Calculation of flue expansion joint length, from thermal expansion to type selection
When engaging in the selection of flue expansion joints, the most headache is actually how to determine the length. Two days ago, a buddy who was doing a desulfurization project asked me, "I copied a length according to someone else's drawings, and it leaked within two months of installing it. Do you think I bought the wrong one?" I said, "It's not that you bought the wrong thing at all, it's that you didn't calculate the right thing at all."
The calculation of the length of the flue expansion joint is actually not so mysterious. The key is five steps. You go through this process, and the length you choose is almost the same. Let's break each step apart and crumble it into pieces today. After reading it, you can calculate it yourself.
Step 1: First figure out how much the flue will expand-thermal expansion calculation
Whether you use a metal rectangular expansion joint or a non-metal expansion joint, the first step is the same – calculate the amount of thermal expansion. The temperature change of flue operation is the culprit of pipe expansion and contraction. This is not accurate, and the rest is for nothing.
Δ L = α × L × Δ T
- AlphaIs the linear expansion coefficient of flue material. Carbon steel is about 12×10⁻⁶/℃ (that is, 0.012mm per meter per degree), and stainless steel is slightly higher, about 16×10⁻⁶.
- LIs the length of the flue between the two fixed brackets, in meters. Not the entire length of the flue, note, but the length of the section of pipe that you intend to have this one expansion joint absorb.
- Δ TIs the difference between maximum operating temperature and installation temperature. For example, the ambient temperature is 25℃ when you install it, and the flue temperature is 350℃ when you run it, so Δ T is 325℃.
A section of carbon steel flue with a length of 10 meters has an installation temperature of 20 DEG C and an operating temperature of 400 DEG C. It is calculated that Δ L =0.012×10× (400-20) =45.6 mm. Well, that means the tube will swell out almost 4.6 centimeters. Remember, this value is the benchmark for you to choose the compensation amount of the expansion joint later.
Tips:If there is an insulation layer in the flue, the actual wall temperature of the pipe will be much lower than the flue gas temperature. Don't be foolish to calculate the flue gas temperature directly. It's best to measure it or discount it according to experience. Otherwise, the calculated compensation amount will be too large, and the length of the selected expansion joint will be extra in vain, which is a waste of money.
Step 2: Select the right expansion joint type according to the compensation amount (metal rectangular or non-metal? Don't get confused)
The amount of thermal expansion is calculated, such as 45.6mm in the example above. Now you have to decide which expansion joint to use.
There's a place where it's easy to roll over-many people think it's all about absorbing displacement anyway, so just buy one. Actually, it's not even close:
- Metal rectangular expansion joint: Suitable for high temperature, high pressure or corrosive media. For example, boiler flue of power station and preheater export of cement industry. The advantages are high temperature resistance (some can reach more than 1000℃) and good pressure bearing. But the disadvantage is also obvious-the single-wave compensation is limited. If the compensation is too large, you may need to connect several ripples in series, or choose a multi-wave structure, and the length will naturally go up.
- Non-metallic expansion joints (fabric fiber expansion joints): Suitable for low-pressure, large-diameter flue, such as desulfurization system, dust collector inlet and outlet. The advantages are large amount of compensation, convenient installation and cheap price. The disadvantage is that the temperature resistance is average (conventional products are only 200-300℃, and special materials can reach 600℃), and they can't bear pressure.
So, you first look at the working condition temperature. If the smoke temperature exceeds 400℃ and the pressure is high, obediently use a metal rectangular expansion joint; If the temperature is not high and the pressure is close to zero, then the non-metallic expansion joint is much more cost-effective. In addition, don't buy circular expansion joints for rectangular pipes and hard join them unless you are willing to add reducer pipes-that's another account.
Step 3: The length calculation formulas of different types of expansion joints are directly applied
How to calculate the length when the type is determined? Different product formulas are different, but the core logic is the same-to ensure that the expansion joint is neither crushed nor pulled when working.
Calculation of length of metal rectangular expansion joint:
For metallic rectangular expansion joints, the length mainly depends on the wavenumber of the corrugation. How much displacement each ripple can absorb will be written on the manufacturer's sample (generally called "single wave compensation amount"). For example, a general-purpose corrugated expansion joint has a single wave compensation of 10mm. You need to compensate for 45.6mm, which requires at least 5 waves (4 waves only 40mm, not enough). However, note that the actual selection cannot be used against the limit, and a margin should be left, usually 1.2~1.5 times the safety factor. So choose according to 45.6×1.3 ≈60mm, which requires 6 waves. Then the actual molding length (wave height + wave distance) of each wave will be given by the manufacturer, for example, the wave distance is 50mm and the wave height is 30mm. The length of six waves is (50+30) ×6=480mm, plus the connecting sections at both ends (generally 100mm each), and the total length is about 680mm.
