If the position is not selected correctly, the expansion joint is equal to white installation- -the installation logic from the thermal displacement
To put it bluntly, the metal expansion joint is a "cartilage head", which is specially used to absorb the displacement caused by thermal expansion and contraction of the pipeline. However, many people only focus on model selection, but neglect the most crucial link-Metal expansion joint position。 The position is wrong, even if you are using the best general-purpose corrugated expansion joint, it should crack or crack, and it should leak or leak. Let's talk about the basic logic first: where the thermal displacement occurs, the expansion joint should be placed. The pipe is elongated by heat, and the fixed bracket divides the pipe into several sections, and the thermal elongation of each section is concentrated at the weakest place in the section and released. The expansion joint is that "weak point". If placed too far away from the fixed bracket, the middle tube will bend; Put it too close, and it can't absorb the displacement. Therefore, before installation, calculate the thermal displacement of the whole pipe system before deciding which fixed bracket to plug the expansion joint. This job seems simple, but there are many rollovers on the scene. Two days ago, I met a customer, and the expansion joint of the steam pipe was directly installed behind the elbow. As a result, when the pipe was hot, the stress at the elbow directly tore the weld joint-this is not called installation, but destruction.
How do fixed brackets and guide brackets determine the position of expansion joints?
The fixing bracket is the "anchor point" of the pipe, which holds the pipe firmly down and prevents it from running around. The thermal elongation of the pipe section between the two fixed brackets must be eaten up by the expansion joint in the middle. So,Metal expansion joint positionIt is naturally between the two fixed brackets. The function of the guide bracket is to ensure that the tube only expands and contracts in the axial direction and does not let it bend laterally. The distance between the guide bracket and the expansion joint is particular: the first guide bracket is generally no more than 4 times the pipe diameter from the end of the expansion joint, and the second one is no more than 14 times the pipe diameter. Beyond this range, the expansion joint will twist around like noodles, and the lifespan is directly discounted. For example, the main steam pipe of a power plant has a diameter of DN600, a temperature of 540℃, a distance of 30 meters between fixed brackets and a thermal elongation of about 120mm. You have to use it at this timeHigh temperature axial expansion jointAnd the guide brackets must be arranged exactly at the above-mentioned distance. And the result? I have seen that in order to save a few brackets money, the distance between the guide frames is pulled to 20 meters, and the expansion joint is fatigued and broken in less than half a year. Save a little money and lose a lot of money, why bother?
Steam, flue gas, cement line... How much difference does the position of the expansion joint differ under different working conditions?
Different media, different temperatures, the installation logic is completely different. The steam pipeline has high temperature and high pressure, and the expansion joint should be as close to the fixed bracket as possible to ensure the stable axial displacement-usually usedUniversal corrugated expansion jointOrExternal pressure single type axial typeThe position is selected on the straight pipe section. The same cannot be said for the flue gas pipes. The flue gas contains dust and sulfur, and the temperature fluctuates greatly, so you have to consider ash accumulation. For example, desulfurization flue, commonly usedNon-metallic expansion jointOrRectangular non-metallic expansion jointThe position tends to be arranged downstream of the elbow or at the interface of the equipment-one to absorb displacement, and two to facilitate dust cleaning. Cement production line is more ruthless, high temperature, high dust, and wear-resistant.Metal Corrugated Expansion Joints in Cement IndustryIt is usually installed on the pipeline at the head and tail of the kiln, and the position is usually next to the fixed bracket, but the ash cleaning port must be reserved. And guess what? Someone installed a non-metal expansion joint at the end of the horizontal flue. As soon as the dust accumulated, the expansion joint was directly buried and unable to move. Therefore, under different working conditionsMetal expansion joint positionNot only the displacement, but also the media characteristics and convenience of maintenance.
Pits stepped on in those years: 3 fatal mistakes in where expansion joints are installed
Mistake 1: Install the expansion joint next to the fixed bracket.The fixed bracket is hard, and the expansion joint is soft. It is placed close to each other. When the tube expands, the expansion joint has no room to stretch at all, which is equal to decoration. Correct practice: Leave the fixed bracket by at least one length of the guide tube (usually 200~300mm).
