Two days ago, I met a purchaser who was doing desulfurization in a power plant. When he came up, he asked me, "How much can your non-metallic expansion joint withstand the temperature?" I knew as soon as I heard it that this guy had fallen into the pit again. Temperature resistance of non-metallic expansion joints is of course important, but just looking at this parameter, nine times out of ten will have to be reworked later. Today, let's make it clear that in the description of the performance indicators of non-metallic expansion joints, which parameters are the vital gates of selection.
Why are performance metrics always ignored? — — The easiest pit to step on when selecting a model
Many people choose non-metallic expansion joints (also called fabric fiber expansion joints), and ask about the temperature first, thinking that as long as the temperature resistance is enough. And the result? Less than half a year after installation, the flange leaked, the fabric layer was torn, and even the whole expansion joint was burst by pressure. Tsk, what's the problem? Because temperature resistance is the only criterion. In fact, according to JB/T 12235-2015 National Standard for Non-metallic Expansion Joints, the performance indicators include at least a dozen parameters such as temperature resistance, pressure resistance, compensation amount, number of structural layers, frame material, sealing form, etc. If you ignore either one, you can lay mines.
Disassembly of core indicators: What do you think of temperature resistance, pressure resistance and compensation?
Let's talk about temperature resistance first. The temperature resistance of non-metallic expansion joints depends on the combination of fabric fibers and sealing materials. Common silicone cloth can withstand about 250℃, polytetrafluoroethylene (PTFE) can reach 350℃, and ceramic fiber composite layer can reach more than 1000℃. However, note that the temperature resistance value is the "medium temperature", not the ambient temperature. If there is 800℃ flue gas in your pipeline, but the outer environment is only 50℃, do you choose an expansion joint with a temperature resistance of 300℃? Then wait until it burns through.
Let's talk about withstand voltage. Non-metallic expansion joints are generally low-voltage equipment, and most of them are used in occasions below 0.1MPa. But have you ever seen the expansion joint attached to the back of the smoke baffle door? The working conditions fluctuate greatly, and instantaneous pressure impact may cause the fabric layer to collapse directly. When selecting the model, we should look at the "design pressure" and "test pressure", and the difference of 1.5 times between them is the bottom line. The frame material — like carbon steel, stainless steel 304 or 316L — directly determines the overall pressure capacity. Don't use thin-walled carbon steel to save money. Corrosion will be over for a year or two.
The amount of compensation (axial, transverse, angular) is the third core. Many people think that non-metallic expansion joints can only absorb axial displacement, but in fact, they are best at lateral and angular compensation. For example, the rectangular non-metallic expansion joint is used in the tail of cement kiln. The thermal expansion direction of the pipeline is complicated, so it has to absorb three-dimensional displacement at the same time. If only a fuzzy parameter of "compensation amount 100mm" is given, then you have to ask clearly: is it axial or horizontal? How many cycles are allowed? Some manufacturers take the maximum displacement as the rated value, and the actual life is halved.
Influence of structural design on performance: fabric layers, frame materials and sealing forms are indispensable
Open a non-metallic expansion joint and you'll see several layers of fabric: the innermost layer is a temperature-resistant layer (ceramic fiber or fiberglass), the middle is an insulation and reinforcement layer (stainless steel wire mesh or aramid cloth), and the outer layer is a protective layer (silicone or fluororubber coating). The more layers, the stronger the temperature and pressure resistance, but the flexibility will decrease. For example, the non-metallic expansion joint used for desulfurization in power plants usually has 4-5 layers, which should be resistant to acid corrosion and absorb pipeline vibration.
The common frame material is Q235 carbon steel, 304 stainless steel and 316L stainless steel. Which one to choose? Look at the media. The desulfurization system has a large amount of SO₂ and condensed acid, and 316L is standard; Cement kiln tail dust, high temperature, carbon steel plus anti-corrosion coating can also be used, but attention should be paid to the problem of coating peeling. The sealing form is more particular: is it flange or plug-in? Single pass seal or double pass seal? The sealing principle of double seal single axis circular baffle door can be used for reference-double seal plus blowing and scavenging, which can effectively prevent external leakage. The same is true for non-metal expansion joints. If the seal is not tight, the flue gas leak will make the pressure loss of your whole system out of control.
