Similar Applications: Fossil Fired Power Plant (Gas Recirculation System), Pulp and Paper Plant (Recovery Boiler to Precipitator), Refinery (Turbo-Expander to CO Boiler and CO Boiler to Precipitator), Cement Plan (Clinker Cooler to Heat Exchanger)
Typical Conditions: 650°F to 850° operating temperature, -10″ to -25″ WG pressure, fuel gas media with heavy particulate, boiler growth contributes to large axial or lateral expansion joint movements depending on the orientation of the joints
Similar Applications: Fossil Fired Power Plant (Air Heater to Coal Pulverizers), Cement (Clinker Cooler to Heat Exchanger)
Typical Conditions: 600°F to 750°F operating temperature, 5″ to 80″ WG pressure, clean air media, boiler growth contributes to large axial or lateral expansion joint movements depending on the orientation of the joints.
Common Design Features:
Fabric Belt: High temperature fabric belt. (FLEXXCEL HT1, HT3, or HT5 depending on maximum temperature.)
Accumulation barrier: 6″ minimum standoff and outboard belt attachment flanges to dissipate heat.
Liner: contoured around expansion joint to allow heat dissipation.
Fabric Expansion Joints and Factors Influencing their Design
Fabric expansion joints perform a function of compensating for duct misalignment and duct thermal growth typical in power plants and other ducting systems. Proper design of these joints starts with asking the right questions about the application, providing the correct answers, and applying design rules to arrive at the appropriate solution.
The guiding principle for fabric joint design is to protect the fabric belt element so that it can absorb movement while retaining the media. The longevity of the belt life can be diminished by many factors. These factors include excessive temperature, harsh corrosives, exposure to abrasive particulate, excessive movements, fly ash weight against the belt, and high internal pressures. All of these problems can be solved if they are anticipated. The quality of the expansion joint design is only as good as the information provided up front. A realistic and accurate analysis of the system is step one. Assuming that is taken care of, these guidelines are a brief introduction to factors that influences the success of the expansion joint.
Fabric gas seal membranes have specific temperature capabilities. When necessary, the addition of insulating materials between the temperature source and the belt will extend the service life. The magnitude of the temperature will determine the thickness of the integral belt insulation and if a separate high density insulation pillow is required.
The belt attachment flanges should be outboard of the cavity and have sufficient standoff from the duct. Care should be taken to avoid external insulation or lagging outside of the belt which prevents proper heat dissipation.
Applications that do not have high temperatures sometimes have a different problem. Relatively low temperatures in flue gas ducting can lead to corrosive condensation. In these situations, a chemical barrier is required to protect the load bearing fiberglass carcass of the belt. External insulation over the joint in these locations can reduce condensation and heat loss.
Generally, movements occur along the axis of the duct (usually compression but occasionally extension) or at right angles (lateral). The key to being able to handle these movements is having the proper width of belt installed in a sufficient span. For compression, a ratio of installed belt span to movement roughly at 4:1 is suggested. The lateral capability is influenced by the amount of belt slack available. Concurrent axial compression will provide the slack thus allowing more lateral. In certain situations, there is lateral offset in the cold installed condition. This may require “pre-compression” of the joint which is in essence just providing extra belt width.
In flue gas ducting with particulate, a liner should be used to protect the belt from direct exposure. If the pressure is negative, the belt stand-off from the gas stream should be increased to keep the belt from being pulled into the gas stream or against the liner. Belt clamping bar edges next to the fabric should be radiused. The belt attachment flange should also be smooth and free of rough surfaces.
Fabric expansion joints exposed to sudden pressure fluctuations, such as near ID fans and dampers, may result in the belt “fluttering”. The fabric will fatigue over time resulting in tears. Using stiffer fabric material, installing a liner and increasing the standoff are steps to take to avoid flutter.
Each location throughout a ducting system can have different conditions that affect the design of expansion joints. As a result, there isn’t one design that can fit all applications. The goal of the expansion joint supplier is to work with engineers and end users to provide the optimum economical solutions.
Fabric expansion joints perform a function of compensating for duct misalignment and duct thermal growth typically in power plants and other ducting systems. Fabric expansion joints are found wherever there is a need to convey hot media in low pressure applications such as “in flowing air” and “out flowing gas” in large combustion processes.
Fabric expansion joints can absorb larger movements than metal expansion joints and do so without spring loads. This is critical to limiting thermally induced stresses in ducting, ducting supports, and related equipment.
How Does a Fabric Expansion Joint Work?
