In the design and operation and maintenance of boiler system, the selection of boiler expansion joint is the key link to ensure the safe operation of equipment and prolong the service life. The operating conditions of the boiler are complicated-the requirements for expansion joints in different parts such as high temperature and high pressure steam pipeline, smoke duct system and pulverized coal conveying pipeline are significantly different. Selection errors can lead to early failure of the expansion joint, air and steam leakage, or equipment damage or even unplanned furnace shutdown. This paper will systematically explain the professional and technical points of boiler expansion joint selection from working condition division, parameter calculation to standard specification.
1. Standard basis for boiler expansion joint selection
The selection of boiler expansion joint needs clear design and manufacturing standards first, which is the basic prerequisite to ensure product quality and safety.
1. Core national standards
The design, manufacture and inspection of metal bellows expansion joints shall comply with GB/T 12777-2019 General Technical Conditions for Metal Bellows Expansion Joints, which was officially implemented on December 1, 2019 and replaced the old GB/T 12777-2008。 In addition, SH/T 3421-2009 "General Rules for Setting and Selecting Expansion Joints of Metal Corrugated Pipe" provides standard guidance for setting and selecting expansion joints in petrochemical industry and other industries。
2. Selection qualification requirements
Formal boiler expansion joint type selection shall require the supplier to hold the Special Equipment Manufacturing License of the People's Republic of China (pressure pipe components), and be able to provide the type test report and material certificate corresponding to the model。
Second, selection strategy according to boiler part division
The selection of boiler expansion joint must be "treated in zones" according to different parts of the boiler system, and the working conditions of each part are significantly different:
1. High temperature and high pressure steam pipeline
Characteristics of working conditions: The temperature of main steam pipe of power plant boiler can reach above 540℃, the pressure can reach above 10MPa, and the thermal displacement is large.
Key points of selection:
- Material: Select high temperature resistant alloys such as Inconel 625, 316L stainless steel
- Structure: Multi-layer bellows design to improve pressure capacity and fatigue life
- Sealing: Use metal seal or high temperature graphite seal to ensure no leakage
- Standard: Designed to follow ASME or EJMA standards
Example: DN200 steam pipe expansion joint of a thermoelectric boiler, design temperature 1100℃, axial compensation 120mm, material selected 310S stainless steel, connecting pipe and guide pipe thickness 10mm, wave thickness 2mm。
2. Boiler smoke duct system
Characteristics of working conditions: Wide range of flue gas temperature (normal temperature-600℃), large dust content, and corrosive media in some areas.
Key points of selection:
- Metal expansion joint: suitable for high temperature section (> 400℃), the waveform is divided into full height type and half height type
- Non-metallic expansion joint: suitable for low temperature section, corrosion resistance, large compensation, no reverse thrust
Waveform selection:
| Waveform type | Wave height | Wave pitch | wall thickness | Compensation per wave | Applicable section |
|---|---|---|---|---|---|
| Semi-high type | 108mm | 59mm | 1.5mm | ±11mm | |
| Full height | 216mm | 86.4mm | 2.6mm | ±24mm | Arbitrary section |
3. Pulverized coal conveying pipeline
Characteristics of working conditions: gas-solid two-phase flow, flow rate 15-30m/s, temperature 60-450℃, there is a risk of wear.
Key points of type selection: The special three-dimensional corrugated compensator for pulverized coal pipeline adopts multi-layer one-time hydroforming process, which can compensate the composite displacement in axial, angular, transverse and various directions, and has extremely strong wear resistance。
Model example: MSW450×8J/200-L indicates a special three-dimensional corrugated compensator for pulverized coal, with a nominal diameter of 450mm, 8 waves and a compensation amount of 200mm。
Installation requirements: For units above 200MW, the expansion joint should be installed as close to the burner as possible, and a fixed bracket should be set to isolate the deformation of the pipeline outside the bracket。 In order to reduce wear and flow resistance, the product is installed in a cold tight state (also called partial installation).
4. Economizer ash transportation pipeline
Characteristics of working condition: The ash hopper of economizer expands under heat, and the vertical displacement can reach 250mm, accompanied by transverse displacement.
Key points of selection: Adopt double metal expansion joint, improve the corrugation appropriately and adopt double layer reinforcement structure to meet the expansion and performance requirements。
Example: In the transformation of an economizer ash pipeline of a 600MW unit, a compound metal expansion joint with a total length of 1200mm φ 325 is adopted to absorb the three-way expansion displacement of the economizer ash hopper 250mm downward, 125mm backward to the furnace and 75mm outward。
5. TRT/Cigarette Hood Import and Export Sensitive Equipment
Characteristics of working conditions: Sensitive equipment such as turbines have strict limitations on pipeline thrust and torque.
