Square expansion joint (also called rectangular expansion joint or square compensator) is widely used in low-pressure large-section pipelines such as flue air duct, metallurgical hot air duct and chemical ventilation system in power plants because it can effectively absorb multi-directional displacement, compact structure and relatively low cost. Unlike circular expansion joints, the installation of square expansion joints involves many key links such as frame alignment, skin laying, insulation layer filling, platen fastening, etc. Deviation in any one step may lead to leakage, deformation or early failure. Therefore, mastering the correct installation method of square expansion joint is the basis of ensuring the long-term stable operation of smoke duct system. This paper will systematically expound the installation process and quality control points of square expansion joint from five aspects: preparation before installation, frame in place, non-metal skin installation, metal square expansion joint installation and acceptance criteria.

1. Preparation work before installation of square expansion joint

The installation of a standard square expansion joint starts with adequate preparation work. The following items need to be completed before installation:

1. Unpacking inspection and model verification

• Check whether the model, specification, pressure level and compensation amount of the expansion joint are consistent with the design drawings
• Check the outer dimensions (length, width, height), flange bolt hole spacing and hole diameter of the square expansion joint
• For non-metallic expansion joints, check whether the skin material is in good condition and free of damage, scratches or moisture
• The metal expansion joint shall be checked for the absence of pits and scratches on the bellows surface and the correct direction of the guide tube

2. Confirmation of installation conditions

• The flue or air duct has been installed in place, and the interface is cut smoothly without burrs
• The installation position of the square expansion joint has been cleaned, and there is no welding slag or oil stain
• The ambient temperature meets the installation requirements (generally not less than-10℃)
• Ready to install tools: torque wrenches, levels, sealant guns, cutting tools, etc

3. Expansion joint pre-adjustment

For metal square expansion joints that need to be pre-stretched or pre-compressed, they should be pre-deformed according to the design value before installation, and the amount of pre-deformation is generally 50% of the total compensation amount.

2. Positioning and Alignment of Square Frame

The frame is the skeleton of the square expansion joint, and its installation accuracy directly affects the sealing effect and service life of the expansion joint. Frame alignment is a top priority in square expansion joint installation.

1. Baseline Settings

• Eject the cross center line at the flue interface to ensure symmetry up, down, left and right
• Calibrate the horizontality and verticality of the frame using a level or laser wire caster

2. Frame installation

• Align the frame (or flange) of the square expansion joint with the flue interface, ensuring that the two flange surfaces are parallel and concentric
• When the side length of the square expansion joint is large, multi-point temporary fixation (spot welding or bolt) should be adopted to prevent deformation
• Controlling the diagonal length difference: when the side length is ≤2m, the diagonal difference is ≤3mm; When the side length is> 2m, the diagonal difference is ≤5mm

3. Gap Control

• The gap between the frame and the flue interface should be uniform, and the gap on one side should not exceed 2mm
• When the clearance is too large, it is necessary to add gaskets for adjustment. It is strictly prohibited to forcibly pull the bolts for correction

3. Installation of skin and insulation layer of non-metallic square expansion joint

Non-metallic square expansion joints are most widely used in power plant flue and desulfurization systems, and their installation focuses on the correct laying of skin and insulation.

1. Insulation Installation

The role of the insulation layer is to protect the skin from direct burns from high-temperature smoke. The installation steps are as follows:

• Lay aluminum silicate fiber blanket or ceramic fiber blanket evenly on the inside of the square frame
• The thickness of insulation layer shall meet the design requirements, generally 50-100mm
• When laying multiple layers, the joints should be staggered to avoid thermal bridge effect
• Use stainless steel wire mesh or stainless steel tape to secure the insulation to prevent collapse or displacement during operation

2. Skin laying

The skin is the core sealing component of the non-metallic square expansion joint, which is usually composed of multiple layers of composite materials (in order from the inside to the outside: fluorine adhesive tape layer, glass fiber cloth layer, silicone cloth layer, and outer protective cloth). Laying points are as follows:

• The skin should be cut according to the expansion size of the square expansion joint, with a margin of 30-50mm for overlapping joints
• The contact surface between the skin and the frame shall be painted with special sealant to ensure that there is no air gap
• The skin at the square corner shall be naturally transitioned and shall not be forcibly folded. Arc transition or 45° miter joint may be adopted
• The multi-layer skin shall be laid layer by layer without wrinkles or air bubbles between the layers

