

Main data of rubber expansion joint

Construction Features & Design Patterns

A rubber expansion joint generally consists of inner tube, cover and carcass. The inner tube shall be made of natural rubber, synthetic rubber, or blend of synthetic rubber. It is a seamless protective, leak-proof lining that extends through the bore to the outside edges of the joint. Since the inner tube is in direct contact with the flowing media, it shall be designed to cover service conditions for chemical, petroleum, sewage, gaseous and abrasive materials. The purpose of the tube is to eliminate the possibility of the materials being handled penetrating the carcass and weakening the fabric. The cover is the exterior surface of the joint that is formed from natural or synthetic rubber. The prime function of the cover is to protect the carcass from outside damage caused by atmospheric chemicals, oils, air, sunlight, vapor, etc. The carcass or body of the rubber expansion joint is the flexible and supporting member between the tube and cover. It is the structural framework of the joint and is made from multiple plies woven fabric or tire cord impregnated with synthetic rubber. Steel wires or solid metal rings are often embedded in the carcass to provide additional reinforcement to the body fabric.

Function & Performance

As illustrated in the picture below, rubber expansion joints are primarily designed to absorb and compensate for various movements and vibration in a piping systems: (A) Axial Compression – The dimensional reduction or shortening of in face-to-face parallel length of the joint measured along the longitudinal axis. (B) Axial Elongation – The dimensional increase or lengthening of face-to-face parallel length of the joint measured along the longitudinal axis. (C) Lateral Movement – The movement of relating displacement of the two ends of the joint perpendicular to its longitudinal axis. (D) Vibration – Mechanical oscillations in the piping system, usually in high frequency. (E) Angular Movement – The angular displacement of the longitudinal axis of the joint from its initial straight line position, measured in angle degrees. One half of the joint has axial elongation and the other has axial compression. (F) Torsional Movement – One end of the expansion joint is twisted around the longitudinal axis while the other end is held fixed.

The rubber expansion joint also exhibits excellent noise absorption characteristics. Its relative impedance to sound based on rubber is 1.0, while the relative impedance based on steel is 500. The low relative impedance number corresponds to low sound transmission. The joint can significantly limit or interrupt the transmission of a sound/ noise from operating equipment to the piping system.

Material & Manufacture

The material selection shall be determined by service conditions. Generally, it includes the material selection for the elastomeric body as well as the end connector (if not integrally formed). As introduced previously in this article, the elastomeric body consists of inner tube, cover and carcass. Both the tube and cover shall be made from a series of elastomers including pure gum rubber, natural rubber, neoprene, butyl, chlorobutyl, styrene-butadiene, CSM, PTFE, nitrile, hypalon, EPDM, FKM, etc. The carcass made of woven fabric or tire cord shall be selected from nylon, polyester, fiberglass, or aramid. For the floating end connector, if it is flange, generally the material shall be carbon steel (ASTM A105) or stainless steel (SS304 or SS316); if it is threaded union, the material shall be malleable cast iron.

The manufacture or fabrication of a rubber expansion joint includes the following salient processes: (1) Compounding – Add necessary ingredients to rubber to form the elastomer that can meet service requirements. (2) Mixing – It involves the mastication process and incorporation of additives. The rubber will become soft and tacky, and articles could be built up from multiple pieces of the masticated rubber. (3) Forming of the joint body on a die mold. (4) Vulcanization of the rubber joint to give the material its elastic recovery behaviour. (5) Assembly – Install the rubber joint with end connector as well as control units. (6) Inspection & Testing. (7) Cleaning & Packaging.

The function of a rubber expansion joint and the absorption of various movements: A-axial compression, B-axial elongation, C-lateral or transverse movement, D-vibration, E-angular movement, F-torsional movement.

Compensation of axial movement with a compensator without tie rods

Compensation of lateral and axial movement with compensators without tie rods on each pipe

Compensation of movements in 2 planes with 3 angular compensators. The advantages are the absorption of large movements, weak adjustment force, weak movement resumption

Compensation of axial and lateral movement with compensators without tie rods

Compensation of a large axial movement with two hinge compensators

Compensation of lateral and axial movement with compensators without tie rods on each pipe

Application area

Rubber expansion joints provide flexibility and concurrent movements, isolation of vibration and noise, resistance to corrosion, abrasion and erosion in various piping systems. They are widely used in process piping systems, urban heating & air conditioning systems, water supply systems, sewage treatment, food industry, ship building industry, power generation, as well as pharmaceutical industry.

• Heating and cooling devices
• Materials handling technologies
• Water pipes
• Desalination plants
• Compressors
• Blowers and fans
• Cement industry
• Chemical industry

• Glass industy
• Wood-processing industry
• Pulp and paper industry
• Railed vehicles
• Refineries
• Shipbuilding
• Steel mills
• Sugar industry

Rubber expansion joint with underground cover and mating flange

Size: DN1800 DN1600 DN900

Cracking:

Exterior surface cracking is most commonly the result of aging or elevated temperature. Because of rubber ages, it becomes hard and brittle and loses inherent flexibility and resilience. Cracking or crazing may not be serious if only the outer cover is involved and the fabric is not exposed. If necessary, repair onsite with rubber cement where cracks are minor. Carefully inspect cracks to determine if underlying fabric reinforcing plies are compromised.

Exposure of Metal Reinforcement:

If the metal reinforcement of an flexible rubber expansion joint is visible through the cover, the rubber expansion joint should be replaced ASAP.

Dimensions:

Any inspections should verify that the installation is correct; no excessive misalignment between the flanges exists; and the installed face-to­-face dimension is correct. Check for over-elongation, over­ compression, lateral or angular misalignment. If incorrect installation has caused the expansion joint to fail, adjust the piping and order a new expansion joint to fit the existing installation.

Rubber Deterioration:

If the joint feels soft or gummy, plan to replace the flexible rubber expansion joint as soon as possible. Chemical attack is the most likely cause.

Leakage:

It is most important to determine where the leak originated prior to implementing any corrective action. If leakage or weeping is occurring from any surface of the expansion joint, except where flanges meet, replace the joint immediately.

Rubber expansion joints have been specified and successfully used for many years to accommodate pressure loads, relieve movement stresses, reduce noise, isolate vibration, compensate for misalignment after plants go on stream and prolong the life of pumps and other motive equipment.

Note bolts need to be installed in correct way!

Insert the bolts in the flanges with the head loward the joints. Tighten the bolts in opposite pairs. The bolts and nuts shall be tighten in progressive and crosswise with bolting pressure evenly distributed. Tighten opposing Nuts/bolts gradually according to the fullwing sequence.