What exactly is a non-metallic rectangular compensator? What's the difference with metal ones?
Don't be confused by the name yet. We call this thing in our stationRectangular non-metallic expansion joint, also called fabric fiber compensator. It is completely different from rubber compensators and metal rectangular expansion joints. The rubber compensator is deformed by elastomer, and the metal rectangular expansion joint is expanded and contracted by corrugation. The non-metallic rectangular compensator we are talking about today uses flexible fabric skin to absorb displacement. In short, it is a joint piece specially designed to "loosen" rectangular pipes.
Compared with metal bellows, what is the difference? Metal bellows can't be made of large-size rectangles-in the process, they are either coiled or stamped. The cost of rectangular bellows is sky-high, and the stress concentration after bearing pressure is severe. Non-metal, on the contrary, the larger the size, the more advantageous it is. It can also absorb three-dimensional displacement (up and down, left and right, front and back), and by the way, eliminate vibration and noise. Tsk, isn't that a little counter-intuitive?
The core structure consists of three components: guide tube, insulation layer and skin
Take apart a non-metallic rectangular compensator, and you will find that it is actually quite straight and white. At the very innermost isguide tubeGenerally, stainless steel or carbon steel is welded and formed, and its function is to guide the airflow smoothly to avoid direct washing of the skin. The deflector must be welded firmly, otherwise the high-speed airflow will break the skin in a few weeks. A layer in betweeninsulation layerTypically aluminum silicate fiber cotton or ceramic fiber, the thickness is determined according to the temperature of the medium. The outermost layer isskinThis is the lifegate of the whole product. The skin is made of multi-layer composite materials. The common ones are silica gel cloth, fluorine tape, polytetrafluoroethylene (PTFE) film, and some are reinforced by stainless steel wire mesh. The temperature resistance level depends on the skin-silica gel cloth at 300℃, fluorine tape plus PTFE layer at 500℃. You ask why not use all fluoroadhesive tape? It's expensive, and silicone cloth has a longer fatigue life. Selection is a trade-off problem.
Why do you have to use this for rectangular pipes?
Two days ago, I met a customer. The desulfurization flue of the power plant has a rectangular section of 3 meters ×2 meters, a working temperature of 180℃ and a pressure of only a few kilopascals. His first reaction was to use a metal rectangular expansion joint. An inquiry, the manufacturer's quotation is twice as expensive, and the delivery time is still long. Why? Metal rectangular expansion joints must be stacked with multiple layers of corrugated plates, and each layer must be stamped and welded. Moreover, large-size corrugated plates are easy to become unstable, so reinforcing ribs must be added. Eventually he changedRectangular non-metallic expansion jointThe cost has dropped by 40%, and the installation is fast. And guess what? I used it for two years and didn't come out for a single leak. Instead, the metal cracked once because of weld fatigue. Therefore, in the case of low pressure and large cross section, non-metal is a solid preference.
The displacement caused by thermal expansion and contraction of rectangular pipes can reach tens of millimeters, and the metal bellows can't absorb the displacement in axial, transverse and angular directions at the same time. The non-metallic skin is a flexible fabric that can theoretically withstand displacement in any direction, as long as sufficient margin is left in the design. In addition, the fabric itself has a damping effect, which can attenuate the vibration of the pipe by 70% to 80%. Noise test data: After installing the non-metallic compensator, the noise near the flue is reduced by more than 15dB (A).
Select the three pits that are most prone to rollover
Pressure rating.Non-metallic rectangular compensatorGenerally, it is only suitable for low-pressure systems, within 0.1MPa. Beyond this number, the skin will bulge or the flange bolts will be pulled off. You have to use it on 0.3MPa pipes? Stop thinking about it, change it for metal.
Temperature. The skin material directly determines the upper temperature resistance limit. Silicone cloth ≤300℃, fluorine cloth ≤350℃, plus PTFE layer can reach 500℃. However, it is noted that the overall temperature resistance of the composite skin is subject to the lowest temperature resistance layer. Some manufacturers claim to be resistant to 500℃ with silicone cloth skin, which is nonsense. You have to ask about each layer of material.
Direction of the guide tube. The deflector is usually inserted into the pipe and must be welded in the direction of the airflow. If it is installed backwards or the weld is not firm, the airflow directly impacts the inside of the skin, and it will be scrapped in a few months. We have seen a case: the guide tube was not welded through during installation. Three months later, the skin was torn open by a hole, and the whole flange surface leaked. The labor cost of replacing a set of non-metallic expansion joints is more expensive than the product itself, so why bother?
Maintenance and longevity: 3-5 years for skin replacement, low cost but don't be lazy
Quite frankly, nonmetals have a shorter lifespan than metals. The design life of metal bellows is generally 10 years, and the non-metal skin has to be replaced in 3-5 years because of aging and wear. But the advantage is that the performance can be restored by changing the skin, without dismantling the whole pipe. Once the metal bellows is cracked, it can only be replaced as a whole. So from a life-cycle perspective, non-metals save money — provided you maintain them according to the rules.
The most overlooked thing when changing the skin is the flange bolt torque. Screwing too tight crushed the skin and loosened the air leak. The national standard "Non-metallic Expansion Joint" JB/T 12235-2015 clearly stipulates the range of bolt preloading force, for example, the recommended torque of M16 bolt is 100-120N·m. In practice, it is recommended to tighten it diagonally in three times with a torque wrench. In addition, after the old skin is removed, the flange surface must be cleaned, and the residual glue slag and rust will affect the sealing effect of the new skin.
In addition, the insulation layer should also be checked. If the insulation cotton is damp or coked, it will affect the temperature resistance. In some projects, in order to save money, ordinary carbon steel is used for the guide tube. As a result, the oxide scale falls off and blocks the insulation layer at high temperature, and finally the skin is partially overheated and burned through. You say it was wrong or not?
So,What is a non-metallic rectangular compensator？ It is a flexible joint in a rectangular pipe system that specializes in coping with thermal expansion and contraction and three-dimensional displacement. The structure is simple, the cost is controllable and the installation is fast, but the premise is that the type selection is not stepped on the pit, and the maintenance is not lazy. Are you in the middle of a model selection or are you already having problems? You might as well check yourself against the above points.