In the flue gas system of flame heating furnace and chemical combustion furnace of oil refining plant, the expansion joint of flue gas pipe of combustion furnace is the key component to connect the outlet of furnace body with the subsequent flue, absorb the high temperature heat displacement and ensure the safe operation of the system. The high flue gas temperature, large pipe diameter and frequent start-and-stop of the combustion furnace put forward extremely strict requirements for the selection, material and installation of the expansion joint. Once the expansion joint of the smoke pipe of the combustion furnace fails, it will lead to smoke leakage and thermal efficiency decrease, or fire or unplanned shutdown in the worst case. This paper will systematically explain the professional technical knowledge of the expansion joint of smoke gas pipe of combustion furnace from the characteristics of working conditions, selection specifications, installation points to fault prevention.

1. Characteristics of working conditions of flue gas pipeline of combustion furnace

Combustion furnace (flame heating furnace) is the core heating equipment in oil refining, chemical industry and metallurgy industry. The working environment of its flue gas pipeline is extremely harsh, so the following special requirements are put forward for the expansion joint of combustion furnace flue gas pipe:

• Ultra-high temperature: The flue gas temperature at the outlet of the furnace can reach 800-1200℃, which is much higher than the working conditions of ordinary industrial pipelines
• Large temperature difference alternating: from normal temperature start-up to high temperature operation, the temperature difference can reach over 1000℃, and the thermal expansion is large
• Dusty flue gas: The flue gas contains catalyst dust or unburned carbon particles, which causes erosion and wear on the inner wall of the expansion joint
• Frequent start and stop: The combustion furnace may start and stop frequently according to the production load, and the expansion joint needs to bear repeated thermal stress shock
• Corrosion risk: The flue gas may contain acidic substances such as SO₂ and SO₃, and there is a risk of dew point corrosion in the low temperature section

Based on the above characteristics, the selection and design of the expansion joint of the flue pipe of the combustion furnace must adopt the technical scheme of higher grade than that of the ordinary flue.

2. Main points of selection of expansion joint of smoke pipe of combustion furnace

1. Selection of Expansion Joint in High Temperature Zone

For the high temperature flue from the outlet of the furnace to the waste heat recovery section, the expansion joint of the combustion furnace flue gas pipe should be preferred to the metal bellows expansion joint. Focus on the following parameters when selecting the model:

According to the provisions of Section 13.3 of National Standard GB/T 51175-2016 "Technical Specification for Flame Heating Furnace Engineering of Refining Unit", the design of the expansion joint shall meet the thermal displacement compensation requirements of the flue duct system, and fully consider the corrosiveness and temperature of the medium。

2. Case analysis of material selection

Fault analysis of expansion joint of ignition air duct of 410t/h boiler in a factory shows that the metallic properties of the part in direct contact with flue gas should not be lower than 0Cr25Ni20Si2 (equivalent to 310S). However, the factory actually uses ordinary stainless steel, which causes the expansion joint to fail rapidly under high temperature。 This case fully shows that the material selection of the flue gas pipe expansion joint of the combustion furnace in the high-temperature area must be strictly required, and it should not be downgraded.

3. Applicable scenarios of non-metallic expansion joints

In the section where the flue temperature is below 400℃ (such as after waste heat recovery), non-metallic fabric expansion joints can be selected. The non-metallic expansion joint can absorb three-way displacement at the same time, and has good vibration isolation effect and relatively low cost. For the low temperature section of sulfur-containing flue gas, the skin should be made of fluororubber (FKM) material to resist acid corrosion.

Installation Specifications and Key Control Points

1. Installation direction confirmation

When installing the expansion joint of the flue gas pipe of the combustion furnace, the flow direction mark must be strictly confirmed:

• Expansion joint with guide tube, the small end of guide tube must be facing the direction of flue gas flow
• The end of the inner welded seam shall be located at the inflow end of the medium on the horizontal pipe

2. Pre-stretching and cold tightening

Because the flue of the combustion furnace is at normal temperature during installation and high temperature during operation, in order to reduce the stress of the expansion joint under hot state, pre-stretching or pre-compression should be carried out according to the design requirements. For expansion joints requiring "cold tightening", the auxiliary components used for pre-deformation shall be removed after the pre-deformation of the expansion joints。

3. Temporary restraint device

When installing the expansion joint of the combustion furnace flue gas pipe, a temporary restraining device shall be installed, and then removed after the pipe is installed and fixed。 After installation, the auxiliary positioning mechanism and fasteners used for installation and transportation protection on the expansion joint must be removed immediately, so that the expansion joint can expand and contract freely.

