Why can't flue systems live without expansion joints?
In the flue at the outlet of a power plant boiler, the flue gas temperature is always five or six hundred degrees. How much can the pipe extend from cold to hot? According to the linear expansion coefficient of carbon steel of 12×10⁻⁶/℃, a 50-meter-long flue can expand 300 mm-almost one foot long-when the temperature is raised by 500℃. Without any compensation measures, the thermal stress of the pipe wall directly pulls the weld, the baffle door is damaged, and even the flue support is pushed askew. It's not like this never happened. The expansion joint (also called compensator) is specially designed to absorb these thermal displacements, reduce the thrust of the pipeline, and at the same time protect the flue gas baffle door and manual plug-in insulation door from being abruptly damaged. To put it bluntly, the flue system expansion joint setting is not optional, but a rigid requirement.
Selection decision: metal vs non-metal, rectangular vs round
What type of expansion joint to choose depends on the cross-sectional shape of flue, medium temperature, corrosiveness and dust content. Rectangular flues mostly use metal rectangular expansion joints or rectangular non-metallic expansion joints-the latter is suitable for occasions where the temperature is not high but large compensation is needed; Circular pipelines are commonly used in power station industry with corrugated expansion joints or general-purpose corrugated expansion joints. Medium temperature is a hard index: high-temperature flue gas (such as above 600℃) honestly choose high-temperature axial expansion joint or metal corrugated expansion joint, and don't expect non-metallic ones to bear it. Sulfur-containing wet flue gas (the kind at the inlet and outlet of desulfurization tower) has to be lined with PTFE hose or PTFE compensator. Ordinary metal bellows will perforate within a few days of touching acid.
The direction of displacement also determines the type. Axial displacement dominated? Upper external pressure single axial type expansion joint or straight tube pressure balance type expansion joint. Lateral or angular displacement? Consider a double hinge transverse type expansion joint or a curved tube pressure balance type expansion joint. Note: The expansion energy saving of the large tie rod is subject to pressure thrust and lateral displacement at the same time, but whether the tie rod nut should be disassembled after installation depends on the type-when the tie rod of the general corrugated expansion joint leaves the factory, the lock nut protects the bellows, and the pipe must be loosened after it is in place, otherwise the expansion joint will not work; The large tie rod expansion joint retains the tie rod to bear the pressure thrust. Many posts on the Internet confused this matter, and as a result, it collapsed as soon as it was installed.
Arrangement principle: cooperation between fixed bracket and guide bracket
Expansion joints cannot be installed blindly. It has to be combined with the fixed bracket and the guide bracket. The fixed bracket divides the flue into several sections, and the thermal displacement of each section is independently absorbed by the expansion joint in the section; The guide bracket restricts the pipe from moving only in a predetermined direction, preventing it from twisting and twisting. Usually the expansion joint is mounted between two fixed brackets and as close to one end of the fixed bracket as possible-you can read it as letting the expansion joint "eat" most of the displacement rather than letting it swing around in the middle. For large flues (such as desulfurization systems), the cooperation of flue gas baffle doors and expansion joints has to be considered. Circular baffle doors (double seals) or single-axis double baffle doors can withstand a certain thrust by themselves, but they will still deform if they are too large. Therefore, when arranging the expansion joint, it is necessary to calculate the blind plate thrust, and don't let the baffle door be the guy.
Installation Pit Avoidance Guide: Guide Direction, Tie Rod Adjustment, Pre-Stretching/Pre-Compression
Look at the direction of the arrow on the expansion joint. The guide tube is facing the inflow end of the medium, which is used to block the airflow from flushing the bellows directly-guess what? Some masters install it backwards to save trouble, the guide tube is blown down, and the bellows are worn out for half a year. Alas, a wrong direction is a loss of tens of thousands of dollars.
Look at the pull rod again. The tie rod and nut of the universal corrugated expansion joint are only transportation fixings when they leave the factory. After the pipeline is in place, the adjusting nut must be loosened to leave a margin for expansion and contraction; However, the tie rods of large tie rod expansion joint and curved tube pressure balance expansion joint are structural parts, which cannot be disassembled. Only the nut is adjusted to set the pre-deformation. How exactly to adjust? Refer to the question and answer of this site "How to adjust the tie rod nut of expansion joint". There are also pre-stretching and pre-compression: during hot installation, the installation temperature is lower than the working temperature, and the pipeline will be elongated in the future, so the expansion joint should be pre-stretched (the elongation is calculated according to the temperature difference); Cold installation is the opposite, pre-compressed. Don't be too troublesome. Skip this step, and the expansion joint will either be insufficient or over-compressed, which will compromise its life.
In addition, a maintenance channel should be reserved around the manual plug-in insulation door or the electric plug-in insulation door-don't install the expansion joint tightly, and it can't be removed when replacing it.
Treatment under special working conditions: Optimization of schemes under high temperature, corrosion and dusty flue gas
For high temperature flues (> 600°C), non-metallic expansion joints (fabric fiber expansion joints) or high temperature axial type expansion joints with thermal insulation are preferred. The creep speed of metal bellows above 600℃ is fast, and the life cliff falls; Instead, non-metallic materials can hold up, and they are cheap. Sulfur-containing wet flue gas (inlet and outlet of desulfurization tower) is highly corrosive. Rubber compensator, rubber PTFE compensator and PTFE-lined metal hose are conventional options. At the same time, the drainage structure should be designed to prevent liquid accumulation corrosion. Dusty flue gas (such as cement mill flue) must be installed with flue gas baffle door or circular baffle door (double seal) upstream of the expansion joint, so that it can be isolated during maintenance; Add a guide tube inside the expansion joint to reduce dust accumulation-once the dust accumulation hardens, the expansion joint will strike. Large-diameter thick-walled expansion joints are used in high-pressure flues, such as air-cooled island vacuum pipes, so double-hinged expansion joints of air-cooled island vacuum pipes should be used, taking into account both vacuum sealing and displacement compensation. There is also a rotary compensator, which is suitable for long-distance direct buried pipelines. The flue system is rarely used, but special layout can also be selected.
Maintenance and life management: inspection cycle, common faults and countermeasures
Flue expansion joints are not installed once and for all. Regularly check the bellows for cracks, corrosion and perforations, and the non-metallic layer for aging and brittleness. The fatigue life of metal expansion joints is calculated according to the number of design cycles, but abnormal vibration or limited displacement will be scrapped in advance in actual use. Common faults: the guide tube falls off (mostly installed in the opposite direction); Loose tie rod nut causes excessive compression of bellows; The rubber compensator is aging and cracked. During minor repair, the tie rod nut can also be adjusted to restore pre-deformation, and if it is seriously damaged, it can only be stopped and replaced. Make a preventive replacement plan: Generally, the power station industry replaces non-metallic expansion joints every 2-3 overhaul cycles, and the metal corrugated expansion energy saving takes 5-8 years, but it is shortened according to the working conditions. Don't forget, the expansion joint is often next to the baffle door. When changing the expansion joint, check the sealing surface of the double-seal single-axis circular baffle door, killing two birds with one stone.