Therma-Flex Fabric Expansion Joints

fabric expansion joints

Fabric Expansion Joints for High Temp Flue Ducts

ThermaFlex Fabric Expansion Joints are non-metallic flue duct expansion joints that provide stress relief for piping and ducting systems by absorbing thermal growth and shock, isolating mechanical vibration, and allowing for misalignment.  ThermaFlex expansion joints are precision engineered specifically for your application.

ThermaFlex Flue duct expansion joints are custom engineered to handle low pressure (±3 psig) applications with temperatures ranging from -100°F to more than 2000°F.

Service Features of Fabric Expansion Joints

  • Absorb axial, transverse and torsional movements
  • Corrosion and chemical resistance
  • Negligible spring rates and loads
  • Easily repairable and installable
  • High cycle life
  • Unique application solutions
  • Vibration dampening and sound attenuation
  • More movement in shorter face-to-face
  • Less force to flex than metal expansion joints
  • Excellent corrosion and chemical resistance
  • Temperature capability range (-110°F to more than 2,000°F)
  • Lower shipping and installation costs
  • Variety of flexible belt materials available:
    • Single-layer and composite belts
    • Elastomers have excellent abrasion resistance
    • Custom designed for each specific application

Design Considerations

  • Media: Internal and External Environment
  • Temperature: Constant operating and maximum excursion
  • Pressure: Normal positive or negative
  • Movements: Axial and lateral, simultaneously or independently
  • Installation: Existing equipment and duct flanges
  • Assembly: Factory-assembled or field-spliced


Baffles are usually not necessary with elastomeric expansion joints due to the tube resistance to most abrasive particular matter. However, baffles must be used with fabric composite joints to protect the inner surface form particulates in the gas stream. UIP International, Inc. can supply baffles fabricated from carbon steel, stainless steel, or other materials as required.

Retaining Rings

Standard construction calls for 2″ wide, 3/8″ thick bars with slotted holes for easy installation. Material of construction can be carbon steel, corten or stainless steel.


  • U-type
  • Belt type
  • Square
  • Rectangular or round
  • No size restriction
  • Choice of FF dimension, depending on movements
  • Associated Metal Work: Backup bars, frames, flanges and baffles

Design Advantages of Non-Metallic Ducting Expansion Joints

  • Large movement capability / Multi-plane movements
  • Corrosion / Chemical Resistance
  • Range of Design Temperature Capability (-110° F -+2000° F)
  • Negligible Spring Rates / Loads
  • Vibration Dampening & Sound Attenuation
  • Lower Overall Costs (Design, Installation, Replacement & Repair)
  • Easy to Install / Repair
  • High Cycle Life
  • Unique Application Solutions


Industrial applications can be separated into general categories based on the media composition (Air or Gas) and temperature.  The following section is designed to aid in the selection of the appropriate expansion joint for the specific application range.  All plants are different, therefor the service locations and temperatures may vary.  This section is only a guide and should be confirmed with one our industry experts.

Product Applications

Industrial applications can be separated into general categories based on the media composition (air or gas) and temperature. The following section is designed to aid in the selection of the appropriate expansion joint for the specific application range. All plants are different, therefore, the service locations and temperatures may vary. This section is only a guide and should be confirmed with one of our industry experts.

Ambient Air Services

Ambient temperature clean air without particulate or chemicals to damage the flexible element. Expansion Joint is used frequently for vibration and sound attenuation from fan equipment.

A UIP International, Inc. integrally flanged elastomeric joint is suggested, using either the Therma-Flex or Mighty Span styles. Neoprene or EPDM single layer belts are frequently used.

Hot Air Services

Clean air, after coming into contact with hot flue gases at the air pre-heater where temperatures are elevated with minimal particulate and or gas carryover. Expansion joint will see thermal movements and vibration. Elevated temperatures require a composite flexible element and a flow liner.

