Expansion joints, selection

The movement-absorbing devices used in semirigid and nonrigid piping systems are usually called expansion joints. Based on the method by which the pressure seal is effected, joints are either of the packed type or the packless or bellows type. Selection depends not only on the required movement but also on the severity of service in terms of pressure and temperature, tolerabiUty of leakage, and the number of service cycles. 

The selection and appHcation of an expansion joint is not as simple as selecting a pipe fitting or a valve and requires a sound understanding of the joint s capabihties and limitations. Improper appHcation of any type of joint can result in serious or damaging effects. However, when properly selected and integrated into the piping system, satisfactory service and safe operation can be expected. Selection and appHcation of beUows expansion joints require special attention to design and installation. 

When selecting a bellows valve, it is important to pay some special attention that the material selection is in accordance with the process conditions. Some SRV manufacturers use as standard bellow material INCONEL alloy 625LCF-UNS N06625 (ASME SB0443). This material is not perfect either but, compared to simple stainless steel, has an enhanced resistance to mechanical fatigue and sour gases it is commonly used in refinery FCC systems for expansion joints. 

A major food, detergent, and consumer products company had defined some piping expansion problems. The solution seemed obvious and several flexible elastomer expansion joints were selected and installed to meet the process condilions within a soap manufacturing plant. [10]... 

Figure 52-4 Note the unacceptable variation in the width of expansion joints and poor selection and placement of joint filler.

The first step in selecting a material for a particular application is to define the properties that are essential to the performance of the component. In some cases, the requirements limit the choice to one or two materials. For example, expansion joints in some chemical installations require a combination of flexibility with resistance to extremety corrosive environments the bellows shown in Figure 8.1 are made from FIFE, despite its high price and difficulties in processing, because no materials other than fluorocarbon polymers meet the requirements. On the other hand, several less expensive polymers have sufficient resistance to dilute sulfuric acid to make a satisfactory battery case toughened potypropylene is chosen for its combination of chemical resistance, stiffness, toughness, processability, and moderate price. 

Differential thermal expansion between various components of calandrias, especially shell-and-tube types, has an important effect on the mechanical design of the equipment. Some types of tubular exchangers incorporate into the basic design and fabrication means to provide for thermal expansion. Other types, specifically fixed tubesheet units, must often be provided with expansion joints in the shell to meet specified differential thermal expansion between the tubes and shell. Another critical area affected by thermal expansion in fixed tubesheet units is the shell-to-tubesheet juncture. Gasketed joints may also be affected by thermal expansion therefore both the flange design and the gasket selection in... 

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. 

The expansion joint designer has many factors to consider when selecting materials, configurations, and methods of manufacture. One condition that is often overlooked is the fact that the steel frame that the joint is mounted on expands radially at the same rate as it does axially. For example a stainless steel frame with internal duct dimensions of 5 m x 5 m operating at 550 C would expand radially 50 mm, which in effect is stretching the materials of the joint by this amoiuit. To... 

Piping joints shall be selected to suit the piping material, with consideration of joint tightness and mechanical strength under expected service and test conditions of pressure, temperature, and external loading. Layout of piping should, insofar as possible, minimize stress on joints, giving special consideration to stresses due to thermal expansion and operation of valves (particularly a valve at a free end). 

There are several possible solutions to the expansion mismatch problem. One is to use a resilient adhesive that deforms with the substrate during temperature change. The penalty in this case is possible creep of the adhesives, and highly deformable adhesives usually have low cohesive strength. Another approach is to adjust the thermal expansion coefficient of the adhesive to a value that is nearer to that of the substrate. This is generally accomplished by selection of a different adhesive or by formulating the adhesive with specific fillers to tailor the thermal expansion. A third possible solution is to coat one or both substrates with a primer. This substance can provide either resiliency at the interface or an intermediate thermal expansion coefficient that will help reduce the overall stress in the joint. 

Metal fillers for high-temperature adhesives must be carefully selected because of their possible effect on oxidation, as indicated in the previous section. Carrier films, such as glass cloth, are generally used to facilitate the application of the adhesive, but they also provide a degree of reinforcement and lowering of the coefficient of thermal expansion. Thus, they reduce the degree of internal stress experienced at the joint s interface. 

Sandwich structures have found wide application in bus and high-speed railway constructions. The key issue for the application of sandwich structures is the careful selection of a suitable joining technique to connect the large-scale sandwich components with each other or with other parts of the structure. Elastic adhesive joints provide uniform load distribution and can compensate for relatively large geometric and production tolerances, and also for the thermal expansion of different substrate materials. Therefore, a smooth and continuous transfer of the interfacial forces into the sandwich structure is enabled, with comparatively low stiffness. These are significant benefits compared with the... 

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