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Design and fabrication requirements of expansion joints for use in low pressure air and flue gas service?

asked Oct 15, 2015 in Mechanical by cchang219 (2,310 points)
Can someone recommend the design and fabrication requirements of expansion joints intended for use in low pressure air and flue gas service associated with fired heaters, boilers, and other fired equipment?

2 Answers

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answered Oct 16, 2015 by TPham (5,010 points)

You should specify the followings when request for cost estimate:

  • Maximum operating pressure and temperature (continuous and short duration).
  • Number of cycles
  • Movement in all planes
  • Permissible leakage
  • Joint orientation
  • Requirements for blinding or purging
  • Flue gas composition and acid dewpoint

The design life should be the greater of 10 times the anticipated number of cycles in 20 years, or 1000 cycles.

The design pressure should be the greater of:

  • 10 in. water gauge (250 mm H2O).
  • 1.5 x the expected maximum operating pressure (or fan shut-in pressure).
  • 5 psig (0.34 bar gauge) for metal bellows.

The design temperature should be 50°F (28°C) above the maximum short duration temperature.

All expansion joints should have an external shroud to protect them from damage.

An internal baffle should be provided to protect the flexible element from debris in the air or gas stream.

Ceramic fiber insulation, encased in a ceramic fiber cloth bag, should be installed in the void area between the metal bellows or fabric belt and the internal sleeve.

Metal temperature of expansion joint parts in contact with the flue gas should be maintained above the acid dewpoint.

Attachment of fabric belts should be such that they can be replaced without dismantling the joint or removing it from the ducting.

Slip joints may be used where motion is <1 in. (25 mm) and will occur in one plane only.

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answered Oct 18, 2015 by sicalamanto (1,560 points)
Materials of construction of each layer of belting should be in accordance with the appropriate Fluid Sealing Association Technical Handbook referenced in Section 2.4. Metal bellows should be fabricated from austenitic stainless steel, Type 321.

For applications with design temperature above 1000°F (538°C), austenitic stainless steel bellows after cold forming should be fully solution annealed followed by rapid air cooling.
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