In industrial flue system, expansion joint is a flexible connection component, and its sealing performance directly affects the operating efficiency and environmental protection standards of the system. However, as the running time increases, the expansion joint will inevitably experience some degree of air leakage. So, how much air leaks from flue expansion joints? The answer to this question is related to the increase of energy consumption of induced draft fans, distortion of emission data and shortening of equipment life. This paper will systematically analyze the problem of expansion joint air leakage from three dimensions: the range of air leakage rate, quantitative calculation method and influencing factors.
1. Typical numerical range of air leakage rate of expansion joint
Regarding the amount of air leakage in flue expansion joints, there are significant differences between different types and states of expansion joints. According to industry statistics and on-site test results, it can be summarized as follows:
| Expansion joint state | Air leakage rate range | Description |
|---|---|---|
| Newly installed qualified products | Good seal, negligible leakage | |
| 1-3 years in operation | 0.5%~2% | Slight aging with less impact |
| 4-6 years in operation | 2%~5% | Obvious leakage, recommended monitoring |
| More than 6 years of operation | 5%~15% | Serious leak, must be replaced |
| Failure damage | >15% | Emergency shutdown treatment |
Taking a typical 300MW unit desulfurization system as an example, when the expansion joint air leakage rate reaches 5%, it is equivalent to thousands of cubic meters of untreated flue gas (or air) leakage per hour, which has a significant impact on the system operation.
2. Quantitative calculation method of air leakage rate
To accurately answer how much air leaks in flue expansion joints, it is necessary to master scientific detection and calculation methods.
2.1 Oxygen balance method
This is the most commonly used on-site estimation method, using the change in oxygen content in the flue gas to calculate the air leakage rate:
Δα = (O₂ ''-O₂ ') / (21-O₂'') ×100%
Among them:
- O₂' — — Oxygen content upstream of the expansion joint (before air leakage), %
- O₂ ''-oxygen content downstream of the expansion joint (after air leakage), %
Calculation example: The oxygen content before a certain expansion joint is 3.5%, and the oxygen content after the expansion joint rises to 5.2%, then the air leakage rate:
Δα = (5.2-3.5) / (21-5.2) ×100% =1.7/15.8×100% ≈ 10.8%
2.2 Pressure differential estimation method
For non-metallic expansion joints, the air leakage rate is roughly related to the pressure difference:
| Pressure differential before and after expansion joint (Pa) | Estimating air leakage rate |
|---|---|
| 50-200 | 1%-3% |
| 200-500 | 3%-8% |
| >500 | >8% |
This method is suitable for rapid judgment, but the accuracy is low, and it is recommended as a preliminary screening method.
2.3 Flowmeter comparison method
When flowmeters are installed upstream and downstream of the expansion joint, the air leakage can be directly calculated:
Q_leak = Q_downstream-Q_upstream
Air leakage rate = Q_leak/Q_upstream ×100%
This method has the highest accuracy, but requires flow measuring points. For the flue system of power plant, the induced draft fan current reverse flow change can be used as auxiliary.
3. Comparison of air leakage volume under different working conditions
The amount of flue expansion joint air leakage is also significantly affected by system pressure. The flue is usually divided into positive pressure section and negative pressure section, and the air leakage mechanism of the two is different:
3.1 Expansion joint of positive pressure section
When the positive pressure section (such as the front of the induced draft fan and the outlet of the booster fan) is running, the internal pressure is higher than the atmospheric pressure, and the air leakage is manifested as smoke leakage:
- Hazard: Direct emission of SO₂ and dust in flue gas, great environmental risk
- Leakage estimation: Q_leak = C × A × √ (2 Δ P/ρ)
Where C is the flow coefficient (take 0.6-0.8), A is the leakage area (m²), and Δ P is the pressure difference between inside and outside (Pa).
For example: the leakage hole area is 1cm², the pressure difference is 5000Pa, the flue gas density is 0.85kg/m³, and the estimated air leakage volume is about 0.25m³/s (900m³/h).
3.2 Negative pressure section expansion joint
When the negative pressure section (such as the rear of the induced draft fan and the clean flue at the outlet of the absorption tower) is running, the internal pressure is lower than the atmospheric pressure, and the air leakage is manifested as air suction:
- Hazards: Air diluted flue gas, high O₂ and low SO₂/NOx concentration measured by CEMS, distortion of emission data
- The leakage estimation method is similar to that of positive pressure, but the direction is opposite
In practice, the air leakage rate of the negative pressure section is usually slightly higher than that of the positive pressure section at the same pressure differential, because the negative pressure is more likely to draw outside air in through the tiny gaps.
