Rubber expansion joints

Improved service water reliability by elimination of rubber expansion joints in the recirculated cooling water system and minimization of their use in the raw service water systems... 

Tyres are normally cured in a modification of the compression mould where a bladder or an inflated airbag forces and holds the green rubber stock of the tyre against the mould surface during vulcanization. This force reproduces the design of the tyre tread and the heat from the steam is introduced into the bladder to effect the vulcanization. Small size rubber expansion joints used in piping systems are moulded by compression moulding... 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

Figure 14.1 A hand fabricated rubber expansion joint for the fertilizer industry pipelines.

Stability, fluid compatibility and fatigue resistance also are associated with wear resistance. Vibration and pulsation in mechanical systems, or pipelines are common in chemical process industries and can lead to leakages of corrosive fluid. The vibration fatigue is an unavoidable factor which is further aggravated by improper installation of the moving system. Leakages due to these pulsations can be prevented by the installation of flexible rubber expansion joints. 

A recent report [1] studied by the author suggests that installation of rubber expansion joints in pipes will prevent breakage and leaking, by absorbing swings and vibrations ... 

Rubber expansion joints offer engineering advantages over metallic joints. Rubber expansion joints, which consist of flanged ends and a flexible section, can absorb within its free length more lateral movements, than any other similarly sized joint Expansion bellows are joints that have several sections. The flexible section of the rubber expansion joint/bellows is often a single fold which, because of the inherent flexibility of the rubber, can deal with large lateral movements with low force, a phenomenon which would require multiple folds in a similar metal component. 

The flexible section of a rubber expansion joint often has a single fold or arch (Figure 7.1). A double arch expansion joint is shown in Figure 7.2. 

The major difference between rubber joints and metal bellows is in the way they absorb pressure loads. Circumferential loads (hoop stress) due to pressure are carried by the folds themselves in a metal bellows. In a rubber expansion joint, the convolution is incapable of resisting pressure by itself, but is supported by the adjacent tube with its internal fabric or fabric reinforcement or by the adjacent flanges. Expansion joints have integrally molded flanges, which are drilled to match standard pipe flanges. AU expansion joints require metallic retainer rings behind the flanges as back up. 

The unique advantages of rubber expansion joints make them used in industrial settings where fluid and gas handling is a primary part of the process, e.g. waste management, or air pollution control. Wherever movements or vibrations from thermal expansion or mechanical equipment expansion occur, joints provide low cost and efficient solutions to leakage problems. 

For piping systems conveying hydrocarbons, emulsions, saline solutions, air, gas, steam, liquids, hydraulic fluids and fuels, suitably designed rubber expansion joints are usually installed within the piping system to accommodate forces due to movement and thermal expansion, and so prevent leakages. 

Rubber expansion joints are successfully used in several industrial plants, nuclear and military installations to control leaks in hot and cold water distribution systems, chilled water and condensed water piping, and suction and discharge sides of pumps adjacent to compressors. 

Bellows can vibrate both from internal fluid flow, and externally imposed mechanical vibrations. At high flow, velocities and flow induced resonance produces bellows fatigue. Multiple bellows are less susceptible to vibration failures because of the damping effect of the interplay friction. The benefits of multiple rubber expansion joints are given below ... 

A catastrophic, sudden failure is virtually impossible when using rubber expansion joints, since the multiple inner core prevents instantaneous bursting. If a crack does occur due to... 

Cover the cover of the rubber expansion joint is made out of a rubber which resists weathering attack and mechanical damage, if any, caused during handling and installation. In general. 

The temperature ratings and application limits of certain rubbers used in the manufacture of rubber expansion joints are given in Table 7.1. 

TABLE 7.1 Temperature ratings and application limits of rubbers used in rubber expansion joints... 

In the manufacture of rubber expansion joints, material selection and formulation should be considered against the constructional characteristics required in the product. Rubbers, compounds, adhesives and solvents are all thoroughly checked for quality, to avoid possible layer separation or delamination of the layers while the joints are being built. 

When minimum movement capability is required, the arch is sometimes filled with soft rubber using a suitable adhesive. The maximum amount of movement (axial extension and compression, lateral deflection and angular rotation) that an expansion joint is capable of absorbing is called the rated movement. This rating depends on various factors, such as the size of the expansion joints, the thickness of the tube, arch or convolution, and the type and properties of rubber compound and fabric used in construction. Rated movements are established by manufacturers of expansion joints theoretically, or are based on actual load deflection curves of each size of joint. Rubber expansion joints are generally subjected to hydraulic and vacuum tests at 1.5 times the operating pressure. No internationally accepted standard technical specification for rubber expansion bellows is available, since they are mostly custom built to specific operational requirements. The Expansion Joint Manufacturers Association in New York has laid down standards for rubber expansion joints, which are called EJMA standards [2]. 

Pathway Bellows Inc Design Manuals for Rubber Expansion Joints. Pathway Bellows Inc, California, 1978. 

A chapter on Rubber Expansion Joints is included since the author feels that the function of such expansion joints as pipe connectors is indirectly linked with leakage and prevention of fluid flow through pipes. 

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