8 key elements for media containment

There are several key elements that go into deciding the best membrane for your expansion joint.

1) Flexible membrane concept . The flexible component of a fabric expansion joint is engineered using proven design methods.  The primary goal is to provide a structural membrane capable of absorbing the thermal growth and movements of a ducting system while containing internal pressures and media.  At the same time,the membrane material must provide maximum longevity when exposed to the system operating environment.

2) Thermochemical stability. While simultaneously meeting mechanical requirements, an expansion joint membrane must perform in severe heat, weather, and chemically hostile environments.  Similar to high alloy metals, the cost of a membrane material is directly related to its ability to resist high temperatures and chemical attack.

3) Flexibility. Flexibility is a primary design requirement which greatly affects the performance of a membrane.  Flexibility rapidly decreases as membrane thickness increases.

4) Permeability. A gas seal membrane must be non-permeable to pressurized gases and liquids. This quality must be maintained despite continued exposure to the harsh thermal, mechanical, and chemical environment of a given duct system.

5) Toughness. A membrane material must be resistant to mechanical abuse such as abrasion, puncture, and tearing.  The adhesion of coatings to reinforcing materials must resist delamination when subjected to high stress.

6) Wicking resistance. In non-homogeneous membranes, wicking of liquids between coating and reinforcing materials may cause accelerated deterioration, especially in a high acid or caustic environment.  Wicking of such liquids may break down the adhesion of different materials, reduce the mechanical properties of one or more materials or both.

7) Strength. For a given load, the strength of a reinforcing material is used to determine the required thickness of a membrane. The use of exceptionally high strength materials allows thinner membranes to be engineered and helps optimize all design requirements.

8)Stress. “Membrane stress” becomes a primary design factor when the thickness of a material is small in comparison with the other dimensions.  “Hoop stress” and “longitudinal stress” are the two elements of membrane stress representing the average tension over the thickness of a membrane, and are considered to act in the plane of the fabric surface. Stresses imposed by bending are neglected when calculating membrane stress, making it easy to determine the required thickness of the material when its mechanical properties, geometric shape and system pressures are known.

Bending stresses are more difficult to calculate and are extremely sensitive to changes in thickness. In areas of severe bending or folding, such as the corners of a fabric expansion joint, the fivers farthest away from the neutral axis (at the surface of the membrane) are subjected to the highest stress and elongation. These high bending stresses cause the surface of the membrane to deteriorate more rapidly than the center fibers. This phenomenon can be observed on the sidewall of a flat tire after a prolonged period of time.

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