Classification and impact assessment of air leakage rate
For the convenience of on-site judgment, how much air leakage of flue expansion joint can be divided into five grades:
| grade | Air leakage rate | Air leakage phenomenon | Handling recommendations |
|---|---|---|---|
| Excellent | No visible leakage | Normal use, regular inspection | |
| qualified | 1%-3% | Trace leakage, instrument only | Strengthen monitoring and plan maintenance |
| attention | 3%-5% | Occasional wet marks or slight smoke | Arranging maintenance within half a year |
| seriousness | 5%-10% | Significant smoke/inhalation sound | Replacement within three months |
| critical | >10% | Smoke columns or sharp howls are visible | Immediate shutdown treatment |
5. Differences in air leakage of expansion joints in different parts
Due to different installation locations, the actual impact of the same air leakage rate varies:
- Denitrification inlet expansion joint: Air leakage reduces flue gas temperature and affects denitrification efficiency. For every 1% increase in the air leakage rate, the denitrification efficiency decreases by about 0.5%-1%.
- Expansion joint in front of dust collector: Air leakage increases the load of induced draft fan and the power consumption increases. 1000m³/h air leakage increases power consumption by about 5-10kW.
- Expansion joint at absorption tower inlet: air leakage causes oxygen content to rise, which affects desulfurization reaction. When the air leakage rate is> 3%, the desulfurization efficiency begins to decrease significantly.
- Net flue expansion joint: Air leakage near CEMS measuring points can cause data distortion. When the air leakage rate is 5%, the measured SO₂ concentration is about 8%-12% lower.
6. Variation law of air leakage rate with time
The air leakage of the expansion joint is not a constant value, but increases rapidly with the running time. The typical evolution law is as follows:
- 0-2 years (run-in period): Air leakage rate
- 2-4 years (slow growth period): the material starts to age and the air leakage rate rises to 1%-3%
- 4-6 years (accelerated period): Bellows fatigue or skin powder, air leakage rate rises to 3%-8%
- More than 6 years (expiration period): Local damage occurred, air leakage rate> 8%
Especially in units with frequent start-and-stop, the above cycle may be shortened by 30%-50%. This is because each temperature cycle accelerates the fatigue and aging of the expansion joint material.
Measures to reduce air leakage
After knowing how much air leaks from the flue expansion joint, it is more important to take steps to reduce the air leak:
7.1 Selection stage
- Select high-quality aging-resistant materials (fluororubber instead of silicone rubber)
- Increase the number or thickness of sealing layers
- For the negative pressure section, non-metallic expansion joint is preferred (better sealing performance)
7.2 Installation Phase
- Flange bolts are tightened to design torque in three diagonal sequences
- Apply high temperature resistant sealant to the flange surface
- Install the guide tube correctly to avoid airflow washing the sealing surface
7.3 Operation and Maintenance
- Establish a regular inspection system for air leakage rate (once per quarter)
- When the air leakage rate is found to be> 3%, use infrared thermography or ultrasonic leak detector to locate the leakage point
- Small leakage points can be plugged online (high temperature resistant tape/sealant), and large leakage points can be shut down for treatment
VIII. Summary
There is no fixed answer to the question of how much air leaks from flue expansion joints. It depends on the type of expansion joints, operating years, working conditions and maintenance conditions. Comprehensive industry data, the following conclusions can be drawn:
- Air leakage rate range: newly installed qualified products15%
- Detection methods: Oxygen balance method is the most practical, pressure difference method is suitable for quick judgment, and flowmeter method is the highest accuracy
- Impact quantification: For every 1% increase in air leakage rate, the energy consumption of induced draft fan increases by about 1%-2%, and the desulfurization/denitrification efficiency decreases by 0.5%-1%
- Hierarchical management: air leakage rate10% subject to emergency treatment
To master the accurate value of air leakage rate and establish a regular inspection system is the basic work to ensure the economic, environmental protection and safe operation of flue system. Expansion joints whose air leakage rate has exceeded 5% should be included in the near-term replacement plan to avoid more serious equipment failure or environmental protection incidents caused by small losses.