In coal-fired power plants, metallurgy and chemical industries, pulverized coal pipelines undertake the important task of transporting pulverized coal to boiler burners. Because the gas-solid two-phase medium flows in the pipeline, and the working temperature is high and the wind speed is high, the problem of thermal expansion compensation in the pipeline system is particularly prominent. The metal expansion joint of flue pulverized coal pipeline is the core compensation element specially designed for such working conditions. This paper will systematically explain the professional knowledge of metal expansion joint of flue pulverized coal pipeline from the definition of structure, performance characteristics, type selection calculation to installation and maintenance, and provide comprehensive reference for engineers and technicians.

1. What is the metal expansion joint of flue pulverized coal pipeline?

The metal expansion joint of flue pulverized coal pipeline, also known as special three-dimensional corrugated compensator for pulverized coal pipeline, is a special compensator designed for pulverized coal pipeline。 It is manufactured by multi-layer one-time hydraulic forming process, which can compensate the composite displacement in axial, angular, transverse and various directions。

The expansion joint is suitable for displacement compensation in any direction of primary air, secondary air and tertiary air ducts with pulverized coal and hot air-gas-solid two-phase fluid with flow velocity of 15~30m/s, working temperature of 60℃ to 450℃ and wind pressure ≤0.25MPa。 It is widely used in 100600MW generator set, which fundamentally solves the problem of pulverized coal leakage and ensures the normal and safe operation of the generator set。

2. Structural composition and model representation

2.1 Main structural components

The core structure of the metal expansion joint of the flue pulverized coal pipeline includes：

• Corrugated pipe: Hydroformed with multi-layer stainless steel (1Cr18Ni9Ti or SUS304), it is the core elastic element that absorbs displacement
• Guide tube: Made of thickened wear-resistant steel (such as low carbon alloy steel), with good welding and wear resistance
• Connectors and flanges: connecting parts of the same material as pipes
• Tie rods and nuts: for installation positioning and transport protection

2.2 Model Representation Example

Take the model "MSW450×8J/200-L" as an example：

• MSW: Codename of 3D Corrugated Compensator for Pulverized Coal
• 450: Nominal diameter 450 mm
• 8J: Wave number is 8 waves
• 200: compensation amount 200 mm
• L: Product length

III. Performance characteristics and technical advantages

Compared with ordinary pipeline compensator, the metal expansion joint of flue pulverized coal pipeline has the following outstanding characteristics:

3.1 High temperature resistance and no powder leakage

The corrugated pipe made of 1Cr18Ni9Ti or SUS304 material, matched with the thickened wear-resistant steel guide tube, can run stably at 60℃ ~450℃ for a long time, and has excellent sealing performance to effectively prevent pulverized coal leakage。

3.2 Three-dimensional compensation capability

It can absorb axial displacement (X) and lateral displacement (Y) in any direction, and the axial compensation amount (Xo) and lateral compensation amount (Ymm) can meet the requirements of all kinds of units。

3.3 No powder accumulation and low stiffness

The advanced structure design makes coal powder not easy to accumulate in the expansion joint, and at the same time, the displacement stiffness is small, and the additional load on the pipe system is small, so that the boiler and the pipe system can expand freely。

4. Type selection calculation method

4.1 Displacement Compensation Calculation

The following displacement calculation requirements should be met for the selection of metal expansion joint of flue pulverized coal pipeline：

1. Axial compensation amount (Xo): determined according to the thermal expansion calculation of the pipeline
2. Lateral compensation amount (Ymm): determined according to the relative displacement of the boiler burner to the pipe

For large thermal power units with power above 200MW, the vertical displacement of the interface between the boiler burner and the pulverized coal pipeline can reach more than 200mm, and the horizontal displacement can reach more than 40mm。

4.2 Precautions for Model Selection

• Install the expansion joint as close to the burner as possible
• Set a fixed bracket to isolate the deformation of the pipeline outside the bracket
• So that the compensator absorbs primarily the displacement of the boiler rather than the overall deformation of the piping system

4.3 Material selection

V. Technical requirements for installation

5.1 Installation Method

There are two main ways to install metal expansion joints in flue pulverized coal pipeline：

