In selective catalytic reduction (SCR) denitrification system, bypass flue is a critical channel when used for overhaul of denitrification system, catalyst replacement or flue gas overtemperature protection. However, the SCR bypass expansion joint installed on the bypass flue has to bear the impact of high-temperature flue gas instantly under emergency conditions because it is in "cold standby" state for a long time. Its design and maintenance logic are completely different from those of the main flue expansion joint. This paper systematically analyzes the core technical points of SCR bypass expansion joint from function positioning, special working conditions, selection parameters to typical fault prevention, and helps the operation and maintenance personnel to avoid the passive situation of "not using at ordinary times, but breaking when used".
1. Functional positioning and working conditions of SCR bypass expansion joint
1. What is an SCR bypass expansion joint?
The SCR bypass expansion joint refers to a flexible compensation element installed on the bypass flue of the denitrification system, usually connected in parallel between the inlet and outlet of the reactor. When the SCR reactor requires offline maintenance (such as catalyst replacement, ash cleaning, ammonia injection grid maintenance) or the flue gas temperature exceeds the upper catalyst tolerance limit (usually> 430℃), the bypass baffle door opens and the flue gas bypasses the reactor directly into the downstream equipment. At this time, the bypass flue causes violent thermal expansion due to the sudden temperature rise. The function of SCR bypass expansion joint is to absorb this transient displacement and prevent the flue structure from being damaged.
2. Three major differences with the expansion joint of the main flue
| Contrast dimension | Main flue expansion joint | SCR bypass expansion joint |
|---|---|---|
| Operating frequency | Continuous operation, long-term hot state | Intermittent operation, long-term cold standby, occasional rapid heat |
| Temperature change | Slow fluctuation (±50℃/h) | Instantaneous temperature rise (normal temperature → 400°C in 10 minutes) |
| Corrosive environment | Continuously attacked by SO₃ and fly ash | Condensed acid is accumulated in cold state and briefly washed in hot state |
| Consequence of failure | Decreased denitrification efficiency due to air leakage | The bypass could not be put into operation, resulting in the forced shutdown of the denitrification system |
It can be seen that the selection logic of SCR bypass expansion joint is completely different from that of main flue, and it must be specially designed for the working condition of "alternating hot and cold, rapid heat and rapid cooling".
2. Special working conditions and medium environment of SCR bypass expansion joint
To correctly select the SCR bypass expansion joint, it is necessary to deeply understand the four extreme working conditions it faces:
1. Long-term cold standby + instantaneous hot
The bypass flue is at normal temperature most of the time (the baffle door is closed and no smoke flows through). When it is necessary to open the bypass, high temperature flue gas (300-420℃, up to 450℃ in emergency) fills the entire bypass flue in 5-15 minutes. This means that the bellows material of the expansion joint needs to withstand the sharp temperature increase from 20℃ to above 400℃, and the thermal stress impact far exceeds the continuous operation condition.
Material requirements: Bellows must have excellent thermal fatigue resistance. In the conventional 304 stainless steel, the precipitation of grain boundary carbides is accelerated under such a vigorous thermal cycle, resulting in a decrease in toughness. It is recommended to use titanium-containing stabilizing elements (such as 321 stainless steel) or molybdenum-containing nickel-based alloys (Inconel 625).
2. Cold Condensed Acid Corrosion
When the bypass is not in operation for a long time, air and moisture will accumulate in the flue. When SO₃ in the flue gas of the main flue infiltrates into the bypass flue in trace amounts through the loosely closed baffle door, it reacts with the condensed water to generate sulfuric acid (pH 1 – 3). The bellows of SCR bypass expansion joint is immersed in this acidic condensate for a long time, which is prone to uniform corrosion or pitting.
Protective measures: set a drainage hole at the bottom of the bypass flue; The material of the expansion joint is at least 316L, and the surface is subjected to pickling and passivation treatment; Non-metallic expansion joints are more advantageous in this condition because PTFE/fluororubber is completely acid resistant.
3. Alternating dry and wet at the moment of operation
When the bypass is suddenly opened, the high-temperature flue gas will quickly evaporate the condensate film on the surface of the bellows. This "wet to dry" abrupt process will lead to the formation of locally concentrated acid on the surface of the trough, accelerating the corrosion rate. At the same time, the vapor pressure generated by the rapid evaporation of the liquid film may cause the interlayer adhesive interface of the non-metallic expansion joint to blister and peel off.
Design countermeasures: The metal expansion joint should choose the structure of large wave distance and deep wave trough to avoid the dead angle of liquid accumulation; The non-metallic expansion joint should ensure that the layers fit completely and no air bubbles remain.
4. Short high dust scour
During bypass opening, the raw high concentration flue gas (containing 30-50g/Nm³ of dust) that has not been dusted by the reactor directly flushes the bypass flue. While each scour time is short (hours to days), cumulative wear cannot be ignored.
Wear-resistant design: Wear-resistant guide tubes must be provided, and the length of the guide tube should extend at least 100mm beyond the bellows crest. The material can be 16Mn + surfacing wear-resistant layer or integral ceramic lining.
3. Selection parameters and structural points of SCR bypass expansion joint
Based on the above working condition analysis, the following five quantitative parameters should be considered in the selection of SCR bypass expansion joint:
1. Temperature Parameters
- Design temperature: Take the maximum flue gas temperature that the bypass can reach (usually 20-30℃ higher than the main flue, it is recommended to design at 450℃).
- Number of thermal cycles: estimated according to the number of bypass openings in the whole life cycle of the power plant (it is recommended to design 200 times/year, corresponding to the fatigue life of bellows ≥5000 times).
