ファブリックエキスパンションジョイントと金属製エキスパンションジョイントの比較：どちらが優れているか？

選択するファブリック伸縮継手そして金属製伸縮継手どちらの製品がより丈夫か、どちらの材料がより高温に耐えられるかといった単純な問題ではありません。適切な選択は、システムの動作方法、作動圧力、通過する媒体、ダクトやパイプのサイズ、接続機器に伝達できる負荷量などによって決まります。

一般的に、布製の伸縮継手は、熱風、排気ガス、または煙道ガスを運ぶ大型の低圧ダクトシステムにおいて非常に効果的である一方、金属製のベローズは、圧力保持と機械的強度が重要な加圧配管およびプロセスシステムにおいて一般的に選択される。

以下の比較では、それぞれの設計が最も優れた性能を発揮する場面と、伸縮継手を選定する前に評価すべき要素について説明します。

布製伸縮継手と金属製伸縮継手の比較（概要）

最も重要な違い：ダクトシステムと配管システム

選定プロセスを開始する最も簡単な方法の1つは、伸縮継手が大型の工業用ダクトに設置されるのか、それとも加圧配管システムに設置されるのかを判断することです。

A非金属製ファブリック伸縮継手特に大型の空気・ガスダクトに適しています。これらのシステムは、発電所、セメント工場、製鉄所、工業炉、集塵システム、燃焼装置、排気設備などでよく見られます。

プロセス媒体は通常気体であり、圧力は加圧されたプロセス配管に比べて比較的低い場合が多い。

金属製伸縮継手は、薄く成形された金属製の波形部材を用いて、圧力保持を維持しながら柔軟性を実現します。金属製ベローズは、蒸気、プロセス配管、石油化学、化学、その他圧力が重要な設計要件となるエンジニアリングシステムにおいて広く使用されています。

このアプリケーション環境の違いが、両技術間のパフォーマンスの違いの多くを説明している。

1. 耐圧性能

圧力は、布製伸縮継手と金属製ベローズを区別する最初の要因となることが多い。

ファブリック伸縮継手

布製ジョイントは、主に低圧ダクト用途向けに設計されています。その柔軟な構成要素は、圧力を保持する金属製ベローズではなく、コーティングされた布地、補強層、シーリング膜、および断熱材で構成されています。

これらは一般的に以下の場所で見られます。

• 排気ダクト
• 燃焼用空気システム
• 工業用排気ダクト
• ファン吸気口および排気口システム
• 炉のダクト
• 集塵システム
• 大気汚染防止装置

金属製伸縮継手

金属製のベローズは、かなり高い内部圧力に耐えられるように設計できる。その複雑な形状の金属壁は、柔軟性と圧力保持の両方を提供するように設計されている。

これにより、金属製伸縮継手は以下のような用途に適しています。

• 蒸気配管
• プロセスパイプライン
• 石油・ガスシステム
• 化学処理
• 熱交換器の接続
• 加圧ガスシステム

選択ルール：相当なシステム圧力を保持する必要がある場合は、通常、まず金属製ベローズを検討すべきです。布製伸縮継手は、耐圧性のある金属製ベローズの直接の代替品として扱うべきではありません。

2. 移動能力

動きのある生地こそ、デザインが特に魅力的に映る部分だ。

布製のベルトは比較的広い範囲で変形することができる。そのため、複数の成形された金属製の波状構造に頼ることなく、軸方向、横方向、および角度方向の大きな変位に対応できる。

熱膨張が複数の方向に発生するような大型ダクトシステムの場合、これにより伸縮継手の配置を大幅に簡素化できる。

典型的な動作の種類

• 軸方向圧縮
• 軸方向の伸展
• 横方向のオフセット
• 角度運動
• 多方向への複合的な動き

金属製の蛇腹もこれらの動きに対応できますが、蛇腹の構造はそれらの動きに合わせて特別に設計する必要があります。

For example, larger lateral displacement may require a universal metallic expansion joint with multiple bellows and a center pipe rather than a single bellows element.

Therefore, when large lateral or combined movement is required in a low-pressure duct, afabric expansion jointis often the more practical design.

3. Reaction Forces and Spring Rate

This is an important engineering difference that is often overlooked during purchasing.

A metallic bellows behaves like a mechanical spring. When it is compressed, extended or laterally displaced, it generates reaction forces that must be considered in the piping design.

These forces may influence:

• Anchors
• Guides
• Equipment nozzles
• Pipe supports
• Duct supports

Fabric expansion joints generally produce much lower spring forces because the flexible textile element is considerably softer than formed metal bellows.

For large duct systems connected to fans, furnaces or lightweight structural components, reducing these reaction loads can be a major advantage.

4. Vibration Isolation

Fans, blowers, turbines and other rotating equipment can transmit vibration into connected ductwork.

