In flue gas treatment systems in iron and steel, metallurgy, electric power and other industries, clean steel flue expansion joints are key flexible components that connect desulfurization equipment with chimneys, absorb heat displacement and prevent corrosive gas leakage. After desulfurization and washing, the temperature of the flue gas in the clean steel flue drops to 45-55℃, which is completely saturated and contains a large amount of acidic condensate, which puts forward extremely high requirements for the selection, installation and use of expansion joints. It is the key to ensure the safe operation and prolong the service life of the equipment to correctly master the use method of the expansion joint of the clean steel flue. This article will systematically explain the use of clean steel flue expansion joint from installation specification, cold adjustment, operation monitoring to maintenance.
I. Preparation before installation
Proper use of net steel flue expansion joints, the first step is standard installation. Before formal installation, the following inspections should be completed:
1. Product acceptance inspection
- Check whether the model, specification and nominal diameter of the expansion joint are consistent with the design drawings
- Check the flexible extension joint (non-metal skin) for signs of damage, scratches, and aging
- Verify that the guide tube is oriented correctly-the small end of the guide tube must be oriented in the direction of flue gas flow
- Check whether the anti-corrosion coating of the metal frame is intact and the sealing surface of the flange is free of scratches and rust
2. Interface size review
At the installation site, the center lines of the two interface flanges of the smoke and air duct should be reviewed to ensure that they remain on the same axis. The coaxiality error should be controlled within ±5mm, which is the basic prerequisite for ensuring the normal operation of the expansion joint。
Core Installation Specifications and Deviation Control
In the use method of net steel flue expansion joint, the deviation control in the installation stage directly determines the compensation ability and sealing performance of the expansion joint.
1. Installation length control
The installation length refers to the distance from the flange face of the expansion joint to the flange face (or between the end face of the connector). According to the industry specification, the allowable deviation of the installation length of the expansion joint and expansion joint is L ± 5mm。 When installing, measure accurately with a tape measure or laser rangefinder to ensure that the length is consistent with the design.
2. Coaxiality and Torsion Control
| Deviation Item | Allowable range | Detection Methods |
|---|---|---|
| Coaxiality error | ±5mm | Pull wire or laser centering |
| Torsion angle | ≤10° | Angle ruler measurement |
| Offset angle | ±0.1° | Level + Angle Ruler |
During installation, the center line of the two interface flanges of the smoke and air duct shall be kept on the same axis center line。 If the coaxiality exceeds the standard, the expansion joint will bear additional bending moment and accelerate fatigue failure. Torsion angle deviation refers to the rotation angle of the expansion joint around the pipeline axis, which should be controlled within 10°; The up-down or left-right offset angle shall be controlled within ±0.1°。
3. Flange connection requirements
For flanged net steel flue expansion joints, the flange bolts should be tightened gradually and evenly under pressure, and the tightness of all bolts should be as consistent as possible. It is recommended to adopt the method of "diagonal tightening and fractional tightening":
- First tightening: Pre-tighten to 30% target torque in diagonal order
- Secondary tightening: tighten to 70% target torque in diagonal order
- Final Tightening: Tighten in diagonal order to 100% target torque
For the wet smoke environment with bad working conditions, spring washers can be installed to prevent bolts from loosening due to vibration during operation.
3. Key operation after installation: Remove the mounting bolts
This is the most overlooked link in the use method of net steel flue expansion joint.
When the expansion joint leaves the factory, for the convenience of transportation and installation, mounting bolts or transport rods are usually set as temporary fixtures. After the expansion joint is installed, it must be completely removed。 These temporary fixtures are set up in different locations:
- Inner nut marked yellow: represents the cold preload position, which should be adjusted to the design position after installation
- Transport tie rod: Must be completely removed after installation so that the expansion joint can expand and contract freely
IMPORTANT WARNING: If the transport rod or positioning rod is not removed after installation, the expansion joint will be locked in a fixed position and cannot absorb the thermal displacement of the pipe. When the pipe is expanding under heat, the thermal stress cannot be released, which will lead to pipe deformation, bracket damage and even expansion joint cracking failure. This mistake is common in actual projects, so it must be checked in the acceptance process.
4. Cold preloading and adjustment
In the use method of clean steel flue expansion joint, cold preloading is the key step to ensure that the expansion joint is in the best working state under hot state.
Principle of preloading
The net steel flue rise in temperature during operation and the pipe will develop thermal expansion. If the expansion joint is pre-compressed or stretched for a certain amount (i.e. pre-tightened) in the cold state (installation state), the bellows can be in the optimal stress state when working in the hot state, thus improving the fatigue life and compensation effect.
Operation Procedures
- Look at the identification mark (usually yellow) on the expansion joint to clarify the preload direction and amount
- Adjust the position of the inner nut according to the design requirements so that the expansion joint is in the pre-tightened state
- Record preload amount as reference data for operation monitoring
Use Monitoring in Operation
The Clean Steel Flue Expansion Joint method of use not only includes installation, but also covers continuous monitoring during the operation phase.
