Operational & Design Considerations

The following background information on the plant and application can have significant influence to the design life of the expansion joint.

• Plant type
• Location of expansion
• Environmental conditions
• Base or peak load plant
• Relevant history of existing and adjacent equipment

Flow Media

In defining system media, the designer should specify the type of fuel, sulfur content, and any additives, cleaning agents or catalysts, which may be used in the system. Ph in the vicinity of the expansion joint should be specified, if known, especially if the media is extremely acidic or caustic.

The probability of condensation should be determined by comparing dew point and normal operating temperatures. Even in systems which normally operate quite hot, shutdowns and startups may produce condensation resulting in excessive corrosion. This condition can also occur on the outside of the steelwork and should be considered.

Abrasive components of the media such as flyash should be identified. Flyash buildup within a flue gas duct should be estimated, and the dead weight calculated to determine the maximum load on an expansion joint element. Plant operating procedures such as duct cleaning (wash-down or vacuuming) are also considered in the design.

Temperature

Temperatures affect the selection of fabric expansion joint materials. The following temperatures for the system should be provided to the engineer.

• Normal operating temperature
• Maximum continuous operating temperature
• Design temperature
• Excursion temperatures and duration
• Ambient temperatures (the lowest, highest and average)
• Dew point temperatures

Pressure & Expansion Joint Leakage

Fabric expansion joints are typically designed for low pressure duct systems with maximum rating at +5 psig for elastomeric and +3 psig for fluoroplastic and fluoroelastoplastic belts.

Fabric expansion joints are designed to be as leak tight as practical. When an unusual amount of liquid is present within the ducting, or leakage requirements are specified, special caulking or gasket materials can be used when attaching the fabric element to attain the desired results. In industrial applications, minor leakage, detectable by soap bubble solution, is considered acceptable.

When replacing a fabric element, leakage through bolt holes is minimized if holes are aligned and punched in the field as opposed to prepunching the holes at the factory. Backup bar bolts should be tightened to 35 to 45 ft. lbs. (45-61 Nm) to ensure optimum clamping pressure.

Movements

Various movements resulting from thermal expansion of the breeching, both Maximum Continuous System Operating Temperature (MCSOT) and excursion temperatures should be specified. Contractions of the breeching due to cold winter shutdowns, mechanically induced movements such as fan and equipment vibrations, and structural deflections due to wind loads and seismic events must also be specified.

Movements in various directions should be analyzed to determine if they occur simultaneously or if they can occur individually without a corresponding deflection in another direction. The designer to make certain that the expansion joint design is capable of absorbing a combination of the total maximum movements should specify maximum installation misalignment.

System Geometry/Installation

The geometry of the expansion joint system, including duct size, orientation, material and attachments all have an effect on the selection of expansion joint frame configuration. The installation of an expansion joint can be performed either internally or externally. Specific frames are designed for each application depending on preferences or accessibility.

External Environment

Correct operation of high temperature expansion joints requires that a portion of the system heat be dissipated to the external environment. Abnormally hot ambient conditions or an adjacent heat source, reflective surface, or duct insulation may create temperatures that exceed the limits of the gas seal membrane and should be considered when designing the system.

An external cover may be desirable to help protect against failing objects or the accumulation of combustible materials such as coal or saw dust. Covers should be designed by the expansion joint manufacturer to ensure that proper air circulation requirements are satisfied.

Expansion Joint Flutter

The inherent flexibility of expansion joints can cause fatigue problems as a result of physical fluctuations known as flutter. Flutter is a result of either flow induced or operating pressure oscillations. Both conditions can cause a premature failure and must be identified in the quotation/evaluation stage. This condition is commonly found adjacent to turning vanes, ID fan and stacks. This pressure oscillation typically exists in ductwork locations where the static pressure is very low +-1 to +-2" w.c (+-25 to +-50 mm H2O).

Installation Considerations

The system designer must consider all installation requirements such as mating duct flanges, adjacent ducting equipment, insulation, painting/ finishing, field assembly, erection and necessary qualified supervision.

Responsibility

The system designer must evaluate whether the lines of responsibility are clear between the ducting supplier, erector, and the expansion joint supplier, and if they will result in a system that meets specification requirements at an optimum overall cost. It is usually in the best interests of the end user to specify and/or order the expansion joints themselves.