In waste heat power generation systems such as cement kiln, glass kiln, steel sintering machine and dry coke quenching, high-temperature flue gas recovers heat through waste heat boiler and generates steam to drive steam turbine to generate electricity. The whole flue gas pipeline is discharged from the kiln head or the kiln tail, passes through the dust collector, waste heat boiler, induced draft fan and then to the chimney. The length of the pipeline can reach hundreds of meters, and the fluctuation range of flue gas temperature is from normal temperature to above 600℃. Under such severe thermal cycling conditions, the problems of thermal displacement and thermal stress of pipeline are very prominent. The expansion joint of waste heat power generation is the core component to solve this problem. Based on the characteristics of waste heat power generation system, the common types, selection points and installation and maintenance specifications of waste heat power generation expansion joints are systematically introduced.
1. Special requirements for expansion joints in waste heat power generation system
The biggest difference between waste heat power generation and conventional thermal power plants is that the source of waste heat flue gas is unstable, the temperature fluctuates greatly, the dust content is high and it is corrosive. Taking cement kiln waste heat power generation as an example, the waste gas temperature at the kiln tail is between 300-400℃, but the fluctuation range can reach ±50℃, and it contains a certain amount of SO₂ and chloride ions. These working conditions put forward four special requirements for the expansion joint of waste heat power generation:
- Temperature fluctuation resistance fatigue: Frequent start-stop and temperature fluctuation cause expansion joint bellows to withstand high cycle thermal fatigue
- Wear resistance: The clinker dust contained in the flue gas will scour the guide tube and the inner wall of the expansion joint
- Low temperature corrosion resistance: When the flue gas temperature is below the acid dew point, the condensate can corrode metal parts
- Large compensation capacity: long-distance flue produces significant thermal elongation under temperature difference
Therefore, ordinary industrial expansion joints are often difficult to meet the working conditions of waste heat power generation, so they must be designed specifically.
Classification and structure of expansion joints commonly used in waste heat power generation
According to the difference of installation position and working condition, waste heat power generation expansion joints are mainly divided into the following categories:
1. Metal bellows expansion joint
Metal expansion joints are the main products of waste heat power generation systems, accounting for more than 70%. Its core component is the bellows, which is usually formed by hydraulic or rolling of multi-layer thin-walled stainless steel plates. According to the different compensation direction, it can be divided into:
- Axial type: absorbs thermal expansion and contraction in the straight direction of the pipe, for longer straight pipe sections
- Transverse: Absorbs lateral displacement perpendicular to the pipe axis, for use in elbows or space-constrained areas
- Angular: Absorbs angular variations, usually used in pairs
- Universal type: simultaneously absorbs composite displacements in multiple directions
In the waste heat power generation, the large tie rod transverse expansion joint is usually used in the inlet and outlet flue of boiler, because there are both axial displacement and transverse displacement, and the internal pressure thrust is large.
2. Non-metallic expansion joint
The non-metallic expansion joint is made of multi-layer flexible fabric materials (silicone cloth, fluorine adhesive cloth, glass fiber cloth) and heat insulation layer. Its biggest advantage is that it can absorb three-way displacement (axial, transverse and angular) at the same time, and has good vibration isolation effect and lower cost than metal type. In waste heat power generation, non-metallic expansion joints are widely used in:
- Rectangular flue at inlet of kiln head waste heat boiler
- Inlet and outlet connection of induced draft fan
- Dust collector inlet and outlet pipeline
- Parts requiring frequent disassembly and maintenance
However, it should be noted that the temperature resistance of non-metallic expansion joints usually does not exceed 400 °C and the pressure bearing capacity is less than 0.1 MPa, so they cannot be used in high pressure or ultra-high temperature areas.
3. Special expansion joint for corrosion resistance
For scenarios with high sulfide and chloride ions in waste heat flue gas (such as waste incineration waste heat power generation and high sulfur fuel cement kiln), corrosion-resistant expansion joints need to be selected. Common protocols include:
- High nickel alloy expansion joint (Inconel 625, C-276)
- All non-metallic expansion joint (fluorine tape skin)
3. Key points of key parts selection
A complete waste heat power generation flue system usually contains multiple expansion joints, and the selection strategy of different parts is different.
1. Waste heat boiler inlet flue
There is the highest flue gas temperature (350-600℃), and there are airflow disturbance and dust impact. It is recommended to choose metal axial expansion joint with guide tube or large tie rod transverse expansion joint. The role of the deflector is to protect the bellows from direct flushing of high-speed dust while preventing eddy-induced vibrations. It is recommended to choose heat-resistant stainless steel (309S, 310S or Inconel 625) as a material, and the design fatigue life should not be less than 2000 cycles.
