In the design and construction of high temperature flue expansion joint, expansion joint nail (also called anchor and target nail) is a often neglected but vital component. It anchors the castable firmly to the expansion joint metal shell like reinforcement in concrete. Although the nail is small, once it fails-it falls off, rust breaks or the layout is unreasonable, the castable will crack and fall off, and the high-temperature flue gas will directly wash the metal bellows, which will eventually lead to the expansion joint burning through and scrapping. This paper will systematically explain the professional technical knowledge of the expansion joint nail scraping from the function to the construction specification.
1. The role of the expansion joint scraping nail: why is it essential?
The core function of the expansion joint scraping nail is to firmly connect the castable with the metal shell, and prevent the castable from peeling under high temperature and vibration conditions.
1. Anchorage and Fixation
The expansion joint scraping nail shall be installed in place before pouring, without interference with other structures, so as to ensure that the poured concrete can fully wrap the scraping nail, and the buried depth shall not be less than 2/3 of the scraping nail length。 The nail is fixed on the metal shell of the expansion joint by welding, and the castable is wrapped after pouring to form the overall structure of "nail-castable", which can resist the erosion of high-temperature flue gas and the damage of thermal stress.
2. Prevent castable from peeling
In the high temperature dusty environment such as circulating fluidized bed boiler and dry coke quenching, high-speed air flow scour will produce continuous wear and impact on the castable surface. Through multi-point anchor, the expansion joint scraper firmly "grasps" the castable on the metal shell, and even if micro-cracks appear on the surface, the whole piece will not fall off.
3. Adapting to thermal expansion differences
The coefficient of thermal expansion of metal shell and castable is different, and the relative displacement will occur when the temperature changes. This difference needs to be considered in the design of the expansion joint scraper, and the material selection and welding quality should ensure that it does not break or loosen during the thermal cycle。
2. Material selection of expansion joint scraping nail
The material selection of expansion joint scraping nail directly affects its service life in high temperature environment. Nails of different materials should be selected in different temperature ranges:
| Operating temperature | Recommended nail material | Corresponding grade | Applicable scenarios |
|---|---|---|---|
| ≤900℃ | 1Cr18Ni9Ti | 321 stainless steel | Conventional high temperature flue |
| 900-1100℃ | 0Cr25Ni20 | 310S Stainless Steel | High temperature flue, boiler furnace |
| 1100-1300℃ | Nickel-based alloy | Inconel600/601 | Extra-high temperature and strong corrosion conditions |
| 1300-1400℃ | Special heat-resistant alloy | Cobalt-based alloy | Extreme high temperature conditions |
In the high-temperature equipment of cement, metallurgy, electric power and other industries, the commonly used materials of expansion joint scraping nails are 1Cr18Ni9Ti (321 stainless steel) and 0Cr25Ni20 (310S stainless steel), with specifications covering Φ 6-Φ 18, shapes including Y-shape, V-shape and other forms, and the operating temperature can reach 1000℃-1400℃。
Key points of selection:
- The material of the scraping nail should be matched with the expansion joint body or the pouring medium to avoid electrochemical corrosion
- The diameter, length and size shall comply with the requirements of the drawings; the deviation shall be ≤0.5mm
- The surface of the nail should be cleaned to remove oil, rust and oxides, and rust removal paint can be applied if conditions permit
3. Arrangement spacing and quantity of expansion joint scraping nails
The layout density of the expansion joint scraping nail directly affects the anchoring effect. If the spacing is too large, the castable will be anchored insufficiently and easily peeled off; If the spacing is too small, it wastes materials and labor.
1. Spacing requirements
Expansion joint scraping nails shall be evenly distributed in the circumference and axial direction of the expansion joint, and the spacing between adjacent scraping nails shall be consistent with the deviation ≤5mm。 The specific spacing shall be carried out according to the design requirements.
2. Quantitative standards
The number of scraping nails for expansion joints should meet the demand of pouring structure, generally arranged according to 4-6 per square meter。 High-temperature and high-scour areas can be appropriately densified, and low-temperature areas can be appropriately relaxed.
