What are non-metallic fabric expansion joints? What's the difference between it and a metal expansion joint
Non-metallic fabric expansion joint, to put it bluntly, is a pipeline compensation element made of fabric fiber and non-metallic materials such as rubber and fluoroplastics. And metal expansion joints (like the one in our stationUniversal corrugated expansion joint、The biggest difference is that it does not rely on the elastic deformation of the metal corrugated sheet to absorb displacement, but relies on the flexible bending and stretching of the fabric layer itself. It's like the difference between wearing a suit and sportswear-the metal expansion joint has strong rigidity and high pressure, but the displacement compensation angle is limited; Non-metallic fabric expansion joints are soft, light and corrosion-resistant, especially good at absorbing multi-directional displacement and vibration reduction.
For example, in the flue duct and desulfurization system of power plants, where the medium temperature is high and corrosive gas is contained, the pipeline turns around, and the metal expansion joint is prone to stress concentration or even fatigue cracking.Non-metallic expansion joint (fabric fiber expansion joint)It's much more flexible. Moreover, the expansion energy saving of non-metallic fabric can withstand greater axial and angular displacement, and the installation space is more compact.
JB/T 12235-2015: What does this national standard govern? Scope of Application Don't Make a Wrong
The criterion we are focusing on isJB/T 12235-2015 Non-metallic fabric expansion joint。 This standard was released in 2015 and officially implemented in 2016. It is the most authoritative standard for non-metallic fabric expansion joints in China at present. What does it stipulate? Technical conditions, test methods, inspection rules, marks, packaging, transportation and storage-all are managed in one stop. However, there is a key point: its application scope is locked in "expansion joints with fabric fibers as reinforcement materials and rubber or fluoroplastics as sealing materials".
That is, if you are using a pure rubber compensator (like the one in the stationrubber compensator) or polytetrafluoroethylene compensators, although similar in structure, this standard is not necessarily fully applicable. In addition, the pressure level mentioned in the standard is usually no more than 0.1 MPa, and the temperature range is generally-40°C to 450°C (depending on the specific material). Don't take this standard to cover the metal expansion joints on high pressure steam pipes, that's another matter.
Large disassembly of material structure: fabric layer, sealing layer and heat insulation layer are indispensable
A qualifiedNon-metallic fabric expansion jointThe internal structure is generally divided into three layers:
- Fabric reinforcement layer: Usually fiberglass cloth, ceramic cloth or polyester cloth, which is responsible for withstanding tension and providing strength. There are clear requirements for the breaking strength and temperature resistance grade of fabrics in the standard.
- Sealing layerPolytetrafluoroethylene film, silicone rubber cloth or fluororubber coating are used, and the main function is to prevent media leakage. In the standard, it is particularly emphasized that the sealing layer must be continuous and complete, without pinholes or bubbles.
- Thermal insulation(Some are called insulation layer): Aluminum silicate wool, rock wool, etc. are commonly used to fill between fabric layers to reduce the outward transfer of heat. If it is a high temperature working condition (such as above 400℃), the thickness of the insulation layer needs to be determined according to thermal calculation.
What levels do I have to pass before leaving the factory? Leakage test, displacement compensation, temperature resistance test
According to JB/T 12235-2015, non-metallic fabric expansion joints must pass at least three hard bars before they leave the factory:
Level 1: Leak test.The test pressure is usually 1.5 times of the design pressure, and the pressure is held for 5 minutes to observe whether there is bubbling and leakage. The standard clearly states that there must be no visible leakage at any part under the test pressure. Two months ago, a customer told me that a batch of expansion joints they bought leaked after three days of installation. When I asked, I found out that there was no airtightness test at all-that is, they didn't even pass the factory standard.
Level 2: Verification of displacement compensation ability.Although it is soft, it has design values for how much axial displacement and how much lateral displacement it can compensate. In the standard, it is required to use a simulated displacement device to load, and check whether the fabric layer has delamination and cracks after at least 10 cycles. Many suppliers only give a theoretical value, but it gets stuck when it is actually installed. The problem lies in the lack of test.
The third level: temperature resistance test.According to your working temperature, put the sample in a heating box for 30 minutes, and then check whether the surface is carbonized and the sealing layer is cracked. Like the one in our stationHigh temperature axial expansion joints andNon-metallic expansion joints (fabric fiber expansion joints) have passed strict temperature resistance tests before leaving the factory, and are directly scrapped if they don't meet the standard.
Those standard red lines that are easily overlooked in installation and operation
There are also many installation details buried in the standard, which many people don't pay attention to. Something like:
-The flow direction of the conduit (guide tube) must be consistent with the flow direction of the medium at the time of installation. If the direction is reversed, the guide tube will not play a protective role, but will hinder the displacement. A schematic diagram of typical installation directions is found in Appendix B of the standard.
-Expansion joints must not be stacked and squeezed during transportation and storage to prevent permanent deformation of the fabric layer. If the crease is found to exceed 2mm after unpacking, it cannot be used.
-During operation, welding and cutting on the expansion joint body are strictly prohibited, and knocking with tools with sharp corners is not allowed. What you think is just to protect the appearance may actually break the integrity of the seal.
Another point: the standard clearly states that "non-metallic fabric expansion joints should not be used in situations that bear the weight and vibration of pipes", but in actual projects, many people regard expansion joints as supports, and as a result, they are torn in a few months. Remember, it is a compensation member, not a force member.
Practical operation of type selection: How to select rectangular non-metallic expansion joint and round shape-look at the standard according to working conditions
When it comes to type selection, first look at the pipe cross-section shape. Rectangular air ducts (such as power station flues) are generally usedRectangular non-metallic expansion joints, circular pipes (such as the inlet and outlet of desulfurization tower) choose circular non-metallic expansion joints. However, the technical requirements for these two shapes in the standard are actually slightly different:
-The rectangular expansion joint emphasizes the arc transition radius of the corner, which is not less than 30mm according to the standard, otherwise the fabric layer is prone to stress concentration at the edges and corners and will be damaged in advance.
-Circular expansion joints pay more attention to the sealing performance of flange connections, and the standard requires that the flatness of flange sealing surface should not exceed 0.5mm/m.
In addition, the table of recommended material combinations under different working conditions is given in the standard appendix. For example, the flue gas temperature is below 200 DEG C, and it is sealed with polyester fiber + EPDM; Glass fiber + silicone rubber for 200-350 DEG C; Above 350℃, it has to be ceramic fiber + fluoroplastic. Select materials according to this table, and you can't go wrong basically.
Finally, national standards are the bottom line, not the ceiling. In actual projects, the demands of media corrosion, installation space and compensation amount may be more demanding than the "typical working conditions" in the standard.