1. Special working conditions and construction difficulties of flue expansion joint of sulfur incinerator

In the production of sulfur acid, sulfur incinerator is the core equipment for combustion of liquid sulfur to generate SO₂. Its outlet flue gas temperature is as high as 1000-1200℃, and it contains SO₂, SO₃ and a small amount of unreacted O₂. The flue expansion joint of sulfur incinerator not only has to withstand extreme high temperatures and strong corrosive media, but also compensate for the thermal displacement between the furnace body and the downstream waste heat boiler. Therefore, compared with the common industrial pipeline expansion joint, the construction of flue expansion joint of sulfur incineration furnace puts forward more stringent requirements on material temperature resistance grade, lining structure reliability and welding quality. In case of improper material selection, failure of anchors or cracking of liners during construction, the expansion joint will be partially overheated and deformed, and the high-temperature flue gas leakage will be caused in the worst case, resulting in production shutdown or even safety accidents. From material preparation, lining construction to welding acceptance, this paper systematically expounds the complete construction technical key points.

Preparation before construction

2.1 Technical information and protocol preparation

Before formal construction, the following technical preparations must be completed:

• Review the manufacturing drawings of the expansion joints and confirm whether the design temperature, pressure and compensation amount match the working conditions of the sulfur incinerator
• Prepare the construction scheme of flue expansion joint of sulfur incinerator, and specify the construction sequence, welding technology, lining technology and inspection standard
• Conduct technical briefing to construction personnel, emphasizing material characteristics and quality requirements under high temperature conditions
• Confirm that the site has the construction conditions: the furnace body and flue have been cooled to normal temperature, the scaffolding has been erected, and the ventilation and lighting are good

2.2 Material approach inspection

The inspection items of the expansion joint body and supporting materials include:

2.3 Pre-inspection of expansion joint

The expansion joint body shall be inspected in detail before installation:

• No mechanical damage, arc burn or indentation on the bellows surface
• The guide tube is in correct position and firmly fixed, and the free end is not stuck
• Transportation rod is intact, record pre-tension/pre-compression magnitude
• Flange sealing surface is flat and free of radial scratches

3. Installation of the expansion joint body in place

3.1 Installation positioning and alignment

The first step in the construction of a sulfur incinerator flue expansion joint is to accurately install the expansion joint to the designed position:

• Determine the installation centerline and elevation of the expansion joint according to the design drawing
• Use a hand-pulled hoist or crane to lift the expansion joint to the installation position. It is strictly prohibited to collide with the bellows
• Adjust the alignment of the expansion joint with the flange of the outlet of the sulfur incinerator and the inlet of the waste heat boiler, and the allowable deviation is ≤3mm
• If the deviation of flange surface parallelism is ≤1.5mm, forced matching is strictly prohibited

3.2 Flanged connections (if any)

For flanged connection structures:

• Sealing gasket should be high temperature resistant type, such as stainless steel covered graphite pad or ceramic fiber pad
• Tighten the bolts symmetrically in diagonal order, and gradually tighten them to the specified torque in 2-3 times
• After tightening, check that the flange clearance is uniform and the deviation is ≤0.5mm

3.3 Welding connections

Most sulfur incinerator flues are welded:

• The groove type is prepared according to the welding process specification, and the oil and corrosion are cleaned
• Butt clearance 2-3mm, blunt edge 1-1.5mm
• Recommended welding method: argon arc welding bottoming + manual arc welding filling cover
• Strictly control the interlayer temperature ≤150℃, adopt small parameters, fast welding

4. High-temperature lining construction-the key link

4.1 Verification of liner structure design

The lining of the flue expansion joint of sulfur incinerator is usually a multi-layer composite structure, and it should be confirmed before construction:

• Backing layer: ceramic fiber blanket, thickness 25-50mm, bulk density 96-128kg/m³
• Main insulation layer: high aluminum castable or mullite castable, thickness 80-120mm
• Hot surface layer: corundum castable or wear-resistant plastic, thickness 50-80mm

Each layer shall be laid in staggered seams, and the staggered distance of joints shall not be less than 100mm.

