How Do Metal Expansion Joints Work? A Practical Engineering Guide
Every pipe that carries a hot or cold fluid changes length. A 100-metre carbon steel steam line heated from ambient to 250 °C grows by roughly 280 mm. If that growth has nowhere to go, it creates enormous forces that can crack welds, overload nozzles on pumps and turbines, buckle pipe supports and distort equipment. A metal expansion joint gives the piping system a controlled place to absorb that movement safely.
This guide explains how metal expansion joints work, the types of movement they absorb, the main designs, the critical role of pressure thrust, anchors and guides, and best practices for installation and maintenance.
What Is a Metal Expansion Joint?
A metal expansion joint is a flexible connector made around one or more bellows, thin-walled, corrugated cylinders usually formed from stainless steel or nickel alloys, welded to end connections such as flanges or weld ends. The corrugations (convolutions) act like a spring and allow the joint to extend, compress, bend and offset while still containing the full system pressure and temperature. Additional parts such as liners, covers, tie rods, hinges and gimbals control or limit its movement.
How Does a Metal Expansion Joint Work?
When the pipe expands, the bellows convolutions compress, much like an accordion closing. When it contracts, they open. Because the bellows wall is thin compared with the pipe, it flexes easily and absorbs movement without transmitting large forces to the piping and equipment. At the same time, the circular corrugated shape gives the bellows the hoop strength to retain internal pressure.
The design balances two opposing needs: a thin, flexible wall for movement and fatigue life, and enough thickness and convolution geometry to resist pressure and avoid instability (squirm). Expansion joints are designed to the Expansion Joint Manufacturers Association (EJMA) standards and ASME B31.3 Appendix X, which set out methods for calculating stresses, cycle life, spring rate and stability.
Types of Movement Absorbed
- Axial movement: Compression or extension along the pipe axis, the most common movement from thermal expansion.
- Lateral movement: Sideways offset of one end relative to the other, perpendicular to the axis, often from expansion in an adjacent pipe leg.
- Angular movement: Bending or rotation about an axis, from equipment rotation or piping geometry.
- Torsion: Twisting about the pipe axis. Bellows are very weak in torsion, and it must be avoided.
- Vibration: Small-amplitude, high-frequency movement from pumps, compressors and engines.
Most real installations combine several movements, and the joint must be designed for the combined case and the expected number of thermal cycles.
Pressure Thrust: The Critical Concept
Internal pressure acting on the bellows creates an axial force, called pressure thrust, equal to the system pressure multiplied by the effective area of the bellows. In an unrestrained expansion joint, this force tries to extend the bellows and push the pipe ends apart. For a 600 mm pipe at 10 bar, the thrust can exceed 30 tonnes.
This thrust must be resisted either by main anchors in the piping system or by restraining hardware within the joint itself, such as tie rods, hinges or gimbals. Failure to account for pressure thrust is one of the most common and dangerous expansion joint design errors.
Main Types of Metal Expansion Joints
Single (Unrestrained) Expansion Joint
One bellows between two end connections. It absorbs mainly axial movement with some lateral and angular movement. Main anchors must resist the pressure thrust, and pipe guides keep the line aligned.
Universal Expansion Joint
Two bellows joined by a centre pipe spool. The arrangement absorbs large lateral movements and is often fitted with tie rods to restrain pressure thrust.
Tied Expansion Joint
Tie rods connect the end fittings and carry the pressure thrust, so main anchors are not needed. The joint can absorb lateral movement but no external axial movement.
Hinged Expansion Joint
A pair of hinges restricts movement to angular rotation in one plane while carrying pressure thrust. Typically used in sets of two or three to absorb large movements in a single plane.
Gimbal Expansion Joint
A gimbal ring with two pairs of hinges allows angular rotation in any plane while restraining pressure thrust. Used in sets for multi-plane movement.
Pressure-Balanced Expansion Joint
Uses additional bellows and a balancing arrangement to cancel pressure thrust while absorbing axial movement. Ideal where heavy anchors are impractical, such as at turbine and compressor connections.
In-Line and External Pressurised Designs
Externally pressurised joints place the pressure on the outside of the bellows, improving stability and allowing very long axial strokes, common in long district heating and steam lines.