A Therma-Flex flat composite belt with a bolt or weld in frame design and a flow liner is suggested. The weld-in outboard angle frame design with field welded flow liner (TWCP600VIFL) is shown.

Low to Moderate Temperature Flue Gas Services

Flue gas which has passed through an air pre-heater and dust collector to reduce the temperature and particulate level.Flue gas may cycle near the dew point where condensation can occur and chemicals are present. Expansion joint may see thermal movement vibration and chemical attack.

A UIP International, Inc. single-layer belt with chemical barrier is suggested in either integrally flanged or flat belt type—such as the Therma-Flex weld in outboard angle frame design and PTFE-coated single-layer belt with gas film layer (TWFPR500TA) shown.

Hot Flue Gas Services

Flue gas directly after combustion stage at elevated temperatures with possible particulate present. Expansion joint is used for possible large thermal movements at elevated temperatures.

Therma-Flex high temperature composite flat belt style with setback frames, cavity pillow and flow liners are suggested. The standard “Z” frame design with telescoping flow liners (ZZWCP1000FPRP shown) or “J” frame with shop liner are two designs used in these applications.

Special Applications

The UIP International, Inc. expertise extends to applications where service conditions require special designs / considerations such as:

  • Gas turbine exhaust and HRSG EJs—large axial movements, thermal shock and radial growth
  • Cyclone inlet, outlet and loopseals at CFBs—high flow velocity or turbulence and elevated temperatures
  • Stack and penetration Seals (HRSG)—lateral movement, field Installation and splicing
  • Cement plant applications—high particulate loading and cementous media
  • Pulp and paper plants—severe chemical attack and vibration
  • Petrochemical plants—elevated temperatures and chemical attack
  • Fabric wrap of existing metal expansion joint—quick inexpensive replacement and possible on-line repair

Anatomy of An Expansion Joint

A. Gas Seal Membrane

The gas seal membrane is designed to withstand system pressure and provide resistance to chemical attack from the interior and the exterior. The gas seal needs to have the flexibility to absorb thermal movements. Depending on system temperature, it may require additional thermal protection.

B. Insulating Layers

The insulating layers provide a thermal barrier to ensure that the inside surface temperature of the gas seal membrane does not exceed its maximum service temperature. The insulating layer can also reduce condensation that would be caused by the gas stream if it were to come in contact with the cool surface of an uninsulated gas seal membrane.

C. Insulating Retainer Layer

This layer keeps the insulating layers in place and maintains thermal integrity. The retaining layer must be able to withstand gas stream temperatures and must be chemically compatible with system media.

D. Back up Bars

Back up bars, positioned at the belt frame attachment, use clamping pressure to create the fabric-to-frame seal and restrain the fabric when it is subjected to the system pressure. The thickness and width of the back up bars should be sufficient to perform this function with the bolt spacing being used. The edges of the back up bars will have a radius to preclude cutting of the fabric.

E. Metal Liner or Baffle or Pillow

A liner is designed to protect the gas seal membrane and insulating layers of the flexible element from abrasive particles which may be present in the gas stream. A liner is also used to reduce flutter of the fabric element caused by turbulence, to help control the accumulation of dust or ash in the expansion joint cavity, and to reduce the temperature of the flexible element.

F. Expansion Joint Frame

An accumulation bag is intended to deter fly ash from building up in the expansion joint cavity. It is typically used, in conjunction with a liner, in duct runs from boilers to air clean-up equipment such as precipitators, scrubbers and bag houses, or whenever high amounts of dust or ash are present in the gas. A fly ash barrier must be capable of retaining its strength and flexibility while being exposed to maximum system temperatures and media.

G. Fabric Expansion Joint Frame

The expansion joint frame design controls the effectiveness of the connection of the flexible element to the ductwork. In some designs they can be attached directly to the duct work and thus eliminate the necessity for an adjoining duct flange. Frames can be designed with a “landing bar” duct attachment which allows some installation misalignment without affecting the flexible element. The frames establish the stand off height of the fabric, which is necessary to achieve thermal integrity during all movement conditions. The edges of the flanges in contact with the gas seal membrane should also have a radius to prevent damage.

