What is the difference between non-metallic rubber expansion joint and metal expansion joint? First understand why it can withstand high temperature and displacement at the same time
Many people will be confused when they contact non-metallic rubber expansion joints for the first time: isn't rubber afraid of high temperature? How can it be used in high-temperature occasions such as smoke duct and desulfurization tower? Here we have to break down a concept first-non-metallic rubber expansion joint, with rubber in its name, but it is not made of pure rubber at all.
The metal expansion joint absorbs the displacement by the deformation of the bellows, and the material is basically stainless steel such as 304 and 316L. The advantage is high strength and good pressure resistance, but when encountering large-size pipes (such as flues with a diameter of more than two meters), metal bellows either can't be made or the cost is ridiculously high. What is more headache is that the metal fatigue problem is difficult to avoid under vibration conditions.
The logic of non-metallic rubber expansion joints is completely different. It does not rely on a single material to carry hard, but combines rubber compensator and fabric fiber expansion joint into a composite structure, and each layer of material does its own work. The rubber is responsible for sealing and low-frequency vibration absorption, and the fabric layer is responsible for temperature resistance and withstanding pulling. Therefore, it can work normally at hundreds of degrees of high temperature, and at the same time, it can easily eat tens of millimeters of multi-directional displacement without delay on both sides.
Disassembly to see the structure: How are rubber compensators and fabric fiber expansion joints combined into non-metallic rubber expansion joints
Cut a non-metallic rubber expansion joint and see it in the same order from the outside to the inside: the outermost layer is stainless steel wire mesh or sheath to protect the internal structure from mechanical damage. Inside is the temperature-resistant layer, which is made of glass fiber cloth, ceramic fiber cloth and other materials. The upper limit of temperature depends on this layer. Further inside is the sealing layer, which is the core part of the rubber compensator-usually fluororubber, EPDM or neoprene, depending on the medium and temperature. There is also a layer of flow guide tube in the innermost part, which is made of metal and directly faces the medium to wash.
Note that this deflector is not an optional item. The speed of dust particles in the flue gas pipe is very high, and without the guide tube, the non-metallic layer can be worn through in a few months. Many early failure cases on the spot are unreasonable in design when they are disassembled. Therefore, when selecting a model, don't just stare at the rubber layer and fabric layer outside. Ask how long and what material the guide tube is made, which is more realistic than anything else.
The national standard for non-metallic expansion joints is JB/T 12235-2015, which has rigid regulations on structural design, material selection and inspection rules. When buying products, ask the manufacturer directly: Do you want to go according to JB/T 12235? One sentence can sieve out many unreliable small workshops.
Don't only focus on diameter and pressure in model selection. Temperature, medium and displacement direction are the key parameters
When choosing non-metallic rubber expansion joints, of course, the pipe diameter and pressure should be reported, but these two parameters are only the entry threshold. The following three really determine the service life:
- Temperature.This determines what material is used for the fabric and rubber layers. EPDM rubber can be selected below 150℃, fluororubber and glass fiber cloth can be replaced around 300℃, and ceramic fiber composite structure should be considered over 500℃. The temperature is reported low, and the product is loaded on, which is equivalent to chronic suicide. It is recommended that the actual working condition temperature increase by 20% be reported to the manufacturer, and a margin is allowed.
- Medium.Flue gas, hot air and corrosive gas, the three media have different requirements for materials. Sulfur-containing flue gas will accelerate the aging of rubber. At this time, the rubber layer has to be made of fluororubber or PTFE composite material (similar to the rubber PTFE compensator of this site), and ordinary neoprene can't stand it. Some customers take general products to install desulfurization flue, and air leakage occurs in less than a year. This is the reason.
- Direction of displacement.Which direction the pipe moves and how many millimeters it moves directly determines the structural form of the expansion joint. For axial displacement, ordinary type should be used, while for lateral displacement and angular displacement, hinge type or universal type structure should be considered. When reporting the displacement, don't just say a total number, but make the components in the three directions of X, Y and Z clear. Where is the installation position and what is the direction of the pipeline? Only when this information is given to the manufacturer can a reliable scheme be given.