3. Positioning accuracy
Based on the sealing surface of the flange of the corrugated expansion joint, the centerline of the crest and trough of the wave, ensure that the center deviation of the nail of the expansion joint is ≤1mm, so as to avoid the uneven pouring force caused by the deviation。 Advance calibration with deviation-free measuring tools (tape measure, level, etc.) to ensure measurement accuracy。
4. Requirements for welding and fixing of expansion joint scraping nails
The welding quality of expansion joint scraping nail is the core guarantee of anchoring effect. If the welding is not firm, the scraper nail loosens at high temperature, and the castable will lose its anchor.
1. Welding Method
The expansion joint is usually fixed by welding. It is necessary to pay attention to the compatibility of nail material and expansion joint material during welding, so as to avoid cracks generated during welding。
- When the non-welded parts are connected with bolts, anti-loosening nuts should be used, and torque control should be paid attention to
2. Weld Quality Standards
| Check Items | Standard Requirements |
|---|---|
| Weld height | Not less than 1/2 of the diameter of the nail |
| Weld length | Not less than 1/3 of the nail length |
| welding defect | No defects such as pores, slag inclusions and edge biting |
Appearance inspection is required after welding and penetration test is performed if necessary。
3. Verticality and firmness
After installation, ensure that the expansion joint nail is vertical, firm and free from loosening。 A skewed nail will cause the castable to wrap unevenly and form a weak spot.
5. Matching of the thickness of the expansion joint scraping nail and the castable
The length of the expansion joint nail must match the design thickness of the castable. This is an easily overlooked but crucial design point:
| Castable thickness | Requirements for nail length | consequence |
|---|---|---|
| 100mm | About 150mm (buried depth ≥2/3) | Normal anchoring |
| 200mm | About 300mm | Normal anchoring |
| 100mm | 300mm | Unable to pour properly, nail exposed |
| 200mm | 100mm | Insufficient buried depth, anchoring failure |
Core principle: The thickness of the castable should be accurately matched with the length of the nail to ensure that the castable can fully wrap the nail, and the buried depth is not less than 2/3 of the length of the nail。
VI. Installation deviation and quality control
The construction quality of the nail scraping of the expansion joint directly affects the overall service life of the expansion joint, and the following links should be strictly controlled:
1. Check before installation
- Verify that the specifications and materials of the nail are consistent with the drawings
- Clean the oil and rust stains on the installation part
- Calibrating Measuring Tools
2. Installation Process Control
- Locate and lay out the line according to the drawing, and mark the position of scraping nails
- Control welding parameters to ensure weld quality
- Sampling the verticality and spacing of the nails
3. Acceptance after installation
| Acceptance items | Criteria |
|---|---|
| Center deviation | ≤1mm |
| Spacing deviation | ≤5mm |
| Perpendicularity | No obvious tilt |
| Weld appearance | No pores, slag inclusions, biting edges |
| firmness | Hammer check no loosening |
sum up
Expansion joint scraping nail is a "steel reinforced iron bone" connecting castable and metal shell in high temperature flue expansion joint. Its selection and construction quality directly determine the service life of castable and the overall reliability of expansion joint. The core points can be summarized as follows:
Material matching: Select the nail material according to the working temperature-select 1Cr18Ni9Ti below 900℃, select 0Cr25Ni20 at 900-1100℃, and select nickel-based alloy at ultra-high temperature
Spacing specification: Even arrangement, adjacent spacing deviation ≤5mm, 4-6 per square meter, center deviation ≤1mm
Welding quality: the height of the weld seam is ≥1/2 of the diameter of the nail removal, the length is ≥1/3 of the length of the nail removal, and there are no pores, slag inclusions and biting edges
Length matching: The thickness of castable is accurately matched with the length of the nail, and the buried depth is ≥2/3 of the length of the nail
Installation sequence: Nailing installation must be completed before castable pouring to ensure no interference with other structures
An expansion joint nail scraping system with reasonable design and welding specification can firmly anchor castable on the expansion joint shell, and run stably for a long time under harsh working conditions such as high temperature and vibration. It is recommended that the materials, specifications and layout requirements of nails be clarified in the design stage, and the welding quality inspection should be carried out by professionals in the construction stage.