In the construction of industrial flue gas pipeline system, the installation specification of flue expansion joint is the technical cornerstone to ensure the normal compensation function of the expansion joint and the safe operation of the system. Irregular installation can lead to early failure of expansion joint, air and smoke leakage, and pipeline deformation, equipment damage and even safety accidents. This article will systematically explain the professional technical points of the flue expansion joint installation specification from the pre-installation preparation, core installation requirements to acceptance criteria.

1. Preparation before installation: the first step of standardized installation

Following flue expansion joint installation specifications, preparation before installation is essential.

1. Model specification check

Before installation, check whether the model and specification of the corrugated compensator and the support configuration of the pipe meet the design requirements。 The checks include:

• Is the nominal diameter (DN) consistent with the pipeline
• Whether the design pressure level meets the requirements of working conditions
• Whether the wave number and length conform to the design drawings

Taking the actual procurement case as an example, the metal expansion joint shall meet the national standard GB/T 12777-2019, and the quality certificate and test report shall be provided at the time of delivery。

2. Appearance and flow inspection

• Appearance inspection: Check the bellows/skin surface for mechanical damage, scratches and rust
• Flow Direction Identification: For compensator with liner, care should be taken to make the direction of liner cylinder coincide with the direction of media flow (install according to the flow direction sign of compensator)
• Inner sleeve direction: The end of the bellows expansion joint with the welded seam, which should be located at the inflow end of the medium on the horizontal pipe

3. Stent Configuration Check

Before installing the expansion joint, confirm that the pipe fixing bracket and guide bracket have been set in place according to the design requirements. According to the requirements of the specification, the distance between the telescopic pitch fixed bracket shall be ≤4 times the pipe diameter。

2. Core installation requirements: hard indicators must be complied with

The specification for installation of flue expansion joints puts forward clear and rigid requirements for the installation process.

1. It is strictly prohibited to use expansion joints to adjust pipeline deviation

Except for pre-tension or "cold tightening" pre-deformation which is required by design, it is strictly prohibited to deform the bellows to adjust the installation deviation of the pipeline。 This is the most common and dangerous wrong operation at the installation site-for the convenience of the drawing, the construction personnel use the method of stretching or compressing the expansion joint to align the pipe flange, which will directly affect the normal function of the compensator, reduce the service life and increase the load of the pipe system, equipment takeover and supporting members.

2. Pre-stretch/pre-compression requirements

Before installation, the bellows expansion joint shall be pre-stretched or pre-compressed according to the provisions of the design documents, and the force shall be uniform。 For compensators requiring "cold tightening", the auxiliary components used for pre-deformation shall be removed after the pipe system is installed。

Special requirements of SCR flue gas system: In order to ensure the uniformity of flue gas flow field, the expansion joint shall be 100% pre-biased during installation, that is, the flue on both sides of the expansion joint shall be misaligned during installation, and the flue axes on both sides shall be on a straight line during operation。

3. Coaxiality and orientation requirements

The bellows expansion joint shall be concentric with the pipe and shall not be skewed or circumferentially twisted。 Installation deviations can cause the bellows to bear additional bending moments, accelerating fatigue failure.

4. Welding protection

Welding slag is not allowed to splash onto the bellows surface and other mechanical damage to the bellows during installation。 Apply a cover to cover the bellows/skin surface during welding operations to prevent damage.

5. Protection of moving parts

All the original moving parts of the compensator shall not be stuck by external components or restrict the normal operation of their moving parts。 After installation, it should be confirmed that the movable parts of the expansion joint can be freely expanded and contracted.

3. Transportation and limiting device treatment: the most easily overlooked link

In the installation specification of flue expansion joint, the handling of transport protection device is the most easily overlooked link.

1. Removal of transport tie rod

After the pipe system is installed, the auxiliary positioning mechanism and firmware used for installing transportation protection on the compensator shall be removed immediately, and the limiting device shall be adjusted to the specified position according to the design requirements, so that the pipe system has sufficient compensating ability under environmental conditions。

Consequence warning: If the transportation tie rod is not removed, the expansion joint will lose its compensation ability, and the thermal stress of the pipeline will not be released, which may lead to pipeline deformation and bracket damage.

2. Limit device adjustment

The limit tie rod nut shall be adjusted in place according to the design drawing. The function of the limiting device is to prevent the expansion joint from generating excessive deformation exceeding the design value during operation, rather than restricting normal expansion and contraction.

Special installation requirements for non-metallic expansion joints

The specification for installation of flue expansion joints of non-metallic expansion joints has its particularities.

1. Dust-proof and diversion design

For the expansion joint of SCR flue gas system, it is necessary to withstand axial, radial and angular displacement and absorb vibration effectively, so the non-metallic expansion joint with inner baffle and high temperature resistance should be preferred。 Stainless steel wire mesh needs to be set in the material of the expansion joint to withstand the flue gas pressure.

2. Drainage hole setting

The expansion joint contacting the wet flue gas and located in the horizontal flue section will be drained through the expansion joint frame with a drain hole of at least DN150 and located on the centerline of the horizontal flue section。

3. Bolt tightening

The skin of the non-metallic expansion joint is fixed by a platen bolt. Due to the large length of the platen (up to 4-6 meters), the distal bolt will loosen due to the compression deformation of the skin after one-time tightening. The operation should be repeated in the mode of "diagonal fastening and fractional tightening".

V. Hydraulic pressure test and airtightness inspection

1. Precautions for hydraulic test

When performing hydraulic pressure tests on compensators for gaseous media and their connecting pipelines, consider whether the compensator needs to be inspected when filling with water。 The water used for hydraulic test shall be clean and non-corrosive, and the content of chloride ions in the water shall not exceed 25ppm. After the hydraulic test, drain the accumulated water in the corrugation as soon as possible, and blow dry the inner surface of the corrugated shell。

2. Airtightness requirements

All expansion joints are designed to be leak-free and can withstand the maximum design positive/negative pressure of the system plus a margin of 1000Pa。 After installation, air tightness test shall be performed to confirm that there is no leakage.

3. Connection requirements to equipment

The expansion joint on the flue is connected by welding, and the expansion joint connected with the absorption tower is connected by flange on the equipment side, and the arrangement can ensure that the expansion joint can be replaced。

VI. Special Installation Specification for FRP Flue

For FRP flue gas pipes, there are additional requirements for the installation specification of flue expansion joints:

• Drainage slope ≥3‰ should be set when horizontal pipeline is installed
• Guide brackets are required for vertical pipe installation
• Parallelism deviation of flange sealing surface ≤0.2mm/m
• Pressure test shall be performed 24 hours after installation

Acceptance criteria

After the implementation of the installation specification of flue expansion joint, it shall be accepted according to the following standards:

The acceptance criteria for flue expansion joints shall include: the specifications meet the design requirements, the installation is firm, the seal is free of leakage, the compensation performance is good, and the appearance is free of defects。

sum up

The core of the flue expansion joint installation specification can be summarized as "eight hard requirements":

A flue expansion joint that strictly follows the installation specifications can run stably for a long time under harsh working conditions such as high temperature and corrosion. It is recommended to be guided by professional technicians on the spot during the construction stage to ensure that every step of installation meets the requirements of the specifications, and ensure the safety and reliability of the flue gas system from the source.