Error 2: The guide bracket is installed backwards.Someone put the guide bracket on the other side of the expansion joint, causing the expansion joint to be forced to bear bending moments. And the result? The bellows is laterally unstable and directly scrapped. The guide brackets must be mounted at both ends of the expansion joint and axially centered.
Mistake 3: Wrong expansion joint type at the elbow.For example, if an axial expansion joint is used near a 90 ° elbow, the thermal displacement direction is oblique. How can the axial type stand it? This is the time to useCurved tube pressure balance expansion jointOrCompound hinge transverse expansion joint。 Last year, there was a project in which a general-purpose expansion joint was forced into the right-angle turn of the flue gas pipe, and it leaked after one week of startup. Check the reason: the position was not selected correctly, the type was not selected correctly, and it was double critical hit.
Straight pipe sections, elbows, near valves-use product examples to explain where to put it
The straight pipe section is the simplest, and the two fixed brackets are in the center position.Universal corrugated expansion jointOrStraight pipe pressure balanced expansion jointDone. Note: If the pipe diameter is large and the pressure is high, considerLarge diameter thick wall expansion jointBut the position still follows the neutral principle. It's complicated at the elbow. The elbow itself will absorb a portion of the angular displacement, and the expansion joint should be arranged on a straight section upstream of the elbow, at least 1.5 times the pipe diameter from the elbow. If you must compensate at the elbow, chooseCurved tube pressure balance expansion joint, it can absorb the combined displacement in the axial and lateral directions. More sensitive near the valve. The valve switch will generate additional thrust, and the expansion joint should not be too close to the valve, otherwise the impact force when the valve acts directly hits the bellows. Generally, it is recommended that the distance from the valve is at least 2 times the pipe diameter, and a short pipe transition is used in the middle. If the valve opens and closes frequently, you can also install limit rods on both sides of the expansion joint-i.e.expansion joint tie rodOne of the functions: to prevent over-stretching. What about product instances? We made it for the air-cooled islandDouble hinge expansion joint for air-cooled island vacuum pipelineSpecially arranged at the elbow of the inlet and outlet of the condenser, it can absorb the heat displacement without affecting the vacuum degree. If you choose the right position, nothing will happen for ten years.
On-site operation: How to deduce the best position according to the displacement amount and the direction of pipe system
It's better to take a pen and calculate it again. In the first step, draw the pipe path diagram and mark all fixing brackets and guide brackets (not yet determined? First determine the spacing of fixing brackets according to the pipe diameter and temperature, refer to GB 50235 or ASME B31.3). In the second step, the thermal elongation of each pipe segment is calculated: Δ L = α · L · Δ T. α is the coefficient of linear expansion (about 12×10⁻⁶/℃ for carbon steel), L is the length of the pipe section, and Δ T is the difference between the operating temperature and the installation temperature. The third step is to determine the rated compensation amount of the expansion joint. For example, the calculated thermal elongation is 100mm, selectUniversal corrugated expansion jointWhen the rated compensation amount is ≥120mm (leave the margin). Step 4, push the installation position backwards: place the expansion joint in the middle of the two fixed brackets to the left or right? If one end of the pipe is connected to equipment (e.g. fan, pump), the fixing bracket should be close to the equipment, and the expansion joint should be placed at the other end, so that the equipment is stressed minimally. If the pipe system has multiple elbows, first useCompound hinge transverse expansion jointOrSleeve type pipe expansion jointTo absorb the combined displacement, the position is selected on the straight section between the elbows. Finally, don't forget to check: Is there enough operating space around the expansion joint? Are there any obstacles that prevent displacement? There was once a project in which the expansion joint was installed, and a pillar was erected next to it. After thermal expansion, it was directly held up, and the pipe was deformed. Alas, we have seen this kind of thing a lot. So,Metal expansion joint positionInstead of just drawing a circle on the drawing, it should be deduced step by step in combination with on-site measurement, fixed bracket position, pipe system direction and actual space. Installed correctly, the equipment will run steadily for ten years; Wrong installation, the rework cost is enough to buy several batches of new expansion joints. Do you say it's worth the extra half day to confirm on site?