Before choosing the non-metallic square expansion joint, make clear these four things
As soon as the name of non-metallic square expansion joint came out, those square and large-section pipes of power plant smoke duct and desulfurization system immediately appeared in the minds of knowledgeable people. This kind of expansion joint is highly photographed in electric power, steel and chemical projects, but not many people really understand it. Today, break up these four things and explain them clearly, so you can avoid a lot of detours when choosing a model.
First, what problem does the non-metal square expansion joint solve? Compared with the metal rectangular expansion joint, what is the difference
Metal rectangular expansion joints can do the work, and most non-metal square expansion joints can be connected; But on the other hand, metal parts may not be able to withstand what non-metallic expansion energy saving does.
The core difference is in the compensation mechanism. The metal rectangular expansion joint absorbs displacement by corrugation deformation, can bear higher pressure, but its fatigue life is limited times. However, the rectangular non-metallic expansion joint is compensated by the flexible deformation of fabric fibers, and there is no concept of fatigue life. As long as the material does not age and wear, it can basically follow the equipment to the overhaul cycle.
Let's spread out a few more hard indicators. In terms of temperature, the metal rectangular expansion joint should be added with heat insulation layer and guide tube at high temperature, which has a heavy structural weight and a sharp increase in cost; The non-metallic square expansion joint is made of ceramic fiber as heat insulation layer, which has a temperature resistance of over 1000℃ and low thermal conductivity. In terms of pressure, metal rectangular parts can withstand tens of kPa or even higher, while non-metal parts are generally designed within tens of thousands of Pa. However, most working conditions in the flue duct system do not require such high pressure at all. There is another point that is often overlooked: the stiffness of non-metallic square expansion joints is extremely low, and the reaction force on pipe supports is an order of magnitude smaller than that of metal parts, so the bracket design can save a lot of trouble.
Therefore, the first multiple-choice question in the selection is not "which is better", but "what exactly does this system bear". Low pressure, large cross section, high temperature flue gas, multi-directional displacement-these scenes are the main battlefields of non-metallic square expansion joints.
Second, fabric fiber layer, heat insulation layer, stainless steel wire mesh... what work do these structures carry respectively
Take apart the cross section of a non-metallic expansion joint (fabric fiber expansion joint), and you will see several layers of material, each with its reason for existence. Don't expect a layer of cloth to overcome the world.
The outermost layer is a layer of fabric fibers, usually coated with silicone rubber with a fiberglass cloth or a fluorine rubber cloth. It is responsible for airtightness, blocking smoke leaks, and at the same time resisting corrosive media in the environment. The choice of this layer directly follows the medium-the acidic flue gas in the desulfurization system, you have to use fluororubber coated cloth with stronger acid resistance, so don't be greedy for cheap.
In the middle is the insulation layer, which is generally made of ceramic fiber felt or aluminum silicate cotton. It carries the temperature, blocking the high temperature of hot smoke from the outside of the structure. The thickness of the insulation layer is not determined by patting the head. It depends on the smoke temperature, the allowable temperature of the outer wall of the pipe, and the temperature resistance limit of the material of the inner skin. Only when these things are clear can the thickness of the insulation layer have a basis.
Don't ignore this layer of stainless steel wire mesh. Many engineering accidents are due to it-the wire mesh plays a reinforcing role to prevent the fabric layer from bulging and tearing under positive pressure. For rectangular cross-section pipes with slightly higher pressure, the wire mesh layer has to be added to multiple layers or partially strengthened at the four corners.
The stainless steel skin of the inner layer, usually a thin sheet, is directly next to the hot smoke. It does not undertake sealing, but can reduce the direct heat radiation and erosion of high temperature smoke to the fabric layer. Many non-metallic expansion joints are damaged, not burned through, but the smoke directly burns the fabric layer behind the skin after it is blown through.
Therefore, when looking at the structure, we can't only look at the number of layers. The quality, thickness and collocation of each layer of materials determine how long this expansion joint can last on the spot.