A flexible fabric connector can help isolate some of this vibration because the textile belt does not create the same rigid mechanical path as solid ductwork.

This makes fabric expansion joints especially useful around:

• Industrial fans
• Blowers
• Air handling equipment
• Exhaust systems
• Large combustion-air ducts

Metal expansion joints can also accommodate vibration, but the movement amplitude, frequency and expected cycle life must be considered carefully in the bellows design.

For high-cycle vibration, neither product should be selected solely from a general product specification. Actual vibration data should be supplied to the manufacturer.

5. Large Rectangular Ducts

Large rectangular ductwork strongly favors fabric construction in many industrial systems.

Consider an exhaust duct measuring several meters across. Producing a metallic flexible element for such a large cross-section requires substantial metal fabrication and careful control of corner stresses.

A rectangular fabric joint can use a flexible belt installed around the perimeter of the duct, making very large dimensions more practical.

This is whyrectangular fabric expansion jointsare frequently seen in:

• Power station ducting
• Cement kiln systems
• Steel plant exhaust lines
• Large furnace ducts
• Flue gas treatment equipment

For these applications, size alone can significantly influence the final choice.

6. Temperature: Which One Is Better?

It is incorrect to assume that metal is always better at high temperature or that fabric automatically has a lower usable process temperature.

The two systems manage temperature differently.

Metal Expansion Joint Temperature

Metallic bellows temperature capability is determined primarily by the selected alloy, design pressure, material strength at temperature and expected fatigue life.

Stainless steels and nickel-based alloys can be selected for elevated-temperature service.

Fabric Expansion Joint Temperature

A high-temperature fabric expansion joint may use several thermal layers rather than exposing one flexible membrane directly to the process gas.

A typical construction may include:

• Hot-face protection
• Insulation layers
• Reinforcement fabric
• Gas sealing membrane
• Outer protective cover

The insulation package can reduce the temperature reaching the external sealing materials.

Therefore, ahigh temperature fabric expansion jointshould be selected based on the full temperature gradient through the joint, not simply on the temperature rating of one fabric.

7. Flue Gas and Exhaust Applications

Hot flue gas and exhaust systems are among the most common applications for non-metallic designs.

These installations often combine:

• Large duct dimensions
• Low internal pressure
• High operating temperature
• Thermal cycling
• Lateral movement
• Fan vibration

That combination closely matches the strengths of fabric expansion joints.

Typical installations include:

• Boiler exhaust ducts
• Industrial furnaces
• Gas turbine exhaust systems
• Cement process ducts
• Incinerator systems
• Flue gas treatment equipment

For customized designs, BSTFLEX manufacturesNon Metallic Fabric Expansion Jointsfor industrial ducting, hot-air, flue-gas and exhaust applications.

8. Chemical and Corrosive Environments

Corrosion resistance cannot be judged simply by comparing “fabric” with “metal.”

The actual materials must be compared.

A metallic expansion joint may use corrosion-resistant stainless steel or nickel alloy when aggressive media are present.

A fabric expansion joint may use chemical-resistant barrier layers such as PTFE-coated textiles or other compatible membrane systems.

Selection should consider:

• Gas composition
• Acid concentration
• Alkaline components
• Moisture
• Condensation
• Operating temperature
• Cleaning chemicals

Condensation is particularly important in flue-gas systems because chemical attack may become more severe when corrosive gases condense on cooler joint surfaces.

9. Flow Velocity and Abrasion

Neither fabric nor metallic bellows should necessarily be exposed directly to severe turbulent flow or abrasive particles without protection.

A fabric joint may use:

• Internal flow liner
• Baffle
• Insulation pillow
• Abrasion-resistant hot-face layer

A metallic bellows may also require an internal liner to prevent erosion, turbulent excitation or direct impingement on the convolutions.

This means gas velocity, dust loading and flow direction should always be included in the expansion joint specification.

10. Installation Space

Available space can strongly affect the decision.

A fabric joint can often absorb considerable lateral movement within a relatively short flexible span.

Metallic systems can also accommodate large displacement, but may require more complex arrangements such as universal joints, hinged joints or gimbal configurations depending on movement direction.

However, there are also piping layouts where a compact metallic bellows is the more appropriate design.

Installation space should therefore be considered together with pressure and movement rather than as an isolated factor.

11. Weight and Structural Loading

Fabric flexible elements are generally lighter than comparable large metallic assemblies.

This difference becomes increasingly important as duct dimensions increase.

Lower component weight can reduce loads during:

• Transportation
• Handling
• Installation
• Maintenance
• Replacement

In very large rectangular duct systems, this can be an important practical advantage.

12. Replacement and Maintenance

Fabric expansion joints are often designed with replaceable flexible belts.

