Why do power plant pipes prefer non-metallic compensators? What is it better than metal expansion joints?
If you go to the scene and walk around those large-diameter smoke ducts, dust collector inlet and outlet, and desulfurization tower connections in thermal power plants and thermal power plants, nine times out of ten will be equipped with non-metallic expansion joints (that is, what we often call fabric fiber expansion joints). Why not use metal? It's not that the metal is bad, it's that the working conditions are different. The flue gas pipeline of power plant has high temperature, strong corrosion, and a large amount of dust and vibration. Although the metal corrugated expansion joint has strong pressure bearing capacity, it can't withstand acid-alkali corrosion and frequent thermal fatigue, and the cost is high and the installation space is limited.
Non-metallic compensators have a unique skill-multi-dimensional compensation. It can absorb axial, radial, angular displacement, and even torsional deformation of the pipe can be pocketed. Moreover, it is not deformed by corrugation, but by stretching and compressing the flexible fabric layer, so the thrust on the pipeline is minimal, and the fixing bracket does not need to be made very thick. In addition, the non-metallic body is light in weight, and the high-altitude work is good for construction. But there is a premise: the installation must be standardized, otherwise no matter how good the material is, it will be in vain.
Before installation, calculate the accounts clearly: temperature, pressure and displacement. If one parameter is wrong, all will be dry
I met a customer two days ago, and the non-metallic compensator tore less than three months after being installed. After checking, the design temperature was 150℃, and the actual operating temperature soared to 200℃. The upper limit of temperature resistance of the fabric layer was broken, and aging and cracking. You say it was wrong or not? So three core parameters must be checked before installation:
- Temperature:The temperature resistance grades of non-metallic expansion joints are divided into many grades. Ordinary silicone cloth can withstand 250℃, fluororubber cloth can withstand 300℃, and ceramic fiber composite layer can withstand more than 1000℃. Be sure to look at the maximum temperature of the actual medium (including the instantaneous peak), not only the design value.
- Pressure:Non-metallic compensators are generally used in low-pressure pipelines (usually ≤0.1MPa), but the pressure in the outlet section of fans in some power plants may reach 0.2MPa. At this time, a structure with reinforcing ribs or steel wire rings must be selected.
- Displacement Amount:Many projects only give total displacement, but not axial and transverse. The compensation ability of non-metallic compensator for different directions is very different, so the manufacturer must be required to calculate the compensation amount in each direction according to the actual pipe system, and pre-deformation should be reserved during installation.
Even if one of these three parameters is wrong, the compensator becomes a decoration. Don't expect to rely on on-site adjustment. If you are wrong, change it. You can't afford to delay the construction period.
Practical installation: The direction of the guide tube can't be reversed. Will the screw be disassembled or not?
Non-metallic compensators usually come with a guide tube (also called an inner liner tube), which guides the media stream and prevents high-speed airflow from washing the fabric layer. The installation direction of the guide tube must be consistent with the flow direction of the medium, and the direction of the arrow is the flow direction. If it is installed backwards, the medium directly impacts the fabric and insulation cotton, and it will leak in less than a month.
Let's talk about the screw. When leaving the factory, the compensator usually has transport screws (also called tie rods or limit rods) at both ends, which are used to prevent excessive deformation during transportation and hoisting. Some installation teams saved trouble, and directly welded it with screws. As a result, the compensator completely lost its compensation ability, and the pipeline was deadpulled when it expanded and contracted thermally, which cracked the equipment interface. The correct way to do this is: after installation in place, loosen the screw nut and allow the compensator to expand and contract freely. But pay attention! In some compensators with pressure balance, the screw is a permanent structure (such as a large tie rod expansion joint) and cannot be disassembled. The criteria for judgment are simple: look at the product description, or ask the manufacturer. If the screw passes through the flange ear plates on both sides of the non-metallic body, it is mostly a transportation limit and must be dismantled; If it is the tie rod on both sides of the metal bellows, it may be a load-bearing part, so don't move around.
The 3 installation mistakes that are most prone to rollover-damaged lining, no pre-deformation done, and random welding of fixing brackets
Mistake 1: Broken lining.Non-metallic expansion joints usually have temperature-resistant lining (aluminum silicate cotton felt or ceramic fiber) inside. If they are punctured by tools or splashed by welding slag during installation, high-temperature flue gas will directly pass through the notch and burn the outer fabric after operation. Therefore, it is necessary to check that the lining is intact before installation, and protect it with a fire blanket when welding.
Mistake 2: The predeformation was not done.When the pipe is installed cold, the compensator should be pre-stretched or pre-compressed in the actual displacement direction. For example, when the pipeline is running, there will be an axial elongation of 20mm, so the compensator should be compressed by 10mm (according to the pre-deformation amount given by the manufacturer) during cold installation, so that it will just return to the middle position after running in place, and the compensator will have the longest life. If directly loaded into free state, the maximum compensation amount may be exceeded during operation, resulting in tearing.
Mistake 3: The fixed bracket is welded indiscriminately.The non-metallic compensator itself does not bear pipe weight, it only absorbs displacement. However, in some sites, in order to save trouble, the fixed bracket is directly welded to the flange or connector of the compensator, which leads to force deformation of the compensator. Correct practice: the fixed bracket is welded on the side of the pipeline, the main fixed bracket must be set at one end of the compensator, and the guide brackets must be set at intervals to ensure that the displacement direction is controlled.
How to check and accept after installation? Pressure holding, displacement test and daily inspection are indispensable
The completion of installation is not the end, acceptance is the key. First thing: do a pressure holding test. Although the design pressure of the non-metallic compensator is low, it must be tested for air tightness or hydraulic pressure according to the regulations to check whether there is any leakage on the flange sealing surface. Second thing: displacement testing. Record the data by measuring the actual installed length and pre-deformation of the compensator with a scale before inflating or heating the pipe. After the temperature is raised to the operating temperature, measure again to see if the actual displacement is within the design range. If the deviation exceeds 10%, check whether there is a problem with the pipe support.
What do you see for daily inspections? Look for bulging, scratching, or whitening (signs of aging) on the surface of the fabric layer; Listen for any abnormal whistling sounds (possibly lining falling off); Feel the flange bolts to see if they are loose. The life of a non-metallic compensator is usually 5-8 years, but it can last up to 10 years with good maintenance.
The non-metal compensator looks simple-isn't it just a few layers of cloth and a flange? But every detail is a pit. If the parameters, installation and acceptance are correct, it can serve safely and steadily. Making a mistake is equal to white clothing, and the rework cost far exceeds the little effort saved.