Method 1: Install horizontally near the burner inlet p>

• The compensator mainly absorbs the vertical and arbitrary displacement of the boiler through transverse deflection and a small amount of axial expansion and contraction
• In order to reduce wear and flow resistance, the product is installed in a cold tight state (also known as partial installation)
• The cold tightening amount is the vertical displacement Δ y at the inlet to ensure that the compensator can work in the horizontal state

Mode 2: Install vertically near the burner inlet

• The compensator absorbs displacement mainly through expansion and contraction, as well as a small amount of lateral deflection
• In order to reduce the force on the nozzle, the compensator is installed in the pre-tensioned state
• The pre-stretching amount is calculated according to the formula, and the length of the two sets of bellows can be increased by 1/2 by the double-ended studs and nuts of the product

5.2 Key Installation Precautions

1. Confirmation of flow direction: Pay attention to the flow direction mark of the guide tube during installation, and do not install the flow direction of the guide tube in reverse direction with the flow direction of the medium
2. Coaxiality requirement: The expansion joint shall be coaxial with the pipeline during installation, and the axiality deviation shall not exceed 3mm
3. Forced centering is strictly prohibited: the other end of the expansion joint is not forcibly twisted by external force in order to align the bolt hole
4. Protect bellows: Do not allow welding slag to splash onto bellows surface during installation to avoid mechanical damage
5. Treatment of tie rods: After installation, except for transportation fixtures, the removal of other tie rods needs to be consulted with the manufacturer; For axial internal pressure expansion joints, loosen the nut on the inner side of the tie rod to the middle position
6. Cold tightening/pre-tensioning: If cold tightening or pre-tensioning is required, the auxiliary components used for pre-deformation shall be removed after the pipe system is installed

VI. Common failure causes and prevention

6.1 Main reasons for failure

High-speed friction of pulverized coal and unreasonable product structure are the main causes of premature damage of metal expansion joint of flue pulverized coal pipeline。 Therefore, we should pay attention to the wear-resistant design and material selection of the guide tube when selecting the model.

6.2 Precautionary measures

• Select thickened wear-resistant steel guide tube
• Rational design of corrugated structure to prevent pulverized coal accumulation
• Check the bellows surface regularly for wear or cracks
• Ensure that the guide tube and the connecting pipe are made into one body and have equal thickness, so that the wear-resistant life can reach the same strength as the pipeline design

VII. SUMMARY

Metal expansion joint of flue pulverized coal pipeline is the key equipment to solve the problem of thermal expansion compensation of pulverized coal pipeline. Its selection and installation quality are directly related to the safe operation of boiler system. The core points are summarized as follows:

1. Special design: adopts multi-layer bellows thickened wall wear-resistant guide tube structure, specially designed for gas-solid two-phase flow conditions, high temperature resistance, leakage prevention, no powder accumulation.
2. Three-dimensional compensation: It can absorb axial, transverse and angular displacements simultaneously, and meet the compensation requirements of vertical displacements up to more than 200mm at the interface between the burner and the pipe of the boiler of large units.
3. Scientific installation: Install close to the burner, choose horizontal installation (cold tight/offset installation) or vertical installation (pre-stretched) according to the working conditions; Pay attention to flow signs, coaxiality requirements and bellows protection during installation.
4. Material matching: 1Cr18Ni9Ti/SUS304 is selected for bellows, and stainless steel 321 is selected for high temperature working conditions; The guide tube is made of thickened Q345 wear-resistant steel to ensure the same strength life as the pipe.
5. Standard maintenance: Regularly check the surface state of corrugated pipes, and deal with abnormalities such as wear and cracks in time to avoid safety accidents caused by pulverized coal leakage.

It is recommended that engineers and technicians refer to D-LD2000 "Typical Design Manual of Smoke, Air and Pulverized Coal Pipeline Parts" and GB/T 12777 standard when selecting the model, and determine the model and specification of the expansion joint according to the actual working condition parameters (temperature, pressure, wind speed and compensation amount)。 Strictly follow the operating specifications during the installation process to ensure the long-term stable operation of the expansion joint and the safety of the boiler and pulverized coal pipe system.