Expansion Joints and Bellows

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Fabric Expansion Joints

Selection Criteria

 

 

Expansion Joints and Bellows

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Expansion joints are one of the most important, and at the same time relatively vulnerable, components in duct systems and pipe lines. It is therefore of vital importance, in order to avoid costly shutdowns of systems and technical problems in general, that each individual expansion joint is designed exactly in accordance with the operational conditions present. The most important factors, of which several are interdependent, are described in the following. When expansion joints are required the following points should be taken into consideration at the design stage.

Typical Installations

 

 

 

Installation Position
First of all, the mounting position for each expansion joint should be evaluated to find the best technical and economical solution. The most suitable style and profile of expansion joint has to be determined and the type of duct flanges and dimensions are designed accordingly.

Where access for installation is restricted,  a complete expansion joint unit could be considered, this would include all the steel parts and fitted with mechanical restraints ready for bolting into the ductwork. Alternatively, some of the steel parts, e.g. the backing flanges only, can be supplied along with the expansion joint.

It may be preferred to order only the fabric element of the expansion joint, in either a “closed” version, or as an “open” version including a joining kit for joining of the expansion joint on site. Fabric Joints can be supplied  with the bolt holes pre-drilled or without bolt holes for drilling on site.

Medium 

The composition of the layers of an expansion joint is highly dependent on the medium, which is to be conveyed. It is therefore of the utmost importance that the contents of the gas is known. The influences on the fabric layers could be acids, solvents, dust, solid matter and many others.
If there are abrasive particles or solid matter in the gas stream, an inner metal sleeve should always be installed, and preferably in combination with an insulation bolster, if there is a risk of particles or dust accumulating between the inner sleeve and the expansion joint.

When the medium is flue gas, an analysis of the gas is normally required in order to make the proper choice of expansion joint. Especially when conveying highly aggressive, or otherwise dangerous gases, gas tightness of the expansion joint can be a requirement. In general, LBH expansion joints can be described as being gas tight. However, it is not possible to guarantee that the clamping area of a standard fabric expansion joint is totally gas tight.

A solution can be found for the majority of special requirements, either by the use of special fabrics, using various sealants or by supplying a special clamping arrangement, where boltholes in the clamping area can be dispensed with. Alternatively an LBH AMANTEK elastomeric expansion joint could be considered.

Temperature

The basic selection criteria are operating, design and peak temperature, together with the ambient temperature and the duration of the peak temperature.

Depending on the temperature involved, one or more insulation layers are
used in the construction of the expansion joint and where necessary, in conjunction with an insulation bolster. For some applications, external insulation of the expansion joint can be an advantage, in order to prevent the temperature of an aggressive medium from continuously dropping below the dew point,  forming a potentially corrosive condensate.

However, it is important to note that expansion joints may only be covered with external insulation in the low-/medium temperature range, and only after approval from LBH. For instructions regarding external duct insulation, see LBH Installation Manual, form SM-0400.

Pressure

The pressure of the medium is another important factor, which determines the optimal type of expansion joint. The LBH range of expansion joints profiles includes types for pressure as well as for vacuum applications.

The design criteria for pressures are operating ,design and peak pressure, together with information regarding the possibility of pulsations occurring in the gas stream. Damage to the expansion joint due to pulsations can be avoided by a special composition of the fabric layers, by metal support rings, or by choosing a suitable inner sleeve. An inner sleeve is always
recommended, but can be dispensed with under certain conditions, e.g. in applications with a relatively low gas velocity.

Movements

The profile, and in particular the building length (F/F), of an expansion joint is highly dependent on the movements of the duct or pipe line, which are to be absorbed by the expansion joint. The possible movements of non-metallic expansion joints, except for vibrations, are illustrated below

(left to right): axial compression (-) / axial extension (+), lateral offset, angular offset, and torsion. The need for a relatively large F/F-length, due to excessive movements, can sometimes be reduced by installing the expansion joint in a “pre-set” position.

Note that non-metallic expansion joints are not self-supporting and therefore to function properly correctly dimensioned fix-points and supports in the ductwork are essential.

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