What is displacement? Why is the flue so important
To put it bluntly, the displacement of the flue metal expansion joint is the distance that the compensator (that is, the expansion joint) needs to be able to "get out of the way" when the pipe expands and contracts thermally or the equipment vibrates. You can think of it as a spring, and as soon as the temperature goes up and the tube becomes longer, the expansion joint has to be compressed; When the temperature drops, the tube shrinks, and the expansion joint has to be pulled back. The amplitude of this expansion and contraction is the amount of displacement.
Why is it important in the flue? Because the flue gas temperature fluctuates greatly-from normal temperature to hundreds of degrees, and even acid-alkali corrosion in the desulfurization system. As soon as the temperature difference comes up, the steel pipe can be extended by several centimeters. If the displacement of the expansion joint is not accurately calculated, it will either be broken or crushed to death. Two days ago, I met a customer, and the flue was equipped with a universal corrugated expansion joint. As a result, the corrugated pipe cracked within a few days of startup. Only after asking did I know that the actual operating temperature of their displacement calculated according to normal temperature was twice as high.
Several key factors affecting the displacement amount: temperature, pressure, pipeline direction, none of which can be missed
Calculating the displacement, not slapping the head. Three hard indicators must be clarified:
- Temperature: The maximum temperature and the minimum temperature of the flue medium, the larger the temperature difference, the larger the displacement. The formula is simple: Δ L = α × L × Δ T, α is the coefficient of linear expansion of steel (about 0.012mm/m·℃), L is the length of pipe, and Δ T is the temperature difference. For example, a 30-meter-long flue, with a temperature difference of 400℃, calculates the axial displacement of 0.012×30×400=144mm. By doing this, you will know why power station flues are often usedCorrugated expansion joint for power station industryOrHigh temperature axial expansion jointYeah.
- pressure: Positive or negative pressure in flue? The pressure will deflate or bulge the expansion joint, indirectly affecting the effective displacement. Negative pressure flue (such as behind the induced draft fan), the expansion joint is easy to deflate, so stability must be considered in design, otherwise the displacement is not in place, and the bellows will become unstable first.
- Pipeline Strike: Straight pipe sections mainly produce axial displacement, but if they encounter elbows, tees or equipment interfaces, they will produce lateral displacement and angular displacement. At this timeCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointThat would come in handy. Don't be stupid and use the axial type, the lateral force will twist the bellows into a twist.
There's another thing that's easy to overlook-the installation temperature. If you load it at 20℃, and if you load it at 40℃, the amount of pre-stretch is much different. Many manufacturers will give a pre-stretch value when they leave the factory, and you have to adjust it according to the actual ambient temperature, otherwise the displacement will exceed as soon as the temperature difference comes up.
What happens when the amount of displacement exceeds the standard, the easiest pit to overlook when you choose an expansion joint?
Tsk, I've seen this pit too many times. Many people take the pipeline layout diagram, just calculate the displacement and ask the manufacturer to order, but the result is that there is a problem when installing it.
If the displacement exceeds the standard, the fatigue life of the expansion joint will drop sharply-it was originally designed to last 10 years, but it leaked after two years. In serious cases, the bellows will break, high-temperature smoke will spray out, the equipment will shut down, and even people will be injured. Remember the accident report of a cement plant the year before last? It isMetal Corrugated Expansion Joints in Cement IndustryThe displacement amount of does not take into account the horizontal thrust, causing the expansion joint tie rod to be bent and the bellows to tear.
Also, if you are usingNon-metallic expansion joint (fabric fiber expansion joint)If the displacement exceeds the standard, it will not only damage, but also cause the fiber layer to tear and leak air instantaneously because of excessive displacement. Although non-metallic expansion joints are cheap, they usually have less displacement than metallic ones and cannot withstand axial pressure, so they need to be installed with more care.
Practical case: Difference in displacement between flue of power station and flue of cement industry
Let's talk about two real scenes.
