Two days ago, I met a customer, and I opened my mouth and asked, "Is your non-metal expansion joint interface flanged or welded?" I was happy when I heard it. This question looks simple, but it can really lead people into a ditch. What exactly is a non-metallic expansion joint interface? Many people mix it with flange and skin into a pot of porridge. Today, we'll break it apart and crumble it into pieces.
What exactly does the non-metallic expansion joint interface mean-don't confuse it with flange and skin
The non-metallic expansion joint interface refers to the position where the expansion joint body and the pipe (or equipment) are connected, that is, the structure that holds the fabric fiber expansion joint and the air duct together. It is neither the skin itself nor the flange alone. The skin is the layer of flexible material in the middle of the expansion joint, which is responsible for absorbing displacement; The interface is the circle of rigid connection structure at the edge, which is responsible for securing the skin to the pipe.
What happens if you get mixed up? You just stared at the skin material and chose a temperature-resistant 800℃. As a result, the interface form was not selected correctly. After two months of installation, the bolts were loose and air leaked, and it was blind. Therefore, the interface form and the skin material are two different things, and they have to be taken care of when selecting the type.
Three common interface forms: flanged flange, butt welding, clamp type, what are the advantages and disadvantages of each
Flanged flange, butt welded, clamp type. No one is absolutely good, only who is right for you.
flanged flangeIs the most common. The edge of the skin of the expansion joint is flanged to enclose a steel plate frame with bolt holes opened in the frame. The advantage is that it is convenient to install on the spot, just screw the bolts on the pipe, and it can be dismantled for maintenance. The disadvantage is that there are many bolt holes and it is difficult to screw them up; Moreover, it requires high flatness of the flange surface, and it leaks if it warps slightly.
Butt weldingIs to weld the metal frame of the expansion joint directly to the pipe. This structure has the highest strength and good sealing performance, and is suitable for places with high temperature and high pressure or corrosion of media, such as desulfurization flue gas system. But the disadvantages are obvious-it can't be removed, and it can only be cut for maintenance. In addition, on-site welding is easy to burn out the non-metallic skin next to it, so it has to be protected.
Clamp typeStainless steel clamps are used to "hoop" the expansion joint on the outer wall of the pipeline. The fastest installation, easy disassembly and assembly, suitable for working conditions that require frequent cleaning or skin replacement. However, the pressure bearing capacity is weak, and it is generally only used for ventilation ducts with low pressure and normal temperature. If you use it on the air duct with high positive pressure, it is equivalent to laying a mine for yourself.
Interface seals and bolt spacing, air leakage and ash leakage are often bad in small details
Many people think that the interface, anyway, has bolts to fix, how big a deal can it be? Alas, air leakage and ash leakage are precisely bad in small details.
Let's start with bolt spacing. If the spacing is too large, the flange edge between the two bolts will "bulge", the sealing strip will not be compressed, and air will leak directly. Generally, the bolt spacing of rectangular non-metallic expansion joint interface is relatively stable within 120mm, and the circular shape can be appropriately relaxed to 150mm. This data is not determined by patting the head. There is a statement in JB/T 12235-2015. When you select the model, you don't mention the spacing, but some manufacturers will give you 200mm row, saving bolt money and pitting your air leakage rate.
Let's talk about the sealing surface. The interface of the flange should be padded with a sealing gasket or sealant, but some of them are installed on site to save trouble, and the flange surface is uneven and not treated. As a result, when the bolts are tightened, the skin edge is twisted, and the sealing gasket is squeezed out. There is also the bolt tightening sequence, which must be twisted alternately diagonally and twisted in two or three times to the torque value. Just screw it off in a circle? The flange surface must be skewed, and leakage is inevitable.
Examples of interface selection under different working conditions: desulfurization flue gas, cement kiln and power station flue
It's boring to just talk about theory. Let's take the actual working conditions and dismantle it.
Desulfurization flue gas: The medium contains SO₂ and chloride ions, strong corrosiveness, low temperature but large pressure fluctuation. In this kind of working condition, I generally recommend the non-metallic expansion joint of butt welding joint, plus the idea of lining PTFE hose for corrosion prevention. The welded structure avoids the corrosion of bolts and kills them. The flange bolts rusting into a lump after two years can't be disassembled. We have done the expansion joint matching the desulfurization flue gas baffle door, the interface is welded to death, and the skin is made of acid-resistant PTFE fabric. There has been no leakage point after five or six years of operation.
Cement kiln: High temperature, high dust, and abrasiveness. The temperature of the kiln duct can reach above 900℃, and the skin must be high-temperature resistant silicone cloth + ceramic fiber. What about the interface form? Most flanged flanges, but the bolts must be made of high-temperature resistant alloys, and the flange surface should be stopped to prevent dust from drilling in through the gaps. Frequent maintenance of cement plants, flange can be removed than welded friendly. We give the flange flange matched by the metal corrugated expansion joint in the cement industry, and the bolt spacing is encrypted to 100mm, just for fear of dust wearing through the sealing strip.
Flue of power station: Typical large cross-section, low pressure, large thermal displacement. The compensators on both sides of the flue gas baffle door are basically rectangular non-metallic expansion joints, and flanged flanges are recommended for the interface. Because of the large size of the flue, welding can't be done, so it can only be connected with bolts. However, the air leakage rate of the flue of the power station has been assessed, so the sealing strip should choose temperature-resistant and aging-resistant silicone rubber, and ensure that the flatness of the flange surface is within 2mm during installation. In addition, the outer skin bulge should be paid attention to in the negative pressure condition of the flue, and the pressure strip should be added to fix the interface.
The most common mistakes at the interface during installation, and how to check and accept them
The level of the installer can directly determine the life of the expansion joint. There are a few of the most common mistakes made at the interface, and you can compare them.
Do not remove transport fixtures. The skin of many non-metallic expansion joints is fixed by pressing strips when they leave the factory to prevent transportation deformation. If you forget to dismantle it when installing it, the expansion joint can't move, which is equivalent to buying an iron knot.
The bolts are tightened too tight. The tighter the bolt on the flange is not the better. The skin is a flexible material, and the strip is deformed by overscrewing, but the seal is not tight. Generally, the torque is according to the value given by the manufacturer. If it is not given, refer to the M10 bolt 25N·m.
Skin burned during construction of welded joints. Sparks splash on the skin during butt welding, and a small hole can scrap the entire expansion joint. Therefore, the skin must be covered tightly with asbestos cloth before welding.
How to check and accept? Teach you three tricks. 1. Visually inspect whether the bolts at the interface are complete and symmetrically tightened, and whether there are obvious gaps on the flange surface; Second, check the gap of the flange sealing surface with a gauge, and it has to be re-adjusted if it exceeds 0.5mm; Third, if conditions permit, do a lighting or smoke test. After the air duct is sealed, light a flashlight on the outside of the interface to see if there is any light leakage inside. If there is light leakage, it means that the sealing strip is not pressed properly, so rework it.
The selection of non-metallic expansion joint interface depends not on which "advanced", but on whether the working conditions can be used. Flanged flange, butt welded, clamp type, each with its own track. You should make the interface form clear first, and then talk about skin materials and anti-corrosion. Don't reverse the order. If you are still unsure, throw the parameters of medium temperature, pressure, dust concentration and displacement over, and we will calculate it carefully.