In the energy recovery system of catalytic cracking unit, the expansion joint of flue gas turbine is the key component to connect the inlet and outlet pipes of the hood, absorb the heat displacement at high temperature and ensure the safe operation of the unit. The flue gas temperature at the inlet of flue gas turbine is as high as 700℃, the pipe diameter is large (usually above DN800), the thermal expansion is large, and the hood body has extremely strict limitation requirements on the pipe thrust. Once the expansion joint of flue gas turbine is improperly selected or incorrectly installed, it will lead to deformation and leakage of the expansion joint, and in the worst case, the vibration of the hood exceeds the standard or even the shutdown accident. This paper will systematically explain the technical key points of flue gas turbine expansion joint from working condition characteristics, type selection principle, common failure causes to installation and maintenance.

1. Special requirements of flue gas turbine working conditions for expansion joints

Flue gas turbine is the core equipment of energy recovery system of catalytic cracking unit, which uses high temperature flue gas expansion to do work to drive compressor or generator. The working environment of its inlet and outlet flue is extremely harsh, and the following special requirements are put forward for the expansion joint of flue gas turbine:

• Ultra-high temperature: The flue gas temperature at the inlet of the hood can reach 650-720℃, far exceeding the working condition range of ordinary industrial pipelines
• Large pipe diameter: Flue diameter is usually above DN800, up to DN2000 for large units
• Large displacement: High temperature causes large thermal expansion of pipeline, and there are longitudinal and transverse bidirectional displacements
• Force sensitivity: The thrust and torque of the inlet and outlet pipelines are strictly restricted by the hood body, and exceeding the standard will lead to collision between the rotor and the shell
• Corrosion risk: The flue gas contains SO₂, SO₃ and catalyst dust, which may cause polysulfuric acid or chloride ion stress corrosion after shutdown

Based on the above characteristics, the selection and design of flue gas turbine expansion joint must adopt completely different technical scheme from ordinary flue gas pipeline.

2. Selection principle of expansion joint of flue gas turbine

1. Inlet pipe: planar three-hinge expansion joint

According to the practical experience of catalytic cracking unit in domestic oil refining enterprises for many years, the expansion joint of flue gas turbine should choose the plane three-hinge expansion joint on the inlet pipeline。 The core logic of this selection is as follows:

Why triple-hinged?

• The inlet pipe of the hood is usually arranged in an L-shape, with thermal displacement in both longitudinal and transverse directions
• The three-hinge expansion joint is composed of three hinge expansion joints, which can simultaneously absorb both vertical and horizontal displacements
• No blind plate force: This is the most critical advantage-ordinary axial expansion joints will produce huge blind plate force (internal pressure action area × pressure), while hinged structure bears internal pressure thrust through hinge plate, and the blind plate force is not transmitted to the hood body
• The deformation reaction force is small, and the thrust and torque on the interface of the cigarette hood are controlled within the allowable range

Arrangement points:

• A hinge-type expansion joint is arranged at each of the two straight arms and the turning point of the L-shaped pipe
• The hinge plates of the three expansion joints must be located in the same plane (planar triple hinge)
• The hinge structure should have sufficient rigidity and strength and at the same time be easy to install

This scheme has been successfully applied in Luoyang Petrochemical General Plant, Golmud Refinery and other projects, and the import force of cigarette hoods has not exceeded the rated index。

2. Outlet pipeline: free-type expansion joint plus constant force spring crane

The selection of flue gas turbine expansion joint on the outlet pipe is different from that on the inlet. The flue gas temperature at the outlet of the hood has been greatly reduced, and the displacement is mainly longitudinal (axial). The following two schemes can be adopted：

Recommended scheme: The form of free-type expansion joint plus constant-force spring crane is preferred. The constant force spring crane can bear the self-weight of the pipeline, so that the expansion joint can only absorb the axial displacement, and at the same time reduce the height of the factory building and the civil construction cost。

3. Analysis of common failure modes and causes

1. Stress corrosion cracking

This is the most common failure mode for flue gas turbine expansion joints. According to the failure analysis, the damage of the expansion joint of the pipe of the hood is mainly caused by stress corrosion cracking：

• Chloride ion stress corrosion: Chloride ions in flue gas interact with tensile stress in a specific temperature range (about 60-200℃), resulting in transgranular cracks in stainless steel bellows
• Polysulfuric acid stress corrosion: During the shutdown process, sulfur-containing flue gas reacts with moisture and air to form polysulfuric acid, which causes stress corrosion cracking along crystal form
• Catalyst deposition: Catalyst particles in dust are deposited on the bellows surface, exacerbating localized corrosion