A fabric expansion joint is inserted into a gap in the ductwork where movement will occur. A fabric expansion joint has two main components — the fabric gas seal and the metal frames. The fabric gas seal is a closed loop, like a belt, with its two edges clamped all around to the metal frames that are in turn connected to the end of ducting. As the ducting moves the fabric belt deforms. The fabric material must do this without tearing or leaking while sometimes being exposed to high temperatures and/or corrosive media.
In some instances, additional components such as insulation pillows, accumulation barriers or flow liners are utilized to help protect the fabric material. The following section describes the basics of fabric expansion joint components and how they are designed.
Design Integration for Fabric Expansion Joints
In addition to fabric expansion joints, U.S. Bellows is a major designer and fabricator of ducting. Design Integration is the design, manufacture and shipping of expansion joints integrated into the ducting as a complete unit directly from U.S. Bellows. This enables U.S. Bellows to offer optimum system design and the lowest installed cost.
Design Integration Advantages:
Elimination of flanged connection gasketing and potential leaks.
Elimination of the risk of installing sensitive assemblies at the job site.
Significant costs savings of both manufacturing and installation labor.
Delivery of the largest “shippable” duct and piping sections to the job site to eliminate as many filed connections as possible, further reducing installation labor.
Minimize the number of flanged expansion joint connections.
Allows integration of ducting to serve as expansion joint flow liner.
Allows expansion joint frames to take the place of duct stiffeners.
Elimination of labor to install flanged expansion joint assemblies at the job site.
U.S Bellows has considerable experience in design and fabrication of integrated ducting with metal and fabric expansion joints. U.S. Bellows is also very knowledgeable with transportation capabilities for wide and heavy loads and can make firm commitments “up-front” for the largest shippable size and heaviest weight.
The drawing below shows a cross section of an expansion joint designed to allow the ducting to serve as a flow liner. The joint frame takes the place of a stiffener flange. The complete duct/expansion joint ships as one factory assembled component.
Neoprene fabric expansion joints were custom designed for a ventilation fan intake duct in a power plant. They are 42″ in diameter and are 65″ in overall length. They were designed for 1/4″ axial movement, 1/8″ lateral deflection and a 100″ water column at 200°F. The expansion joints are fabricated with a neoprene reinforced belt with stainless steel clamps, carbon steel spool pipe and angle flange ends. Each joint was dye penetrant examined prior to shipping.
A total of thirty-six fabric expansion joints were custom designed for a lignite coal processing and gasification plant in Mississippi. They are 12″ diameter, 14″ overall length and designed for 1/4″ axial movement and .8″ lateral movement. The expansion joints are fabricated with carbon steel flange ends, stainless steel clamps and a PTFE coated fabric belt. They are designed for hot air circulation flow at 600°F and a pressure of 30″ water column.
Fabric expansion joints are often used in ducts which carry hot gases at low pressures. The major design parameters are the temperatures and flow rates of the gases and the amount and abrasiveness of solids suspended in the gases. Layers of different fabrics insulation can be combined to accommodate the temperatures and pressure in the system. We specialize in all types of fabric expansion joints from high temperature furnace bags to fabric expansion joint and duct work assemblies.
A special type of high-temperature fabric expansion joint developed by U.S. Bellows, Inc. is the furnace sealing bag. The objective of the bag is to seal the air inlet conduits’ penetrations into a furnace and thus prevent heat loss. Because of thermal expansion of both the conduits and the furnace, the bag must be able to expand and contract during normal cycles of operation.
The bag consists of a tapered sleeve formed from layers of flexible, flame and heat-resistant impervious fabric, which is connected at its upper end to another sleeve in the furnace floor. The lower end of the bag is connected to the air inlet
conduit (usually a pipe) so as to form an airtight seal. The connection to this pipe is made such that the fabric is collapsed when the furnace is cold and extended when the furnace is hot. When required, a tapered coil spring formed from suitable metal is installed around the conduit inside the bag. This prevents the fabric from collapsing inward during vacuum conditions inside the furnace. For most applications, band straps may be used to attach the bag at both ends.
U.S. Bellows is proud to release our online fabric expansion joint catalog!
Fabric expansion joints perform a function of compensating for duct misalignment and duct thermal growth typical in power plants and other ducting systems. Fabric expansion joints are found wherever there is a need to convey hot media in low pressure applications such as “in flowing air” and “out flowing gas” in large combustion processes.
See an example of how fabric expansion joints are being used with ducting. The figure below represents a typical balanced draft system with a “cold” precipitator. The black components represent the locations of fabric expansion joints throughout the system.
**The examples above are representative and should not be used for design. The user should obtain actual values for the particular system being considered