Anyone who has worked in flue gas purification in power plants, cement plants or metallurgical industries knows that there is never a rigid direct connection between the electrostatic precipitator body and the inlet and outlet flues. There must be a section of connection in the middle that can "breathe". Once there is a problem with this connector, air leakage, dust accumulation, deformation and even tearing and cracking will follow. Today, I will break up the metal expansion joint of electrostatic precipitator and explain it clearly.
Why do you have to pretend? Chain reaction of thermal displacement, vibration and seal failure
The inlet smoke temperature of electrostatic precipitator is usually between 120℃ and 180℃, and some kiln heads and sintering machines can reach above 200℃. From cold to hot flue, the pipeline elongation of tens of meters is tens of millimeters casually calculated. If you use hard connections, the flange bolts can't stand it first, then the flue wall deforms, and finally the sealing welds at the inlet and outlet of the dust collector crack. Once the flue gas leaks in, the air flow distribution inside the electric field is messy, and the dust removal efficiency falls straight down. This is not counting the continuous vibration caused by the induced draft fan. That thing is an alternating load, and no matter how thick the steel is, it can't withstand long-term fatigue.
Absorbs displacement and vibration while holding the seal. It is not an optional accessory, but the first line of defense on the sealing chain of the entire electrostatic precipitator system.
Rectangular, circular and high temperature types, matched by flue path and interface size
The vast majority of flues at the inlet and outlet of electrostatic precipitator are rectangular in section, especially large electrostatic precipitators, whose width can easily be five or six meters or even wider. This occasion uses a metal rectangular expansion joint, also called a rectangular corrugated compensator. It is the same as the circular expansion joint used in pipelines, but it is much more complex in structure. Why? The four corners of rectangular section are the stress concentration areas, and the corrugated plate is prone to fatigue cracking at the corners, so the arc radius at the corners and the quality of weld are very high in manufacturing. Better manufacturers will do integral molding or strengthening welds at the corners, instead of simply welding.
Circular electrostatic precipitator metal expansion joints are generally used for flue branch pipes, bypass pipes or circular pipes connecting the inlet and outlet of induced draft fans. Under the premise that the interface size is determined, the pressure bearing capacity of circular structure is much better than that of rectangular structure, the wall thickness can be thinner, and the number of corrugations can be less.
As for the high-temperature type, such as electrostatic precipitator of sintering machine head or waste incineration flue gas treatment system, when the flue gas temperature exceeds 250℃ or even 400℃, the allowable stress of ordinary austenitic stainless steel will be significantly reduced. At this time, high-temperature axial expansion joints have to be used. Inconel 625 or heat-resistant alloy are often selected as materials, and the wave number and layer number have to be recalculated. Instead of replacing it with an ordinary expansion joint, when the temperature is high, the creep rate of the material rises sharply, and the fatigue life of the bellows may be shortened by an order of magnitude.
Temperature, pressure, displacement-three parameters determine success or failure
The most prone problem when selecting is that the parameters are not completely given, or the average value is given instead of the extreme working condition value. Let's talk about the design temperature first. It is necessary to take the highest smoke temperature that may occur during operation, not the average temperature under normal working conditions. In order to clean dust, some electrostatic precipitator systems will raise the flue gas temperature for a short time, and some even reach above 250℃. For every 50°C higher temperature, the fatigue life of austenitic stainless steel may decrease by 40% to 50%.
In terms of pressure, electrostatic precipitator generally runs in a slight negative pressure state, and-3000Pa to-5000Pa are common. But don't forget the positive pressure impact of pneumatic ash transport and bypass flue switching. The design pressure should be taken according to the most unfavorable working condition, and the vacuum instability check should be considered. If the internal pressure is too large, the bellows will bulge, and if the external pressure is too large, the bellows will deflate. Both cases have been seen on the spot.
The amount of displacement is the most easily underestimated. Many purchases only focus on axial displacement, neglecting lateral displacement and angular displacement. In the actual installed pipe system, the thermal expansion direction of the flue is rarely pure axial, and the lateral displacement will be forcibly applied to the expansion joint if the support and hanger are not well arranged. This is why, under the same working condition, some expansion joints are fine after ten years, while others leak after two years-80% of them are because the displacement combination is not calculated.
What happens if the deflector is in the wrong direction? Common Errors and Prevention in the Field
When installing an electrostatic precipitator metal expansion joint, the deflector direction is an extremely easy detail to roll over. The function of the guide tube, to put it bluntly, is to block the airflow and prevent the high-speed dusty smoke from directly washing the corrugated valley bottom. The direction of smoke must follow the bell mouth of the guide tube, and the small mouth end of the guide tube should face the direction of incoming flow. What happens if you put it backwards? One is dust accumulation, the airflow forms a vortex in the trough area, and the dust is deposited at the root of the corrugation; Second, wear. When the wind speed is high, the dusty airflow directly hits the bellows, and it will wear out in a few months.
Another common mistake is forgetting to remove the transport tie rod. Some expansion joints come from the factory with transport fixtures that must be loosened or removed after installation. In order to save trouble, many sites directly keep the tie rod. As a result, the expansion joint completely loses its compensation ability, and the thermal displacement is all suppressed to the flue and equipment. It is still a small matter to run and leak, and it can rip the shell apart.
Check the flow direction mark before installation, remove the tie rod after installation, and check whether there is any additional force between the connector and the expansion joint. Welding slag, even the bolts left behind by construction stuck in the corrugated gap, will also lead to local stress concentration and affect the life.
From Expansion Joints to Flue Baffle Doors: The Complete Logic of Seal Fit
For the stable operation of the electrostatic precipitator system, it is not enough to rely on the expansion joint seal. The flue gas baffle door and bypass baffle door at the inlet and outlet also bear the functions of isolation and sealing. If the baffle door is not closed tightly, the smoke will rush in during maintenance, and people can't go in to work; During operation, the sealing air is not adjusted properly, and the cold air leaks into the electric field, which will make the local flue gas temperature drop below the dew point, and the equipment corrosion will be aggravated. By the way, double-sealed single-axis circular baffle door and desulfurization flue gas baffle door have their own emphasis on sealing structure. The former is suitable for frequent switching conditions, while the latter pays more attention to corrosion prevention and zero leakage. If the standards of expansion joint, baffle door and compensator are not uniform, and the installation matching clearance does not match, the tightness of the whole system is the "barrel effect"-which board is short, the water will leak from.
Systematically look at the sealing fit of electrostatic precipitator, you will find that the expansion joint is a flexible buffer, the baffle door is a rigid partition, and the non-metallic expansion joint is more suitable in some positions with severe low-temperature corrosion. Don't underestimate these "middleware", their working environment can be much more rigorous than the device itself.
So how to choose electrostatic precipitator metal expansion joint? First calculate the displacement, then determine the material and wave number, and finally put the direction and installation details of the guide tube in place. With the right parameters, the right direction and the right details, this expansion joint can be silent for ten years. On the other hand, which link is sloppy, it will put a cold gun on you when you pay the least attention-air leakage, dust accumulation, shutdown, emergency repair, delayed power generation and production time, enough for you to buy dozens of expansion joints.