1. Appearance inspection
- Check the surface of the expansion joint for water seepage, air leakage and bulge every week
- Check for looseness and corrosion of pressure plate bolts
- Observe whether the drainage hole is unobstructed and blocked
2. Temperature monitoring
- Monitoring External Surface Temperature of Expansion Joint by Infrared Thermometer
- If local abnormal temperature rise (much higher than normal value) is found, it indicates that the internal insulation layer may be damaged or the smoke flow may be biased
- Under normal circumstances, the external temperature of the expansion joint with heat insulation packing should be ≤50℃
3. Displacement observation
- Set a displacement reference point near the expansion joint
- Regularly observe whether the actual expansion and contraction of the expansion joint is within the design range
- If over-limit displacement is found, check for failure of guide bracket and fixed bracket
4. Leak Detection
- Check for acid drip at the bottom of the expansion joint with pH test strip
- Check for smoke leakage with a portable gas detector
- Discovery of leakage should be recorded in time and arranged for treatment
Drainage Management
For horizontally installed net steel flue expansion joints, drainage management is an important link in daily use.
Drain hole setting
The net steel flue is exposed to wet smoke, and horizontally mounted expansion joints must be provided with drainage holes at the lowest point of the frame. The drain holes are usually located on the centerline of the horizontal flue section, and the bore diameter is determined according to the expansion joint specifications (general DN50-DN150).
Key points of drainage operation
- The drain valve shall be kept normally closed (to prevent outside air from being sucked in during negative pressure operation)
- Turn on drainage for 1-2 minutes per shift (8 hours)
- Observe the properties of the discharged liquid:
- Clear, colorless: normal condensate
- Yellow or reddish brown: Frame starts to corrode, anti-corrosion coating needs to be checked
- Cloudy or particle-containing: Insulation may be broken
- Close valve in time after drainage
Prevent clogging
- Stainless steel dustproof net (hole diameter 3-5mm) shall be installed on the inside of the drainage hole
- Inspect and clean drain holes monthly to prevent dust from accumulating and clogging
Prohibited Matters and Safety Warnings
In the use method of clean steel flue expansion joint, the following behaviors are strictly prohibited:
1. Load bearing is strictly prohibited
Expansion joints cannot be used as pipe supports, tools, materials must not be stacked on expansion joints, or as work platforms. Hoisting should be tied to the metal frame with a hoisting belt, not to the skin or bellows.
2. Welding damage strictly prohibited
If there is on-site welding during installation, fire-proof cloth should be used to cover the expansion joint body to prevent welding slag from splashing and damaging the skin。 Welding temporary supports to bellows or skin is strictly prohibited.
3. It is strictly prohibited to use expansion joints to forcibly adjust pipeline deviation
It is strictly prohibited to adjust the installation deviation of the pipe by deforming the bellows. If there is a large deviation in the pipe joint, it should be solved by adjusting the pipe support or adding transition joints, and the expansion joints should not be forcibly stretched or compressed.
4. Prevent mechanical damage
It is strictly prohibited to impact the expansion joint by heavy objects or pierce the telescopic ring belt by acute angles of metal。 Protective measures shall be taken to protect the expansion joints during construction crossing operations.
5. Prevent open flames
The fabric part of non-metallic expansion joint is flammable, so attention should be paid to prevent open flame ignition during installation and use. Expansion joints containing thermal insulation cotton especially require attention to fire protection.
Regular maintenance and replacement cycle
The final link in the use method of clean steel flue expansion joint is to establish a regular maintenance system:
1. Bolt retightening
- 1 month after commissioning: Full tightening of all platen bolts
- 3 months after commissioning: tightening again
- Quarterly thereafter: Check fastening once
2. Inspection cycle
| Check Items | Frequency | Contents |
|---|---|---|
| Appearance inspection | Weekly | Whether the skin is damaged, aged or bulged |
| Drainage operation | per shift | Turn on the drainage and observe the draining liquid |
| Bolt tightening | Quarterly | Tighten bolts according to specification |
| Anti-corrosion inspection | per annum | Check Frame Coating Integrity |
| Comprehensive testing | Each shutdown for maintenance | Wall thickness testing, non-destructive testing |
3. Replacement cycle judgment
- The normal service life of non-metallic skin is about 3-5 years
- It should be replaced if:
- Large-area aging brittleness of skin
- Repeated leak repair more than 2 times in the same location
- Metal frame corrosion thinning more than 30% of design thickness
sum up
The core of the use method of clean steel flue expansion joint can be summarized as "seven steps of installation, five monitoring of operation and three regular maintenance":
| Stage | Core Essentials |
|---|---|
| Installation Phase | Check model specifications, coaxiality ±5mm, torsion ≤10°, length L±5mm, diagonal tightening of flange bolts, correct direction of guide tube, remove transport tie rod after installation, prevent mechanical damage and welding splash |
| Cold-state regulation | Pre-tighten or pre-bias according to the design requirements, and adjust the inner nut to the specified position |
| Operation Monitoring | Appearance inspection, temperature monitoring, displacement observation, leak detection, regular drainage (per shift) | r>
| Regular maintenance | Bolt re-tightening (1 month, 3 months for the first time, and subsequently every quarter), anti-corrosion inspection, and comprehensive inspection of furnace shutdown |
| Lifetime management | Skin replacement in 3-5 years, timely replacement when the replacement standard is reached |
One-time standard installation and correct use can ensure the long-term stable operation of the net steel flue expansion joint under the harsh working conditions of high temperature and corrosion. It is suggested that all enterprises should incorporate the use specifications of expansion joints into the operating procedures, and list expansion joints as key inspection items during each shutdown for maintenance.