What exactly is Non-Metallic Expansion Joint ffl? Understand its structural logic first
Two days ago, a customer came over with drawings and said that he wanted to order "that kind of cloth expansion joint". When I looked at it, he was talking about non-metallic expansion joints ffl — specifically, fabric fiber expansion joints. The name of this thing sounds fancy, but the structure is actually not complicated: the outside is a temperature-resistant layer and a sealing layer, the middle is a reinforcing layer, and the inside is filled with thermal insulation cotton. When in use, cooperate with the guide tube to separate the high-temperature medium from the ring belt.
Why does it absorb thermal displacement and dampen vibration at the same time? Because it is a "flexible body". The displacement caused by thermal expansion and cold contraction is absorbed by the deformation of the fabric band; The vibration from the operation of the equipment was also eaten up by this composite structure. No matter how the metal expansion joint is designed, the bellows itself is a rigid part, and it can only rely on the elastic deformation of the corrugation to compensate for the displacement. The vibration energy is basically not lost after transmission, and the vibration reduction effect is naturally incomparable.
In terms of structural logic, the stress models of rectangular non-metallic expansion joints and circular non-metallic expansion joints are completely different. Rectangular is mostly used in flue and air duct, with large cross-section and low pressure, which depends on the flexibility of fabric layer and the division of labor and cooperation of multilayer composite materials. This logic is understood, and all the following selection problems are easy to say.
Core advantages: high temperature resistance, corrosion resistance and large compensation
To put it bluntly, the non-metallic expansion joint ffl can stand up by three hard indicators.
The first one is high temperature resistance. Silicone-coated fiberglass cloth can carry it to about 300℃, and fluororubber composite material can carry it to 200℃. If matched with a temperature-resistant layer and thermal insulation cotton, no matter how high the temperature of the flue gas is, the heat can be kept out of the ring belt. If you replace it with metal, you have to choose high-nickel alloy for bellows at this temperature level. Not to mention the doubling of the cost, the fatigue life of thick-walled bellows will drop like a cliff.
The second is corrosion resistance. The net flue gas at the outlet of the desulfurization tower has high humidity and acidic medium, and the spray of condensed water is dilute sulfuric acid. In this environment, metal bellows may pit and perforate in less than a year. The non-metallic ring belt is coated with fluororubber or PTFE, which is resistant to acid and alkali. The surface is smooth and difficult to hang materials, and the influence of corrosive media on it is much less. We have done a comparison, and the non-metallic expansion joint behind the same desulfurization flue gas baffle door can continue to serve after five years of use.
The third is that the amount of compensation is large. The single wave compensation amount of metal expansion joint is only a few millimeters to more than ten millimeters, while the non-metal expansion joint can achieve an axial displacement of tens or even hundreds of millimeters, and the lateral deflection angle is much larger. Think about it, a 30-meter-long flue has a thermal expansion of 150mm based on 5mm/m. You carry it hard on metal bellows? How many waves do you have to connect in series? Non-metallic expansion joints are solved by one.
The easiest pit to step on in selection: not the softer the better
Cold water must be poured here. Many people think that non-metallic expansion joints are made of fabrics. The softer, the more "flexible" and the stronger the compensation ability. The idea is dead wrong.
If the band is too soft, it will bulge under the action of internal pressure. As soon as the pressure goes up, the circumferential stress bearing by the fabric layer directly exceeds the standard, ranging from deformation to collapse. When selecting, the pressure level and structural design should be looked at together. The setting of the guide tube is also very critical-it not only diverts the flow, but also plays a role in isolating high-temperature media and protecting the ring belt. The clearance, insertion depth and fixing mode of the guide tube should be checked according to the working temperature. When the temperature exceeds 300℃, a heat insulation layer must be added outside the guide tube.
Let's talk about flange spacing. The installation length of non-metallic expansion joints is not randomly determined. The flange spacing is too large, the effective length of the ring belt is not enough, and the compensation amount cannot be reached; If the spacing is too small, the thermal displacement will get stuck by the flange edge. We have seen a project where the customer used the old flange in order to save money, and the spacing was 20mm less than the drawing. As a result, the bolt holes didn't match up during installation, and finally reworked it.
The pressure level has to be matched. Although the working pressure of the expansion joint behind the flue gas baffle door is only a few kPa, there will occasionally be positive pressure impact in the desulfurization system. If you select the type according to zero pressure, the ring belt will be broken as soon as the impact comes. It is safe to check at 1.5 times the working pressure.
Installation and maintenance: Bolt pre-tightening is the line of life and death
Install that little thing, talk about it too much is tears. What are non-metallic expansion joints ffl most afraid of? I am most afraid that the bolt preload force is not well controlled.
The preloading force is too small, there is a gap between the flange surface and the ring belt, and the smoke leaks as soon as it runs. If the preloading force is too large, the ring belt will be pressed out of the strangulation mark, the fiber layer will be damaged, and the life will directly shrink. Many on-site workers are done with wind cannons, which is equivalent to joking about the life of expansion joints. The standard practice is to tighten symmetrically with a torque wrench in multiple times, and control it according to the torque value required by the drawing.
The daily inspection looks at three parts. Look at the surface of the loop band for scratches, wear or bulging; Second, see whether there are corrosion marks or media leakage at the flange connection; Third, look at the state of the tie rod nut-the tie rod of the non-metallic expansion joint is used for transportation limit, and it should generally be loosened after installation. There is an important difference with the metal expansion joint here: the tie rod of the metal expansion joint restrains the internal pressure thrust and cannot be disassembled during operation; The tie rod of non-metallic expansion joint only prevents the loop belt from being overstretched during transportation and hoisting, and too much restraining force will hinder the normal displacement during operation. There are quite a few people who have reversed this perception.
National standard and inspection: What are the hard indicators of JB/T 12235-2015
The national standard for non-metallic expansion joints is JB/T 12235-2015. This standard specifies the tensile strength, tear strength, temperature resistance, air tightness and other technical requirements of the loop belt.
There are three tests that are required before leaving the factory. Pressure test to verify that the ring belt and flange connection can withstand 1.5 times the design pressure without leakage. Airtightness test, check the welds and joints for air bubbles with soapy water at rated pressure. Displacement compensation test, simulate the axial and transverse displacement under actual working conditions, and observe whether the ring belt has abnormal deformation.
When purchasing, don't just look at the quotation. Ask the manufacturer for three things: material certificate, factory inspection report (pressure and air tightness data), and type test report (displacement fatigue test results). If these three things are missing the same, you have to weigh it.
One more sentence. Many people don't pay attention to the detail of the sealing surface. The flange sealing surface of non-metallic expansion joint must be smooth and clean, and leveling treatment should be done after welding. If the flange surface is warped and deformed, no matter how big the bolt preload force is, it will not be compacted, and finally air and ash will leak. Always check this before installing.
Non-metal expansion joint ffl This thing looks simple and has a lot of doorways. The working conditions are clear, and the selection, installation and maintenance are according to the rules. It can be used safely and steadily for ten years; If you want to save trouble and choose according to your feeling, I will miss it for you in three months. Is that the truth?