2. Displacement Parameters
The bypass flue is usually long (20-50 m) and the amount of thermal expansion is not negligible. Calculation formula:
Axial expansion Δ L = α × L × Δ T
Where α is the linear expansion coefficient of flue steel (about 12×10⁻⁶/℃), L is the distance (m) from the fixed point of the bypass flue to the expansion joint, and Δ T is the temperature rise (calculated at 350℃). For the case of L =20 m, Δ L ≈ 84 mm.
Selection suggestion: The axial compensation amount of a single expansion joint should not exceed 100mm, and when it exceeds, multiple expansion joints should be set up for segmented absorption.
3. Pressure parameters
The bypass flue design pressure is typically ±5 kPa to ±10 kPa. Note: When the bypass baffle door is not closed tightly, the expansion joint may bear the reverse pressure difference, so reinforced bellows with sufficient stiffness should be selected.
4. Structural selection control
| Characteristics of bypass conditions | Recommended expansion joint types | Key Structural Requirements |
|---|---|---|
| Low temperature bypass ( | Non-metallic fabric expansion joint | PTFE inner layer + fluororubber outer coating, with drainage hole |
| High temperature bypass (350-450℃), turned on 10-50 times a year | Metal single layer bellows (321 stainless steel) | Double-layer guide tube + trough reinforcing ring |
| High temperature + high sulfur coal (SO₃> 100ppm) | Metal bellows (Inconel 625) | Electric heat tracing + insulation, maintaining wall temperature> 160℃ |
| Ultra-low emission retrofit (bypass needs to be tightly sealed) | Metal + non-metal composite type | Metal bellows under pressure, sealed with non-metallic layer |
4. Typical faults and preventive measures of SCR bypass expansion joint
According to the investigation of many accidents of SCR bypass failure to operate normally, the main failure modes and preventive measures of SCR bypass expansion joints are as follows:
1. Thermal fatigue crack of bellows (accounting for 45%)
Phenomenon: After several times of bypass opening, circumferential micro-cracks appear in the bellows trough, and in severe cases, they leak through.
Root cause: The thermal fatigue resistance of the material is insufficient, or the thermal shock stress generated by rapid heat (up to 2-3 times the steady-state stress) is not considered in the design.
Prevention: Choose materials with excellent thermal fatigue resistance (such as Inconel 625 or RA330); Do 100% penetration detection after manufacturing; Control the temperature rise rate when the bypass is open (by adjusting the opening of the bypass baffle door, the smoke temperature slowly rises to the target value within 30 minutes).
2. Condensate acid corrosion perforation (accounting for 30%)
Phenomenon: After inspection after long-term discontinuation, it was found that there were dot-like corrosion pits or even perforations at the bottom of the bellows trough.
Root cause: the bypass baffle door is not closed tightly, and hot flue gas infiltrates to form condensed acid; Or a clogged drain hole.
Prevention: Check whether there is liquid accumulation at the bottom of the bypass flue during monthly inspection; An automatic drain trap is set at the lowest point of the expansion joint; Non-metallic expansion joints can fundamentally solve the problem of acid etching.
3. The guide tube falls off and gets stuck (accounting for 15%)
Phenomenon: After the bypass is opened, the flue vibrates abnormally, and the expansion joint cannot be expanded or contracted.
Root cause: The guide tube is fixed by spot welding, and the solder joint breaks due to the difference of thermal expansion under rapid thermal shock.
Precaution: The guide tube and the connection pipe should be connected by full welding + plug welding, and the thermal expansion coefficients of the two materials should be close (avoid stainless steel guide tube with carbon steel connection pipe).
4. Aging failure of sealing elements (accounting for 10%)
Phenomenon: The non-metallic expansion joint is coated with fluororubber hardening and cracking, and the air leakage rate exceeds the standard.
Root cause: Ozone aging or condensate acid attack in long-term cold state.
Prevention: Choose neoprene or fluororubber with excellent ozone resistance and acid resistance; Compulsory replacement of sealing layer every 5 years.
V. Key points of operation and maintenance inspection of SCR bypass expansion joint
In order to ensure that the SCR bypass expansion joint can be put into operation reliably when required, it is recommended to establish a quarterly special inspection system:
- Cold inspection (at each shutdown):
- Visually check the bellows surface for cracks, corrosion pits and dust accumulation hard lumps.
- Check that the drain hole is clear (can be dredged with iron wire).
- Measure the position of the displacement pointer and confirm that there is no permanent plastic deformation.
- Thermal verification (after each bypass commissioning):
- After the bypass is closed and cooled, check the bellows for new cracks.
- The number of bypass openings was recorded and compared with the design fatigue life of the bellows.
- Preventive replacement:
- When the metal expansion joint has been operating for 8 years or the cumulative thermal cycle exceeds 80% of the design value, it shall be Replacement.
- Non-metallic expansion joints Replace all flexible sealing elements every 5-6 years
- VI. CONCLUSIONS AND CALL TO ACTION
Although the use frequency of SCR bypass expansion joint is much lower than that of main flue expansion joint, its reliability is directly related to whether the denitrification system can be safely maintained off-line under emergency conditions. Because it is faced with the special working condition of "long-term cold standby + instantaneous rapid heat + condensation acid corrosion", the thermal fatigue resistance, acid corrosion resistance and reliable fixation of the guide tube must be emphasized when selecting the model.
Act Now: Please inspect the expansion joints of your power plant's SCR bypass flue – when was the last inspection? Is there an internal leak in the bypass baffle door? Is the drainage hole unobstructed? If you are unable to confirm whether an existing SCR bypass expansion joint meets the requirements of rapid heat, or if you plan to replace an older expansion joint, please contact our engineering and technical team today.