When Expansion Bellow is Required in a Piping System?
Expansion Joints or Expansion bellows are mechanical devices that absorb thermal movement. It consists of one or more metallic or non-metallic bellows. In general expansion bellows or expansion joints (Fig. 1) is not recommended in a piping system. But sometimes, the bellows are employed in a piping system in one of the following situations:
- When the space constraints do not permit providing adequate flexibility by conventional methods (e. g. expansion loops etc.) for maintaining the system stresses within acceptable limits.
- When conventional solutions (e.g. expansion loops etc.) create unacceptable process conditions (e.g. excessive pressure drop).
- When it is not practical to limit the piping-induced loads on the terminal nozzles of the connected equipment within admissible limits by conventional methods.
- When equipment such as Compressors, Turbines, Pumps, etc. necessitates isolating the mechanical vibrations from being transmitted to the connected piping.
Design Considerations for Piping Systems with Expansion Bellow
For the bellow to effectively work the following design considerations shall apply to the Piping System that employs a Bellow.
Expansion Joint Standard: The design of the piping system employing a Bellow shall take into account the recommendations made by the Expansion Joint Manufacturer Association (EJMA) Standard. The specification for the Bellow shall as a minimum provide technical information as per the Specification Sheet given in Appendix-A of EJMA.
Type of Expansion Bellow: The type of the Bellow shall be selected depending on the Magnitude of the thermal movement of the connected piping and the configuration of the piping profile in relation to the location of the Bellow.
Expansion Bellow under Cyclic Loading: In general, caution shall be exercised in employing a Bellow in a piping system operating under cyclic loading conditions (e. g. due to temperature cycle). If no solution is available as an alternative to Bellow then the number of temperature cycles anticipated during the design life of the system shall clearly be indicated in the Bellow Specification for the Vendor to confirm the Fatigue Life Expectancy.
Stress Relieving of Expansion Bellows: Under normal circumstances stress relieving or annealing shall not be specified for the Bellow since the life of a Bellow greatly relies on its improved fatigue strength due to work hardening of Austenitic Stainless Steel during the forming operation of the convolutes.
Piping Support: The supporting of the piping system employing a Bellow shall be such that the movement of the connected piping at the Bellow ends is positively predictable. The support shall be adequately designed for the highest pressure either during operation or hydro test conditions.
Torsional Rotation: The Bellow shall not be subjected to Torsional Rotation.
Expansion Bellow Compatibility: The Bellow material shall be compatible with the flowing medium. Specific consideration shall be given to the following.
- Series 300 Stainless Steel against Chloride Stress Corrosion.
- Nickel Alloys against Caustic induced Stress Corrosion.
- Nickel Alloys against the presence of Sulphur.
Compatibility of Bellow material against pipeline cleaning chemicals if they are dozed in the piping system at regular intervals.
Leaching of the Bellow material from the insulation material.
- The requirement of Internal Sleeves shall be specified in an application involving fluid velocities which can induce resonant vibrations. The evaluation of acceptable flow velocity limits (based on EJMA, Section C-3) is covered in Section (Internal Sleeve Requirement) of this chapter. The sleeves are normally installed in the direction of flow.
However, in the case backflow is anticipated during operation the same shall then be highlighted clearly in the Specification to allow the Bellow manufacturer to provide an extra heavy sleeve and thereby prevent possible damage to the Bellow resulting from the buckling of the sleeve.
Bellow Design Pressure: The Design Pressure of the Bellow shall be based on the actual maximum working pressure of the pipeline under question. No ad-hoc cushion shall be allowed in specifying the Design Pressure.
Test Pressure of Expansion Bellow: The Hydraulic Test Pressure of the Bellow shall be 1.5 times the design pressure (1.1 times in case of pneumatic test pressure) with due allowance for temperature correction factor. However, the Test Pressure calculated in the above manner must not produce membrane stresses in excess of Yield Strength at room temperature.
Expansion Bellow Design Temperature: The Maximum, Minimum, and Installation temperature shall be specified accurately and a suitable pre-compression of the Bellow shall be considered and specified accordingly if ambient temperatures may vary significantly during pipeline construction.
Insulated Bellows: The Insulation Guard Sleeve shall be specified in the case of insulated Bellows.
Displacements absorbed by Expansion bellow: The displacements to be absorbed by the Bellow shall be based on the expected piping growth including the terminal movements. In addition to the above, the specified displacements shall also account for the following eventualities
The actual metal temperature of the piping may not necessarily attain the design temperature used for the calculation of the expected thermal growth of the piping.
The installed location of the Piping Supports (e.g. guides/ anchors etc.) in the field may not necessarily be exactly in accordance with the design.
The temperature range attained during the actual operation cycle may not necessarily be exactly the same as anticipated in the design.
The Bellows when handling process fluids that can solidify or scale to pack the convolutions shall be provided with necessary means to avoid the same. The use of internal sleeves, purge connections (for gas/vapor mediums), and drip holes (for trapped liquid) are some of the ways to achieve the same.
Extra Safety Features of Expansion Bellows
Depending on the severity of the process/ design condition the Bellow may be specified with extra safety features as below:
- Extra-heavy external cover which would impede the effect of the jet/ splash of escaping fluid in case of a Bellow failure.
- Limit Rods employed from the consideration of dynamic loading can be designed for restraining the pressure thrust in case of anchor failure.
- A two-ply or concentric double-shell Bellow with both the ply’s capable of withstanding the full line pressure.
It is normally a good practice to provide a sketch of the piping system with details of all supports and other design data as part of the Bellow Specification for establishing a clear understanding with the Bellow Manufacturer.
The supports on a piping system employing a Bellow shall have clear space around the supports to allow access for their inspection during the hydro test and during plant operation.