Calculation of length of non-metallic expansion joints:
Non-metallic expansion joints are much simpler. It consists of fabric loop straps and a metal frame, the loop straps themselves fold. Generally, manufacturers will give the compensation ability of the ring belt per unit width, for example, every 100mm width can absorb 30mm displacement. You need 45.6mm, which is 55mm by 1.2 times the factor, then the band width needs to be 55/30×100 ≈ 183mm. Then add the platen width and frame width on both sides, and the final length is usually about 250-350mm. The non-metallic expansion joint has another advantage-it is not afraid of multi-directional displacement, and can compensate in both radial and axial directions, so the length requirements are less demanding.
Step 4: Cold tightening and pre-stretching during installation. If this step is missed, accidents will happen
After calculating the length, you think it's over? Naive. There was a pit in the installation and people who stepped on it cried afterwards-cold tightening and pre-stretching.
What do you mean? The flue is cold when installed, but hot when operated. If you weld the expansion joint directly at the free length, the expansion joint will be compressed as soon as the temperature rises (because the tube has expanded). Not a big problem for non-metallic expansion joints, which are soft by themselves. However, if the metal rectangular expansion joint is over-compressed, the corrugation may be unstable, and the life of the metal rectangular expansion joint may be greatly reduced.
Pre-stretching。 Simply put, the expansion joint is artificially stretched a little bit during installation, so that it is in a stretched state when it is cold, and returns to a free state or even slightly compressed when it is hot. How much to stretch? Generally take half of the amount of thermal expansion. For example, if you calculate that the expansion amount is 45.6mm, then pre-pull 22.8mm (rounded to 23mm).
How to achieve it? Adjust with tie rod bolts. In our stationUniversal corrugated expansion jointAndHigh temperature axial expansion jointAll with pull rods, just screw the nut to the designated position when installing. Note that the tie rod should be loosened after the pipeline heats up, otherwise the expansion joint can't deform freely.
Note:Non-metallic expansion joints generally do not need to be pre-stretched, but be careful not to let the fabric ring strap be twisted during installation, otherwise it will be easy to tear apart during operation.
Step 5: Take an actual case and go through the process to see if the calculation is correct
With all that said, let's have a real real battle. Suppose you have a desulfurization flue gas pipeline (rectangular section 1.5m ×1m), two fixed brackets are 8 meters apart, the pipeline material is Q235 carbon steel, the installation temperature is 25℃, the normal operating temperature is 150℃, and the internal negative pressure of the flue is.
The first step is to calculate the amount of thermal expansion:Δ L =0.012×8× (150-25) =12 mm. It's only 1.2 centimeters, which is quite small, right?
The second step is to select the type:With low temperature and negative pressure, it is obvious that non-metallic expansion joints are the most cost-effective. And the pipe is rectangular, corresponding to the one in our stationRectangular non-metallic expansion jointJust right.
The third step is to calculate the length:The compensation amount is 12mm, which is 14.4mm according to the coefficient of 1.2. Check the sample of rectangular non-metallic expansion joint. Every 100mm band width can compensate for 25mm axial displacement. The band width needs to be 14.4/25×100=57.6mm, and the round is 60mm. Add the frame and platen width on both sides (80mm each), total length =60+160=220mm. The closest in the manufacturer's specification is 250mm product, so choose 250mm.
Step 4 Installation Adjustment:The non-metallic ones do not need to be pre-stretched, but be careful to let the fabric loop strap be loose and not stretched when installing. In addition, check that the guide brackets on both sides of the flue are aligned, so that the expansion joint does not suffer additional torsional force.
You see, the whole process goes down, and that's all. The key to calculating the length of flue expansion joint is to calculate the thermal expansion amount accurately, then select the product type according to the compensation amount, and apply the corresponding formula. Don't forget to cold tighten and pre-stretch when installing. If you follow every step of the way and something goes wrong-there is a high probability that you bought a substandard product. Remember that in our stationDesulfurization flue gas baffle doorAndNon-metallic expansion joint (fabric fiber expansion joint)Quality is guaranteed.
Don't try to save trouble and find a calculator on the Internet. The product parameters of different manufacturers are quite different. It is best to directly find the manufacturer's technology and calculate the length according to the amount of compensation they give. After all, if the expansion joint is chosen to be short and cracked in operation, the downtime loss is not a small number.