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Pressure Balanced Expansion Joints

April 25, 2019

Pressure Balanced Expansion Joints

Expjt pressubal 1 Expansion joints are often included in industrial piping systems to accommodate movement due to thermal and mechanical changes in the system. When the process requires large changes in temperature, metal components change size. Expansion joints with metal bellows are designed to accommodate certain movements while minimizing the transfer of forces to sensitive components in the system.

Pressure created by pumps or gravity is used to move fluids through the piping system. Fluids under pressure occupy the volume of their container. The unique concept of PRESSURE BALANCED EXPANSION JOINTS(PBEXJ) is they are designed to maintain a constant volume by having balancing bellows compensate for volume changes in the bellows (line bellows) which is moved by the pipe. An early name for these devices was “pressure-volumetric compensator”.

Pressure Balanced: Configurations

Two general configurations of constant volume expansion joints are those which are installed where the pipe changes direction and those installed where the pipe does not change direction. (often called “in-line”). Figure 1 shown below is an example of a PBEXJ used where the pipe changes direction. The two bellows are mechanically linked so any change in volume in the line bellows due to change in its length, either compression or extension, is compensated by an opposite change in the balancing bellows. In this PBEXJ the two bellows have the same diameter so the volume does not change during compression or extension of the line bellows. Pressure thrusts are transferred to the members which tie the bellows together rather than in the piping system.

Figure 2 shows PBEXJ used when the direction of the pipe does not change. In this example two smaller bellows have opposite movement to the single larger bellows. When they are compressed, it extends. When they extend, the larger bellows is compressed. The sizes are chosen so the ratio of the mean diameter of the larger bellows to the diameter of the smaller bellows is equal to the square root of 2 and this PBEXJ maintains a constant, volume when the bellows change length. The piping designer sizes the line bellows to accommodate changes in length expected in the piping system at the point where the PBEXJ is installed. The spring rates of both PBEXJs in Fig. 1 and Fig. 2 are defined by the spring rates of the mechanically linked bellows.

Pressure is used to describe the force transmitted by a fluid to the solid boundaries it contacts. It is expressed as a force per unit area and this force is at right angles to the surface at each point. Pascal’s Principle states that pressure (a force) applied to an enclosed fluid is transmitted to every portion of the fluid and to the walls of the containing vessel. Piping designers must consider all forces operating on the system. When the fluid is imcompressible, forces can be transmitted rapidly. With a PBEXJ properly placed, forces which change the length of the bellows will not be transferred to sensitive connections of the piping to the system.

Mechanical changes caused by various factors are a major problem in piping design. During operation of an industrial process the following steps occur:

Process changes are made including changes in temperature, pressure, and flow rate. Certain mechanical changes such as opening or closing a valve may be part of the control system used to make process changes. This often changes the INTERNAL WORKING PRESSURE inside the piping system.
Other mechanical changes, such as thermal expansion of the pipe, result from the process changes.

Pressure Balanced: Applications

Engineers often specify a pressure balanced expansion joint in the piping connecting the boiler feed-pump turbine exhaust to the condenser in conventional fossil-fueled electric power generating plants. [1] Variations in operating conditions cause frequent changes in temperature, pressure and steam quality. Condensers are designed to withstand steam generator overloads without causing significant loss of electric power, but the piping system must accept the forces associated with these variations.

The major advantage of a pressure balanced joint is its ability to absorb externally imposed axial movement without imposing additional pressure loading is objectionable. [2] The force resulting from the bellows spring rates is not eliminated, but it is relatively small when compared to the pressure thrusts.

Examples included in the EJMA standards [2] point out the relation of the guides to the function of the anchors and guides to the function of the pressure balanced expansion. Figure 3 below illustrates an example using a pressure balanced universal expansion installed at a change of direction between two anchors. The pipes are guided so expansion of both pipes is toward the joint. In this example, the bellows, which are components of the universal joint, accommodate movement in both the horizontal and vertical directions.

