Metal Bellows

If you’ve ever placed a bellows order that came back wrong — or spent three weeks in back-and-forth emails with a manufacturer trying to establish what you actually need — you already know that bellows specification is more nuanced than it looks.

This guide gives you the complete list of information a metal bellows manufacturer needs to design and build a part that will perform in your application. Whether you’re a first-time buyer or an experienced procurement engineer switching to a new supplier, this is the specification checklist to use before you submit any RFQ.

A bellows designed for the wrong pressure rating fails by buckling or yielding. A bellows designed for the wrong temperature range experiences creep or fatigue. A bellows designed without proper movement data gets over-cycled and fails early. A bellows with the wrong end configuration doesn’t fit the piping.

Every one of these failures results in replacement cost, downtime, and often emergency procurement at premium lead times. The investment in getting the spec right upfront is trivially small compared to the cost of getting it wrong.

State the maximum operating pressure in PSI or bar, and indicate whether it’s internal pressure or external pressure (some applications, like externally pressurized expansion joints, have pressure on the outside of the bellows).

Also note any pressure cycling — if the system cycles from 0 to max pressure repeatedly, that affects fatigue life calculations. Include both the design pressure and any pressure spikes or surge conditions.

Provide both the minimum and maximum operating temperatures, and specify whether high-temperature exposure is continuous or cyclic. A bellows that’s at 1200°F for two hours then cools to ambient experiences very different stress conditions than one at 1200°F continuously.

Temperature range directly drives material selection — it’s the primary input for determining whether standard stainless steel grades or high-nickel alloys are required.

This is the most commonly under-specified parameter — and the most consequential for fatigue life. You need to provide:

• Axial movement: compression and extension, in inches or mm
• Lateral offset (angular or parallel): in inches or mm, and the direction
• Whether movements occur simultaneously or independently
• Whether movement is cyclic (happens repeatedly) or is a one-time installation offset

If you’re unsure of the exact movement values, a thermal analysis or pipe stress analysis of your system will generate them. Bellows Systems’ engineering team can assist with piping design and stress analysis if needed.

How many times will the bellows be subjected to full movement, pressure cycling, or thermal cycling over its service life? This might be 50 startup/shutdown cycles per year for an industrial boiler, or 100,000 cycles per year for a pneumatic actuator.

Cycle life requirement is the primary input for determining ply count, convolution geometry, and wall thickness in the bellows design. A part rated for 1,000 cycles looks and costs very different from one rated for 1,000,000 cycles.

What are the bellows in contact with on the inside? What is it exposed to on the outside? This drives material selection and may also affect surface finish requirements.

Be specific: not just ‘gas’ but ‘natural gas with up to 200 ppm H2S.’ Not just ‘acid’ but ‘sulfuric acid at 20% concentration at 180°F.’ The difference between these details can mean the difference between 316SS and Hastelloy C-276 — and a service life of 20 years versus 2 years.

Use Bellows Systems’ chemical compatibility tool to check your process fluid against available materials.

Provide:

• Bore diameter (ID) — the pipe or duct inner diameter the bellows must match
• Overall installed length — the face-to-face dimension in the piping or equipment
• Any restrictions on OD — clearance constraints from insulation, adjacent piping, or structural members
• Cross-section profile — circular, rectangular, oval, or custom shape
• Number of convolutions if specified by your design standard

How will the bellows connect to the adjacent piping or equipment? Bellows Systems manufactures multiple end configurations:

• Standard I-cuff ends — straight tangent ends for welding into pipe
• S-cuff, T-cuff, U-cuff, V-cuff ends — various flange and attachment profiles
• Cut-at-crest or cut-at-root ends — for integration into expansion joint assemblies
• Truncated convolutions — for space-constrained installations

If the bellows will be assembled into a larger expansion joint, flanged connector, or OEM device, include a sketch or drawing of the assembly interface. This prevents the single most common misfit error in bellows procurement.

When you submit an RFQ to Bellows Systems, you can use this structure:

• Application description — one sentence on what the bellows is for
• Operating pressure (max, design, any surge)
• Temperature range (min, max, continuous vs. cyclic)
• Movement: axial (compression/extension), lateral, angular
• Cycle life requirement
• Media in contact (inside and outside)
• Physical dimensions: ID, overall length, OD envelope
• End configuration (or reference drawing)
• Quantity and required delivery date
• Any applicable design standards or certifications

If you have a drawing or model, attach it. If you don’t, that’s fine — BSI’s engineering team can develop one as part of the quoting process.