Typical case: The U-shaped expansion joint on the inlet pipeline of the flue gas turbine of the catalytic cracking energy recovery system of a factory perforated and leaked successively 7 months after it was put into operation, which was analyzed as caused by stress corrosion。

2. Expansion joint deformation and overheating

If the expansion joint of the flue gas turbine is not pre-tightened as required or the constant force spring is not reset after the triple rotation modification or overhaul, the expansion joint will generate excessive rotation angle and local overtemperature in the working state。

Consequences: Excessive deformation of expansion joint → local stress of bellows exceeding standard → fatigue crack → leakage failure.

3. The hinge mechanism is stuck

The hood of a heavy oil catalytic cracking unit starts again after normal shutdown, and the failure of disc not moving car occurs repeatedly during the warming process. After analysis, the reason is that the hinge plate and the pin of the expansion joint cannot produce angular displacement, which makes the pipeline riser unable to expand freely when heated, which causes additional thrust to the hood shell and causes the rotor to get stuck。

Material selection and anti-corrosion measures

1. Bellows material

The bellows of the expansion joint of flue gas turbine must be made of high-grade stainless steel with high temperature resistance and corrosion resistance:

• Inconel 625/825: Nickel-based alloy with excellent resistance to high temperature oxidation and stress corrosion
• SS 316L: Suitable for cooler areas but caution in chlorine-containing environments
• SS 321/347: Contains titanium or niobium stabilizing elements with good intergranular corrosion resistance

2. Anti-corrosion measures

The following protective measures should be taken against the risk of stress corrosion：

• Shutdown protection: Purge the flue with nitrogen in time after shutdown to prevent wet air from entering
• Alkali washing and neutralization: Periodic alkali washing to neutralize the polysulfuric acid that may be formed
• Surface passivation: Pickling passivation treatment after bellows manufacture is completed

Key points of installation and maintenance

1. Installation precautions

• Pre-tightening requirements: The three-hinge expansion joint must be pre-tightened or pre-biased according to the design requirements during installation, otherwise additional stress will be generated under working condition
• Flatness of hinge plates: The three hinge plates of a planar three hinge must be located in the same plane and must not be twisted
• Constant force spring reset: Ensure that the constant force spring hanger has been correctly reset after maintenance
• Bolt tightening: Flange connecting bolts should be tightened evenly by torque wrench

2. Operation Monitoring

• Surface temperature monitoring of expansion joint: If local overtemperature is found, it indicates that the internal insulation layer may be damaged or the flue gas deviates
• Displacement check: periodically check whether the actual displacement of the expansion joint is consistent with the design value
• Vibration monitoring: When the hood body vibrates abnormally, check whether the expansion joint generates additional thrust on the hood

3. Shutdown Inspection

At each planned shutdown, the flue gas turbine expansion joint shall be checked as follows:

• Whether there are cracks, corrosion pits and discoloration (over-temperature marks) on the bellows surface
• Whether the hinge plate and pin shaft are deformed, worn or stuck
• Weld cracking or not
• Whether the internal guide tube is detached or worn by erosion

sum up

The selection and design of expansion joint of flue gas turbine is the key link for the safe operation of energy recovery system of catalytic cracking unit. Different from ordinary flue gas pipe expansion joints, the hood expansion joints must meet the four core requirements of "no blind plate force, low reaction force, high temperature resistance and corrosion resistance":

• Inlet pipeline: adopts a plane three-hinge expansion joint to absorb the vertical and horizontal displacement, and the blind plate force is caused by the hinge Structural commitment
• Export pipeline: free type expansion joint plus constant force spring crane, economical and reliable
• Failure prevention: Focus on stress corrosion cracking, select corrosion-resistant alloy and make shutdown protection
• Installation quality: Pre-tighten strictly as required to ensure flatness of hinge plate and reset of spring crane

An expansion joint of flue gas turbine with reasonable design and standard installation can run stably for a long time in high temperature, large displacement and corrosive environment, and provide reliable guarantee for energy recovery of catalytic cracking unit. If you are planning the pipeline system of the hood or are faced with frequent failure of the expansion joint, it is recommended to entrust a professional organization to conduct pipeline stress analysis and special design of the expansion joint.