Start with a stuck baffle
Last month, I went to a power plant for a return visit, and the maintenance master complained to me that the non-metallic expansion joint on the flue leaked again. When I disassembled it, the fabric fiber layer on the inside of the skin was worn like a broken fishing net. I asked, did you not install a baffle? He was stunned for a long time. A baffle? That sheet of iron?
Alas, this scene is so typical. Many people think that the non-metallic expansion joint baffle is just a layer of iron, and it is done by welding it. But in fact, this layer of "iron sheet" is the vital gate of the whole flue system. In the desulfurization flue, where the dust content is hundreds of mg/Nm³ and the wind speed is more than ten meters per second, if there is no baffle to guide the airflow away, the high-temperature flue gas will directly wash the fabric fiber layer of the skin, and the expansion joint will be scrapped in less than half a year. The role of the baffle is not to support, but to sacrifice itself to protect the skin-it is a wear-resistant layer, a diversion layer, and the key to the longevity of the whole non-metallic expansion energy saving.
Structural dismantling: The baffle and its partners
A typical non-metallic expansion joint (fabric fiber expansion joint) from the outside to the inside is roughly: skin, insulation cotton, baffle, guide tube. The skin is responsible for sealing, the thermal insulation cotton is responsible for heat insulation, the baffle is responsible for separating the airflow and dust from the skin, and the guide tube is responsible for making the medium transition smoothly and reducing vortex. Each of these four layers does its own thing, and one is indispensable.
The material selection of baffle, stainless steel or weather-resistant steel is commonly used in engineering. Stainless steel is corrosion resistant and suitable for low temperature and high humidity environment after wet desulfurization; Weathering steel has high strength and wear resistance, which is suitable for dry flue gas section. The thickness is not determined by patting the head. Generally, it is 3mm to 6mm. Places with high wind speed and large dust have to be thick, but it should not be too thick, otherwise the thermal stress will be large and the weld will easily crack. In the structure diagram of rectangular non-metallic expansion joint of this station, the baffle is usually made into a tapered or circular arc transition, which is to let the airflow go along, not directly against the skin.
Failure cases: How are they all broken?
The baffles fell off, torn, and the welds cracked.
The first, shedding. Most of the bolts were not tightened during installation, or ordinary carbon steel bolts were used, and the smoke flushed and corroded. The second, tearing. The flue gas temperature fluctuates frequently, and the baffle is stuck due to heat expansion, and it is cracked by hard tension. Third, weld cracking. This is the biggest pit-the amount of cold drawing was not controlled during on-site welding, and the expansion joint had already worked hard in the cold state, and the weld collapsed as soon as it heated up.
Low temperature corrosion after wet desulfurization. The working conditions of desulfurization flue gas baffle door are completely different from those of ordinary flue gas baffle door-after desulfurization, the flue gas temperature drops to 50 or 60 degrees, the humidity is close to saturation, the contents of SO₂ and Cl⁻¹ are still very high, and the pH value of condensate is very low. If you choose ordinary carbon steel for the baffle, it will soon be rusty for you. In the product pages of flue gas baffle door and desulfurization flue gas baffle door of this site, the materials and anti-corrosion coatings are marked separately, which is the reason.
Key Points of Model Selection and Design
Rectangular flue and circular flue have different requirements for baffles. Rectangular flue has many corners, and the airflow is easy to deflect. The baffle is best made into an "L" shape or with a certain diversion angle to guide the airflow to the central area. The circular flue has good symmetry, so the baffle can be made into an annular cone, but attention should be paid to the uniformity of the gap with the guide tube, otherwise the local flow rate is too high and it will still wear out.
What working conditions need to add another layer of insulation on the outside of the baffle? Remember a simple judgment: if the inner surface temperature of the skin exceeds 80℃, it is recommended to add a layer of glass fiber cloth or ceramic fiber blanket between the insulating cotton and the baffle to block the heat radiation to the metal baffle. If the temperature of the baffle itself is too high, the strength will be reduced, and the diversion effect will disappear if it is deformed.
Give you a simple selection criterion, three parameters: wind speed, dust concentration and temperature. If the wind speed is greater than 15m/s, or the dust concentration is higher than 5g/Nm³, the thickness of the baffle is recommended not to be less than 5mm, and the material is preferably wear-resistant steel. If the temperature exceeds 350℃, a heat insulation layer must be added, and the baffle should leave sufficient thermal expansion gap. If the temperature is lower than 120℃ and the humidity is high, the material must be changed to stainless steel, so don't hesitate.
Installation and maintenance practice
Install the baffle on site, don't weld it. The direction must follow the direction of the airflow, and the airflow will directly pour into the thermal insulation cotton if it is installed backwards. The gap should be kept according to the design drawing. Generally, the overlap between the front end of the baffle and the guide tube is not less than 20mm, and the expansion gap on both sides is calculated according to the temperature. Don't screw the torque of the bolt-it will fall off too loosely, and it will crush the sealing surface too tightly. It is best to use a torque wrench, beat according to the value given in the drawing, and mark the bolts after beating, so as to facilitate inspection.
How to judge whether the baffle can still be used during shutdown for maintenance? Take a look, touch, tap. Aim whether there is perforation or crimping on the surface of the baffle; Press with your hand to see if there is any obvious deformation; Take a small hammer to tap and listen to the sound. The sound of the empty drum indicates that the interior has been rusted and layered. By the way, there are many ideas worth learning from in the seal design of circular baffle door (double seal) and single-axis double baffle door in this station-the baffle door cuts off the airflow, and the non-metallic expansion joint baffle guides the airflow, but the considerations of corrosion allowance and sealing surface are the same. For each overhaul, go through the following list:
- Are there any wear and corrosion pits on the surface of the baffle
- The presence or absence of cracks in welds and heat-affected zones
- Whether the gap between the baffle and the guide tube is blocked by dust
- Whether the fixing bolts are loose or corroded
- Whether the insulation cotton is collapsed or evacuated
Don't underestimate this list, each one is a lesson gained with real money. The baffle is a thing that is inconspicuous when installed, but kills you when broken. Take it seriously, and the flue system is stable.