Pipe Expansion Joint: Design Pressure of 65 PSIG and Design Temperature of 1076ºF 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. The piping system should be reviewed to determine the location and type of pipe expansion joint most suitable for the application.

1. Metal Bellows Design Guide
2. Rubber Bellows Design Guide
3. Metal Bellows Design Guide

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. 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 steels 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. 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.

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. 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.

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. The movements which are to be absorbed by the pipe expansion joint shall include not only piping elongation or contraction, but also 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. 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. 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 eprovided 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:. 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. Limit rods designed for dynamic loading can be employed to restrain the longitudinal pressure.

Thrust in the event of an anchor failure. Such rods would normally remain completely passive until the anchor restraint is removed. 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 initial start up. 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.

The stock number is used to identify each unique bellows made to standard dimensions. Available stock parts are presented in the Stock Bellows Data List. Stock number nomenclature is composed of four elements separated by dashes (-).

The first element refers to the material; the second represents the outside diameter of the tubing used to form the bellows; the third identifies the wall thickness of the tube; and the fourth indicates the spring rate. An example of the nomenclature is shown below. Example Stock Number: SS-125-46-80 Material: SS (Stainless Steel) Tube OD (inches): 0.125 Wall Thickness (inches): 0.0046 Spring Rate (lbs./in.): 80 Material designation: B = brass, BC = beryllium copper, H = hastelloy, IX = inconel X-750, I600 = inconel 600, I625 = inconel 625, I718 = inconel 718, M = monel, N200 = nickel, NSC = Ni-span C, PB = phosphor bronze, R = rodar, SS = 300 series stainless steel. Stock parts may be modified per customer’s specifications.

A modified stock bellows is signified by an “M” at the end of the stock number (e.g., SS-125-46-80M). The neck of a bellows is located on both ends of the convolutions and is used to attach mating parts. The neck diameter (inches) is based on tooling but can be modified by expansion or contraction.

Stock tolerance is typically ±0.002. Production runs average ±0.001. Custom tolerances of ±0.001 or less are possible. Tight tolerance control is achieved by applying uniform pressure on the outside diameter using a round collet while supporting the inside diameter with a standard plug gage. Care must be taken to not over stress thin-walled necks. The length of a tube neck (inches) is measured from the outer face of the end convolution to the end of the neck. The length of a cup neck is typically measured from the inner surface of the cup to the end of the neck.

Other neck configurations are available for customized products (see “Neck Types” below) and neck lengths are based on customer specifications. Stock tolerance is typically ±0.015.

Custom tolerances of ±0.005 or less are possible. Neck Types The Stock Bellows Data List presents available neck configurations for stock bellows. Stock bellows necks are typically tube or cup neck configurations.

Other neck configurations are available as optional modifications for customized products. Tube Neck: “A” type necks are the standard type and most consistent in size, and therefore preferable when tight tolerances are required. Cup Neck: “C” type necks provide an alternate attachment configuration and are typically selected to meet limited access requirements. Flange Neck.: “F” type necks can be made on one or both ends of any bellows. The flange neck diameter is typically 75% of the convolution outside diameter (Column 3). The flange neck length can be customized upon request.

Root Neck.: “R” necks can be made on one or both ends of any bellows. Root necks are typically incorporated to customize the length of a stock bellows neck.

Metal Bellows Design Guide

Split Convolution Neck.: “S” necks can be made on one or both ends of any bellows. Split convolution necks are also typically incorporated to customize the length or diameter of a stock bellows neck. Custom Necks: Mini-Flex specializes in custom products. Please consult Mini-Flex engineering staff for details.Important Note: Cutting a stock bellows neck and modifying it with a flange, root, or split convolution neck will increase the spring rate and squirm pressure and decrease the convolution free length (Column 7) and maximum deflection in compression (Column 8). The convolution free length is the free relaxed length (inches) of the convoluted section of a bellows and is measured from the outer faces of the end convolutions. For bellows with cup type necks, the convolution free length is measured from the inner surface of the cup at the base of the neck(s).

Overall length equals the convolution free length plus both neck lengths. Stock tolerance is typically held to +0.050 -0.010. Tolerance on production runs average ±0.005. Modified tolerances of ±0.005 or less are possible on custom products. Supplemental information:. The convolution pitch is approximately equal to the convolution free length divided by the number of convolutions (Column 13). The minimum free length or maximum compressed length of convolutions is equal to the convolution free length (Column 7) minus the maximum deflection in compression (Column 8).

The convolution free length can be modified for certain applications. See “ Number of Convolutions” (Column 13), “ Maximum Deflection in Compression” (Column 8) and “ Neck Types” for more information. For flexible hose applications: Stock bellows can be customized to a maximum of 200% of the stock convolution free length.

The maximum deflection in compression is defined as the maximum compressed movement (inches) from the convolution free length position (Column 7) that will not result in permanent deformation of the convolutions. The maximum deflection in compression is determined by compressing the bellows until convolution contact creates a significant increase in force. Additional compression is available when the end convolution face(s) are free to move. The additional compression stroke is equal to approximately 40% of the stroke per convolution per end. Supplemental Information:.