4.2 Welding of Anchors

The anchor is the mechanical connection between the liner and the metal shell, and its welding quality directly affects the construction success or failure of the flue expansion joint of sulfur incinerator:

• Anchors must be made of heat-resistant stainless steel (310S or Incoloy 601), 304 or carbon steel is strictly prohibited
• The spacing is 150-200mm, arranged in plum blossom shape
• Point fixing prior to welding, keeping the anchor perpendicular to the shell surface
• Weld length ≥30mm, weld angle height ≥5mm, no cracks and biting edges
• Anchor ends should be bent in a V-shape or a Y-shape to enhance engagement with castable

4.3 Castable Construction

In the construction of flue expansion joint of sulfur incineration furnace, castable lining is the link that is most prone to quality problems:

• Formwork support: Support the formwork according to the designed thickness. The formwork should be firmly sealed and coated with release agent
• Forced mixing: Adopt a forced mixer, and strictly control the amount of water added according to the manufacturer's instructions (usually 5-7%)
• Pouring and vibrating: Pouring in layers, each layer thickness ≤150mm, vibrating densely with plug-in vibrating rod to avoid vibration leakage
• Reserved expansion joints: Set one expansion joint every 1-1.5m along the circumferential direction, with a width of 3-5mm, and fill it with ceramic fiber paper
• Curing: After pouring, cover with plastic film, moisturizing and curing for ≥24h, and the ambient temperature is not less than 10℃

4.4 Curing and drying

After curing, strict drying (oven) treatment must be carried out:

• Warming curve: Warming to 110°C at 15°C/h, constant temperature for 24h
• Second stage: the temperature is raised to 350 °C at 20 °C/h, and the temperature is constant for 16 h
• Third stage: raise the temperature to 600°C at 25°C/h and keep the temperature constant for 8h
• During the heating process, slight negative pressure of flue is maintained, and water vapor is discharged in time
• After drying, naturally cool to normal temperature, check that the surface of the liner is free of cracks and peeling, and the knocking sound is crisp and qualified

5. Welding quality control and inspection

5.1 Key points of welding process

The welding of the metal part of the expansion joint shall strictly follow the welding process regulations:

• Selection of welding materials: Match with base metal, and use E309L welding electrode for different steel welding (stainless steel + carbon steel)
• Welding sequence: welding symmetrically from the middle to both ends, or jump welding in sections to reduce welding deformation
• Weld appearance: well formed, smooth transition with base metal, no crack, unfused, biting edge
• Interlayer Cleanup: Remove slag and splash after each weld

5.2 Non-destructive testing

• Penetration test: 100% PT for all butt welds and fillet welds, grade I qualified
• Radiographic inspection: Butt welds shall be RT at a proportion of ≥20%, not less than Grade II
• Inspection shall be performed 24 hours after completion of welding to exclude delayed cracks

5.3 Tightness Test

After all lining and welding work is completed, perform tightness tests:

• Install temporary blind plates on both sides of the expansion joint and fill with 0.05-0.1MPa compressed air
• Apply leak detection liquid on welding seam and flange connection, and hold pressure for 30 minutes without bubbles is qualified
• For sulfur incinerator system, flue gas leak detection can also be used after the oven heats up (safety needs to be ensured)

VI. Common construction problems and countermeasures

Special Precautions for Safe Construction

The construction of flue expansion joint of sulfur incinerator involves high-temperature materials, limited space and welding operations, and safety measures should not be ignored:

• Handle hot work tickets and restricted space work tickets before construction
• Sulfur incinerator and flue can only be entered after replacement, isolation, ventilation and qualified testing (oxygen content 19.5-23%, toxic gas is zero)
• The lining materials (refractory fibers, castable materials) will produce dust, workers should wear dust masks and goggles
• Equipped with fire extinguishing equipment and supervised by special personnel during welding operation

VIII. Summary

The construction of flue expansion joint of sulfur incinerator is a professional project with high requirements for materials, technology and quality control. Summarize the key elements of successful construction: "Select refractory materials, weld anchors firmly, pour densely and bake in place". At the material end, the bellows and anchors must be made of heat-resistant stainless steel of grade 310S or above, and the lining castable must be configured according to the operating temperature zone; At the construction end, the anchors are arranged according to the plum blossom shape and welded firmly, and the castables are strictly controlled by water, vibrated in layers, and expansion joints are reserved; At the acceptance end, the drying curve is strictly implemented, and the non-destructive testing and tightness test are implemented item by item. Particular attention should be paid to the fact that the furnace of the flue expansion joint of the sulfur incinerator should not be rushed to success. If the water is not fully discharged, it will be put into high-temperature operation, which will lead to the bursting of the inner lining and cause irreversible damage. Through standardized construction management, the expansion joint can effectively ensure the long-term stable operation under extreme working conditions above 1000℃, and provide reliable guarantee for the safe and continuous production of sulfur acid production plant.