• Metal Bellows Product Overview — full product range
• Metal Expansion Joints — when bellows elements are assembled into expansion joint systems
• Custom Engineering Services — EJMA/ASME design and pipe stress analysis
• Bellows FAQ — common specification questions answered
• Piping Design Guide — piping system bellows selection guidance
• ASME B31.3 Process Piping Standard (External) — standard for process piping design

When you’re specifying a metal bellows, one of the first decisions you’ll face is how many plies — layers of material — your bellows element needs. It seems like a simple technical choice, but it has significant consequences for cost, service life, spring rate, pressure capacity, and fatigue performance.

This guide breaks down exactly what each ply configuration offers, where each falls short, and how to match the right ply count to your actual application.

In metal bellows manufacturing, a ‘ply’ refers to a single layer of material formed into the convolution profile of the bellows. A single-ply bellows has one layer. A two-ply bellows has two concentric layers formed together. A multi-ply bellows has three or more layers.

The plies are formed together as a unit — they move together as the bellows compresses, extends, or deflects laterally. The key differences between ply configurations show up in pressure capacity, spring rate (the force required to move the bellows), cycle life, and manufacturing cost.

A single-ply bellows is formed from a single tube of material. The convolutions are formed using mechanical punch forming or hydroforming, creating a flexible element that can absorb movement in axial, lateral, or angular directions.

• Lowest spring rate — requires the least force to move, minimizing loads on adjacent piping or equipment
• Best flexibility for a given convolution geometry
• Most cost-effective option for straightforward applications
• Easiest to manufacture in a wide range of sizes, from very small OEM components up to large pipe sizes
• Well-suited for small axial movements in low-to-moderate pressure applications

• Lower pressure capacity compared to multi-ply at the same wall thickness
• More susceptible to fatigue failure under high cycle counts
• Not recommended for applications with large movements, high pressures, or demanding thermal cycling

• Mechanical seals and actuators where low spring rate is critical
• Fluid management components: accumulators, volume compensators
• Low-pressure piping expansion joints with small movements
• OEM components in instruments and precision equipment

Learn more about our single ply options: See the Bellows Systems Single Ply Bellows page.

A two-ply bellows uses two concentric layers of thinner material formed together into the convolution profile. The two plies work together mechanically, and the combined structure behaves differently from simply doubling the wall thickness.

• Better pressure capacity than single ply at the same overall diameter
• Improved fatigue life — the load is distributed across two layers, reducing peak stress at any single point
• Good spring rate balance — stiffer than single ply but still flexible enough for piping applications
• Wider operating range — handles larger movements than comparable single-ply elements
• Better redundancy — if a pinhole leak develops in one ply, the second ply continues to contain the media

• Higher spring rate than single ply — can introduce more load into the piping system
• More expensive than single ply due to additional material and forming operations
• Not rated for the highest-pressure or extreme temperature applications

• Process piping expansion joints in chemical plants, refineries, and power generation
• Gas and liquid piping systems with moderate pressure and temperature
• Applications requiring extended cycle life without the cost premium of multi-ply
• Where redundancy against through-wall failure adds safety value

Multi-ply bellows use three or more layers of material — with each ply typically thinner than what would be used in a single or two-ply design. Bellows Systems specializes in multi-ply bellows with high cycle life, using state-of-the-art seam welded tube technology.

• Highest pressure capacity of any bellows configuration at a given diameter
• Engineered for demanding cycle life — used in applications with thousands of thermal or pressure cycles
• Thinner individual plies mean lower stress per ply, extending fatigue life significantly
• Can be designed to specific spring rate, cycle life, and pressure targets
• Essential for high-temperature, high-pressure, and high-vibration environments

• Higher cost due to complex manufacturing and precision material requirements
• More technically demanding to specify — requires engineering input on ply count, thickness, and convolution geometry
• Not necessary (and therefore overcost) for simple, low-demand applications

• Engine exhaust systems (Caterpillar, Waukesha, Solar Gas Turbines) with continuous thermal cycling
• Aerospace and defense applications requiring certified fatigue life
• Subsea and high-pressure oil and gas applications
• Power generation systems with startup/shutdown cycle requirements
• Any application where EJMA design calculations must be documented

If you answer yes to any of the following, step up to multi-ply:

• Operating temperature above 900°F (482°C) continuously
• More than 5,000 thermal or pressure cycles per year
• Operating pressure above the single or two-ply catalog limit for your bore size
• Application is in aerospace, defense, or a safety-critical system requiring documented fatigue life
• Media leakage would be a safety or environmental incident

If none of those apply, two-ply is usually the right balance of performance and cost for process piping and industrial applications. Single ply is correct for low-pressure, low-cycle OEM components.

• Single Ply Bellows — specifications and applications
• Two Ply Bellows — pipe bellows details
• Multi Ply Bellows (High Pressure) — engineered for demanding service
• Testing and Qualification — how BSI validates bellows performance
• EJMA Design Standards (External) — Expansion Joint Manufacturers Association