Who in the dry plumbing business hasn't seen a metal expansion joint? Universal corrugated expansion joint and high-temperature axial expansion joint, these metal parts are indeed the main force in steam and hot water pipelines. But if you put it on the smoke pipe, it will crack for you in less than three months. Why? Because of some working conditions, metal bellows are inherently unable to bear it. At this time, you have to rely on a non-metallic compensator-but don't rush to think of it as a "cheap stand-in", which solves the problem that metal parts can't handle at all.
High temperature and corrosion resistance: the natural enemy of metal bellows, a special session of fabric fibers
The temperature of the flue gas pipeline of thermal power plant, the tail of cement kiln and the original flue of desulfurization system always rises to hundreds of degrees, and the medium is full of sulfide and chloride ions. In this environment, pitting and stress corrosion cracking of metal bellows are common phenomena. If you change to 316L or 254SMO, the cost goes up, but the lifespan is still not good.
The same cannot be said for non-metallic expansion joints (also called fabric fiber expansion joints). Its flexible fabric layer is laminated layer by layer with high-temperature resistant materials such as ceramic fiber and glass fiber, which can steadily hold high-temperature smoke and reduce the surface temperature. A thermal power customer told me that the original metal expansion joint was replaced with a non-metal one once every six months, and there was no air leakage after three years of use.
Non-metallic expansion joint ≠ rubber compensator ≠ PTFE compensator. The temperature resistance of rubber compensator generally does not exceed 100℃, the polytetrafluoroethylene compensator has first-class corrosion resistance but weak pressure, while the energy saving of fabric fiber expansion can be carried above 1000℃, but it can hardly bear the pressure. Before selecting, distinguish whether your pipeline is high-temperature flue gas or corrosive liquid. Don't mix it.
Large displacement compensation and low rebound force: rectangular pipes can only rely on it
For pipes with large thermal expansion, metal compensators are not impossible, but you have to calculate how many waves and how long the guide tube is needed. What if I don't have enough space? Especially for rectangular smoke ducts, have you ever seen a metal bellows with a square section? Even if you do it, that stress distribution is enough for you to have a headache.
The non-metallic compensator has low stiffness, and a fabric loop can absorb tens of millimeters of multidimensional displacement, which can be digested in axial, transverse and angular directions. In the flue gas system of the flue duct of the power station and the tail of the cement kiln, the thermal expansion of the pipeline is so large that the metal compensator can't be put down at all, and the rectangular non-metallic expansion joint is easily handled by the multi-layer structure-the outer fabric holds the medium, the middle insulation layer insulates the heat, and the inner deflector protects the fabric from being washed by the airflow. Think about it, a rectangular air duct is several thousand millimeters wide and has an expansion of several hundred millimeters. How can metal parts compare with it?
One more word, the reaction force of the non-metallic compensator on the pipeline is extremely small, which means that the load of the bracket and the fixed point is small, and the civil foundation doesn't have to be so bulky, which can save a lot of money.
Vibration isolation, noise reduction and tightness: Invisible helpers for operational stability
Pipeline vibration and noise are pain points in the operation and maintenance of power plants and cement plants. When the fan started and the baffle door opened and closed, the metal compensator transmitted the vibration hard, and after a long time, the weld cracked and the flange loosened. The damping characteristics of non-metallic compensators are inherently superior. They can absorb vibration energy and suppress both noise and shock.
And the non-metallic compensator and the baffle door are natural partners. You look at the flue gas baffle door, round baffle door (double seal), electric plug-in insulation door of the desulfurization system, and the flexible connections on both sides of these devices are almost exclusively non-metallic expansion joints. Why? Because the baffle door needs a certain degree of freedom when it moves, the non-metallic compensator is soft and not stuck, and it can also ensure the sealing performance of the system, and the door frame will not be squeezed and deformed due to thermal expansion and contraction. If a metal compensator is used, the displacement when the door panel opens and closes will easily break the bellows.
Pits that are easy to step on when selecting models: Don't just look at the maximum compensation amount on the sample
The non-metallic compensator looks simple in structure, and there are many examples of rollover during model selection.
- Pressure rating: The fabric layer is not a steel plate, and the force it can withstand under positive pressure is limited. If the pressure exceeds 0.1MPa, you have to choose carefully. Use non-metals hard in high-pressure conditions, which is a joke about safety.
- Upper temperature limit: If the sample is written at 500℃, do you think everything will be fine? The medium temperature and the surface temperature of the fabric are two different things. If the thickness of the insulation layer is not correct, the outer surface will still be hot, and the inner fabric will still age.
- Media composition: There are acid and alkali components in the flue gas, the wrong choice of fabric material, and the corrosion is faster than metal. If it contains tar and dust particles, it is necessary to add a deflector plate and a purge port.
- Installation pre-deformation: Non-metallic expansion joints generally have preloading or preloading when they leave the factory. If the on-site installation can't match the temperature, they will be directly hard-installed, and the compensation amount will be completely wasted.
In addition, refer to the national standard JB/T 12235-2015 for non-metallic expansion joints. Don't just focus on the maximum compensation amount on the sample, but also the fatigue life and flange load have to be counted. Some manufacturers give samples that only write "maximum displacement", but that is the limit value, not the allowable value under fatigue life. You choose according to the limit value, and the fabric will crack within a few months of installing it. When the time comes, you will stop working and change parts, and the loss will be great.
To be honest
Non-metal compensators are not universal pieces-high-pressure steam pipes require high elastic reaction force. If you hard on non-metal, it will definitely pull your crotch. On the other hand, non-metals are given priority to pipelines and equipment connections with high temperature, large displacement, low pressure and corrosive media, and the right choice can save a lot of operation and maintenance costs. This is a multiple choice question, not a true or false question. Figure out the working conditions and calculate the parameters. Non-metallic compensators can save you not only money for spare parts, but also the awful time of being woken up by the phone in the middle of the night to deal with leaks.