H. Gasket

Single-ply fluoroplastic and fluoroelastoplastic belt designs such as Ultrachem materials require flexible chemically inert gaskets. Cuff tape can also be applied in this area to protect the belt from conducted frame heat and movement abrasion.

Single-Layer Expansion Joints

A single-layer expansion joint is made of a single layer of material only. Depending upon the area of application, the layer is made from fabric, various (fluoro) elastomers or fluoropolymers accompanied by fabric reinforcement as composite materials. Single-layer expansion joints provide the tightness and chemical resistance necessary when exposed to heavy condensate.

Pre-Assembled Units
Usage of fabric expansion joints that are pre-assembled onto steel components offers a number of advantages:

  • Installing unit into the duct is quick and easy
  • Transport brackets secure the unit into place during the transport and installation process
  • Larger units are transported in sections and are welded on-site for easier handling, transportation and installation
  • Clamping bar, belt and gasket assembly is done by factory-trained technicians which provides maximum pressure sealing capacity
  • Final assembly quality inspections (ISO 9000) insure maximum durability and performance

Multi-Layer Expansion Joints

The design of multi-layer expansion joints consists of four groups of materials. By combining different materials and considering their thermal, chemical, mechanical resistance, in addition to their fatigue properties, we can construct an expansion joint that provides the best solution in both technical and economical respects.

The four essential materials in the construction of fabric expansion joints include:

  • Outer covering material: This material ensures stability and provides protection to the expansion joint from pressure and temperature. Typically, this outer covering material is coated or laminated and can serve as a gas sealing barrier. Specially-designed fabric expansion joints may be constructed with stainless steel wire mesh and steel bands to provide additional mechanical protection and dimensional stability.
  • Gas sealing foil: This is the sealing element typically embedded between the layers of fabric or integrated throughout the outer cover. This element is impermeable and chemically resistant.
  • Insulating materials: These materials are utilized for the protection of both the gas sealing foil and other materials from effects of high temperatures.
  • Temperature-resistant fabrics: These types of fabrics are extremely strong and temperature resistant and are utilized for the protection of the gas sealing foil or insulation materials.

Important variables to consider when deciding the design and type of expansion joint needed:

  • Medium: The type of expansion joint needed is largely determined by possible chemical influences. Effects of abrasion from solid matter are largely preventable through the use of sleeve/baffle construction.
  • Temperature: Reducing temperature effects on an expansion joint require a specific number of insulating materials. Our experts determine this through calculation of temperatures in a complete expansion joint. The temperature range depends upon the expansion joint design.
  • Pressure: Determining whether the expansion joint will be used in a positive pressure or negative pressure area will have a great influence on the type and design selected.
  • Tightness (sealing): Tightness requirements ultimately influence the design and configuration of the expansion joint’s flange area.

Basic Design Configurations

Integrally Flanged “U” Type

The single layer belt can be provided either as a fully hand moulded UIP International, Inc. “Mighty Span” Style 600 Elastomeric expansion joint or a continuous moulded corner THERMA-FLEX Style Elastomeric or Fluoropolymer joint. Style 600 joints are available with or without various arch profiles moulded into the body. The service temperature is limited by the material rating. U-type joints are used frequently in fan applications due to their minimal hardware requirements and vibration/sound attenuation.

Flat Belt Type

Available in single layer or composite belts mounted parallel to the duct on attachment frames. The frames may be either bolted to mating flanges or welded directly to the ductwork. See above for standard frame profiles. Flat belt types are commonly used in high temperature applications and where a setback is recommended or required or required. This configuration can be designed to readily accept various accessories such as telescoping or floating flow liners, cavity pillows and fly ash seals.


Standard Non-Metallic Expansion Joint Profiles