Stress. The pressure bearing capacity of non-metallic rubber expansion joints is one order of magnitude lower than that of metal expansion joints, generally from several kPa to tens of kPa. Low-pressure large-diameter pipeline is its home field, and high-pressure pipeline should not be hard.
Some of the most common mistakes in on-site installation: the tie rod is removed early, the guide tube is installed backward, and the flange surface is not aligned
I met a customer two days ago and called to say that the expansion joint cracked within a month of installation. I went to the site to see that several taboos were committed during the installation process.
First, the tie rod was removed early. Non-metallic rubber expansion joints come out of the factory with tie rods, which are used to fix shipping and installation dimensions. Only after the pipe is aligned and the flange is tightened can the tie rod nut be loosened-but it is not necessary to remove it all. It is better to reserve some limits. This customer is good. On the same day as the pipe was installed, the tie rod was completely disassembled. The system was not running yet, and the thermal deformation of the pipe was fully pressed onto the expansion joint, so it was strange that it didn't crack.
Second, the guide tube is installed backwards. The guide tube has directionality, so it must be installed along the flow direction of the medium. If it is reversed, it will become a baffle. Holding the pressure is a trivial matter, and it is fatal for the medium to directly wash the non-metallic layer. This error occurs quite frequently at the scene. Glancing at the direction of the arrow before installing it can save a lot of trouble later.
Third, the flange surfaces are misaligned. The connection strength of non-metallic rubber expansion joint depends entirely on the flange compression. If the flange surfaces on both sides of the pipeline are not parallel, once the bolts are tightened, the local stress will all be concentrated on one side, and the rubber layer will soon tear from there. Before installation, measure with a horizontal ruler, and control the error within 2mm before attaching the bolts.
The pipes at both ends of the expansion joint must have independent supports or guide brackets. Non-metallic expansion joints can only absorb displacement and can't bear load, which is a principle problem, but it is used as pipe racks in many construction sites.
How long will this thing last? Several signals to judge whether non-metallic rubber expansion joint should be replaced
The design service life of non-metallic rubber expansion joints is generally marked as 8 to 10 years, and how long it is actually used depends on the working conditions. Data speaks: clean room temperature water system, some can last for more than 10 years; On the flue of coal-fired power plants, the environment is harsh and the medium contains sulfur, so many people will change it in three to five years. Therefore, there is no need to be superstitious about the years of the manufacturer's target, and it is more reliable to look at the state.
These signals appeared, indicating that it was time to prepare spare parts: the outer fabric was cracked or fluffed, and powder fell when touched by hand; There are turtle cracks caused by ozone aging on the surface, which can be buckled in with your fingernails; There are leakage marks at the compression of the flange, or obvious air leakage sounds can be heard; The rubber layer is hard, brittle and loses its original elasticity. Also, if it is found that the surface temperature of the expansion joint is higher than that of other parts of the pipe, it means that the insulation layer or internal insulation layer has failed, and in this case, it must be replaced as soon as possible.
How to extend the service life? Routine inspections work better than any repair. Look regularly to see if there is any foreign body accumulation, whether there is any burn from flying sparks, and pay attention to drainage and freezing in winter. For pipelines with particularly large vibrations, a protective cover may be installed outside the expansion joint. In this way, it is common to last for two or three more years.
In the final analysis, the non-metallic rubber expansion joint takes half an hour to clarify the working condition parameters when selecting the type. When installing it, it should be done according to the specifications, and at least three after-sales trips should be fewer. If you are not sure what structure to choose, directly throw the temperature, medium, displacement direction and pipe size to the manufacturer, and let a professional help you calculate it. Don't pat your head to determine the model. Otherwise, it will be easy to install and troublesome to change.