Key points of type selection: The flexibility of the expansion joint should be fully considered when setting the nozzle of the equipment, and the force and moment generated by the thermal expansion and compression expansion joint of the equipment and pipeline should not exceed the ultimate load set by the turbine manufacturer for the equipment。
3. Parameter Calculation of Boiler Expansion Joint Selection
The key to the selection of boiler expansion joint lies in the accurate calculation of compensation amount:
1. Calculation formula of thermal displacement
Δ L = α × L × Δ T
Among them:
- α: Line expansion coefficient of pipeline (carbon steel 12×10⁻⁶/℃, stainless steel 16×10⁻⁶/℃)
- L: length of pipe section between two fixed brackets (mm)
- Δ T: Difference between operating temperature and installation temperature (℃)
2. Selection compensation margin
The rated compensation amount of the selected expansion joint shall satisfy: rated compensation amount ≥1.2× calculated thermal displacement
3. Cold tightness calculation of pulverized coal pipeline
For the pulverized coal pipeline, when vertically installed near the burner inlet, the compensator is installed in the pre-stretched state, and the pre-stretched amount is calculated according to the formula。 The product can be pre-stretched by the manufacturer at the time of ordering or by the user at the time of installation.
4. Comparison table of material selection
| Operating temperature | Recommended Material | Applicable site |
|---|---|---|
| ≤350℃ | 304 stainless steel | Low temperature smoke duct, general duct |
| 350-450℃ | 316L/321 stainless steel | Medium temperature smoke duct, reheater |
| 450-600℃ | 321/316H stainless steel | High temperature superheater, main steam |
| 600-800℃ | 310S/Inconel600 | High temperature flue, SCR inlet |
| 800-1100℃ | 310S/Inconel625 | Boiler outlet, ignition air duct |
The heat-resistant metal corrugated expansion joint purchased by Jiangsu Thorpe Group works at a high temperature of about 1100℃ without any cooling device, and the material is 310S stainless steel。
V. Selection decision-making process
The following steps should be followed in the selection of boiler expansion joint:
Step 1: Clarify the working condition parameters
- Operating temperature (max/min), operating pressure
- Media composition (flue gas, steam, pulverized coal), corrosiveness
- Amount of thermal displacement (axial, transverse, angular)
- Installation space dimensions
Step 2: Judge the type by part
| Part | Type of recommendation | Material |
|---|---|---|
| Main steam pipe | Metal axial type | Inconel/310S |
| High temperature flue (> 400℃) | Metal expansion joint | 310S/321 |
| Cryogenic flue ( | Non-metallic expansion joint | Fluorine/silicone rubber |
| Pulverized coal pipeline | Three-dimensional ripple compensator | Wear-resistant stainless steel |
| Economizer ash pipe | Duplex metal expansion joint | Double-layer reinforced structure |
Step 3: Calculate the compensation amount and determine the wave number
- Calculate Δ L, rated compensation ≥1.2× Δ L
- Determine the required wavenumber according to the compensation amount per wave of the waveform
Step 4: Confirm the connection method and installation requirements
- Flanged connection or welding
- Whether pre-stretching/cold tightening is required
- Whether a guide bracket is required
VI. Common selection errors and correction
| Wrong practice | consequence | Correct practice |
|---|---|---|
| 304 stainless steel is selected for high temperature section | Rapid corrosion failure | Select 310S/Inconel for high temperature section |
| Lateral displacement not considered | Torsional deformation of expansion joint | Calculate the three-way displacement and select the appropriate structure |
| No pre-bias/cold tightening is set during installation | Insufficient compensation | Install strictly according to design requirements |
| Thrust limits are not considered for interface with equipment | Equipment damage | Select hinge type/pressure balance type for sensitive equipment |
sum up
The selection of boiler expansion joint is the key link to ensure the safe operation of boiler system. Correct selection should follow the following core principles:
| Selection dimension | Core Essentials |
|---|---|
| Partition selection | Main steam pipe selects metal axial type; The smoke air duct is divided according to temperature; Special three-dimensional compensator for pipe selection of pulverized coal |
| Material matching | High temperature segment ≥310S/Inconel, low temperature segment optional fluororubber non-metal |
| Parameter Calculation | Δ L = α × L × Δ T, rated compensation ≥1.2× calculated value |
| Standard execution | Follow GB/T 12777-2019 national standard |
| Installation Requirements | The high temperature section is 100% pre-biased, the pulverized coal pipe is pre-stretched, and the guide bracket is provided |
One-time standard boiler expansion joint selection can avoid pipeline failure and equipment damage caused by thermal displacement from the source. It is suggested that professional organizations should be entrusted to conduct pipeline stress analysis and special selection of expansion joints at the design stage, and the acceptance should be strictly according to national standards to ensure the safety and reliability of the whole life cycle.