In the clean flue of wet desulfurization system, the clean flue gas flue expansion joint drain hole is a seemingly unremarkable but crucial detail design. The temperature of the clean flue gas drops to 45-55℃ after being washed by the desulfurization tower, and it is completely saturated, carrying a large amount of water mist and acidic droplets. When these wet flue gases pass through the non-metallic expansion joints, the condensed acid will accumulate in the annular groove formed by the skin and the pressure plate, and long-term immersion will lead to skin penetration and bolt corrosion and fracture-this is the root cause of many expansion joints "leaking again shortly after replacement". This paper will systematically explain the professional technical knowledge of drainage hole of flue flue expansion joint from the necessity of drainage hole, design specification to maintenance management.
1. Why must the flue flue expansion joint be provided with a drainage hole?
1. Groove water accumulation: "innate defect" of non-metallic expansion joint
The non-metallic expansion joint consists of a skeleton and a skin with a multi-layer fabric (fluororubber/silicone rubber coated fiberglass cloth) that is fixed to the frame with platens and bolts. This structure will naturally form an annular groove when installed-this is the "hotbed" of standing water。
Hazard chain: When the unit is running, the condensed acid liquid in the wet flue gas accumulates in the groove → the acid liquid slowly penetrates the skin fabric layer → reaches the position of the fixing screw → the corrosion of the screw causes loosening or breaking → the acid liquid flows out from the broken screw hole and the damaged part of the skin → the serious "small waterfall" phenomenon appears。
2. Consequences of no drainage holes
| consequence | Specific performance | Time Period |
|---|---|---|
| Skin seepage | Moist patches on surface, drip | 6-12 months after commissioning |
| Bolt corrosion | Loose, broken, weak skin fixation | 1-2 years |
| Frame corrosion | Metal frame corrosion perforation | 2-3 years |
| Environmental issues | Acid mist pungent, ground corrosion | Persistence |
From the point of view of operation, expansion joints without drainage holes will generally be corroded and damaged in one to two years。 If the drainage hole is not set and the center hole is opened and the drainage pipe is used instead, the drainage pipe will be misaligned due to the temperature change, which will break the skin and form a new leakage point。
3. Water will also accumulate between layers
There is another problem with multi-layer skin design: water accumulation also occurs between layers during operation. Even if the skin is replaced, water seepage will continue if the drainage between the layers is not resolved。 This analysis just verifies the necessity of setting drainage holes.
2. Design specification for drainage hole of flue expansion joint for clean flue gas
1. Set Location Requirements
According to the technical requirements of several engineering projects, the setting of the drainage hole of the expansion joint of the flue flue gas must follow the following principles:
- Must be located in the horizontal flue section: Only expansion joints that are exposed to wet flue gas and are located in the horizontal flue section need to be provided with drain holes
- At the lowest point: The drainage hole shall be set at the lowest point of the expansion joint frame to ensure that the condensate can be discharged naturally by gravity
- Located on the centerline: The drain hole shall be located on the centerline of the horizontal flue section
2. Pore size and material requirements
| Design elements | Technical Requirements | basis |
|---|---|---|
| Minimum pore size | ≥ DN150 | Technical specification requirements |
| Drainage fitting materials | FRP (FRP), nickel-based alloy steel | Acid corrosion resistance requirements |
| Drainage destination | Return to FGD Area Sump | Environmental protection and system cycle requirements |
The drainage holes are the parts that are in direct contact with the corrosive condensate, so highly anti-corrosive materials must be selected. FRP and nickel-based alloy steels such as Inconel have excellent acid resistance to prevent the drainage holes themselves from being corroded and failed。
3. Design idea of drainage device
Patent technology CN1804465A discloses an automatic drainage device for flue expansion joint, the core design of which is as follows：
Structural composition:
- Low discharge port (set at the lowest point of the expansion joint)
- Drain pipe (soft connection can be added to prevent cracking by thermal displacement)
- Automatic drainage valve (automatically controls drainage by float principle)
How it works:
- The accumulated water enters the drain pipe through the low discharge port
- The float in the automatic drain valve rises and falls with the liquid level
- Automatically start drainage when the liquid level reaches the set value
- Automatically shut off after drainage is completed to prevent smoke leakage
The advantage of this patented scheme is that it realizes "automatic drainage with accumulated water and automatic closure without accumulated water", which not only ensures smooth drainage, but also prevents outside air from being sucked into the flue during negative pressure operation, which can be described as killing two birds with one stone。
4. Thermal insulation and antifreezing
For projects in northern cold areas, heat insulation and anti-freezing measures should also be considered for drainage holes and drainage pipelines, so as to prevent the internal structure of the expansion joint from being damaged by icing and expansion of condensate when the machine is shut down in winter。
3. Remedial scheme of expansion joint without drainage hole
Is there a remedy for expansion joints that have been installed but are not provided with drains? The answer is yes.
