What the hell do deflectors do in non-metallic compensators?
Many people regard the deflector as a supporting role, thinking it is just a windshield. Wrong! It is the first line of defense against media erosion in non-metallic expansion joints (fabric fiber expansion joints). The non-metallic skin is responsible for sealing and displacement absorption, but the internal high-speed airflow or particulate matter directly impacts the skin, and the life will be greatly reduced. The deflector is just to block the front and carry the damage-the equivalent of wearing a body armor on the skin. If you think about it, smoke or dust rushes over at a speed of more than ten meters per second. Without the deflector, the skin may break in a few months.
What are the common deflector materials? Don't be confused by the model
When it comes to choosing materials, staring at 304 and 316L as soon as you come up, it is actually easy to deviate. Let's understand the basics first:
- Stainless steel 304/316L: Good corrosion resistance, suitable for the high-temperature sulfur-containing flue gas at the back end of corrugated expansion joint in power station industry. But expensive and afraid of fluoride ions.
- Carbon steel Q235: Cheap, but afraid of acid and alkali. It can be used in dry normal temperature conditions, but don't use it in places with heavy moisture.
- Galvanized sheet: Suitable for dry low-temperature working conditions, such as normal temperature air ducts. The cost is low, but the galvanized layer rust once it breaks.
- Weathering steel: Specially used in the high humidity and high sulfur environment behind the desulfurization flue gas baffle door. The surface will form a dense rust layer to protect the interior, which is much stronger than ordinary carbon steel.
Two days ago, I met a cement factory customer, and chose an ordinary carbon steel deflector to install it in front of the metal corrugated expansion joint in the cement industry. As a result, it was perforated in half a year. He is still distressed by the difference of several hundred dollars, and the money for a set of compensators later is enough to buy ten good deflectors. Do you think it's a loss or not?
How to determine the material under different working conditions? Three elements determine
Temperature, corrosiveness, particle hardness – all three must be considered at the same time. Just looking at the temperature regardless of corrosion, or just staring at the hardness and ignoring the temperature, it is easy to roll over.
The flue gas at the rear end of the corrugated expansion joint used in power station industry has high temperature and sulfur, and 316L is standard. If the chloride ion concentration is high, such as the wet desulfurization environment behind the desulfurization flue gas baffle door, 316L can't bear it, so duplex stainless steel or even titanium has to be used. The clinker dust hardness in the metal corrugated expansion joint of cement industry is large, the thickness of the deflector plate should be thickened, and the surface should be wear-resistant coated. Some manufacturers use ordinary stainless steel directly to save trouble, and as a result, a pit is worn out in half a year.
By the way, there is another common misjudgment: thinking that 304 is omnipotent. 304 was selected for the deflector of the flue non-metallic compensator in a steel mill, but they ignored the fluoride ions in the medium. As a result, stress corrosion cracks in a few months, and the whole non-metallic expansion joint leaks and stops. Fluoride ion is a chronic poison when it is encountered by 304, and it must be 316L or higher.
What will happen if the material is not selected correctly?
The most typical is that the premature failure of the deflector leads to the skin being washed and damaged, and the whole expansion joint leaks and stops. On-site maintenance is extremely costly-it is common to lose hundreds of thousands a day of downtime. Some people use thin carbon steel to save money, but it rust out in two months. The money for a set of compensators is enough to buy ten good deflectors. Don't you understand this account?
In the production line of a cement plant, the flue gas temperature is about 300℃, and it contains a small amount of SO2 and dust. They chose the Q235 deflector, 3mm thick. After half a year, the deflector is perforated, and the high-temperature flue gas directly washes the skin. After one week, the skin breaks and the non-metallic expansion joint is scrapped. It cost 12,000 to replace a set of non-metallic expansion joints (fabric fiber expansion joints), and with downtime losses, the total cost was over 50,000. At the beginning, if you spent more than 2,000 yuan to upgrade to 304 stainless steel and thicken it to 5mm, it could be used for three to five years.
What other details are easily overlooked when selecting deflector material?
If you choose the right material, it will be useless if the details are not done well. Three key points:
- The thickness must leave sufficient corrosion margin。 It is not that 3mm is enough in theoretical calculation, but there will be corrosion and wear in actual operation, so a margin must be added. It is generally recommended to add 1-2mm. Especially in the high humidity environment behind the desulfurization flue gas baffle door, the corrosion rate is fast, and the margin should be larger.
- Connection mode-Is it bolted or welded to the frame? For rectangular non-metallic expansion joints, the deflectors are typically assembled in sections. The overlap direction should follow the flow direction of the medium, otherwise the airflow recoil will aggravate the wear. Some sites are installed backwards, and as a result, the deflector interface is washed out first.
- There should be a gap between the deflector and the skin。 Don't crush it, or the compensator will tear the skin when it displaces. This gap is generally 10-20mm, depending on the model and displacement of the expansion joint.
In addition, for circular pipelines, the deflector should be made into an annular shape, and pay attention to the position and arrangement of openings to avoid local stress concentration. These details are stipulated in the corresponding product standards, such as the national standard JB/T 12235-2015 for non-metallic expansion joints.
Don't just look at the price. A good deflector can last five years, but a poor one will break in half a year. The money saved is not enough for the labor cost of a single repair. Choosing the right material of non-metallic compensator deflector is to reduce the failure rate from the root source.