Expjt pressubal 4

Accommodating Thermal Expansion in Heat Exchangers

In critical industrial applications, heat exchanger shell bellows are designed to manage the significant change in length called axial deflection, caused by different thermal expansion that takes place between the shell and tubes.

In a shell and tube heat exchanger, the tubes inside heat up faster and operate at higher temperatures than the outer shell. Our heat exchanger shell bellows are flexible components built into heat exchangers to accommodate the thermal expansion and contraction that occur due to the specific design of these heat exchangers.

This differential movement, if not properly accommodated, can lead to severe stresses, potential leaks, and ultimately, system failure.

Choosing the Right Heat Exchanger Shell Bellows: Heavy Wall Bellows vs. Thin Wall Bellows

Selecting the optimal bellows for a shell-and-tube heat exchanger is crucial for ensuring long-term performance and safety.

US Bellows offers two primary types, each with distinct advantages:

Heavy Wall Bellows: These robust bellows feature a wall thickness comparable to the heat exchanger shell itself, making them incredibly durable and often eliminating the need for an external cover.

Formed by welding flanged and flued plates, they are built to withstand demanding conditions, though a drain might be necessary for trapped fluids.
Thin Wall Heat Exchanger Bellows: Characterized by many small corrugations in the bellows and often reinforced with rings for higher pressure capabilities, these bellows offer superior flexibility and a more compact design. Their construction avoids welds around the circumference of the bellows, which contributes to a longer lifespan. An external cover is typically required for mechanical protection.

Both thick- and thin-wall expansion joints can provide reliable performance in shell and tube heat exchangers. However, it is important to note that the hydrostatic pressure and load (PA load) are carried by the tubes, which function like tie rods.

Both thick and thin expansion joints manufactured by US Bellows are designed per the ASME Section VIII code. Heavy-wall bellows are designed per ASME Section VIII, Div. 1 Appendix CC, and thin-wall bellows per Appendix 26 and need an ASME U-stamp to undergo stringent ASME Code inspections.

It is only after passing the pressure testing and a thorough review of the quality documents and design calculations that the expansion joints receive the ASME stamp, as well as the U-2 manufacturer’s partial data report, duly signed by the ASME Authorized Inspector, affirming their compliance with the highest industry standards.

Precision Engineering & Quality for Heat Exchanger Components

Our commitment to excellence in heat exchanger bellows components is rooted in advanced engineering and rigorous quality control. US Bellows’ engineering processes adhere to the latest revisions of EJMA standards, ASME B31.3, ASME B31.1, and ASME Section 8, Division 1 & 2, utilizing proprietary in-house software and finite element analysis (FEA) for precise design.

Our comprehensive quality assurance program includes extensive in-process and final inspections, with capabilities for Helium, Hydro, X-Ray, and Dye Penetrant testing to guarantee the integrity and performance of every bellows used in shell and tube heat exchangers. Trust our experienced team to deliver superior solutions tailored to your specific requirements.

Schedule time with an engineer for a free consultation.

Ready for Reliable Thermal Management? Get a quote for your next heat exchanger project. Request a Quote Today!

Did you know that US Bellows is a Piping Technology Company?

We are proud to be a one-stop solution from expansion joints to pipe supports and engineering services. We work hard to simplify your supply chain, and ensure system reliability with quality assurance.

Air-Cooled Condenser Expansion Joints

Air-Cooled Condenser PDF

Fig0 plant

Modern power plants frequently use air-cooled condensers to return spent steam back into the cycle after it has been used to spin turbine to generate electricity. This is a critical part of the Rankin cycle, which improves power generation efficiency.

Of greatest concern for the piping designer is the thermal expansion that occurs in the major duct system that feeds the condenser from the turbine.

The first expansion joint is required at the turbine case at the exhaust to the condenser duct. This is frequently a rubber dogbone design. It serves to reduce loads transmitted via differential thermal growth from the expanding ducting and turbine casing. Generally, the restrictions on the turbine case which often must comply with NEMA 23A limits preclude the use of metallic bellows expansion joints.

