In the boiler smoke duct system, the expansion joint is the core component to absorb the thermal displacement of the duct, isolate the vibration and ensure the seal of the system. Whether its design meets the standard or not is directly related to the safety and reliability of boiler operation. So, what exactly does the design standard of boiler flue expansion joint include? What specifications should designers follow? In this paper, the design basis, classification standard, key technical parameters and selection principle of boiler flue expansion joint will be systematically sorted out, so as to provide professional reference for engineering designers.

I. Main standard specifications for boiler flue expansion joint design

The core basis of the design standard of boiler flue expansion joint is GB/T 12777 General Technical Specifications for Expansion Joints of Metal Bellows. This standard specifies the classification, requirements, test methods, inspection rules, marking and packaging of metal bellows expansion joints, etc. It is suitable for the design, manufacture and inspection of circular bellows expansion joints installed in pipelines and whose flexible parts are integral forming bellows。

In addition to national standards, the following specifications are also an important basis for design:

• EJMA Standard (American Association of Expansion Joint Manufacturers Standard): Applicable to situations with high requirements for the calculation of compensation amount
• DL/T 5437 Code for Design of Flue, Wind and Pulverized Coal Pipelines in Thermal Power Plants: Specific Design Code for Boiler Flues in Power Plants
• D-LD2000 "Typical Design Manual of Smoke, Air and Pulverized Coal Pipeline Components": Provide typical design and selection reference

It is usually clearly required in the procurement and technical documents that the metal expansion joint shall meet the national standard GB/T 12777-2019, and the quality certificate and test report shall be provided at the time of delivery。

2. Classification standard of boiler flue expansion joint

According to the design criteria for boiler flue expansion joints, expansion joints can be classified according to the following dimensions：

1. Classification by material

2. Classification by use form

• Axial type compensator: mainly used to absorb axial displacement
• Transverse compensator: mainly used to absorb lateral displacement
• Angular compensator: mainly used to absorb angular displacement

3. Classification by waveform (rectangular section)

For rectangular flue ducts, there are two standard forms of rounded wave expansion joints：

3. Key technical parameters and design values

The key technical parameters in the design standard of boiler flue expansion joint include:

1. Allowable compensation per wave

In the absence of cold drawing, the recommended values of the basic expansion per wave of the standard wave expansion joint are as follows：

• Standard full height: Δ A = ±24mm
• Standard half-height: Δ A = ±11mm

The above is a single-wave compensation amount. By increasing the number of nodes or multi-wave combinations, the total compensation capacity can be expanded accordingly.

2. Design temperature and pressure

• Operating temperature: Rectangular metal bellows compensator is suitable for working conditions ≤600℃
• Working pressure: ≤0.05MPa (about 1500mmH₂O)

3. Wavenumber vs. Length

The multi-node compensator is a combination of single-wave and double-wave standards. When the wave number increases, the total length of the compensator increases accordingly. It should be noted that the wave number of a single expansion joint usually does not exceed 6 waves。

4. Cold-drawn design

In order to reduce the number of expansion nodes, cold drawing 50% should be considered in design。 Cold drawing can effectively reduce the wave number required by the expansion joint and save the material cost.

4. Key Points of Design Selection

1. Thermal displacement calculation

The designer shall select the form of expansion joint according to the thermal displacement of the boiler tube system. The following data are required：

• Circulating medium in tube
• Design pressure of smoke duct
• Maximum temperature during operation
• Cross-sectional dimension of smoke duct (length × width)
• Waveform selected (full or half height) and wavenumber
• Type of fuel used in the boiler

2. Compensation capacity calculation

When a single expansion joint cannot meet the compensation requirement, the following scheme can be adopted：

• Two expansion joint pairs: the compensation capacity can be expanded by 5-10 times to achieve universal compensation
• Toggle system: consists of two expansion joints, which can absorb three-way thermal displacement in space

3. Dust-proof design

For dusty flue, a dust baffle (dust baffle) must be set inside the expansion joint to prevent dust accumulation from affecting the compensation function。

4. Expansion Joint Set Pair Use

Spherical compensators shall be used in pairs to form a universal joint structure that can absorb lateral displacement in one or more directions of the pipe。 When the bellows compensator is used in two groups, the compensation capacity can be expanded by 5-10 times.

V. Applicable scenarios of different types of expansion joints

According to the design standard of boiler flue expansion joint, the selection principles of different types of expansion joint are as follows:

VI. Design quality requirements

1. Flange sealing surface protection

After the expansion joint is finished, rust inhibitor should be applied to the sealing surface of the flange, and protective measures should be taken to prevent scratches and impacts 。

2. Inspection and Acceptance

• Factory inspection: Appearance inspection and dimensional measurement are required for each expansion joint
• Type inspection: Conduct according to GB/T 12777
• Quality certification documents: Quality certificate and test report shall be provided upon delivery

3. Confirmation of scope of application

The designer shall confirm that the expansion joint is suitable for boiler smoke duct system with working temperature ≤600℃ and working pressure ≤0.05MPa。

VII. Common problems and precautions in design

1. Non-metallic expansion joints prohibited from pre-stretching: Non-metallic compensators can be pre-compressed and slightly offset, but not pre-stretched
2. Dusty flue must be equipped with dust-proof baffles: When the dust content in the pipeline is large, it is necessary to set dust-proof baffles inside the compensator
3. Fully consider the influence of fuel category: The flue gas characteristics produced by different fuels (oil and coal) vary greatly, so the fuel category data should be provided for structural design
4. Rectangular flue waveform selection: standard half-height type is suitable for occasions where the cross section of flue duct is less than 4.6m²; Full height for larger sections

VIII. Summary

The design standard of boiler flue expansion joint is a technical system with GB/T 12777 as the core, supplemented by EJMA standard and power industry specification. The core points can be summarized as follows:

1. The standard basis is clear: GB/T 12777-2019 General Technical Conditions for Metal Bellows Expansion Joints is the fundamental basis for the design, manufacture and inspection of metal expansion joints; Power plant projects are also subject to DL/T 5437 and D-LD2000
2. Rectangular expansion joints have standard waveforms: half-height type (h =108mm, compensation per wave ±11mm) is suitable for small section flue; Full-height type (h =216mm, ± 24mm per wave compensation) is suitable for large section flue
3. Working condition parameters are the basis of selection: working temperature (≤600℃), working pressure (≤0.05MPa), medium characteristics (dust content, corrosiveness), fuel category, etc. must be clearly defined
4. Expandable compensation capacity design: When the compensation of a single expansion joint is insufficient, the compensation capacity can be expanded by 5-10 times by two sets of expansion joints to realize universal compensation
5. Dust protection and protection can not be ignored: dust flue must be equipped with ash baffle; Protective measures should be taken on the sealing surface of flange

In actual engineering design, it is recommended that the above standard requirements be compiled into design specifications and procurement technical documents to ensure the reliability and service life of expansion joints from the design source. For large-scale projects such as 50~600MW units, you can refer to the size parameter series in relevant technical data for type selection。