In the desulfurization system of thermal power plant and the flue gas treatment device of industrial boiler, the bypass flue expansion joint is the core flexible component connecting the bypass flue with the inlet and outlet flue of the desulfurization system. As the "flexible throat" of the bypass system, it should not only have good sealing performance under normal working conditions, but more importantly, it can withstand the direct impact of high-temperature flue gas and quickly compensate the thermal displacement when the bypass is opened. Once the bypass flue expansion joint fails, it will lead to flue gas leakage and environmental protection exceeding the standard, and in the worst case, the desulfurization system cannot be isolated, the equipment is damaged and even the safety accident. This paper will systematically explain the technical knowledge of bypass flue expansion joint from system function, selection key points to maintenance management.

1. Function of bypass flue system and special status of expansion joint

1. Basic role of bypass system

Flue gas desulfurization (FGD) bypass flue is an important part of power plant desulfurization system. Its core function is to bypass the desulfurization tower and directly introduce the raw flue gas into the chimney when the desulfurization system fails, overhauls or runs under low load. With the increasingly stringent national standards on sulfur dioxide emission of thermal power plants, flue gas bypass is no longer set in newly built units, and the bypass baffles of existing units have been sealed, which is only used as safe passage in emergency situations。

2. Particularity of bypass flue expansion joint

The bypass flue expansion joints in the bypass system are mainly the bypass flue inlet expansion joints, such as the booster fan inlet expansion joints。 Different from conventional flue expansion joints, bypass flue expansion joints face two extreme working conditions:

This extreme working condition of "alternating hot and cold" puts forward higher requirements on thermal shock resistance and fatigue life of expansion joint materials.

3. Impact when booster fan trips

The test data of a power plant show that when the booster fan fails and trips, the flue pressure of the original flue gas will rise sharply. Under 115MW and 150MW loads, the flue pressure of raw flue gas reached 3138Pa and 3379Pa, respectively。 This pressure shock poses a serious challenge to the pressure bearing capacity and impact resistance of the bypass flue expansion joint. It is based on this risk that the plant replaced the flue expansion joint of the booster fan inlet with a high-grade expansion joint with a pressure of about ±7000Pa。

2. Technical requirements of bypass flue expansion joint

1. High pressure capacity

In view of the pressure shock when the bypass is opened, the bypass flue expansion joint must have a sufficiently high pressure bearing capacity. According to engineering practice experience, in the design of bypass system, the bearing pressure grade of expansion joint should be at least 1.5 times of the maximum possible pressure of the system. In a power plant, the expansion joint at the inlet of the booster fan was replaced with a model with a pressure of ±7000Pa, which is the embodiment of this principle。

2. High temperature resistance and thermal shock resistance

The bypass flue expansion joint needs to withstand the direct impact of raw flue gas at 120-180℃ or even higher temperature when the bypass is opened. In terms of material selection, metal expansion joints should be made of high-temperature resistant stainless steel (such as 316L and 321), while non-metal expansion joints should be made of high-temperature resistant fluororubber or ceramic fiber composite materials.

3. High fatigue life

The start-and-stop of the bypass system is infrequent, but each bypass opening creates a complete thermal cycle shock to the expansion joint. Therefore, the design fatigue life of the bypass flue expansion joint should not be less than the expected number of starts and stops within the design service life, and generally requires ≥1000 complete thermal cycles.

4. Sealability Requirements

The bypass flue expansion joint is also required to have good sealing performance when the bypass is closed, so as to prevent the corrosion of the desulfurization system caused by the original flue gas leakage. According to the requirements of GB 50753-2012 and other specifications, the expansion joint should be able to withstand the maximum design positive/negative pressure of the system plus the pressure margin of 1000Pa, and be designed without leakage.

3. Key points of selection of bypass flue expansion joint

1. Material selection

In engineering practice, 316L stainless steel is used to replace the non-metallic expansion joint in the low-load denitrification bypass flue renovation project of No.1 furnace of Chaohu Power Generation Co., Ltd。 This selection reflects the comprehensive consideration of corrosiveness and temperature resistance in bypass conditions.