Anchors and Guides
- Main anchors resist pressure thrust, spring forces and friction. They are placed at changes in direction, at the ends of straight runs and at large equipment connections.
- Intermediate anchors divide a long pipe run into sections, each with its own expansion joint, but do not carry pressure thrust.
- Guides keep the pipe aligned so that axial expansion joints move only axially. The first guide is typically within 4 pipe diameters of the joint and the second within 14 diameters, per EJMA guidance.
Materials and Accessories
Bellows are most commonly made from stainless steel 321 or 316L. Inconel 625, Incoloy 825 and Hastelloy are used for higher temperatures and corrosive or chloride environments. Flanges and pipe ends are usually carbon steel or stainless steel. Accessories include internal liners to reduce turbulence and protect the bellows, external covers for mechanical protection and insulation, shipping bars to hold the joint at its set length during installation, and purge connections for erosive or fouling services. For corrosive services at lower temperatures, alternatives include the PTFE expansion joint.
Where Metal Expansion Joints Are Used
- Steam lines, condensate and boiler feedwater systems
- Turbine and compressor exhaust and inlet connections
- Refinery and petrochemical process piping and fired heater transfer lines
- Heat exchanger shells and pressure vessels
- Exhaust gas, flue gas and hot air ducts
- District heating, LNG and cryogenic lines
- Pump suction and discharge to isolate vibration
Expansion Joint vs Expansion Loop
The alternative to an expansion joint is a pipe loop or offset that absorbs movement by bending the pipe itself. Loops need no maintenance and have no thin-walled parts, but they take up considerable space, add pressure drop and require extra pipe, fittings and supports, especially in large diameters. Expansion joints are compact, have low pressure drop and are often the only practical solution in large-diameter ducts, congested plants and connections to sensitive rotating equipment. The choice depends on available space, pipe size, pressure, temperature and cost over the life of the system.
Installation Best Practices
- Install anchors and guides before the joint and confirm they are designed for the full thrust and spring forces.
- Never use the expansion joint to correct pipe misalignment or to make up fit-up gaps.
- Install with the flow direction arrow matching flow when a liner is fitted.
- Keep shipping bars in place until installation and hydrotest restraint is complete, then remove them before operation.
- Protect the thin bellows from weld spatter, arc strikes, tools and impact.
- Do not pressure test above the joint’s rated test pressure, and ensure anchors are in place before testing.
Common Failure Modes
- Fatigue cracking from movements or cycles beyond design.
- Squirm (instability) from excessive pressure or missing guides.
- Corrosion and chloride stress corrosion cracking from external insulation or process media.
- Torsion damage from twisting during installation.
- Flow-induced vibration in high-velocity gas without a liner.
Metal Expansion Joints from Bliss Flow Systems
Bliss Flow Systems supplies the stainless steel expansion joint in single, universal, tied, hinged and gimbal designs for steam, process, exhaust and utility service. Our wider expansion joint range also includes rubber and PTFE joints, so we can recommend the right flexible connection for each part of your piping system.
Frequently Asked Questions
What does a metal expansion joint do?
It absorbs thermal expansion, vibration and misalignment in piping and ductwork, protecting pipes, supports and connected equipment from excessive stress.
What is pressure thrust in an expansion joint?
It is the axial force created by internal pressure acting on the bellows’ effective area. It must be resisted by main anchors or by tie rods, hinges or gimbals.
What movements can a metal expansion joint absorb?
Axial, lateral and angular movements, plus vibration. Torsion should be avoided.
What is the difference between a hinged and a gimbal expansion joint?
A hinged joint allows angular movement in one plane only, while a gimbal joint allows angular movement in any plane. Both restrain pressure thrust.
How long do metal expansion joints last?
Life depends on movement, pressure, temperature and cycle count. A properly designed and installed joint can last for decades; joints are designed for a specified number of cycles per EJMA.
Conclusion
Metal expansion joints work by letting flexible bellows absorb the movement that piping systems inevitably experience, while containing full pressure and temperature. Understanding movement types, pressure thrust and the role of anchors and guides is the key to a safe, long-lasting installation. With the right design, materials and installation practice, expansion joints protect your piping and equipment for many years.
Contact Bliss Flow Systems
For product selection, technical datasheets or a quotation, reach our engineering sales team at the office nearest to you, or send an enquiry through our contact page.