What is the amount of expansion and contraction of flange plus metal expansion joint? Don't be fooled by the parameter table, look at these three points
Two days ago, I met a customer and asked me with the product parameter table: The expansion and contraction amount marked in this model is ±50mm, and the thermal elongation of our steam pipeline is only 30mm. Should it be no problem? When I looked at the diameter and pressure of the pipe, I told him directly: Brother, if you install it according to this number, it will leak in less than a quarter.
The scaling amount written on the parameter table is only laboratory data under ideal working conditions. In practical application, how much the expansion and contraction of flange plus metal expansion joint can reach depends on the real situation of your pipeline-temperature, pressure and installation posture. If one is wrong, the nominal value will be directly discounted by 50%.
What exactly is the amount of expansion? Why It Matters More Than Caliber
The amount of expansion and contraction, simply put, is the maximum distance that the expansion joint bellows can safely compress or stretch in the axial direction (along the direction of the tube). For example, marked ±25mm means that it can absorb a total thermal displacement of 50mm-25mm compression in the forward direction and 25mm stretching in the reverse direction. However, note that this value is measured at normal temperature, no pressure, and the bellows is free.
Many people only stare at the caliber (how much DN) when choosing a model, and choose a large size if they think the tube is thick and under high pressure. In fact, the amount of expansion and contraction is the vital gate. The thermal elongation of the pipe is calculated according to the formula Δ L = α ·L · Δ T. No matter how thick the pipe diameter is, once the temperature rises, the length change has nothing to do with the diameter, but only with the medium temperature and the length of the pipe. A DN200 steam pipe, with a length of 50 meters, rises from normal temperature to 300℃, and has a thermal elongation of close to 180mm. If you choose an expansion joint with a telescopic amount of only 100mm, it will be hard to carry it when installed, and the fatigue life of the bellows will directly return to zero.
The caliber determines whether it can be installed, and the amount of expansion determines how long it will last。
Three factors influencing the amount of expansion and contraction: temperature, pressure and pipe posture
Temperature – the most hidden killer
The extension and contraction amount on the parameter table is mostly measured at 20℃. If you run 300℃ steam in your pipe, the elastic modulus of the bellows material (usually 304 or 316L stainless steel) will decrease and the fatigue strength will be discounted. In industry practice, when the temperature exceeds 350℃, the safety factor will be amplified by 1.2 times for every 100℃ increase. That is to say, the amount of stretching and contraction corresponding to the actual allowable fatigue life may be only ±35 mm in nominal ±50 mm.
Pressure-the "internal pressure thrust" of the bellows
The pressure will generate an internal pressure thrust (also called blind plate force) through the bellows, which will offset part of the elastic rebound ability of the bellows. The higher the pressure, the "harder" the bellows and the smaller the amount of expansion and contraction it can actually absorb. Take our commonly used general-purpose corrugated expansion joint as an example. When the design pressure is 1.6MPa, the effective expansion and contraction amount is about 70% of the nominal value. If the pressure is higher than 2.5MPa, it may only be 60% left.
Piping Position-Is It Straight
Many on-site pipes are not straight, with elbows and vertical sections. If the flange plus metal expansion joint is installed on a horizontal pipe, its own gravity plus the vibration of the pipe will generate additional lateral displacement and angular displacement. Although these displacements are not written in the parameter table, the axial expansion and contraction capacity of the bellows decreases with every little lateral deflection. There is the most terrible situation: vertically installed and without guide bracket, relying on the expansion joint to support gravity by itself, and the axial expansion and contraction amount is directly discounted by 40%.
How much difference does the amount of telescoping vary between models? Take our products as an example
The expansion and contraction of the expansion joints with the same caliber and different structures can be several times different. Let's talk briefly about the categories on our site:
- Universal corrugated expansion joint: Single-layer corrugation, large expansion and contraction (for example, DN100 can reach ±40mm), suitable for low-pressure normal-temperature pipelines, with the highest cost performance when the temperature is lower than 200℃ and the pressure is lower than 1.0MPa.
- High temperature axial expansion joint: Multi-layer corrugation plus outer sheath, the temperature resistance can reach 550℃, but each layer of corrugation shares the displacement, and the nominal expansion and contraction amount may only be ±20mm. The high temperature can be made up by adding pre-stretching.
