How special are flue operating conditions? Ordinary expansion joints simply can't hold it
What's running in the flue? Thermal shock caused by high temperature flue gas, sulfide, frequent start-and-stop. In this environment, just take a general-purpose corrugated expansion joint to make do, that is, dig a hole for yourself. Two days ago, a customer installed a general-purpose expansion joint on the desulfurization flue, and it was perforated in half a year-the sulfide in the medium was not prevented. Why? Ordinary stainless steel bellows can't withstand wet corrosion, let alone acid dew point corrosion after flue gas condensation.
So the flue system must use special metal expansion joints. For example, the corrugated expansion joint used in the power station industry is specially designed for the high temperature and pressure fluctuation of boiler flue gas; The metal corrugated expansion joint of cement industry used in the smoke exhaust pipe of cement industry can withstand high temperature and dust erosion. There is also a common misunderstanding: some people use rubber compensators on high-temperature flues, and as a result, they will embrittle and crack within two months. Remember, if the flue temperature exceeds 200℃, don't think of rubber, be honest with metal.
Don't only look at the caliber when selecting the type-temperature, displacement and guide tube can't be missed
Many purchases ask "how much is the expansion joint of the DN500" as soon as they come up, which is equivalent to asking "how much is a car"-the configuration is far from the same. To answer how to use the flue metal expansion joint well, the selection parameters must be calculated clearly: the temperature range determines the material of the bellows (304 or 316L, or the more heat-resistant Incoloy 825); Pressure fluctuation affects the number of corrugated layers and wall thickness; The amount of displacement — axial, transverse, angular — determines the wavenumber and structural form. For example, high-temperature axial expansion joints are specially used to absorb the thermal elongation of straight pipe sections, but when used on horizontal flues, don't forget that self-weight will cause sagging, so you have to add guide brackets.
Deflector tube. What exactly does it do? Refer to the question and answer in our station, the guide tube can reduce the direct scouring of the bellows by the medium and prolong the life. Especially in dusty smoke, there is no guide tube, and the dust particles polish the bellows like sandpaper, which will wear out in two or three years. In addition, the metal corrugated expansion joint in the cement industry is aimed at the high-temperature dust environment, and the material of the guide tube is usually thickened or welded with wear-resistant layer, which is not available in general-purpose products.
Installation details decide success or failure-arrow, tie rod, pre-stretch who remembers who worries
Where does the arrow on the expansion joint point? Not the direction of medium flow, but the direction of displacement! Someone installed it backwards, and as a result, the bellows was squeezed in the reverse direction when the heat expanded, which directly became unstable. This is clearly stated in the question and answer of "What is the direction of the arrow of the expansion joint?" Don't feel it. And do you want to remove the tie rod nut? The role of the tie rod of the expansion joint of the large tie rod is to limit the displacement and protect the bellows. After installation, the nut clearance must be adjusted according to the working conditions-it is not welded to death or completely loosened. How exactly to adjust? Look at our question and answer "How to adjust the tie rod nut of the expansion joint". Generally, the thermal expansion margin should be reserved in the cold state.
In one project, the expansion joint of the large tie rod was directly welded to death after installation, without pre-stretching. As a result, as soon as the pipeline heated up, the bellows was hard and cracked in two years. Correct practice: First calculate the amount of cold drawing, and pre-compress or pre-stretch the bellows to the designated position through the tie rod during installation, and then fix it. In addition, the installation direction of the guide tube must be consistent with the flow direction of the medium, otherwise the air flow impacts the end of the guide tube, which will aggravate the vibration.
Don't be a hands-off shopkeeper in routine maintenance-the cleaning cycle determines the life span
Flue metal expansion joint does not need to be served every day, but regular inspection cannot be less. Open the manhole to see if there are cracks and corrosion pits in the bellows, and whether the guide tube has fallen off. When using with smoke baffle doors (such as electric plug-in insulation doors), pay attention to the valve opening and closing sequence-open the baffle door first and then vent smoke to avoid sudden impact. In addition, when non-metallic expansion joints (fabric fiber expansion joints) and metal expansion joints are mixed, the life matching should be considered: non-metallic joints are generally replaced in 3-5 years, while metals can be used for 8-10 years. Don't let non-metallic joints break first, and plan the replacement cycle.
Dust accumulation. Serious dust accumulation in the flue, long-term overweight compression of the bellows, resulting in deformation or even rupture. And guess what? Some users complain that the expansion joint leaks after two years of use. In fact, it is not a quality problem, but a long cleaning cycle. It is recommended to check the ash accumulation once a month, especially at the bottom of the horizontal flue, and install ash discharge ports if necessary.
Troubleshooting-What about chirping, smoke leakage and deformation?
The instability of the expansion joint is 80% because the displacement amount is not enough when selecting the type, or the cold drawing amount is wrong during installation. For example, high-temperature axial expansion joints are used on horizontal flues, and their own weight causes sagging-this requires adding guide brackets to support the bellows. Another situation: when the system starts and stops, you hear a "chirping" sound, which is the friction between the bellows and the guide tube. Check whether the guide tube is offset or falling off.
For smoke leakage problems, first look at whether the bellows has perforations. If corrosion pits are found, it means that the flue gas composition is not considered when selecting materials-the wet flue gas after desulfurization should be made of materials resistant to chloride ion corrosion, such as 316L or duplex steel. If it is only flange surface leakage, check whether the gasket is aged and the bolt torque is uniform.
Finally, talk about matching-combination and matching get twice the result with half the effort
The metal expansion joint in the flue system is not an island. How to match with flue gas baffle door, metal hose and sleeve pipe expansion joint? For example: on the flue that needs to be completely isolated, the double-sealed single-axis circular baffle door distribution station industry corrugated expansion joint not only ensures sealing but also absorbs displacement. In the exhaust pipeline of cement industry, the temperature in the front section is high (400℃ +), and metal rectangular expansion joints are used; The temperature in the rear section drops but it is highly corrosive, so it is more cost-effective to use rectangular non-metallic expansion joints (fabric fibers). Choose the right combination, and the price/performance ratio is much higher.
The pipeline is complicated and needs to compensate for multi-directional displacement. At this time, the double hinge transverse expansion joint or the curved tube pressure balance expansion joint comes in handy. Don't look at the many models, but the core logic is one: find out where your pipeline moves, how much it moves, and how dirty the media is, and then push back the selection. Don't be lazy, calculate the parameter table, and save ten times more money than changing the inflation afterwards.