Flue of power station: Typically, the line from the boiler outlet to the dust collector and then to the desulfurization tower. The pipeline is long, often 40 or 50 meters, and the temperature drops from 350℃ to about 150℃. The displacement is large and the direction is complicated-there are horizontal sections, vertical sections, and climbing sections from the boiler to the chimney. At this time it is often necessary to useCorrugated expansion joint for power station industryCooperateCompound straight pipe bypass pressure balanced expansion jointTo absorb large axial displacement, and then solve the lateral displacement through hinges or tie rods. Power station projects have high requirements, so the displacement calculation must be accurate to millimeters, and a safety margin must be reserved, usually 1.2 times of the calculated value.
Cement industry flue: From the cement kiln preheater to the kiln tail, the temperature fluctuation is not so violent, but there is much dust and great wear. Moreover, cement flue often has turns and changes in diameter, and the pipe is short but has many angles. The amount of displacement is generally not large, butMetal Corrugated Expansion Joints in Cement IndustryIt must be wear-resistant and corrosion-resistant, and the guide tube (that is, the liner inside the expansion joint) must be thick, otherwise the displacement parameters will be completely messed up after wear. You can refer to our question and answer on "The Specific Function of Expansion Joint Guide", which is quite clear.
What's the difference? The power station focuses on large displacement and complex system; Cement focuses on wear and temperature resistance. When you choose a model, you can't take the drawings of the power station directly to apply them to the cement line, and the other way around.
How to communicate the displacement requirements with the manufacturer? Just remember these numbers
Communicate with the manufacturer, don't just say "I want an expansion joint with a displacement of ±100mm". The manufacturer will ask you three numbers:
- Axial displacement: How many millimeters? Positive-negative or one-way? (Usually written as +50/ -30mm)
- Lateral displacement: How many millimeters? Both directions or just one?
- Angular displacement: How many degrees? This is generally only used for large pipe diameters or special directions.
ForUniversal corrugated expansion jointThe displacement absorbed by each wave is about 10-15mm (see the wave height and wall thickness). You calculate the total displacement and divide it by the displacement per wave, and you know how many waves are needed. But don't exceed the maximum wave number recommended by the manufacturer. Too many ripples will unstable.
In addition, be sure to tell the manufacturer about the media ingredients. For example, if there is SO₂ and condensate in the desulfurization flue, you have to useDesulfurization flue gas baffle doorCooperatePTFE-lined hoseOrPTFE compensatorTo embalm. The displacement calculation is exactly the same, but the stiffness of the expansion joint will change because of different materials, so the manufacturer has to re-check it.
On-site adjustment skills of displacement during installation and commissioning
When the expansion joint arrives at the site, what you get is its "free length"-that is, the length at normal temperature without pressure. Before installation, it must be pre-stretched or pre-compressed according to the actual installation temperature. This adjustment depends on the tie rod and the nut:How to adjust the tie rod nut of expansion joint？ The initial length of the expansion joint can be changed by loosening the locking nut and rotating the adjustment nut to push the ear plate. For details, you can flip through our Q&A.
Draw marks on the pipes at both ends of the expansion joint, measure the distance difference before and after installation, and check whether the displacement range given by the manufacturer covers the actual working conditions. If the installation error is found to be too large, adjust the tie rod in time.
One more thing – don't be superstitious about "maintenance-free".Function of expansion joint tie rodIt is to restrain excess displacement, but the locking nut on the tie rod must be loosened after installation is complete (unless it is a fixed type). Some workers don't loosen the nut after installing it, which is equivalent to locking the expansion joint. When the smoke is hot, the pipe directly explodes the expansion joint.
Large diameter, large diameter flue, such as more than 3 meters in diameter, you better useMetal rectangular expansion jointOrRectangular non-metallic expansion jointThe rectangular structure has strong bending resistance, and it is not easy to deform even if the displacement is large. Circular pipes may be consideredLarge diameter thick wall expansion jointThe wall thickness is thicker, the stress is dispersed during displacement, and the life is longer.
Okay, that's all that can be said about the flue metal expansion joint displacement. Only when the calculation is accurate, the right choice is made and the flue system is installed can it run steadily. What do you think?