Standard background and scope of application: Why do non-metallic compensators need a separate set of national standards?
Think about it, can the standard set of metal expansion joints be fitted to non-metal heads? Obviously not. Non-metallic compensators (also called non-metallic expansion joints, fabric fiber expansion joints) are made of flexible materials such as polytetrafluoroethylene, silicone rubber and glass fiber cloth, which is completely different from metal bellows. JB/T 12235-2015 This standard is a rule specially set for this kind of products. The applicable temperature range, pressure level and displacement compensation amount are all listed separately. To put it bluntly, in occasions with low pressure and large displacement such as flue, desulfurization tower and dust collector, metal parts can't bear it, so they have to be non-metal. At this time, standards come in handy.
Disassembly of core technical parameters: temperature, pressure, displacement, how are they specified in the standard?
It is clearly stipulated in the standard that the design temperature of non-metallic compensator should be determined according to the skin material-the long-term temperature resistance of silicone skin is only over 200℃, and fluororubber can reach 250℃. If it encounters high-temperature dusty flue gas, ceramic fiber insulation layer must be added, and the upper temperature limit can be pulled to 1200℃. In terms of pressure, the working pressure of most non-metallic compensators does not exceed 0.5MPa. After all, the flexible structure cannot bear high pressure. The displacement is divided into three kinds: axial, transverse and angular. The standard gives the calculation formula and test method. For example, the transverse displacement is usually calculated according to 1/3~1/2 of the pipe expansion. In actual selection, you can't just look at the maximum displacement number on the sample, but you must leave enough safety factor, otherwise the skin will tear as soon as it is stretched.
Fundamental difference from metal expansion joints: completely different structure, materials, failure modes
The metal expansion joint absorbs the displacement by the deformation of the bellows, and the failure probability is fatigue crack or stress corrosion. What about non-metallic compensators? Structurally, it is a frame plus multi-layer skin, plus thermal insulation cotton and flow guide tube. Once the skin is aged, worn or corroded by acidic gas, it leaks directly. Two days ago, I met a cement factory customer. After more than two years of use, the skin surface was crisp-just because acid-resistant fluororubber was not selected according to the standard requirements. In addition, the installation direction of the non-metallic compensator is also particular, and the guide tube must flow towards the medium, otherwise the high-speed dust will directly wash the skin and perforate it in a few months.
Selection and design points: Special requirements for rectangular non-metallic expansion joints and fabric fiber compensators
Rectangular non-metallic expansion joints are most commonly used in power stations and steel industries, and they are often equipped in front and back of flue gas baffle doors. When selecting, pay attention to the aspect ratio of rectangular section. It is recommended in the standard that it should not exceed 2:1, otherwise the rigidity is not enough and the frame will be distorted during operation. What about fabric fiber compensators, its skin is usually made of four or five layers of materials: the outermost layer is weather-resistant silicone coated cloth, the middle is glass fiber reinforced layer, and the inner layer is corrosion-resistant polytetrafluoroethylene film. The standard has lower limit requirements for the thickness and tensile strength of each layer of material. In addition, the design of the guide tube can't be careless-the specific function of the guide tube is to guide the airflow to reduce the vortex and avoid local overheating or wear of the skin.
Common pits in installation and acceptance: guide tube, tie rod, pre-tension, these details are clearly stated
The biggest pit during installation is that the tie rod is not adjusted. In many projects, the transportation tie rod is regarded as a permanent part, or it is not disassembled at all. As a result, the compensator can't move freely, and the pipeline stress is all suppressed on the equipment. It is stated in the standard that the transport tie rod must be removed or adjusted to the specified position after installation. How to adjust the expansion joint tie rod nut? Loosen the nut so that the pull rod can slide freely, but don't pull it out completely. Keep it as a limit. Pre-stretching, non-metallic compensators generally do not require cold drawing like metal parts, but the standard also gives a special situation-when the temperature difference between the installation temperature and the working temperature exceeds 100℃, it should be pre-compressed or pre-stretched, and the compensation amount is calculated according to the linear expansion coefficient. Don't forget to check the direction of the guide tube during acceptance. The direction of the arrow should be consistent with the flow direction of the medium. There is also a common problem: the flange surface is uneven, the bolt tightening force is uneven, which leads to skin wrinkles, and the air leakage rate exceeds the standard in the later period.
Practical experience beyond the standard: how to flexibly apply it when encountering high-temperature dusty flue gas and desulfurization conditions?
The standard gives the bottom line, and the actual working conditions vary widely. For example, for high-temperature dusty flue gas, the skin structure in the standard alone is not enough, so it is necessary to add anti-scour plates, or stack wear-resistant ceramics on the guide tube. The desulfurization condition is even more headache. The wet flue gas has acidic condensate, and it is standard to select PTFE plus fluororubber for the skin, but the frame anticorrosion should also keep up-carbon steel sprayed with glass fiber reinforced plastic, or 2205 duplex steel should be directly used. We matched the compensator for the desulfurization flue gas baffle door made by a thermal power plant, and added a special anti-corrosion layer for PTFE compensator on the inside of the skin, which didn't leak after five years of use. Another trick: when the displacement of the pipeline is particularly large, don't kill a single compensator, and use the combination scheme of compound hinge transverse expansion joint or straight pipe pressure balanced expansion joint. The pressure is balanced and the skin is stressed evenly. Standards are dead,