1. If the length is wrong, the whole pipeline system will suffer-why can't the non-metallic expansion joint be used by just cutting a section?
The non-metallic expansion joint (fabric fiber expansion joint) on the flue gas pipe just tore from the flange interface, and the whole unit was forced to shut down for emergency repair. Only after asking did they know that when they installed it, they felt that the expansion joint was "soft", so they casually cut an old part that was 150mm shorter than the original design and replaced it. As a result, it collapsed after less than a week of operation. This is not an isolated case-the length of a non-metallic expansion joint directly affects its compensation ability, fatigue life, and seal reliability. When the length is calculated incorrectly, it can leak at the slightest, pull off the pipe support at the worst, and even cause safety accidents.
Isn't it just a soft connection of cloth clip rubber? How long or less can it be different? Hey, it's a lot worse. Unlike metal bellows, the non-metallic expansion joint can adjust the compensation amount by the number of corrugations. Its core compensation layer-polytetrafluoroethylene or silicone coated fiber fabric-the number of layers, thickness and free length are accurately matched according to the design displacement. If the length is short, the fabric will break fatigue in advance if it is stretched too much; When the length is longer, the gap between the end of the guide tube and the inner wall of the pipe becomes larger, and high-speed flue gas erosion leads to local perforation. Therefore, the length is not determined by patting the head, but is calculated according to three parameters: temperature, pressure and displacement. For any miscalculation in between, the plumbing system will have to take the blame for you.
2. The core factors affecting the length of non-metallic expansion joints: temperature, pressure and displacement, none of which can leak
When calculating the length of non-metallic expansion joints, the first hard bone to chew is the collection of working condition parameters. Temperature determines the temperature resistance grade and thermal expansion of fabric material, pressure determines the safety factor of reinforcement layer, and the displacement (axial, transverse and angular) directly determines the compensation length. Take the most common rectangular non-metallic expansion joint as an example. If it is used in hot air ducts, the medium temperature is 350℃, the working pressure is 30kPa, the axial compensation amount is ±80mm, and the transverse direction is ±20mm. Under this working condition, the free length of the fabric is usually 1.2~1.5 times of the compensation amount (1.3 times to be conservative), and then the extension length of the guide tube and the installation margin of the flange are superimposed. Then why not 1x? Because the fabric will creep when under pressure, leaving redundancy can ensure that the compensation performance during the whole life cycle is not eaten up.
Let's talk about pressure. Low pressure (
3. How to set the formula for calculating the length of non-metallic expansion joint? Step-by-step disassembly from axial compensation to guide tube size
The national standard JB/T 12235-2015 actually gives the reference calculation formula, but many people can't understand the pile of symbols. Let me break it apart and tell you: The total length L of the non-metallic expansion joint is generally equal to the length of the guide tube Ld plus the free length of the fabric Lf, plus the thickness of the sealing surface of the flanges on both sides. Where the minimum value of Lf is 1.3 times of the axial compensation amount Δ x (for rectangular expansion joints, it also depends on the aspect ratio. When the aspect ratio is greater than 2, the transverse compensation capacity will decrease, and additional length needs to be increased). If this step feels abstract, you can directly apply the empirical formula: Lf ≥1.3× (Δ x +0.5× Δ y), Δ x is the axial displacement and Δ y is the lateral displacement.
How to determine the length Ld of the deflector? Its role is to guide the medium through smoothly and avoid high-speed airflow directly washing the fabric. Usually, Ld is 0.2~0.3 times the diameter of the pipe, and is at least 30~50mm longer than the free length of the fabric, so as to ensure that the guide tube is always inserted into the pipe, and no vortex erosion is generated. For example: the circular non-metallic expansion joint of the DN800, compensation ±60mm in the axial direction and ±15mm in the transverse direction. Calculate Lf ≥1.3× (60+0.5×15) =87.75 mm, rounded 90 mm; The length Ld of the guide tube is 0.25 times of the diameter of the pipe, i.e. 200mm, plus the thickness of the flanges on both sides is 30mm, and the total length L =90+200+30=320mm. This 320mm is the theoretical design value, but the actual installation margin has to be added. Generally, the installation allowance is 10~20mm, which is used for on-site centering adjustment. Therefore, the final order length can be set at 340mm-don't underestimate this 20mm. Without it, the flange bolt hole may not match, and more fabric slack will affect the compensation efficiency.
