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Sealings/Heat Exchangers How New Materials Ensure Reliable Sealing of Heat Exchangers

| Author / Editor: Christian Wimmer* / Wolfgang Ernhofer

ePTFE gasket tapes for high chemical resistance—The connections to shell and tube heat exchangers pose immense challenges for the seals that are used because of both the chemically aggressive media and the frequent temperature load changes. Despite its excellent chemical resistance, polytetrafluoroethylene (PTFE) is not typically suitable as a sealing material in this case because the creep tendency of this material jeopardizes a reliable seal.

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Progression of the sealing surface pressure over time. Based on EN13555, PQR, using 15mm x 3mm bands laid to form a closed ring (Ø150mm). The values were determined at 30 MPa and 150 ˚C (302 ˚F).
Progression of the sealing surface pressure over time. Based on EN13555, PQR, using 15mm x 3mm bands laid to form a closed ring (Ø150mm). The values were determined at 30 MPa and 150 ˚C (302 ˚F).
(Source: Gore, Graphic: PROCESS; Picture: © Robert Kneschke - Fotolia)

Shell and tube bundle heat exchangers generally include not only several connection pieces but also a shell cover flange with a significantly larger nominal diameter. The shell cover flange creates the seal to the tube bundle flange, which then seals the shell flange.

These connections are subjected to the full operating pressure and test pressure of dozens of bar and seal it from the environment. Oftentimes the tube bundles are arranged in several passes that are channeled by the use of partition plates. This requires a seal at the head of the heat exchanger and between the tube passes at the partition plates where the pressure difference is considerably lower. The numerous potential different arrangement of the passes requires specially adapted seal designs.

Gallery
Gallery with 8 images

The sealing challenges that pertain to the shell cover flange position can be attributed to a combination of different conditions:

  • High temperatures/pressures
  • Temperature load changes caused by starting up and shutting down part of the process
  • Complex sealing geometries including separators, often combined with large nominal diameters
  • Chemically aggressive media
  • Damaged sealing surfaces, due to corrosion or warping
  • The economic necessity of minimizing downtime
  • The requirement to document legal compliance

REM images of the Gore ePTFE
REM images of the Gore ePTFE
(Picture: Gore)

“Creeping“ Gasket Tapes Made of PTFE

Gasket tapes are occasionally used to seal the static connections to shell and tube heat exchangers – sealing materials with a defined width and thickness, but an undefined length. They offer a way of functionally connecting the junction points and can be shaped as desired at the point of installation.

Thermal Processes at Achema 2015
Gallery with 10 images

Since the gasket circumference is completed at the time of installation, it is no longer necessary to completely dismantle the heat exchanger, removal of the tube bundle, for example. It is only important that the sealing surfaces be sufficiently accessible. Gasket tapes have been around in many different industrial fields for several decades.

Additional Information
Characteristics of Gasket Tapes Made of ePTFE
8 Good Reasons for Usage
  • Easy and fast adaptation to suit even complex flange geometries
  • Design flexibility because of the various widths, which then allow for higher surface pressures (higher tightness classes)
  • High creep resistance compared to other PTFE-based materials
  • Excellent adaptability to flange irregularities
  • Not reusable (due to the high compression and hardly any spring back)
  • Non-biodegradable, do not age, UV-resistant
  • Resistant to all types of media (pH 0 to 14)
  • Storable for a long time, limited only in terms of the declining bonding strength of the mounting aid adhesive tape

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