- External pressure single axial expansion joint: The bellows is external, and the bellows bears external pressure under working condition, which avoids the "expansion" problem caused by internal pressure. The expansion and contraction amount under the same diameter can be 15%-20% larger than that of internal pressure type. Suitable for high pressure steam piping.
- Compound hinge transverse expansion joint: This kind of hinge absorbs lateral displacement, and the axial expansion and contraction amount is very small (within ±5mm). However, if the pipeline needs L-shaped or Z-shaped compensation, it can achieve large displacement through the combination of multiple hinges, which is different from the conventional expansion and contraction amount.
When selecting the type, you can't just look at the number, but also the "direction" of pipeline displacement-if there is more axial displacement, choose the axial type, and if there is more lateral displacement, consider the compound hinge or curved pipe pressure balance type.
What will happen if the telescoping amount is large? The role of pre-tensioning and tie rods
Some people think that it is better to choose it bigger. The nominal is 100mm, but the actual use is 30mm. Is it always safe? Wrong. If the expansion and contraction amount is too large, the bellows is "too loose" in the cold state, and the slight vibration of the pipeline during operation will cause unnecessary deformation of the bellows, which will accelerate fatigue. Moreover, the expansion joint with large expansion and contraction amount usually has a large number of corrugated layers and a longer whole, so it is easy to become unstable, so it is necessary to provide additional pull rods or guide brackets.
Then when to reverse the operation – with pre-stretch? When the temperature of your pipe medium is high and the amount of thermal elongation is large, but the nominal compression of the product is not enough, you can pre-stretch the expansion joint to half the nominal value in the cold state. For example, the hot elongation is 80mm, and the maximum compression amount of the product is 60mm, then the cold state is stretched 30mm, so that the hot state is compressed to-50mm (the medium is compressed 50mm after expansion, not more than 60mm). The function of the tie rod is to limit excessive stretching or compression and protect the bellows from exceeding the limit.
How to adjust the tie rod nut specifically? We talked about it specifically in an article before, but the core principle is one:The amount of pre-stretch cannot exceed 60% of the maximum allowable stretch of the productAnd must be mated with the pipe fixing bracket.
Actual Combat Calculation: A Steam Pipeline Case, See How to Determine the Expansion and Contraction Quantity
A medium-pressure steam pipeline in a chemical plant, with a working temperature of 300℃ and a design pressure of 1.6MPa, is made of No.20 steel, with a length of 40 meters. Both ends are fixed, and a flange plus metal expansion joint needs to be installed in the middle.
Step 1: Calculate thermal elongation
The expansion coefficient α of No. 20 steel wire is 12.2×10⁻⁶/℃, and the temperature difference Δ T =300-20=280℃
Δ L =12.2×10⁻⁶ ×40×1000 (mm) ×280=136.64mm
Step 2: Consider pressure and temperature reductions
Under the working conditions of 1.6MPa and 300℃, the actual expansion and contraction amount corresponding to the allowable fatigue life of 304 stainless steel bellows shall be taken as 65% of the nominal value according to experience. Then the nominal amount of expansion and contraction required =136.64/0.65 ≈ 210mm
Step 3: Select Products
Universal corrugated expansion joints with nominal expansion and contraction of ±105mm (total 210mm), the largest in DN200 series is only ±80mm. What to do? Two scenarios:
First, the high-temperature axial expansion joint is used, which is nominal ± 60mm but is pre-stretched by 60mm (cold stretching), and the maximum compression in hot state is 120mm, which is close to the requirements. However, note that a tie rod must be set after pre-stretching.
Second, the external pressure single axial expansion joint is used instead. Under the same circumstances, the effective expansion and contraction amount can reach 80% of the nominal value. Just choose a product with a nominal ± 130mm.
The final customer adopted Scheme 2 because the external pressure bellows does not directly contact the medium, has better fatigue resistance, and does not need pre-stretching, so the installation is hassle-free.
You see, if you only look at the maximum expansion and contraction of DN200 series on the parameter table of ±80mm, and put it on directly, the thermal elongation of 136mm has long been exceeded. But after reduction and selection optimization, the problem is solved.
There is no standard answer about how much the flange plus metal expansion joint expands and contracts. Remember three points: don't believe in the nominal value, it depends on the reduction of working conditions; The direction of displacement determines the model; Pre-stretching and tie rods are remedies not conventional schemes. Next time you get the parameter sheet, ask yourself first: What's my pipe temperature? How much pressure? Is the installation position straight or curved? Do the math before placing the order.