If the surrounding steel frames remain serviceable, maintenance may involve replacing the flexible element rather than removing the complete assembly.

This can be useful in large duct installations where removing welded steel frames would create significant shutdown work.

Metal expansion joints generally require replacement or repair of the metallic bellows assembly when the bellows itself becomes damaged.

The actual maintenance cost depends on system design, accessibility, joint size and failure mode.

13. Expected Service Life

There is no meaningful universal statement such as “metal lasts longer than fabric.”

Service life depends on whether the joint was correctly designed for the application.

A fabric joint can fail prematurely because of:

• Excess temperature
• Chemical attack
• Abrasion
• Incorrect installation
• Overextension
• Unexpected movement
• Condensation

A metal bellows can fail because of:

• Fatigue
• Corrosion
• Stress corrosion cracking
• Excess pressure
• Squirm
• Torsion
• Excess movement
• Flow-induced vibration

Correct engineering matters more than simply choosing one material category over another.

Which Expansion Joint Is Better for Different Applications?

When Should You Choose a Fabric Expansion Joint?

Afabric expansion jointshould be seriously considered when most of the following conditions are present:

• The system is a duct rather than a pressure pipeline.
• The medium is air, exhaust gas or flue gas.
• Operating pressure is relatively low.
• The duct cross-section is large.
• Significant lateral movement must be absorbed.
• Several movement directions occur simultaneously.
• Reaction loads on adjoining equipment should be minimized.
• Fan or blower vibration is present.
• A rectangular or irregular geometry is required.

This is the typical application envelope for anon metallic expansion joint.

When Should You Choose a Metal Expansion Joint?

A metallic expansion joint should normally be evaluated first when:

• The system is pressurized piping.
• Steam or process fluid is being transported.
• Pressure containment is critical.
• The applicable piping design requires metallic construction.
• High mechanical strength is required.
• The system is designed according to metallic bellows engineering standards.

Metallic bellows are engineered pressure-containing components and should be designed around system pressure, temperature, movement, material, fatigue life and piping loads.

Non Metallic vs Metallic Expansion Joint: A Better Selection Method

Instead of asking, “Which expansion joint is better?” engineers should ask, “Which expansion joint architecture matches this system?”

Use the following sequence.

Step 1: Identify the System

Is it a large duct or pressurized pipe?

Step 2: Define Pressure

Specify normal pressure, design pressure and whether the system operates under positive or negative pressure.

Step 3: Define Temperature

Provide both continuous operating temperature and maximum excursion temperature.

Step 4: Separate Every Movement

List axial compression, axial extension, lateral displacement and angular movement individually.

Step 5: Identify the Medium

Specify hot air, exhaust gas, flue gas, steam, liquid or process chemical.

Step 6: Check Flow Conditions

Include gas velocity, dust loading, abrasive particles and turbulence.

Step 7: Evaluate Equipment Loads

Determine how much reaction force can safely be transferred to fans, duct supports, equipment nozzles and anchors.

Step 8: Review Installation Geometry

Confirm shape, dimensions, flange arrangement and available face-to-face installation length.

A Practical Example: Large Flue Gas Duct

Consider a large rectangular flue-gas duct installed downstream of industrial combustion equipment.

The system has:

• Large rectangular dimensions
• Hot gas
• Low operating pressure
• Axial thermal growth
• Lateral movement
• Fan vibration

In this situation, a fabric expansion joint is often an efficient choice because it can combine large cross-section capability, multidirectional flexibility and low reaction forces.

Now consider a high-pressure steam line operating at elevated temperature.

Although thermal movement still exists, the system requires reliable pressure containment. A metallic bellows engineered for the piping pressure, temperature and movement is generally the appropriate technology.

These examples demonstrate why the operating system must determine the expansion joint type.

What Information Should Be Sent to the Manufacturer?

For either metallic or fabric designs, the quality of the engineering recommendation depends heavily on the information supplied.

For a custom fabric joint, BSTFLEX recommends providing:

• Duct width and height or diameter
• Joint face-to-face length
• Operating temperature
• Maximum temperature
• Positive or negative pressure
• Process medium
• Axial compression
• Axial extension
• Lateral movement
• Angular movement
• Gas velocity
• Dust or particulate content
• Chemical composition
• Existing flange details
• Drawings or photographs
• Required quantity

Fabric Expansion Joint Solutions from BSTFLEX

BSTFLEX manufactures custom non-metallic expansion joints for industrial hot-air, exhaust and flue-gas ducting systems.

Depending on operating conditions, flexible constructions can incorporate technical fabrics, coated fiberglass, PTFE-based sealing layers, high-temperature insulation and protective components.

Round, rectangular and application-specific configurations can be manufactured according to customer drawings and operating data.

See theBSTFLEX 非金属ファブリック伸縮継手特注製造オプションについては、お問い合わせください。