2. Waste heat boiler outlet to dust collector flue
The flue gas temperature has dropped to about 200℃, but the dust concentration is still high. Optional non-metallic expansion joints or low-cost metallic expansion joints (304 stainless steel) are available here. If the metal type is selected, it is recommended to add an inner lining wear-resistant layer; If the non-metallic type is selected, it is necessary to ensure that the thickness of the heat insulation layer is not less than 50mm to prevent the ablation of the skin by high temperature conduction.
3. Inlet and outlet of induced draft fan
The inlet and outlet of the fan are the parts with the strongest vibration, which require high vibration isolation performance of the expansion joint. Non-metallic expansion joints or rubber expansion joints are the best choice. At the same time, the fan inlet expansion joint needs to bear negative pressure, and an internal support ring or a reinforced structure should be set to prevent deflation.
4. Flue elbow and diameter reduction
There are often complicated displacements in the front and back of the elbow and the reducing diameter pipe section, and a single axial expansion joint cannot meet the requirements. Hinged or universal expansion joints are recommended, or composite displacement compensation is achieved by combining two axial expansion joints.
4. Material selection and applicable temperature range
The material selection of waste heat power generation expansion joint directly determines its service life. The following are the applicable temperature references for different materials:
| Material Type | Maximum operating temperature | Applicable site | Pros and Cons |
|---|---|---|---|
| 304 stainless steel | 550℃ | Low temperature section flue | Low cost, average corrosion resistance |
| 316L stainless steel | 650℃ | Medium temperature section flue | Good resistance to chloride corrosion |
| 309S Stainless Steel | 800℃ | High temperature section entrance | Oxidation resistance, high temperature strength |
| 310S Stainless Steel | 950℃ | Ultra-high temperature section | Good heat resistance and higher price |
| Inconel 625 | 980℃ | High corrosion high temperature section | Excellent comprehensive performance and high cost |
| Non-metallic fluorine tape | 200℃ | Low temperature section/fan port | Multidirectional compensation, not resistant to high voltage |
| Non-metallic silicone cloth | 350℃ | Medium temperature section flue | Low cost, moderate life |
V. Key points of installation and maintenance
1. Installation precautions
- Pre-tension/pre-compression: the metal expansion joint shall be cold-tightened according to the design calculated value during installation. Generally, the pre-deformation amount is 50% of the total compensation amount
- Direction of the guide tube: The open end of the guide tube must be directed towards the direction of media flow to prevent dust from entering the bottom of the bellows
- It is strictly prohibited to knock the bellows: Do not contact the bellows surface directly with a crow bar or hammer during installation
- Bracket setting: guide brackets must be set on both sides of the expansion joint, and the distance between the first guide bracket and the expansion joint is ≤4DN
2. Daily inspection content
- Check the surface of the metal bellows for corrosive droplet deposition, cracks or wear and thinning
- Inspect non-metallic skin for damage, pinhole leakage, or aging cracking
- Measure the surface temperature of the expansion joint to determine whether the heat insulation layer has failed
- Check for loose or stuck attachments such as pull rods and hinges
3. Replacement cycle recommendations
- Metal expansion joint: Usually, the design life is 8-10 years, and the wall thickness is sampled every 2-3 years in actual operation
- Non-metallic expansion joint: the skin life is generally 3-5 years, and should be replaced as a whole when due
Frequently Asked Problems and Solutions
| Problematic phenomenon | Probable cause | Solution |
|---|---|---|
| Metal expansion joint bellows cracked | Insufficient fatigue life/skewed installation | Re-check the displacement and replace the high fatigue life design |
| Non-metallic expansion joint skin burn through | Moisture or insufficient thickness of insulation | Replace heat insulation layer and add water phobic holes |
| Abnormal deformation of expansion joint | Failure of fixed bracket or incorrect cold tightening | Check bracket and reinstall per specification |
| Excessive flue vibration | Mismatch of expansion joint stiffness with system | Adjust the number of expansion joints or change to vibration damping type |
conclusion
The expansion joint of waste heat power generation is the core component that connects flue system, absorbs heat displacement and ensures equipment safety. The reasonable selection and use of the expansion joint directly affects the continuous and stable operation of waste heat power station. In this paper, three main types of expansion joints for waste heat generation are systematically sorted out- -metal bellows type, non-metal type and special corrosion-resistant type. Specific selection suggestions are given for the key parts such as boiler inlet, outlet, fan inlet and outlet and elbow. The core selection principle can be summarized as follows: heat-resistant metal + guide tube for high-temperature section, non-metal or high-nickel alloy for low-temperature corrosion section, heavy vibration isolation at fan port, and angular/universal type at elbow. At the same time, pre-stretching installation, regular inspection, maintenance and replacement in strict accordance with the specifications can minimize the failure probability of the expansion joint and significantly improve the operating reliability and economic benefits of the waste heat power generation system. It is hoped that this paper can provide practical and professional technical reference for the engineers engaged in the design, operation and maintenance of waste heat power generation.