4. Coaxiality and bracket setting

During installation, the bellows expansion joint shall be concentric with the pipe and shall not be skewed, and circumferential torsion of the expansion joint caused by installation shall be avoided. The guiding and fixing supports shall be reasonably set at both ends of the expansion joint, and the installation error of the pipeline shall not be adjusted by the method of deforming the pipeline or compensating the expansion joint。

5. Welding protection

During installation, welding slag is not allowed to splash on the surface of the bellows, temporary supports are not allowed to be welded on the wave nodes, and hoisting fixtures such as wire ropes are not allowed to be directly tied on the wave nodes, so as to avoid mechanical damage to the wave nodes。

4. Common fault causes and preventive measures

1. Failure due to manufacturing quality problems

Case: In the manufacturing process of the ignition air duct expansion joint of a 410t/h boiler, high-temperature resistant stainless steel (not less than 0Cr25Ni20Si2) was not selected according to the technical agreement, but ordinary stainless steel was used, resulting in rapid corrosion and damage of the expansion joint in high-temperature flue gas。

Precautionary measures: When purchasing the expansion joint of the smoke pipe of the combustion furnace, the material certification documents should be strictly reviewed, and a third party should be entrusted to conduct the material re-inspection if necessary.

2. Failure due to improper installation

Case: In the same boiler project, the installation misalignment amount of 80mm was not reserved according to the design requirements during installation, and the expansion joint bellows was seriously deformed during operation, and the internal pouring material was damaged and fell off。

Precautionary measures: Strictly follow the design drawings and installation specifications. For expansion joints with pre-offset requirements, they must be precisely controlled during installation.

3. Failure due to process operations

Case: During ignition, because the rated output of the burner oil gun is too large, the atomization air pressure is too low, the light diesel oil combustion is insufficient, the temperature of the combustion chamber connected to the expansion joint exceeds the design temperature, and the high-temperature flue gas directly enters the bellows of the expansion joint, which finally leads to the damage of the expansion joint。

Precautionary measures:

• Strictly control the ignition heating rate to avoid sharp temperature change
• Ensure sufficient fuel combustion to prevent secondary combustion
• Set up temperature monitoring points to find over-temperature conditions in time

4. Common faults of metal expansion joints

V. Main points of daily maintenance and inspection

1. Operation Monitoring

• Temperature monitoring: Set temperature measurement points on the surface of the expansion joint. If local overtemperature is found, it indicates that the internal heat insulation layer may be damaged
• Displacement check: periodically check whether the actual expansion and contraction amount of the expansion joint is consistent with the design value
• Leak detection: Use a portable gas detector to check for smoke leakage around the expansion joint

2. Shutdown Inspection

At each scheduled shutdown, the following inspections shall be performed on the combustion furnace flue gas pipe expansion joint:

• Whether there are cracks, corrosion pits and discoloration (over-temperature marks) on the bellows surface
• Whether the guide tube falls off or is worn by erosion
• Whether the internal insulation layer has collapsed or fallen off
• Weld cracking or not
• Whether the fixing bracket and the guide bracket are loose or deformed

3. Replacement cycle

According to the working conditions and maintenance level, the replacement cycle of the expansion joint of the combustion furnace smoke pipe is generally as follows:

• High temperature section metal expansion joints: 3-5 years
• Non-metallic expansion joint in low temperature section: 4-6 years
• Frequent start-and-stop conditions should shorten the replacement period appropriately

VI. Special requirements for connection with equipment

According to the provisions of the Code for Construction of Industrial Metal Pipe Engineering GB 50235-2010, the compensator connected to the equipment shall be connected after the equipment is finally fixed。 This requirement is particularly important in the installation of the expansion joint of the combustion furnace flue gas pipe-the combustion furnace body will be displaced in the hot state, and if the expansion joint is connected when the furnace body is not completely fixed, it may cause additional stress during operation.

sum up

The expansion joint of combustion furnace flue gas pipe is the key component to ensure the safe operation of combustion furnace flue gas system. Proper type selection, standard installation and regular maintenance are the three core elements to ensure its long-term reliable operation:

• Type selection: Metal expansion joints (310S or nickel-based alloy) with temperature resistance ≥1000℃ must be selected in the high temperature section, and the material shall not be degraded
• Installation: Strictly press Install the flow direction marker, set the pre-offset amount, and remove the transportation rod in time after installation
• Maintenance: Check the bellows regularly, monitor the surface temperature, and deal with abnormalities in time

One standard selection and installation can ensure the long-term stable operation of the expansion joint of the flue gas pipe of the combustion furnace under high temperature alternating working conditions.