1. Working characteristics of annular flue of roasting furnace and importance of expansion joint

In carbon, alumina, metallurgy and other industries, roasting furnaces usually adopt annular flue structure, and multiple furnace chambers are connected in series or parallel to concentrate smoke exhaust. This annular flue is subjected to a complex heat load in the working process: the temperature rises from normal temperature to above 1000℃ during the roasting cycle, and then it is cooled slowly, and each cycle will produce a huge amount of thermal expansion. At the same time, the annular flue is affected by the settlement of the furnace foundation, the deformation of the steel structure and the asynchronous temperature increase of each furnace chamber, and it will also appear multi-directional displacement. Therefore, the reasonable selection and reliable installation of the expansion joint on the annular flue of the roaster directly determines the airtightness and service life of the whole smoke exhaust system. Once the expansion joint fails, it will not only lead to hot flue gas leakage and energy consumption increase, but also lead to environmental protection emissions exceeding the standard and furnace condition fluctuation.

2. Analysis of displacement characteristics of annular flue of roaster

2.1 Axial displacement

The amount of axial elongation of the annular flue along the circumferential direction is the primary consideration. Taking an annular flue with a diameter of 20m as an example, when it rises from normal temperature (20℃) to 800℃, the axial thermal elongation of the carbon steel flue can reach:

Δ L = α × L × Δ T =12×10⁻⁶ ×62.8m ×780℃ ≈ 588mm

That is, the entire annular flue will generate a circumferential expansion amount of nearly 600 mm. This value far exceeds the compensation capacity of ordinary pipe compensator, and multiple expansion joints on the annular flue of roasting furnace must be set in sections to absorb it.

2.2 Lateral and Angular Displacement

Because the temperature rise rate of each roasting furnace chamber is different, between two adjacent flue interfaces:

• Vertical misalignment (foundation settlement or support deformation)
• Radial displacement in horizontal direction
• Angular deflection of flange surface

These displacements occur alone or in addition, requiring the expansion joint to have the ability to simultaneously absorb axial, lateral and angular displacements.

2.3 Cyclic Thermal Fatigue

The roaster typically runs 10-12 cycles a year with a roasting cycle of 20-30 days. Expansion joint bellows undergoes a complete stress cycle during each temperature rise and cooling process, which belongs to the typical low cycle fatigue condition. The design value of fatigue life during model selection should not be less than 1000 full displacement cycles, corresponding to the actual service life of about 8-10 years.

3. Types of expansion joints suitable for annular flue

3.1 Large tie rod transverse expansion joint

Large tie rod transverse type is the most commonly used structure of expansion joint on annular flue of roasting furnace. Characterized in that a group of bellows is arranged at both ends, and the middle is connected by a large pull rod, which can absorb large transverse displacement and bear internal pressure thrust at the same time. The large tie rod expansion joint is arranged at the straight section or interface of the annular flue, which can effectively compensate the directional misalignment between adjacent furnace chambers.

3.2 Universal hinge type expansion joint

For the part that needs to absorb the angular displacement in two planes at the same time, universal hinge expansion joint should be selected. The structure realizes universal rotation through two sets of mutually perpendicular hinge plates, and the angular compensation can reach ±8°. Typically used in pairs, the tube segment between two universal hinge expansion joints can absorb lateral displacement.

3.3 Compound axial expansion joint with tie rod

In the annular flue, when some straight pipe sections mainly bear axial displacement and the space is limited, compound axial expansion joint can be selected. Its bellows is connected by an intermediate pipe, and the total compensation amount can reach 1.5-2 times that of a single expansion joint. Limit rods must be installed to prevent internal pressure thrust from being transmitted to the weak bracket or furnace interface.

3.4 Rectangular flue compensator

The annular flue mostly has a rectangular cross section at the branch pipeline connected with each furnace chamber. Non-metallic fabric compensator or rectangular metal corrugated expansion joint are usually used for the expansion joint on the annular flue of rectangular roaster to meet the sealing requirement of variable cross-section interface.

4. Key parameters of type selection design

4.1 Calculation of Compensation Amount

The calculation of the compensation amount cannot simply superimpose the displacement in each direction, but should be vector combined according to the actual working conditions:

• Total axial compensation = thermal elongation of pipe section at design temperature × safety factor (1.15~1.2)
• Required lateral compensation amount = expected misalignment amount + installation deviation
• Angular compensation amount = determined according to the allowable deflection angle of flange surface and pipe flexibility

4.2 Pressure Class and Temperature

The annular flue of the roaster is usually operated under slight negative pressure or slight positive pressure (-5kPa ~ +5kPa), but the pressure fluctuation when the fan starts and stops needs to be considered. The design pressure of bellows is generally selected according to 0.1MPa. As the flue gas often contains SO₂, CO₂ and dust, it is recommended to choose 316L or Incoloy 825 as the material of corrugated pipe, which is significantly better than 304 in high temperature and corrosion resistance.