1. On-site opening scheme
Holes can be opened on site at the lowest point of the expansion joint frame, and drainage stub and valves can be installed.
Operation Points:
- After confirming the lowest point position, use mechanical drilling (gas cutting is strictly prohibited)
- Remove burrs and smooth after drilling
- Install stainless steel or FRP drainage short pipe
- Install ball valves for regular operation
2. Material Upgrade: From Silicone Rubber to Viton Rubber
In addition to adding drainage holes, the choice of skin material is also crucial. If the inner layer of the skin is made of acid-resistant silicone rubber plate, acid water will quickly penetrate into the fixing screws, and leakage may occur after one or two months of operation。
Remedy: Replace with a fluororubber (FKM) skin, which has excellent acid resistance and is able to resist the attack of condensate on the skin.
Drainage Management in Operation and Maintenance
1. Valve Condition Management
It is not recommended that the valve of the drain hole of the flue flue expansion joint be opened normally. The reasons are as follows: the clean flue is usually operated under negative pressure. If the drain valve is normally open, the outside air will be sucked into the flue, which affects the oxygen control of the system and the power consumption of the induced draft fan.
Correct way to do this:
- The valve remains normally closed
- Turn on drainage for 1-2 minutes per shift (8 hours)
- Observe the properties of discharged liquid, and record any abnormalities in time
2. Inspection and dredging
| Check Items | Frequency | Contents |
|---|---|---|
| Drainage patency | Weekly | Open the valve to check for water outflow |
| Clogging cleanup | Monthly | Use fine steel drill to dredge and backblow to clean |
| Anti-corrosion inspection | Quarterly | Check drain root for corrosion |
3. Diagnostic value of draining fluids
The internal state of the expansion joint can be judged by observing the liquid properties during drainage:
| Liquid Character | Probable cause | Treatment measures |
|---|---|---|
| Clear, colorless | Normal condensate | Continue to observe |
| Yellow/reddish brown | Frame begins to corrode | Check the anti-corrosion coating |
| Turbid/particle-containing | Damaged insulation | Replacement during scheduled overhaul |
| Abnormally increased discharge | Poor upstream flue insulation | Check flue insulation |
V. Summary and Suggestions
Although the drainage hole of the flue expansion joint of the purified flue gas is small, it can guarantee the expansion of non-metal Critical design for long-term reliable operation. Every link from design, installation to maintenance cannot be ignored:
Design stage
- Must be provided: Horizontal flue expansion joints exposed to wet flue gas must be provided with drainage holes
- Accurate position: Located at the lowest point of the expansion joint, on the centerline of the flue
- Material anti-corrosion: FRP or nickel-based alloy steel for drainage fittings
- Aperture to standard: DN150 at least
Installation Phase
- Confirm the lowest point on site before opening the hole
- Mechanical drilling is adopted, gas cutting is strictly prohibited
- Drainage pipeline to drainage pit, seal well
- Northern Project Considers Thermal Insulation and Freezing Prevention
Operation and maintenance
- The valve is kept normally closed and the water is drained regularly
- Check for smooth drainage
- Observe the properties of discharged liquid and find abnormalities in time
A drainage system of flue flue expansion joint with reasonable design and standardized management can fundamentally solve the stubborn problem of "leaking after changing the skin", and increase the service life of expansion joint from "leaking in 1-2 years" to "no leakage in 3-5 years". Remember: changing a drain hole is better than ten emergency plugs.