2. Structural form

The structural form of the bypass flue expansion joint shall be determined according to the installation position and displacement direction:

• Straight section bypass inlet: preferably axial type expansion joint, absorbing axial thermal displacement
• At the bypass elbow: Consider hinged or universal expansion joints
• Space-constrained areas: Non-metallic expansion joints are ideal because of their strong three-way compensation ability

3. Pressure Class

The pressure level of the expansion joint of the bypass flue should be higher than the normal working pressure of the system, and the peak pressure shock value when the booster fan trips should be considered. According to engineering experience, the bearing capacity shall reach ±5000Pa to ±7000Pa。

Engineering application cases and technical requirements

Case 1: Upgrade of expansion joint at booster fan inlet

During the test of the desulfurization system of a thermal power plant, it was found that when the booster fan faults and trips, the flue pressure of the original flue gas can increase to about 4000Pa。 In order to ensure the safety of the equipment, the plant replaced the expansion joint of the flue at the inlet of the booster fan with an expansion joint with a pressure of about 7000Pa。

Enlightenment: The pressure bearing capacity of the expansion joint of the bypass system must cover the pressure peak in extreme working conditions such as booster fan trip.

Case 2: Replacement of denitrification bypass flue expansion joint

A total of 4 non-metal expansion joints (material: 316L stainless steel) and 2 metal expansion joints (PN1.6/DN600, 304 stainless steel) were replaced in the renovation project of low-load denitrification bypass flue of No.1 furnace of Chaohu Power Generation Co., Ltd.。

Technical requirements:

• Non-metal expansion joint material: 316L stainless steel
• Specification of metal expansion joint: PN1.6, DN600, length 600mm, wall thickness ≥5mm
• Number of expansion hoops: not less than 6

This configuration fully considers the pressure bearing, temperature resistance and sealing requirements under bypass conditions.

Case 3: Expansion joint replacement of flue gas pipe

In the 2025 sulfuric acid production department project of Dayukou, the expansion joint of flue gas pipe needs to be replaced due to corrosion leakage, and the newly replaced expansion joint is manufactured according to PPJH-Full external pressure bypass straight pipe pressure balanced expansion joint standard of GB/T 12777 of General Technical Conditions for Expansion Joints of Metal Corrugated Pipe。

Standard basis: GB/T 12777-2019 General Technical Conditions for Expansion Joints of Metal Corrugated Pipe is a mandatory national standard for the design, manufacture and inspection of expansion joints of metal corrugated pipe. The selection and acceptance of expansion joints of bypass flue shall strictly implement this standard。

Installation and maintenance management

1. Installation Points

• Confirmation of flow direction: Expansion joint with guide tube, the small end of guide tube must be facing the direction of flue gas flow
• Bracket configuration: guide brackets must be provided on both sides of the expansion joint, and the distance between the first guide bracket and the expansion joint is ≤4 times the pipe diameter
• Welding protection: No welding slag is allowed to splash onto bellows/skin surface during installation
• Removal of transport tie rod: Remove the transport protection tie rod immediately after installation, so that the expansion joint can expand and contract freely

2. Operation Monitoring

It is recommended to install a pressure transmitter near the expansion joint of the bypass flue to send the pressure signal to the DCS system for real-time monitoring。 The range of the transmitter shall be designed to be ±5000Pa or higher, ensuring that the pressure fluctuation range under all operating conditions is covered.

3. Regular inspections

• Check the appearance of the expansion joint for cracks, corrosion and deformation every time the machine is shut down for maintenance
• Check the flange connecting bolts for looseness and tighten them if necessary
• For non-metallic expansion joints, check the skin for aging and cracking
• Check whether the drainage hole is unobstructed (wet smoke section)

4. Replacement cycle

The service life of the bypass flue expansion joint is affected by the start-stop frequency and the corrosive environment. Replacement should be considered when the following criteria are met:

• Apparent cracks or corrosion perforations in skin/bellows
• Pressure bearing capacity is reduced and the system pressure requirements cannot be met
• Repeated leakage in the same location, repaired more than 2 times

VI. SUMMARY

Bypass flue expansion joint is a key flexible component in desulfurization bypass system. The core points of its selection and maintenance can be summarized as follows:

A bypass flue expansion joint with reasonable design and standard selection can ensure reliable isolation and safe operation of bypass system under extreme working conditions such as high temperature shock and pressure fluctuation. It is recommended that the type selection calculation should be carried out strictly according to GB/T 12777-2019 standard at the design stage, and sufficient pressure safety margin should be left for extreme working conditions such as booster fan trip。