What does the non-metallic expansion joint JBT refer to? First clarify the relationship between standards and products
Let's take the word apart first. JBT is the abbreviation of the recommended standard in the machinery industry, and its full name is JB/T 12235-2015 "Non-metallic Expansion Joint"-pay attention to this standard number. Many purchasing masters can't remember it all, and they only know how to call "Non-metallic Expansion Joint JBT". In fact, to put it bluntly, it is a technical specification, which stipulates the terms, classification, technical requirements, test methods and inspection rules of non-metallic expansion joints. It is not a product model, let alone a certain manufacturer's patent.
Then why do people always like to talk about "jbt"? Because in reality, many drawings and bidding documents directly say "non-metallic expansion joint jbt", which is actually a less rigorous but conventional statement. You just have to understand one thing: standards are the bottom line, not the selection manual. The selection depends on the working condition parameters, not on the standard number to set out the product. More on that later.
Incidentally, JB/T 12235-2015 replaces the old version in 2004, and the content makes more detailed constraints on the belt material, insulation layer design, fatigue test, etc. If you meet a manufacturer that still supplies according to the old standard, you should be more careful.
From fabric fibers to rectangular structures: Common forms and applicable scenarios of non-metallic expansion joints
The structure of non-metallic expansion joint, to put it bluntly, is one or several layers of flexible band (commonly used fabric fiber, silicone rubber coated glass fiber cloth, fluororubber composite cloth, etc.), with metal frame, guide tube and thermal insulation layer. Instead of absorbing displacement by corrugation deformation like metal bellows, it "digests" thermal expansion and contraction by deformation of the flexible material itself. Because of this, it is especially suitable for large diameter, low pressure, low temperature but complicated displacement-such as flue duct of power plant, flue gas duct of desulfurization system, or air duct of cement plant.
In terms of shape, the most mainstream are rectangular and circular. There is a rectangular non-metallic expansion joint in this station, which is specially used in rectangular cross-section pipelines, such as the outlet of induced draft fan of power plant, the connection between flue and chimney. Circular ones are common in circular pipelines, like the inlet and outlet of desulfurization towers. There is also a non-metallic expansion joint (fabric fiber expansion joint) with a large DN. The band is made of multiple layers, filled with heat insulation materials such as ceramic fibers in the middle, and the outer layer is covered with a layer of corrosion-resistant skin.
And guess what? Many customers want the "largest model" when they come up, but in fact, the selection logic of non-metallic expansion joints is not according to size, but according to working conditions. The last three parameters are the real life gates.
The three most easily overlooked parameters in model selection: temperature, displacement and media corrosivity
Let's start with the temperature.The endurance temperature of non-metallic expansion joints does not refer to the medium temperature alone, but to the "actual temperature of the ring belt surface". In many working conditions, the medium in the tube is 500℃, but after you choose the insulation layer and the guide tube, the surface of the ring belt may only be 120℃. The difference in the middle is where the design skill lies. When selecting the model, don't look at these two temperatures, only take the medium temperature to set the standard, and there will be problems nine times out of ten. JB/T 12235-2015 clearly stipulates the temperature resistance, and different temperature grades correspond to different material combinations, which must not be taken for granted.
Let's talk about the displacement.The strength of non-metallic expansion joints is that they can absorb multi-dimensional displacements-axial, transverse and angular displacements. But note that you have to give the real displacement, not just write "80mm". It is necessary to distinguish whether it is axial or transverse, hot or cold. There was once a renovation project, and the owner only reported the axial displacement. As a result, the pipeline swung laterally by more than 30 mm after heating up, and the expansion joint was directly torn. This lesson is expensive.
Finally, let's talk about media corrosiveness.The most common thing in flue gas systems is acid dew point corrosion. You have to make clear the composition of the medium: how much sulfur, how much water vapor, and whether there are chloride ions? Don't just write the word "smoke". The non-metallic expansion joints used in the desulfurization flue gas baffle door of this station are all acid-resistant fluororubber coated. Because the flue gas temperature is low and the humidity is high after desulfurization, ordinary glass fiber cloth can be corroded in a few months.
So you see, if you don't understand these three parameters, the expansion joint will be a ticking time bomb when bought back.
Common myths about installation and maintenance: Why do some non-metallic expansion joints not last long?
I bought the right thing, but it leaked within half a year of installing it. What reason? Probably something went wrong with the installation.
Forgot to remove the transport fixtures. When non-metallic expansion joints leave the factory, in order to protect the ring belt, transportation rods or pressure plates are generally installed. Some masters found it troublesome, so they welded it directly with the fixing parts. As a result, as soon as the pipe expanded thermally, the expansion joint did not move at all-it did not work at all, and it was the pipe and bracket that were damaged.
The deflector is installed backwards. The function of the guide tube is to let the medium go along and avoid the high-temperature airflow directly washing the ring belt. Install it backwards, the cold surface becomes the hot surface, and the life of the ring belt is directly cut in half.
I don't think the non-metallic expansion joint needs maintenance. In fact, the surface of the ring belt should be checked regularly to see if there are scratches, aging cracks or foreign matter accumulation. Many expansion joints of power plants are installed in open air or dusty areas, and if there is too much dust accumulation, the heat dissipation of the ring belt will be poor, and the life will still be discounted. There is another point that is easy to overlook-the pipes on both sides of the expansion joint must be provided with independent brackets, so that the displacement can be concentrated on the expansion joint, otherwise it will become a force-bearing member, and no matter how good the skin is, it will not be able to bear it.
In the final analysis, the non-metallic expansion joint will not be used for a long time, and 70% of it is a problem of installation and maintenance, but the product itself is fine.
Non-Metallic Expansion Joints vs Metallic Expansion Joints: When Should You Think Another Way?
Since non-metal expansion joints have so many benefits, why use metal expansion joints for? This question, scoring occasions.
The strengths of metal expansion joints (such as corrugated expansion joints) are high temperature and high pressure resistance and strong corrosion resistance, such as the main steam pipeline of power stations and chemical high-temperature pipelines, with a pressure of more than ten MPa and a temperature of five or six hundred degrees Celsius. Non-metallic fabrics can't bear this working condition at all, so metal corrugated pipes must be used. However, metal expansion joints also have shortcomings: the displacement compensation ability is relatively single, the lateral displacement compensation amount is small, the cost is high and the thrust is large when the diameter is large-you calculate how much a DN3000 metal expansion joint costs, and then calculate the non-metal expansion joint of the same diameter, and the gap is not a little bit.
Air duct, flue duct, cryogenic duct, preference to non-metal expansion joint; For high-temperature and high-pressure liquid or steam pipes, look for metal corrugated expansion joints. Another situation is for mixing-for example, in the tail flue of a power station boiler, the front section uses non-metal to absorb heat displacement, and the rear section is connected in series with a metal expansion joint as insurance. Although this idea is simple, many design institutes will get stuck in the scheme review stage, just because they are used to a set of parameters to dominate the world.
Now go back to the beginning question-how exactly is the non-metallic expansion joint jbt used? Remember the standard number, grasp the parameters in the selection, abide by the rules in the installation, and be sober in the comparison. Just these four sentences. As long as the temperature, displacement and media corrosiveness are clearly answered, you will find that you will never be taken away by the manufacturer again.