Maximum compression per convolution (D c) is equal to the maximum deflection in compression (D m) (Column 8) divided by the number of convolutions (N): D c= D m /N. Movement beyond the maximum deflection in compression or extension beyond the relaxed free length of convolutions will decrease cycle life, change the spring rate and cause permanent deformation which will not allow the bellows to return to its original length without the utilization of mechanical force. Increasing the maximum deflection in compression is achievable by extending the bellows free length (Column 7) by 175% maximum of the convolution free length. It is inadvisable to use the total maximum deflection in compression when long life is required. Hydro-formed bellows function best in the compressed state. Spring rate pounds per inch (lbs./in.) is the dead weight or force in pounds required to compress a bellows one inch.

Stock bellows are usually rated at 30% to 50% of the maximum deflection in compression. Spring rate linearity varies from part to part and within the specified maximum compressed range (convolution free length minus maximum deflection in compression).

Consult Mini-Flex engineering staff when linearity is a concern or when a specific spring rate is required at a designated stroke. Stock tolerance is typically ±20%. Custom tolerances of ±5% or less can be achieved. Supplemental Information:. Force required to compress the bellows (within its specified range) equals the spring rate multiplied by the travel. Spring rate per convolution equals the spring rate multiplied by the number of convolutions.

The effective area is the calculated area in square inches of the effective diameter, which lies approximately halfway between the inside and outside diameter of convolutions. The effective area tolerance is representative of the variation in the convolution outside diameter (the convolution inside diameter is fixed and cannot be modified). Formulas: (A) Mean Effective Area (square inches) (A e) External Effective Area (square inches) (A i) Internal Effective Area (square inches) (O) Convolution Outside Diameter (inches) (I) Convolution Inside Diameter (inches) A= π ((O+I)/4) 2 or simplified; A = 0.1963(O+I) 2 A e =.1963(O+(I+2t)) 2 A i =.1963((O-(2t.8))+I) 2 Volume (V) in cubic inches equals effective area multiplied by length. Bellows volume capacity (less the neck inside diameter) is effective area multiplied by the convolution free length (L): V=AL.

Rubber Bellows Design Guide

Volume displacement is equal to the stroke (D) times the effective area V d=AD. Pressure (P) in pounds per square inch required to compress the bellows any distance within its maximum deflection equals spring rate (R) multiplied by the deflection (D) and divided by the effective area (A): P= RD/A. Critical squirm pressure is measured in pounds per square inch (PSI). Squirm occurs when the convolutions are unrestrained from sideways movement, the necks are fixed, and the bellows is subjected to internal pressure.

Metal Bellows Design Guide

When critical squirming pressure is reached, a slight bow, or sideways movement occurs. When pressure is increased, the bellows will lose stability and eventually enter into the form of a “U” bend.

Squirm will not occur if the bellows is guided internally or externally by a rod stem or in a close fitting bore. Squirm is more likely to occur when the convolution free length exceeds the convolution outside diameter. Supplemental Information:. The squirm rating is provided for reference purposes and will vary with convolution wall thickness (within the tolerance range). Depending on application, the actual squirm pressure may be considered the maximum internal proof pressure (see “Maximum Operating Pressure” below). Higher critical squirm pressure ratings are more desirable when bellows function is critical (greater critical squirm pressure ratings result in increased safety factor and longer bellows life). Exceeding the actual squirm pressure will cause sidewall yield that may cause an increase in spring rate and a decrease in maximum deflection.

In some cases this deformation is minor and will not affect the bellows function. External pressure does not cause squirm regardless of length. Maximum Operating Pressure: The maximum operating pressure of a bellows is considered the proof pressure, and is unique for each application.

The minimum internal or external burst pressure pounds per square inch (PSI) represents the minimum internal or external pressure that will not result in fracture when a bellows is restrained from squirm or sideways movement. At this pressure, severe permanent deformation of convolutions takes place, possibly rendering the bellows useless. This designation is used for design information only and is dependent on the tensile strength of the forming material. Actual internal or external burst pressures are higher.

Modifying a bellows convolution quantity is possible by splitting at the major or minor diameter or cutting a flange from or between any convolutions. This change would then become the bellows neck. See “ Neck Types.” Supplemental Information:. Decreasing the number of convolutions will increase the spring rate and squirm rating and decrease the maximum travel and length. Increasing the number of convolutions will decrease the spring rate and squirm rating and increase the maximum travel and length. Formulas Volume (V) in cubic inches equals effective area multiplied by length. Bellows volume capacity (less the neck inside diameter) is effective area multiplied by the convolution free length (L): V=AL.

Volume displacement is equal to the stroke (D) times the effective area Vd=AD. Pressure (P) in pounds per square inch required to compress the bellows any distance within its maximum deflection equals spring rate (R) multiplied by the deflection (D) and divided by the effective area (A): P= RD/A. A: Effective area of a bellows (nominal) A e: Effective area of a bellows (external) A i: Effective area of a bellows (internal) A d: Effective diameter of a bellows D: Axial movement per bellows (compression) D c: Axial movement per convolution (compression) D m: Maximum axial movement per bellows (compression) F: Force (lbs.) h: Convolution height (external) I: I.D. Of convolutions L: Convolution free length L m: Free length minimum compressed length N: Number of active convolutions O: O.D. Of convolutions P: Pressure (PSI) p: Pitch of convolution R: Spring rate of bellows (lbs./in.) R c: Spring rate per convolution t: Tubing wall thickness V: Volume (cu.in.) V d: Volume displacement (cu.in.).