In the design and construction of industrial flue gas pipeline system, the setting specification of flue gas pipeline expansion joint is the key technical basis to ensure the safe operation of the system and prevent the damage of thermal stress. Improper setting of expansion joints will directly lead to pipeline deformation, equipment damage and even production shutdown accidents. Based on the current national standards and industry norms, this paper systematically sorts out the core requirements of expansion joint setting, and provides operational guidance for engineers and technicians.

First, why do you need to follow the setting specifications of flue gas pipe expansion joints?

The flue gas pipeline is subjected to high-temperature flue gas impact during operation, and the wall temperature of the pipe can reach hundreds of degrees Celsius, resulting in significant thermal expansion. If the expansion joint is not set according to the specification, the thermal stress will not be released, resulting in pipeline weld cracking, support deformation, equipment interface leakage and other serious consequences。

The core value of following the setting specification of flue gas pipeline expansion joint lies in: ensuring that the expansion joint can effectively absorb the thermal displacement of the pipeline; Controlling the thrust of the pipeline to the fixed bracket and equipment within a safe range; Prolong the overall service life of the pipeline system; At the same time, it meets the regulatory requirements of safety production and environmental protection. The mandatory and systematic nature of the specification makes it the inevitable technical bottom line in all links of design, construction and acceptance.

2. Core specification basis for expansion joint setting

1. Design Procedure Rationale

The design of the expansion joint of flue gas pipeline shall comply with the provisions of DL/T 5121-2020 Technical Specifications for the Design of Flue, Wind and Pulverized Coal Pipeline in Thermal Power Plant。 This regulation puts forward clear requirements for the selection, arrangement and calculation of expansion joints. For flue gas pipelines in chemical and non-ferrous metal smelting industries, the relevant provisions of GB 50316 "Design Code for Industrial Metal Pipelines" should also be referred to。

2. Product Standards

The manufacture of the expansion joint body shall meet the requirements of GB/T 12522-2009 "Stainless Steel Corrugated Expansion Joint" standard, which is equivalent to the American Association of Expansion Joint Manufacturers (EJMA) standard, which clearly stipulates the material, design pressure, fatigue life and other parameters of the bellows。 Expansion joints used for ship smoke exhaust pipe systems shall also comply with T/CANSI 79-2023 Requirements for the Arrangement of Expansion Joints and Hangers for Ship Smoke Exhaust Pipe Systems。

3. Installation Acceptance Specifications

In the construction stage, the engineering construction standards such as SH/T 3522-2017 and CECS 129:2001 shall be followed, and specific provisions shall be made on the installation position, pre-offset setting and bolt tightening moment of the expansion joint。

3. Position specification of expansion joint setting

The specification for the setting of flue gas pipe expansion joints puts forward clear quantitative requirements for the installation position:

1. Spacing requirements from the fixed bracket

The expansion joint shall be arranged between two fixed brackets, and the distance from the fixed bracket shall not be greater than 4 times the pipe diameter (≤4D)。 This requirement ensures that the expansion joint can effectively absorb the amount of thermal elongation of the pipe section while preventing pipe instability.

2. Special requirements for high temperature flue

The high temperature flue gas pipeline must be provided with a compensator, and the two ends of the compensator should be provided with brackets。 For the flue of SCR denitrification system, due to the high flue gas temperature and large lateral displacement, non-metallic expansion joint should be preferred, and it should be set according to 100% pre-offset during installation-that is, the flue on both sides is deliberately misaligned during installation, so that the flue axis returns to the same straight line during hot operation。

3. Difference between horizontal and vertical pipes

• Horizontal pipeline: The expansion joint shall be provided with a drainage slope ≥3‰, and the expansion joint located in the horizontal flue section shall be provided with a drainage hole at the lowest point of the frame, with a hole diameter of at least DN150, to prevent condensate accumulation and corrosion。
• Vertical pipeline: Guide brackets and load-bearing devices are required to prevent the expansion joint from bearing the pipeline's own weight.

Key control points of installation and construction

1. Check before installation

Before installation, check whether the model, specification and pipe support configuration of the expansion joint meet the design requirements。 For expansion joints with liner barrels, verify that the direction of the liner barrel is consistent with the direction of media flow (install according to the flow direction sign on the expansion joint).

2. Pre-bias and cold tightening requirements

For expansion joints requiring "cold tightening", the auxiliary components used for pre-deformation shall be removed after the pipe system is installed。 During installation, it is strictly prohibited to deform the bellows to adjust the installation deviation of the pipeline-this operation will seriously affect the normal function of the expansion joint, reduce the fatigue life and increase the additional load on the pipeline and equipment。

3. Timing of removal of protective devices

After the pipe system is installed, the auxiliary positioning mechanism and firmware used for installing transportation protection on the expansion joint shall be removed immediately, and the limiting device shall be adjusted to the specified position according to the design requirements。 This is a link that is easily overlooked in the setting specification of flue gas pipeline expansion joint-if the transportation rod is not dismantled, the expansion joint will lose its compensation ability, and the thermal stress of the pipeline will have nowhere to be released.

4. Welding and protection requirements

No welding slag shall be allowed to splash onto the bellows surface during installation, and no mechanical damage shall be caused to the bellows。 Welding connection is the mainstream connection method of flue expansion joint. The expansion joint connected with absorption tower and other equipment should be connected by flange on the equipment side to facilitate subsequent replacement。

V. Pressure test and acceptance requirements

1. Precautions for hydraulic test

When conducting hydraulic tests on expansion joints and their connecting pipelines for gaseous media, the bearing capacity of expansion joints when filling with water should be considered。 The test water shall be clean and non-corrosive, and the chloride ion content shall not exceed 25ppm-chloride ions will cause stress corrosion cracking of stainless steel bellows. After the hydraulic test, drain the accumulated water in the bellows as soon as possible and blow dry the inner surface。

2. Airtightness requirements

All expansion joints shall be designed to be leak-free and capable of withstanding the maximum design positive/negative pressure of the system plus a margin of 1000Pa。 For expansion joints constructed according to the specification, the qualified rate of air tightness test can reach more than 98.7%。

Common violation settings and corrective measures

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