The flue expansion joint looked simple, but it was full of pits to choose. Two days ago, I met a customer, and when I came up, I asked for a metal rectangular expansion joint. As a result, I asked about the working conditions-the inlet temperature of the desulfurization tower was 150℃, and the humidity was high with acidic condensate. Tsk, the metal one will corrode and perforate within three months of going up, so you have to replace it with non-metal. So today, let's talk about one thing: how to determine the technical scheme of flue expansion joint, so as not to waste money.
Find out the working conditions of the flue first-temperature, pressure and medium determine the selection direction
This is the iron rule. Temperatures exceed 400℃, pressures are higher than 0.1MPa, and the medium is corrosive-each item of data is doing multiple choice questions for you. For example, in the flue of a power plant boiler, the flue gas temperature can rush above 600℃, and ordinary bellows can't bear it. High-temperature axial expansion joints or external pressure single axial expansion joints are required, and heat-resistant alloys (such as Inconel 625) are used as wave bodies. However, the temperature of the net flue after desulfurization drops to 50~80℃, but the acidic condensate (pH value as low as 2~3) brought by wet desulfurization is the killer. At this time, PTFE compensator or PTFE-lined metal hose is the correct solution, and the anti-corrosion layer can separate the medium from the metal body. If you use a universal corrugated expansion joint, the corrosion pit will spread from the inside and leak in half a year. What about the stress? Non-metallic expansion joints are enough for low-pressure large-section flues (such as the inlet and outlet of dust collector), and the pressure of fabric fiber layer generally does not exceed 0.05MPa; However, if it is the rear section of the pipeline pump or the outlet of the fan, the pressure may be above 0.2MPa, so it is necessary to use a metal rectangular expansion joint or a straight pipe pressure balance expansion joint to avoid the risk of blasting.
The working condition parameters are determined first, and the material structure is selected later. Don't take other people's drawings and copy them. The flue working conditions in different positions of the same unit are different, and the temperature difference is 50℃, so the selection is completely different.
Metallic or non-metallic? Rectangular or circular? Matching logic for different scenarios
Sectional shape and displacement capacity. Most flues are rectangular (square air ducts), so metal rectangular expansion joints and rectangular non-metallic expansion joints are the most widely used. Circular flue (such as fan inlet and outlet, pipeline conveying section) is equipped with circular expansion joints, including general corrugated expansion joints, compound hinge transverse expansion joints, etc.
How do you divide metals from non-metals? Metal expansion joint (bellows) is good at absorbing axial displacement, strong pressure bearing capacity, suitable for high temperature and high pressure. The biggest advantage of non-metallic expansion joints (fabric fiber expansion joints) is that they absorb multi-dimensional displacement-they can be eaten in both transverse and angular directions, and they are lightweight and easy to install. For example: the kiln tail flue in the cement industry has high temperature but complicated pipeline direction. The effect is very good by using metal corrugated expansion joints and non-metal expansion joints in the cement industry as compensation. However, if it is the inlet and outlet of desulfurization tower, the pipeline displacement is large and the corrosion is strong, and the rectangular non-metallic expansion joint is almost the only option-because it can solve the three problems of anti-corrosion, large displacement and low cost at the same time.
Can you use all metal? Sure, but at a high cost. Metal rectangular expansion joint needs guide tube, reinforcing ring and multi-wave design, and the material cost doubles; The price of non-metallic expansion joints is only 1/3 of the former, and the life span can be 3~5 years. You have to calculate your own price/performance ratio.
Don't underestimate the installation details-tie rod adjustment, deflector, arrow direction these pits
Choose the right model and install the wrong one for nothing. I have seen too many cases: when the expansion joint arrived at the site, the workers removed the tie rod nut and welded it directly. As a result, the corrugated section was pulled and deformed during pressure test. The function of the tie rod is to prevent the bellows from being over-stretched or compressed. It is in a locked state during transportation. After installation, the nut position should be adjusted according to the expansion amount-for specific practices, refer to the question and answer of "How to adjust the tie rod nut of the expansion joint". Simply put: first loosen the lock nut, twist the adjustment nut to make the tie rod free, and then tighten it after the pipeline pressure test.