Rubber dogbone expansion joints are quite simple, but can be very large and require some care in design and fabrication.

Special clamps are machined to properly mount the rubber belt. In turn these clamps are welded to the frame and provide the sealing point for the dogbone.


VIDEO: Fabrication Process Start to Finish
Fig.1: Typical Air-Cooled Condenser	Fig. 2: Aerial View of Dogbone Expansion Joint Frame

Special clamps are machined to properly mount the rubber belt. In turn these clamps are welded to the frame and provide the sealing point for the dogbone.

Critical analyses are performed on the associated structures to ensure that there is no risk of collapse due to full vacuum conditions, which frequently occur.


Fig. 4: Turbine Exhaust Structure	Fig. 5: Turbine Exhaust Structure Loading Analysis

Tied Universal Expansion Joints

The next expansion joint is required at spent steam risers to the condenser frame. This is frequently a tied universal metal bellows design. It serves to reduce loads transmitted via differential thermal growth from the expanding ducting. Tied universal expansion joints are relatively simple, but can be very large and also require some care in design and fabrication.

Fig. 6: Exhaust Riser Tied Universal Expansion Joints

Several types of structural analyses are performed to ensure conformance with design parameters and special construction loading that may occur during erection.

VIDEO: Loading of Universal
Expansion Joint onto the
Truck for Shipment

Fig. 7: Bellows Design Calculations

Fig. 8: Tie Rod Calculations

Internal pressure calculations are performed for the bellows elements, shell, and the hardware attached to the expansion joint.


Fig. 9: Shell / Hardware Interaction Under Load	Fig. 10: Shell / Hardware Buckling During Vacuum Loading

Special erection considerations must be studied to ensure that the structure as defined by the customer is suitable to withstand lifting loading during installation.


Fig. 11: Structural Loading During Installation	Fig. 12: Actual Structural Loading During Installation

As power plants have grown in generating capacity, so too have the sizes of the ducting that must service them. The units pictured below will be welded together in the field to form a tied universal expansion joint assembly.


Fig. 13: 178″ Dia. Tied Universal Expansion Joints On-Site

Fig. 14: On-Site Assembly

The final expansion joint is required at spent steam lines, which run perpendicular to the risers. This is frequently a single-hinge or single-gimbal metal bellows design. It serves to reduce loads created when the tied universal joint causes foreshortening in the duct system. Hinge expansion joints are moderately complex, but when dealing with a very large size require some care in design and fabrication.


Fig. 15: Single Hinged and Tied Universal Expansion Joints Installed at a Solar Power Plant

The bellows element is analyzed the same way as the tied universal except that the motion is angular instead of lateral. Hardware/shell interaction under loading will be a duplication of those for the tied universal except for the hinge arms and the pins, which are subject to classical design analysis.


Fig. 16: Single Hinged Expansion Joints

Fig. 17: Classic Hinge Hardware Calculation

Fig. 18: Single Hinge Structure Detail

Fig. 19: Single Hinged Expansion Joint Assembly

Fig. 20: Single Hinged Expansion Joints

Fig. 21: Tied Universal Expansion Joint

Expansion Joint vs Pipe Loop: Which Thermal Solution Is Best?

April 24, 2019

Expansion Joints versus Pipe Loop

When designing piping systems, effectively solving for thermal expansion is crucial. Both pipe loops and expansion joints serve this purpose, but understanding the nuances of expansion joints vs. pipe loops is essential to make the best selection.

Pipe loops, while effective, often demand significant space and can lead to increased pressure drop within the system. This can be challenging in facilities with limited footprints or where maintaining consistent flow rates is essential.