How to choose between different working conditions? Differentiated requirements for desulfurization, cement kiln tail and chemical pipeline in power plants
Let's talk separately.
Power plant desulfurization: The flue gas temperature is generally 120-180℃, but it contains a large amount of SO₂, HF and other strong corrosive gases, and the humidity is high. When selecting, the fabric layer of non-metallic expansion joint must contain PTFE or fluororubber coating, and the frame should be made of 316L stainless steel. The sealing form is recommended to be double-pass seal plus expansion joint lining. Look at the desulfurization flue gas baffle door in our station, and the matching non-metallic expansion joints are all in this configuration. The pressure is not high (generally 0.05-0.1MPa), but the transverse compensation amount is required to be large, because the thermal expansion of the flue will cause the lateral displacement of the pipe.
Cement kiln tail: The temperature rushes to 350℃ or even higher, the dust concentration is large, and the wear is serious. At this time, the non-metallic expansion joint should choose high-temperature resistant ceramic fiber layer, and the outer layer should be added with wear-resistant sheath. It is best to use heat-resistant steel (such as 1Cr18Ni9Ti) for the frame, or increase the wall thickness. The amount of compensation is mainly axial and angular, because the pipe runs straight, but the amount of thermal expansion is large. Many metal corrugated expansion joints in cement industry can also be used, but the advantages of non-metal are light weight, easy installation, and can absorb multi-directional displacement.
Chemical pipeline: The situation is most complicated. It may be exposed to organic solvents, strong acids and alkali, and the temperature may not be high but it is extremely corrosive. The fabric layer must be resistant to chemical corrosion when selecting the model-PTFE compensator (PTFE-lined) is typical, but PTFE-lined metal hose and non-metal expansion joint are not the same thing, so don't mix it. Chemical pipelines usually have a slightly higher pressure (0.2-0.5MPa), stainless steel should be used for the frame, and media permeability should be considered for the seals. Sometimes you have to add an anti-static design.
Failure case review: What problems will happen if the indicators fail to meet the standards?
Tell me a real case. The dust removal pipeline of a steel plant uses non-metallic expansion joints. At the beginning, only the temperature resistance (250℃) was selected. As a result, the temperature instantly rushed to 400℃ in actual working conditions. Within a month, the fabric layer was carbonized, perforated, and dust sprayed all over the floor. A ceramic fiber layer with a temperature resistance of 500℃ was changed, but the frame was still carbon steel. As a result, the frame was corroded and perforated the next year. Finally, the 316L frame + ceramic fiber layer + double seal was changed to run stably. All three pits were stepped on: insufficient temperature resistance, wrong frame material, and too weak sealing form.
The rubber compensator of a chemical plant was originally used in room temperature pipelines. As a result, there was a small amount of benzene in the medium, and the rubber swelled, which leaked in three months. Later, it was replaced with a non-metallic expansion joint (PTFE lining), and nothing happened. Therefore, media compatibility is more important than temperature resistance, but unfortunately, many people ignore it.
Summary: A reliable selection list, follow it without mistake
Choose non-metallic expansion joints, don't just focus on temperature resistance. Follow the following 6 steps, and it is basically stable:
- 1. Operating condition parameters: Media composition, temperature range (normal + instantaneous), pressure (design + test), pipeline displacement direction and amount.
- 2. Number of fabric layers and material: Select ceramic fiber, glass fiber, PTFE, etc. according to temperature and corrosiveness, and the number of layers is not less than 3.
- 3. Frame Material: Carbon steel, 304, 316L, selected according to corrosivity, wall thickness ≥3mm.
- 4. Sealed form: Flange type, plug-in type or double-pass seal, with blowing and scavenging if necessary.
- 5. Compensation Amount: Confirm the rated values in axial, transverse and angular directions respectively, leaving a margin of 20%.
- 6. Implementation standards: JB/T 12235-2015 is the bottom line, don't trust verbal promises.
Remember, in the performance index description of non-metallic expansion joints, temperature resistance is only the foundation, while pressure resistance, compensation amount and structural design are the keys to determine life. Next time you select, you can save a lot of money on rework by carefully digging these parameters. Do you have any questions about the selection?