4. What is the difference between the three pits that are most prone to rollover in length calculation: installation margin, flange matching, rectangle and circle?
The first pit is that the installation allowance is left too casually. Some engineering teams think that "it's always right to leave more". As a result, the fabric is too long, and during operation, it is sucked in the negative pressure section and bulged out in the positive pressure section, and the fatigue life is directly discounted. The correct way is to leave 10~15mm pre-compression or pre-stretching according to the actual installation deviation range of the pipeline, and keep the fabric in the optimal working length range. The second pit is the flange match. The flanges of non-metallic expansion joints are usually welded with angle steel or channel steel. If the flange bolt hole spacing does not match the pipe flange, forced installation will lead to twisting of the fabric and local stress concentration. Especially for rectangular non-metallic expansion joints, the number of bolts on the long side and the short side is different, so you have to rework them if you press the wrong hole distance-I have seen a project where the center distance between the flange bolt holes was 5mm different, and a crowbar was used to hard separate them on the spot. As a result, the expansion joint was torn from the corner less than a month after being installed. The third pit is the difference between rectangle and circle. The fabric of circular non-metallic expansion joint bears uniform force, and the length calculation can be simply processed by axial and transverse superposition; However, the stress concentration at the corner of the rectangular expansion joint is serious, and the length of the rectangular expansion joint needs an additional compensation margin of 15% ~20%, and the four corners of the guide tube should be made arc transition, otherwise the fabric at the corner will be worn out quickly by air flow. So don't take a round formula to set a rectangle, and you will flip over.
5. Practical case: How to calculate the length of flue gas pipe and hot air pipe respectively? Reference to national standard JB/T 12235-2015
Let's start with the flue gas pipes. Desulfurization system of a power plant, flue section 2000×1200mm (rectangular non-metallic expansion joint), working temperature 150℃, pressure ±3kPa (positive and negative pressures alternate), axial displacement ±50mm, transverse displacement ±20mm. According to the national standard JB/T 12235-2015, the free length of fabric Lf ≥1.3× (50+0.5×20) =78mm, take 80mm; Considering the large lateral displacement, it is necessary to increase the corner compensation margin by 15%, and the actual Lf is 92mm. The length of the guide tube is 0.2 times of the short side size of the pipe (1200mm), i.e. 240mm, plus the flange thickness of 40mm and the installation margin of 15mm, the total length L =92+240+40+15=387mm. Final order press 390mm. Note that in this case, positive and negative pressures are alternated, and a layer of PTFE film should be added to the number of fabric layers to prevent penetration, but the number of layers has little influence when calculating the length (the thickness of each layer is only 0.5~1mm), which can be ignored.
Let's talk about hot air ducts. Hot air duct DN1000 (circular non-metallic expansion joint) of a cement plant, working temperature 350℃, pressure 50kPa, axial displacement ±80mm, no transverse displacement. Silicone coated fabric with high temperature resistance is selected for high temperature, and the thermal expansion coefficient is large, so the compensation amount needs to additionally consider the thermal elongation of the fabric itself. Calculate Lf ≥1.3×80=104mm, plus thermal elongation (about 0.5mm estimated at 0.5% length, which can be ignored), but the guide tube should be made of heat-resistant stainless steel, and the length should be lengthened to 0.3 times the diameter of the pipe, i.e. 300mm, to avoid direct baking of fabrics at high temperature. The flange adopts thickened type (50mm), leaving the installation allowance of 10mm, and the total length L =104+300+50+10=464mm. This case reminds everyone that under high temperature conditions, the length of the guide tube should be long rather than short, otherwise the life of the fabric will be reduced from 5 years to 1 year.
Nonmetallic expansion joint length calculation is not metaphysics, it is a three-dimensional puzzle based on working conditions, standards and experience. Skip any step and could spend ten times more on the spot to take care of the aftermath. If you happen to encounter an uncertain size at hand, you might as well make a list of temperature, pressure and displacement first, and check it item by item according to the national standard JB/T 12235-2015-every try.