Advantages of Using an Expansion Joint Over a Pipe Loop

Limited Space for Pipe Loops: Expansion joints are ideal when there isn’t enough room to accommodate the large footprint required by pipe loops.
Need for Minimal Pressure Drop: In systems where maintaining flow efficiency is crucial, expansion joints prevent the turbulence and friction losses associated with multiple pipe elbows.
Handling High-Velocity or Abrasive Fluids: Expansion joints are better suited for media with high velocity or abrasive properties, reducing wear compared to traditional loops.
Inadequate Structural Support: When there’s no existing support to carry the weight and shape of a pipe loop, expansion joints provide a lighter, more practical alternative.
Low-Pressure or Large-Diameter Systems: In low-pressure pipelines or systems with large diameters, pipe loops can become unwieldy or impractical; expansion joints offer a more straightforward solution.
Short Construction Timelines: Expansion joints reduce installation time by eliminating the need for extensive welding, fittings, and loop supports.
More Cost-Effective Overall: In most cases, expansion joints have lower material, labor, and support structure costs compared to pipe loops.

Piping System Drawing Example with Expansion Joints

Piping System Drawing Example with Pipe Loops

C:documents and settingsrogersdesktopronsylv model (1)

Choosing between an expansion joint and a pipe loop depends on more than just design preference; it requires careful evaluation of system constraints, budget, and performance goals. In many cases, expansion joints offer significant advantages in terms of space efficiency, reduced material requirements, and faster installation.

For compact, cost-effective, and high-performance piping systems, expansion joints from US Bellows provide a proven solution.

Contact our engineering team today to get expert guidance on the best thermal expansion solution for your next project.

Ready to move forward? Request a quote and submit your design specs today to get started with a custom expansion joint solution for your next project.

Did you know that US Bellows is a Piping Technology Company?

Fabric Expansion Joint Applications

The figure below represents a typical balanced draft system with a “cold” precipitator and the fabric expansion joints being used with ducting.
**Black components are fabric expansion joints.

Fabric Expansion Joint Application Example

The main sections of the ducting are as follows:

FD Fan to Air Preheater
Air Preheater to Boiler
Air Preheater to Pulverizer
Air Preheater to Inlet from Boiler
Air Preheater to Precipitator or Bag House
Precipitator or Bag House to ID Fan
ID Fan to Scrubber
Scrubber to Stack

**The examples shown are representative and
should not be used for design. The user should obtain
actual values for the particular system being considered.

Fossil Fired Power Plant

High Temperature Dirty Flue Gas Expansion Joint Applications

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 Plant, 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.

Sample Data Sheet for Listed Applications

Data Sheet for High Temperature Dirty Flue Gas Expansion Joint Applications

Recommended Expansion Joint Designs

Common Design Features:

Fabric Belt: Un-insulated fabric material. (FLEXXCEL HD7)
Accumulation barrier: Fills expansion joint cavity to minimize the accumulation of particulate.
Liner: Flow liner to retain the accumulation barrier and protect the belt from abrasion

Style 200W

Design Performance: *****
Manufacturing Cost: $$$
Installation Cost: $$

Unique design features: Integral telescoping liners to retain the accumulation barrier and protect the belt from abrasion.

Style 100W

Design Performance: ****
Manufacturing Cost: $$
Installation Cost: $$$

Unique design features: Field weld liner.

Style 300W

Design Performance: ****
Manufacturing Cost: $$$$
Installation Cost: $$$

Unique design features: Integral liner.

Style 600W

Design Performance: **
Manufacturing Cost: $$$$
Installation Cost: $$$$

Unique design features: Field weld liner, Ease of pre-compression for installing in breech opening, Requires belt attachment nuts to be tack welded, Leakage around belt attachment fasteners possible.

Style 700W

Design Performance: ***
Manufacturing Cost: $$$
Installation Cost: $$$

Unique design features: Belt installed and replaced from inside the duct, Field weld liner, Ease of pre-compression for installing in breech opening, Requires belt attachment nuts to be tack welded, Leakage around belt attachment fasteners possible.