Criterias for choosing the right type of expansion joint
Expansion joints are vital components in most industrial plants. They are installed as flexible connections in duct and pipe systems to compensate for the inevitable thermal expansion, vibrations, and minor misalignments.
A number of important factors have to be taken into consideration when choosing the right expansion joint. These factors include: the operating conditions of the plant, the necessary chemical or thermal resistance levels, requirements for pressure resistance, noise suppression, and absorption of vibrations, etc.
The range of rubber, metal and fabric expansion joints is wide and range from flexible single-layer joints, e.g. used in HVAC duct systems, to very complex multi-layer composite constructions with bolster used in conventional power plants, the petro-chemical industry, flue gas cleaning, oil, gas and offshore installations, etc. This type of expansion joint give optimum performance at operating temperatures of up to 1400 C.
An area for experts
The design and production of expansion joints is for specialists and except for standard types for very simple purposes, the supplier/manufacturer should always be involved in the dimensioning and final choice. Expansion joint solutions are often custom-made solutions for a specific purpose. To avoid unnecessarily difficult and expensive solutions, it is advisable to involve the supplier already during the planning for advice on optimum allocation of movements, ets. A fabric expansion joint is made of various layers and the function of the joints depends on the properties and composition of those layers. In principle, the typical manufacturer has more than 50 well defined materials types to choose from. In order to ensure optimum functioning and life under the given conditions, combinations of these are tested in the laboratories and at pilot plants to document the properties.
Criterias for choosing the right expansion joint
Whether it is a new construction, where the conditions require a technical/economical optimum solution, or an existing plant, where a less optimum installation may have to be accepted, a number of criteria are decisive in order to make the right choice.
1. Some of the planners of projects may be inclined to consider placement and space as secondary. However, it is important to provide sufficient accessibility for installation and later maintenance and testing. In cases where the placement of the expansion joint means that it is exposed to a high ambient temperature (e.g. 760 C) or bad cooling conditions, this should be taken into consideration already when determining the design. It may be necessary to use a special type of insulation or reflecting covering.
2. The primary factors deciding the build up of the materials are the type of medium and the plant´s form of operation. Air can be clean, dust-laden, chemical-laden, or radiated. Flue gas varies according to type of fuel and can be damp (dew point is not met), contain soot, or fly ash, etc. Especially chemical resistent materials are used when the expansion joint is installed in plants where it is exposed to very aggressive medias or varying PH-values. Under those conditions optimum density is ensured, i.a. due to the special properties of the material and the special flange gaskets.
3. The contents of solids influences not only choice of material composition and surface treatment but also the expansion joint construction itself (internal sleeves, pre-expansion joint, abrasion plate, etc. may be required).
4. The temperature has considerably influence on the material choice and composition (insulation), building form (P- or V-flange), and construction (e.g. placement of the flange).The outer cover materials (PTFE®, Viton®, silicone, etc.) are protected against thermal overload by insulation layers. Thickness and number of layers is dependent on the temperature.
5. The pressure is also decisive when it comes to choosing the build up of layers (sufficient strength) and construction (e.g. with support rings). Apart from dimension and test pressure; positive and negative pressure, surge, and pressure pulsations influence the choice.
6. The requirements for density depend on application field and medium. The density of the bellows and installation units are distinquished. Pressure and temperature influence the construction and in general, it is easier to insulate a flange expansion joint.
7. Formations of condensate may occur if the dew point is not met. The cause may be the process itself or it may occur during the stop of the plant or operation with reduced capacity. The dew generated increases the chemical stress on both expansion joint and duct system. To protect the expansion joint from rain, snow, sandstorm, etc. plate covering is used. This also protects from contact.
8. Flow velocities above a certain size can cause vibrations/pulsations which are normally compensated for by using internal sleeves, bolsters, and similar.
9. Mechanical stress, such as vibrations, oscillations, and sound waves, is accommodated by fabric and rubber expansion joints. Naturally, it is important to avoid over load (any form changes apart from the ones projected), abrasion caused by contents of particles (internal sleeves), rubbing against other components in the plant, etc. That way, vibrations will not be carried through a fabric expansion joint.
10. Movements are accommodated in axial direction (compression/elongation), lateral and angular directions (misalignments), and torsion movements. Contrary to steel expansion joints, fabric and rubber expansion joints can accommodate combinations of the above. Reaction forces in fabric expansion joints are normally regarded as insignificant and the flexibility of the fabric expansion joint makes it considerably less bulky than a similar steel expansion joint.
11. Installation of a fabric expansion joint is fairly easy and very quick. In principle there are 2 ways to do it: a smaller expansion joint can be delivered pre-assembled on the steel parts and ready to install on the duct flanges.
As for larger expansion joints, it is necessary to carry out the installation on site due to transportation. Where necessary, a supervisor can be used to lead the installation work.
CAE for design optimization
Expansion joint constructions are exposed to considerable influences in many installations, not least combinations of influences and peak load periods, e.g. gas turbines.
In expansion joints with integrated steel parts for flange or welding installation on site, cracks in these steel parts occur from time to time – sometimes after only a short time of operation. The reasons for this are, e.g. poor steel design, wrong steel quality, bad welding, and poor insulation of steel parts.
Modern computer systems such as CAE are obvious solutions for construction and analysis of complex expansion joints and the influences to which they are exposed. Based on the ANSYS® finite element method (FEM), KE-Burgmann has developed their own models for use in analysing the construction of which expansion joints are part. In particular:
– temperature distribution in steel parts and insulating material
– stress/strain arising in steel parts
– heat loss throughout the expansion joint unit
As a result of this analysis, we are able to
– calculate the life expectancy of expansion joint units
– optimise the construction
– recommend the best steel quality
FEM is part of the package solution that can be supplied, and it is used to carry out trouble-shooting in existing plants and to propose design changes.
Expansion Joints Division
Park Allé 34
Tel. +45 75361811
Fax. +45 75361532