A design specification shall be prepared for each pipe expansion joint application. Prior to writing the pipe expansion joint design specification it is imperative that the system designer completely review the piping system layout, flowing medium, pressure, temperature, movements, and other items which may effect the performance of the pipe expansion joint. Particular attention shall be given to the following items.
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The piping system should be reviewed to determine the location and type of pipe expansion joint most suitable for the application. Both the EJMA Standards and most reliable pipe expansion joint manufacturers’ catalogs provide numerous examples to assist the user in this effort. The availability of supporting structures for anchoring and guiding of the piping and the direction and magnitude of thermal movements to be absorbed must be considered when selecting the type and location of the pipe expansion joint. Torsional rotation of the bellows should be avoided or special hardware should be incorporated into the design to limit the amount of torsional shear stress in the bellows.
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The bellows material shall be specified by the user and must be compatible with the flowing medium, the external environment, and the operating temperature. Consideration shall be given to possible corrosion and erosion. The 300 Series stainless sheets of steel may be subject to chloride ion stress corrosion. High nickel alloys are subject to caustic induced stress corrosion. The presence of sulfur may also be detrimental to nickel alloys. The material chosen shall also be compatible with the environment surrounding the pipe expansion joint, water treatment and cleaning materials. In some cases, leaching of corrodents from insulating materials can be a source of corrosion.
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Internal sleeves shall be specified in all applications involving flow velocities which could induce resonant vibration in the bellows or cause erosion of the convolutions resulting in premature failure.
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The system design pressure and test pressure shall be specified realistically without adding arbitrary safety factors. Excess bellows material thickness required for unrealistic pressures will often produce an adverse effect on the bellows fatigue life or increase the number of convolutions required which may reduce the stability of the bellows. In the case of high-temperature applications, it may not be possible to test the expansion joint to 1.5 times the equivalent cold pressure rating of the system. This results from the various materials employed in the construction of the pipe expansion joint, temperature gradient utilized in the design, pressure stability criteria, anchor strength, etc. The manufacturer must be consulted.
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The maximum, minimum and installation temperatures shall be accurately stated. Where the ambient temperature can vary significantly during pipeline construction, pre-positioning of the pipe expansion joint at installation may be required.
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The pipe expansion joint manufacturer shall be advised if the pipe expansion joint will be insulated. Insulation details shall be furnished to the manufacturer in order to properly design the component parts.
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The movements which are to be absorbed by the pipe expansion joint shall include not only piping elongation or contraction but also the movement of attached vessels, anchors, etc. and the possibility of misalignment during installation. Unless included in the design requirements, misalignment of the pipe expansion joint must be avoided. Where movements are cyclic, the number of cycles expected shall be specified. Similar to pressure, the movements specified must be realistic. An excessive safety factor can often result in an expansion joint which is unnecessarily flexible; thus its stability under pressure is unnecessarily reduced.
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If the flowing medium can pack or solidify, provisions shall be made to prevent entrapment or solidification of the material in the convolutions which could result in damage to the pipe expansion joint or pipeline.
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Internal sleeves are usually installed in the direction of flow. If the stagnant flow medium trapped behind the sleeve is undesirable, drain holes in the sleeve, purge connections, or packing shall be specified. Where backflow will be encountered, an extra-heavy sleeve shall be specified to prevent buckling of the sleeve and possible damage to the bellows.
The predicted amplitude and frequency of external mechanical vibrations to be imposed on the bellows, such as those caused by reciprocating or pulsating machinery, shall be specified. A resonant condition in the bellows will result in a grossly reduced fatigue life and must be avoided. The pipe expansion joint designer will attempt to provide a non-resonating design; however, the ability to always assure non-resonance is impossible. Therefore, field modifications to the pipe expansion joint or other system components may be necessary.
The piping system drawings shall specify the location of all anchors, guides, supports and fixed points. Both the anchors and guides must be suitable for the highest pressures to be applied to the system. IN MOST CASES THE TEST PRESSURE WILL BE SIGNIFICANTLY HIGHER THAN THE SYSTEM OPERATING PRESSURE.
The system designer shall specify those special features which best accomplish personnel protection in his particular system. Piping systems containing high pressure and/or hazardous materials which are located in close proximity to personnel shall be provided with additional safety features which will protect such personnel in the event of a failure in the system. Pipe Expansion joints can be furnished with special features including, but not limited to, the following:
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Extra-heavy covers which could serve to impede the effect of a jet flow produced by a failure; however, such covers will not prevent the escaping medium from expanding and filling the surroundings in which it is located.
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Limit rods designed for dynamic loading can be employed to restrain the longitudinal pressure
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Thrust in the event of anchor failure. Such rods would normally remain completely passive until the anchor restraint is removed.
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A two-ply or two concentric bellows design may be employed with each ply or bellows designed to contain the full line pressure. The annular space between the plies or concentric bellows can be monitored continuously for leakage by means of suitable instrumentation. A change in pressure in the annulus could be used to detect bellows leakage.
The system designer shall provide for the accessibility of components (anchors, expansion joints, guides, etc.) in the piping system for periodic inspection after the initial startup.
PIPE EXPANSION JOINT DESIGN
The pipe expansion joint design shall conform to the requirements of the EJMA Standards, the ANSI Piping Codes and the ASME Boiler and Pressure Vessel Codes as applicable. The design of structural attachments shall be in accordance with accepted methods, based on elastic theory.
PIPE EXPANSION JOINT MANUFACTURING QUALITY
The pipe expansion joint manufacturer shall be required to furnish, on request, a copy of their Quality Assurance Manual.