What exactly is a non-metallic expansion joint? Compared with metal expansion joints, where is it stronger and weaker
Non-metallic expansion joints, also called fabric fiber expansion joints, are flexible compensation elements made of non-metallic materials to put it bluntly. Different from the metal corrugated expansion joint that absorbs displacement by corrugated deformation, it mainly depends on the flexibility and interlayer slip of fiber fabric, rubber and polytetrafluoroethylene to digest the thermal expansion and contraction of pipelines. You don't think it is inferior because it has a "non-metal" in its name. In many working conditions, it is much more reliable than metal expansion joints.
Let's start with the advantages. Non-metallic expansion joints have strong corrosion resistance, especially against the acid and wet flue gas behind the desulfurization tower. It won't take long for the metal bellows to pit and perforate, but the non-metallic skin can carry it for several years. Furthermore, its displacement compensation methods are varied. Besides axial direction, it can also absorb a large transverse and angular displacement, and it has low stiffness and small reaction force to pipe support. In addition, it is a composite structure, which has good thermal insulation performance, unlike metal expansion joints, which require extra package insulation.
The pressure capacity is poor, generally from several kPa to tens of kPa, and high-pressure pipelines can't be used; The upper limit of temperature resistance is limited by fiber materials, which usually does not exceed 600℃. Ultra-high temperature conditions can only rely on metal. Another-the non-metallic expansion joint is afraid of being scratched by sharp objects, so it is slightly careless during transportation and installation. It will be a hole if it is rubbed, and it will leak air within a few days of installation.
Huinong's local desulfurization and dust removal pipeline working conditions, why is it more suitable to use non-metal expansion joints
There are many thermal power, steel and coking projects in Huinong. The media in desulfurization and dust removal pipelines are basically sulfur-containing flue gas and dust particles. The temperature is not particularly high, but the corrosiveness and wear are superimposed, which is a test of materials. Metal expansion joint In this environment, the outer wall of the bellows is easily corroded by chloride ion stress, and the inner wall is washed by particulate matter, so it may have to be replaced in one year. The non-metallic expansion joint is different. The acid-resistant rubber and PTFE membrane on the outer layer of the skin can separate the corrosive medium, and the stainless steel wire mesh on the inner layer can withstand dust erosion. In addition, it is a flexible body, so the vibration and thermal displacement of the pipeline are "soft landing" by it. Therefore, non-metallic expansion joints are the mainstream choices for Huinong's local desulfurization island, dust collector inlet and outlet, and fan connection section.
Judging from the actual project, the desulfurization flue of the power plant in Huinong area was originally made of metal rectangular expansion joints, but after two or three years of operation, the weld cracked and the bellows corroded and thinned. Later, it was replaced with a rectangular non-metallic expansion joint, and the double-sealed circular baffle door was used for isolation, so there were much fewer problems. This is not to say that metal must not work, but that the working conditions are not appropriate.
The four parameters to be really fixed when selecting the model: temperature, pressure, medium and displacement
Don't make a mistake in one of these four parameters. If you make a mistake, it will be the fate of rework and scrapping.
Temperature.The skin of the non-metallic expansion joint is a multilayer structure, which is divided into heat insulation layer, temperature resistance layer, sealing layer and outer sheath layer from inside to outside. How much temperature you quote, directly decide to use aluminum silicate cotton or ceramic fiber, fluororubber or silicone rubber. When the temperature is reported to be low, the skin aging accelerates; If the price is high, the cost will go up a lot in vain.
Stress.Don't just look at the design pressure of the pipeline, but look at the actual operating pressure where the expansion joint is located. The desulfurization flue is usually slightly negative pressure or slightly positive pressure, but the positive pressure in the outlet section of the fan may reach more than ten kPa. Once the pressure exceeds the pressure limit of the skin, it bulges like a balloon, and finally rips open with a "pop".
Medium.The corrosiveness of sulfur-containing wet flue gas, dust-containing hot air, clean flue gas and raw flue gas is very different. If it is the net flue gas after wet desulfurization, the acid dew point is low, and the pH value of the condensate is very low. The inner layer of the skin must be made of acid-resistant PTFE membrane or acid-resistant rubber. If there is particulate matter in the medium, the wear layer of the inner liner has to keep up.
Displacement.This is the soul of selection calculation. What is the amount of thermal expansion of the pipe? Is the device interface displacement direction axial or lateral? What is the difference between the installation temperature and the target operating temperature? These data were inaccurate. Even if the skin material was selected correctly, the skin would still be torn apart if the displacement exceeded. Therefore, don't be lazy when selecting the type. Take out the stress analysis report of the pipe system and fill it according to the displacement above.
Details that are easiest to be overlooked during installation and maintenance, if not, they will be scrapped directly
First, the direction of the guide tube. Non-metallic expansion joints are generally equipped with a guide tube, which must face the flow direction of the medium. If it is installed backwards, the medium directly washes the inner fibers of the skin, and it will be worn out in a few days. There are flow arrows on the pipes, so take a look before installing.
Second, it is strictly forbidden to tie the skin directly with wire ropes. You unload the goods from the hook of the crane, and the wire rope is strung on the skin. It looks fine outside, but the inner PTFE membrane has been dented. If you work with injuries, it will undoubtedly leak. Use a sling, or padding a plank.
Third, the bolts cannot be tightened at once. The bolts of the connecting flange should be tightened symmetrically and multiple times, and the torque should be controlled within the standard range. When screwed too tightly, the pressure plate on the edge of the skin fractures the PTFE layer; Screw too loosely, leak air and ash. Also, many projects are installed with expansion joints and ignored. After half a year's operation, the bolts are loose due to thermal cycle, and no one tightens them, so they still leak.
Fourth, after installation, remove the transportation rod for transportation. This thing is used to prevent deformation during transportation. If it is not disassembled, the expansion joint cannot be freely displaced, and its function is completely wasted. There are too many people stepping on this pit.
Common failure modes of non-metallic expansion joints and how to avoid them at the source
Skin corrosion and leakage, joint tearing and cracking, frame corrosion and deformation.
Skin corrosion leakage. When selecting materials, the corrosiveness of the medium is misjudged, or the number of skin layers is insufficient, which leads to the acid condensate penetrating into the fiber layer, and the yellow liquid seeps out from the outer surface over time. The countermeasure is to clearly explain the media composition, pH value and dew point temperature when selecting the type, so that the manufacturer can configure the skin structure according to the actual working conditions.
Ripped and cracked. Most of the displacement is not enough, and the thermal displacement of the pipeline exceeds the maximum compensation amount of the skin, which forcibly cracks the skin. There is also the cold draw during installation, and the pre-deformation amount that should have been adjusted in the cold state was not done, and it directly exceeded the limit when it was heated. The countermeasure is to order strictly according to the displacement given by the design institute, and leave enough cold tightness during installation.
Frame corrosion. The frame and pressure plate of the non-metallic expansion joint are all metal parts, which are in the desulfurization environment. The anticorrosive coating is not thick enough or bruised during installation, and it will be quickly corroded by flue gas. The countermeasure is to choose suitable stainless steel material (such as 316L) or reinforced anticorrosive coating, and the damaged parts must be repainted during installation.
In the final analysis, choosing non-metallic expansion joints is not about the price, but about who knows the working conditions better. Huinong's local project environment is relatively harsh. Find a manufacturer with desulfurization performance, report the four parameters, and keep an eye on the installation details. This thing can run safely for several years. If you just buy a bargain to save trouble and repair it three times a year, that's a loss.