3. How to determine the on-site working conditions: temperature, pressure, medium corrosiveness and displacement, none of which can be leaked
Fill in one wrong selection parameter, and the back is full of pits. This is not an exaggeration. The accident case of turning over the expansion joint is most likely due to the fact that the working conditions are not clearly mentioned.
Let's start with the temperature. Continuous operating temperature, instantaneous peak temperature, and temperature distribution uniformity are distinguished. In some systems, the temperature rises sharply when the furnace is turned on and off, and the ceramic fiber insulation layer is fine, but the aging rate of the fabric layer will be significantly accelerated. If the flue gas temperature fluctuates in a large range, it must be calculated according to the highest temperature instead of the average temperature when selecting the model.
Pressure is easy to understand, but many people ignore "pressure pulsation" and "negative pressure conditions". The air duct behind the induced draft fan may be slightly negative pressure. At this time, the expansion joint is subjected to external pressure, and the problems of structural stiffness and deflation of the sealing layer should be considered. Never use the design of positive pressure condition to directly set negative pressure.
The corrosiveness of the medium is the most prone to problems. The wet flue gas in the desulfurization system contains corrosive components such as SO₂, SO₃ and Cl⁻¹, and the pH and condensation temperature should be considered comprehensively in the material selection. Common Myth: Just look at the media name, not the actual concentration and temperature combination. The corrosion rates at 80°C and 150°C are completely different things for the same media.
The amount of displacement is the most easily underestimated. The square expansion joint usually absorbs axial displacement, transverse displacement and angular displacement at the same time, and the displacement in these three directions does not occur independently in operation, but is superimposed. In addition to the displacement value of normal working conditions, the displacement of extreme working conditions such as wind load, earthquake and equipment foundation settlement should also be put out. Leaving more margin is not a bad thing.
4. Three details that are most prone to rollover in installation and maintenance: direction of guide tube, cold pull amount and frame bolts
No matter how detailed all the previous selection work is, the installation link is as useless as it is. I have seen more on-site rollover cases, and summed up three high-frequency minefields.
Install the guide tube in reverse direction. The function of the guide tube is to guide the flow of the medium, reduce turbulence and wear, and at the same time protect the heat insulation layer from being directly washed by the air flow. When installed, the opening direction of the guide must be toward the direction of the medium flow-that is, the large end is upstream. Install it backwards, which can increase the resistance at the slightest, and the airflow can directly scour the inner wall of the expansion joint at the worst, and the skin and insulation layer can be penetrated in a few months. The specific function of the expansion joint guide tube, in detail, can also prevent the accumulation of solid particles and reduce flow-induced vibration, but the premise is that the direction is right.
Improper handling of cold pull quantity. Non-metallic square expansion joints are generally pre-stretched (i.e. cold-drawn) when they leave the factory, so that the expansion joints can establish a certain amount of pre-compression under low-temperature installation, so that the displacement during hot operation is within the allowable range. Some on-site drawings save trouble, directly remove the fixing parts for transportation and install them hard, and the amount of cold drawing is completely not adjusted according to the drawings. The result is that the pipe has not yet reached its design temperature, and the expansion joint has been jacked to death.
"One-time tightening" of frame bolts. Don't underestimate this detail. The frame bolts are connected to the expansion joint and the pipe flange, which will be loosened due to thermal expansion and contraction during operation. The correct method is: first symmetrically preload during installation to achieve the design torque; After the operation heats up, make up again while it is hot; Check again every other week at the beginning of the run. In many leakage accidents, it was found that the bolts were not tightened again and the sealing surface was detached.
One more sentence-don't rush to work before installation, first match the product certificate, installation instructions and design drawings, and confirm that the model, batch and technical parameters are consistent before starting. This is not a waste of time, and it can help you avoid low-level incidents such as suppliers shipping the wrong model. Anyway, in the engineering business, one more confirmation and one less rework.
A few days ago, I met a customer. The flue expansion joint leaked after less than a year of use. When I removed it, I found that the direction of the guide tube was reversed, and the insulation layer was washed out of a hole by the flue gas. Check the drawings, they are clearly drawn on them. You say this can blame the poor quality of the product? Obviously not. Only when installed, used and maintained well can the equipment work for you to the design life.