On the side of the smoke duct of power plant or the ash discharge pipe of cement plant, you often see a flat "cloth bag" joint with bolts pressing the flange. That is the non-metallic compensator, also called the non-metallic expansion joint (fabric fiber expansion joint). Many purchasers ask with pipeline parameters: What is a non-metallic compensator? What's the difference from the metal expansion joint? Let's say that it is a flexible section that can absorb thermal displacement, vibration and installation errors of the pipe, but the main force is not a bellows, but a layer of composite fabric skin.
Start with the name: What is the difference between metal expansion joint and corrugated expansion joint?
Metal expansion joints and corrugated expansion joints, essentially the same thing, absorb displacement by the deformation of stainless steel bellows. They have strong pressure bearing capacity and high temperature resistance, making them particularly useful on circular steam pipes. However, on large-section rectangular pipes such as smoke ducts in power plants and ash discharge pipes in cement industries, metal ones are not effective-high cost, limited displacement, and fear of acid-alkali corrosion. Masters prefer to use non-metallic expansion joints for three reasons: they are cheap, corrosion-resistant and can absorb three-dimensional displacement. Especially the fabric fiber skin has great elasticity, and the lateral, vertical and angular displacements can all be eaten. The metal bellows can only give an axial direction, and it will be blinded when it encounters complicated displacements.
Pick open to see the structure: Non-metallic compensators are not as simple as a piece of cloth
When disassembled, the non-metallic compensator consists of a skin (non-metallic elastic material), thermal insulation layer, stainless steel wire mesh, frame flange and flow guide tube. Each layer has a division of labor: the skin is the sealing core, and fluororubber, silicone rubber or polytetrafluoroethylene are commonly used; The insulation layer is made of ceramic fiber blanket to block the high-temperature smoke, otherwise the high temperature will directly burn the skin; The stainless steel wire mesh is the skeleton, which enhances the compression and tear resistance of the skin; Frame flanges are responsible for pipe connections; The guide tube is placed inside to allow the airflow to pass smoothly and avoid the medium directly washing the skin. Rectangular non-metallic expansion joint and circular non-metallic expansion joint have the same structural law, that is, the flange shape is different, and the rectangular one is more suitable for large-section air duct.
Take the working conditions for themselves: Where can big things happen without non-metallic compensators
Behind the desulfurization flue gas baffle door, the flue gas is wet and contains sulfur, and the metal bellows corroded and wore out within a few months after being installed. At this time, the non-metallic skin is on top, and it is resistant to acid and alkali and chloride ions. There are also rectangular air ducts that can't be reached by high-temperature axial expansion joints, such as the flue at the tail of the boiler, which has a large cross section, high temperature and more than one displacement direction, and there is no way for metal at all. What about the rubber compensator? Afraid of high temperature, it will age and crack when it exceeds 150℃. The non-metallic compensator is insulated with ceramic fiber, and the skin surface can carry it to 600℃. Do you know how to choose?
Don't underestimate the selection: JB/T 12235-2015 is full of hard indicators
The national standard JB/T 12235-2015 for non-metallic expansion joints frames the temperature resistance grade, displacement calculation and fatigue life. The standard is divided into several grades according to the working temperature. When selecting the type, the thermal expansion of the pipeline should be calculated first, and then the corresponding fatigue life requirements in the standard should be checked. What to choose for skin material? Depending on the temperature and corrosiveness of the medium, the flue gas has a high sulfur content and a fluororubber composite layer, which ordinary silicone rubber can't bear. What about insulation thickness? According to the inner wall temperature backward, let the outer surface temperature of the skin be controlled in a safe range. There is an on-site case: when selecting the ash discharge pipe in a cement plant, it was greedy for cheap, and the insulation layer was thinned. After half a year's operation, the surface temperature of the skin soared to more than 80 degrees, which directly burned and cracked. Another case is the compensator at the desulfurization inlet. The skin is made of ordinary silicone rubber, which ages and leaks after three months. It stops after replacing the fluororubber PTFE composite skin. The price of the wrong choice is not just as simple as air leak.
Both installation and repair are particular: only when you can install and repair can you use the full life
Install the non-metallic compensator, first look at the direction of the guide tube, which must follow the flow direction of the medium. Install the guide tube backwards for a ride, and the skin will be torn apart after a few strokes. Do you want to do pre-stretching? The non-metallic compensator has low stiffness, so it generally does not need to be pre-stretched like metal, and it can be kept in a free state when installed. But the tie rod nut must be adjusted properly-the tie rod is locked when transported, and it must be loosened after installation, so that the expansion energy saving can expand and contract freely, and guess what? Many sites forget to loosen the nut, the compensator becomes a rigid connection, and the displacement of the pipeline is all stuck in the equipment, which cracks within a few days.
After two or three years of operation, the skin ages and cracks. Look at the damage area first. Small breaks can be repaired locally and treated with special glue and patch cloth; If a large area is torn or the flange is detached, replace the whole skin directly. Compared with the metal expansion joint, the non-metal replacement cycle is short, about 3-5 years, and the metal bellows can last 10 years. However, non-metal is cheap, lightweight, quick to disassemble and assemble, low labor cost, and lower comprehensive maintenance cost in the corrosive smoke environment. Alas, understanding what a non-metallic compensator is is much more important than choosing a model in a daze.