Deflectors are also often overlooked. The specific function of the expansion joint guide tube is to guide the flow direction of the medium, avoid the high-speed air flow directly washing the ripples, and reduce the flow resistance at the same time. If it is installed backwards (the direction of the arrow is away from the flow direction), the guide tube becomes a throttle, the pressure loss spikes, and the bellows is washed and aggravated. Therefore, the direction of the arrow of the expansion joint refers to the flow direction of the medium, which must be consistent with the flow direction of the pipe when installed. This is clearly explained in the question and answer of "What is the direction of the arrow of the expansion joint?"
When the non-metallic expansion joint is installed, the fabric fiber layer cannot be deadpulled, and it should be fixed with a special pressure plate; The rubber compensator should leave sufficient bolt spacing to prevent the sealing surface from crushing. Together, the installation specifications are tighter than the selection parameters.
How do matching baffle doors and expansion joints work together? Talk about the special needs of desulfurization flue
Desulfurization flue is recognized as the "hardest hit area". The flue gas contains sulfur, water and acid, and requires frequent start-stop switching. Expansion joints and baffle doors must work together-for example, double-sealed single-axis circular baffle doors with non-metallic expansion joints can achieve zero leakage when closed, while the expansion joints absorb the thermal expansion and contraction of the pipe. If the baffle door is selected as a single-axis single-baffle door, the sealing performance is poor, and air leakage leads to condensation and corrosion on the inner wall of the expansion joint, which will enter a vicious circle.
Desulfurization flue gas baffle door and electric plug-in insulation door are common partners. The former is used for flue gas switching and the latter for complete isolation. Pressure fluctuations and purge media must be taken into account after the expansion joint is installed in front of the baffle door. There is a case: when the desulfurization bypass baffle door is closed, the flue pressure suddenly rises by 0.05MPa, and the fabric layer of the non-metallic expansion joint is almost torn. Later, it was replaced with a metal rectangular expansion joint and a double seal structure. Therefore, when selecting the model, the opening and closing logic of the baffle door and the displacement compensation ability of the expansion joint should be calculated uniformly, and we can't only focus on one component.
Rectangular non-metallic expansion joints (fabric fiber + polytetrafluoroethylene layer) are preferably selected for desulfurization flue pipe, with desulfurization flue gas baffle doors or double-sealed single-axis circular baffle doors. If the temperature exceeds 200℃ or the pressure exceeds 0.1MPa, consider the high temperature axial expansion joint lined with PTFE anti-corrosion scheme.
Maintenance and longevity – how often to change? Which signals indicate it's time for overhaul?
The typical life of non-metallic expansion joints (fabric fibers) in desulfurization flue is 3~5 years; Metal corrugated expansion joints can be in the boiler flue for up to 8~10 years, but only if they are checked regularly. Which signals indicate it's time for overhaul? First, visible cracks or orange peel-like wrinkles on the bellows surface — a precursor to fatigue failure. Second, the tie rod nut is loose or the tie rod is bent and deformed, indicating that the displacement exceeds the design value. Third, the fabric layer of the non-metallic expansion joint appears powdered, delaminated or perforated, which must be replaced at this time, otherwise the leakage will cause dust accumulation and collapse. Fourth, the surface of the rubber compensator is sticky and cracked, which represents aging.
There are cracks in the welds at both ends of the expansion joint (especially on-site welds), the wear degree of the guide tube, and the state of the lock nut of the tie rod. If conditions permit, use ultrasonic thickness measurement to judge the remaining wall thickness of metal bellows. Don't wait for smoke to change, overhaul downtime costs are far more expensive than parts.
In the final analysis, there is no standard answer to the technical scheme of flue expansion joint, but there is an iron law: starting from the working conditions, matching the material structure, knotting the installation details, and doing preventive maintenance. If you follow this logic, there is a high probability that you won't step on the pit.