4.3 Guide tube and wear resistance measures

Scour of bellows trough by dusty flue gas is a common failure mode. A guide tube must be set inside the expansion joint on the annular flue of the roasting furnace. The inner diameter of the guide tube should be slightly smaller than the inner diameter of the flue, and the thickness should not be less than 4mm. The material can be wear-resistant steel plate or wear-resistant layer on the surface. The upstream end of the guide tube should be fixed, and the downstream end should slide freely to guide the airflow through smoothly and avoid dust directly washing the bellows.

V. Technical points of installation and construction

5.1 Inspection and pre-adjustment before installation

Before installing the expansion joint on the annular flue of the roaster, the following work should be completed:

• Check whether the expansion joint model, compensation amount and pressure grade are consistent with the design drawings
• Check the bellows surface for scratches, indentations, and arc burns
• Measure the actual length of the installation and confirm whether the amount of pre-stretch/pre-compression meets the site temperature conditions
• Record the protection position of the transport pull rod, and release or adjust it according to the requirements after installation

5.2 Counterpart Accuracy Control

Because of the large span and many brackets of annular flue, the actual position of flue interface may deviate from the design. When the expansion joint is matched, it shall meet the following conditions:

• The interface clearance is uniform, and the difference between the maximum clearance and the minimum clearance is not more than 3mm
• Radial staggering not more than 10% of tube wall thickness (maximum not more than 2mm)
• During flange connection, bolts shall be able to penetrate freely, and holes shall not be forcibly pried

It is strictly prohibited to use expansion joint deformation to compensate the flue counterpoint deviation, otherwise it will greatly reduce the fatigue life of bellows.

5.3 Welding and Heat Treatment

The welding joint between the expansion joint and the flue shall be carried out according to the qualified welding process. For austenitic stainless steel bellows, attention should be paid to when welding:

• Argon arc welding is used to prevent root oxidation
• Control interlayer temperature ≤150℃ to avoid intergranular corrosion and hot crack
• Dissimilar steel welds connected to carbon steel flue are transitioned by E309L electrode
• Generally, the whole heat treatment is not performed after welding, but the weld can be subjected to local pickling and passivation treatment

5.4 Final setting of pull rod and limit device

After installation is in place and welding is complete, the tie rod shall be adjusted according to the design requirements:

• For tie rods for transport protection only: Remove or loosen the nut completely to allow the expansion joint to expand and contract freely in working condition
• For structural tie rods carrying internal pressure thrust: after pre-displacement adjustment, tighten the nut to transmit the thrust to the fixed bracket
• For limit pull rod: Set the maximum expansion and contraction protection value. When it is exceeded, the limit will act

Operation and maintenance and failure prevention

6.1 Key points of daily inspection

The expansion joint on the annular flue of the roasting furnace after commissioning shall be included in the scope of daily inspection:

• Visually inspect the surface of the bellows for cracks, corrosion pits or dust accumulation
• Monitor for abnormal whistling sounds (which may indicate seal failure or deflector detachment)
• Infrared Temperature Measurement and Comparison of Temperature Differences in Different Positions of the Same Expansion Joint
• Check whether the tie rod nut is loose and the hinge is stuck

6.2 Common Troubles and Handling

6.3 Lifespan Management

It is suggested to establish a life management ledger of the expansion joint on the annular flue of roasting furnace, and record each loading time, cumulative cycle times, problems found in inspection and maintenance records. When the cumulative number of cycles is close to 80% of the design fatigue life, it should be included in the scheduled replacement list to avoid unscheduled shutdown caused by sudden failure during operation.

VII. SUMMARY

The core of the selection, installation and maintenance of the expansion joint on the annular flue of roasting furnace is to accurately understand the unique multi-directional, large displacement and thermal fatigue conditions of the annular flue. Practice has proved that the key to successful application can be summarized as: "segmented setting, universal compensation, wear-resistant diversion and reliable limit". In the selection stage, the expansion joint of large tie rod transverse type or universal hinge type is preferentially recommended; In the installation stage, strictly control the counterpart accuracy according to the measured data and set the tie rod according to the design regulations; Establish a life management system based on the number of cycles in the operation and maintenance stage. Through the standardized management of the whole life cycle, the problems of flue gas leakage, rising energy consumption and excessive environmental protection caused by expansion joint failure can be effectively avoided, which provides a solid guarantee for the long-term stable operation of the roasting furnace. For the new roaster project, the maintenance space and replacement channel of the expansion joint should be considered together in the design stage to reduce the difficulty of later operation and maintenance.