How to calculate the skin size of non-metallic compensator? Full process disassembly from thermal displacement to blanking
Two days ago, I met a customer who made dust removal pipes. When I came up, I asked, "Is the skin size of your non-metal expansion joint directly enlarged by 50mm according to the inner diameter of the pipe?" I almost choked after listening to it. If this algorithm works, then the friends of the design institute have changed to selling skewers. Today, let's make this clear-the core logic of non-metallic compensators is different from that of metal expansion joints when calculating the skin size.
The starting point of the calculation of skin size: the inner diameter of the pipe is only a reference, the effective area and thermal displacement are the core
The inner diameter of the pipe is DN1000, so the skin will be 1100 wide? Wrong. The skin is a layer of flexible fabric fibers that covers the "effective flow area" of the non-metallic expansion joint structure, rather than a simple set of pipe sizes. In actual calculations, the first step is to determine the total amount of thermal displacement of the piping system – including axial, lateral, and angular displacements. For example, a 30-meter steam pipe, with a temperature increase of 200℃ and an axial elongation of at least 50mm, with a lateral swing, may be displaced to 80mm in one direction. The width of the skin must accommodate these displacements while ensuring that the edge of the skin does not break away from the flange sealing surface at maximum displacement. How to take the effective area? Generally, take the inner diameter of the pipe plus twice the one-sided reserved displacement space. This space is determined according to the displacement amount, not patting the head.
The number of layers, thickness, and material choices-they directly determine the actual spread size of the skin
The skin is not a layer of cloth, it is a composite laminate structure. The common non-metallic expansion joint (fabric fiber expansion joint) skin consists of four to six layers: the innermost layer is a temperature-resistant layer (ceramic fiber or PTFE film), sandwiched with a moisture barrier layer (fluororubber cloth), and the outer layer is a weather-resistant layer (silicone-coated fiberglass). The thickness of each layer is different, and the total thickness on stack may be between 5 mm and 15 mm. And guess what? This thickness creates a "superimposed radius effect" at the corners. A rectangular non-metallic expansion joint, if the inner diameter of the corner is R =50mm, each layer is superimposed by 2mm, six layers are 12mm, and the outer arc expansion size is nearly 80mm more than that of the inner arc. When cutting, if it is lofted directly according to a flat rectangle, it will be tense when installed, and it will tear in a short time. Therefore, the thickness and bending radius of each layer must be taken into account when calculating the expansion size, so we can't be lazy.
Length is inferred backwards by displacement: Don't forget fatigue life and installation reserve
How to determine the length of the skin (that is, the effective compensation length of the compensator)? Remember one principle: the length cannot be less than 1.5 times the maximum displacement, and the fatigue life should be considered. For example, if the design displacement is 100mm in X direction, it should be at least 150mm long. Why? Because the skin is repeatedly stretched and compressed, the fibers will wear out. It is recommended in JB/T 12235-2015 that the strain of a single layer of skin should not exceed 15% under 1000 cycles of life. You calculate, if the length is only 120mm, pulling 100mm has reached the limit, and it will break twice. In the actual project, the installation reserve has to be added-the on-site flange is not necessarily completely aligned, and there may be a 5mm deviation; Cold state pre-stretching should also be considered. Therefore, the reliable approach is: theoretical length = maximum displacement ×1.5+ installation margin (20~30mm).
Width and Bolt Hole Spacing: Flange Matching Matters More Than You Think
The width of the skin, in addition to accommodating displacement, has to fit into the bolt hole spacing of the flange. Many problems on the spot are not the wrong calculation of the skin size, but the mismatch of the hole spacing. The diameter of the center circle of the bolt hole on the flange surface is generally 30~60mm larger than the outer diameter of the pipe. However, the skin edge of the non-metallic compensator should be flanged and beaded, and the bolts should be tightened through the beaded. The width of the strip is usually 40~50mm, and the hole distance should strictly match the flange bolt hole. For example, the flange of DN1000 has a diameter of 1140mm in the center circle of the bolt hole, and 24 holes are evenly distributed. Your skin is 1100mm wide, and the pressing strip can't cover the hole at all. So blanking width = flange hole center circle diameter +2× (bead width + bolt via allowance). It is best to take the flange drawing directly, and don't listen to the customer saying that it is "the same as the standard"-the deviation of the hole spacing in actual processing is often 1~2mm, so it has to be measured on site.
Don't step on pits: Take non-metallic compensators as metal expansion joints, and the results are all wrong in size
Some people regard the non-metallic compensator as a metal rectangular expansion joint, and directly use the deployment formula of metal bellows to cover the skin. It turned out to be something completely rigid and impossible to bend. The corrugation of the metal expansion joint depends on the elastic deformation of the material to compensate the displacement, while the non-metal skin is a flexible fabric by folding and stretching. The former calculation focuses on wave height, wave pitch and wall thickness, while the latter focuses on the number of layers, fiber direction and temperature resistance grade. For specific data: the same DN500, the axial displacement is 50mm, the metal expansion joint may be 200mm long, and the non-metal expansion joint (fabric fiber expansion joint) only needs to be 150mm long, but the width should leave more lateral displacement margin. You have to use the formula of metal, and it is wrong to calculate the length directly.
After all, when calculating the skin size of non-metallic compensators, the core is two sentences:The displacement determines the length, the flange hole spacing determines the width, the effective area and layer thickness determine the unfolding shape。 Don't directly enlarge the inner diameter of the pipe by 50mm, and that method will fool yourself. If you really want to calculate accurately, bring me the drawings, and we will follow them step by step.