3. Pressure plate installation

The role of the platen is to press the skin tightly against the frame, creating a reliable seal. Installation requirements:

• The bolt holes of the pressure plate shall correspond to the bolt holes of the frame one-to-one, and shall not be misaligned
• The bolts shall be tightened evenly and sequentially, symmetrically from the middle to both sides, to prevent wrinkling of the skin
• The tightening torque shall meet the design requirements. Generally, the torque of M12 bolts is 40-60N·m
• After tightening, check the clearance between the platen and the frame; it should be uniform and consistent

4. Installation points of metal square expansion joint

Metal square expansion joint is the first choice for high temperature and high pressure or large compensation. Its installation is similar to that of circular metal expansion joints, but special attention is paid to the stiffness characteristics of square sections.

1. Bellows Protection

• The wall thickness of the metal square corrugated pipe is relatively thin, so knocking and scratching are strictly prohibited during installation
• Wet cloth should be applied to cover the surface of the bellows during welding operation to prevent burning of welding slag

2. Confirmation of the direction of the guide tube

• The open end of the guide tube must be directed toward the flow direction of the medium to guide the flue gas through smoothly and prevent the dust from directly washing the bellows
• Sufficient clearance shall be left between the guide tube and the bellows to avoid friction during operation

3. Pull rod and limit device

• Transportation fixtures should be removed after installation, otherwise they will limit the deformation of the expansion joint
• The limit pull rod shall adjust the nut position according to the design requirements to allow normal displacement and prevent over-limit displacement

4. Welding requirements

• The welding of the joint between the square expansion joint and the flue shall be made by symmetrical segment welding to avoid deformation caused by local overheating
• The welding electrode shall be matched with the base metal, and the weld shall be continuous, free of pores and slag inclusion
• Remove weld slag after welding is completed and perform appearance inspection

Inspection and acceptance after installation

The installation quality of square expansion joints ultimately needs to be confirmed by acceptance. Inspection items include:

1. Appearance inspection

• The skin surface is flat, free of wrinkles and damage
• The pressure plate bolts are complete, and the tightening is not loose
• Metal bellows without dents and scratches
• Square frame without twist deformation

2. Sealability Check

• Fill the inside of the expansion joint with compressed air (0.01-0.05MPa), and apply soapy water to the welds and joints. If there are no air bubbles, it is qualified
• Conduct airtightness test of system before commissioning to confirm that there is no leakage

3. Dimensional review

• Measure the length, width and height dimensions of the square expansion joint after installation, and compare the deviation with the design value
• Check whether the positions of guide brackets and fixed brackets on both sides of the expansion joint meet the design requirements

4. Pre-deformation inspection

• For metal square expansion joints requiring pre-tension or pre-compression, verify that the amount of pre-deformation meets the design value

VI. Common installation errors and preventive measures

Final confirmation before commissioning

After all installation work is completed, the following confirmations shall be completed before commissioning:

• Verify that there are no impurities around the expansion joint and that there is sufficient deformation space reserved
• Verify that guide brackets and fixed brackets are set as required
• Record the installation date, model, pre-deformation amount and other parameters of the expansion joint, and file them for future reference
• For non-metallic square expansion joints, the temperature should be increased gradually when put into operation for the first time, so as to avoid skin damage due to sharp heat

sum up

The installation of the square expansion joint is a delicate work with strong technicality and interlocking, and its quality directly relates to the sealing, safety and service life of the smoke duct system. This paper systematically expounds the complete process of square expansion joint installation: unpacking inspection and condition confirmation should be done before installation; During installation, we should focus on controlling the accuracy of frame alignment (diagonal difference ≤5mm), the thickness of insulation layer laying (50-100mm with staggered butt joints), the flatness of skin laying (corner arc transition) and the fractional symmetrical tightening of pressure plate bolts (moment 40-60N·m). For metal square expansion joints, attention should also be paid to the direction of the guide tube, welding protection and removal of the transportation fixtures. After the installation is completed, it must pass the triple acceptance of appearance inspection, sealing test and dimensional review. Strict adherence to the above installation specifications and acceptance criteria can effectively avoid common problems such as leakage, deformation and early ablation, and ensure the reliable operation of the square expansion joint during the design life. It is hoped that this article can provide a set of clear and practical operation guidelines for site installation engineers and maintenance personnel.