What are exhaust non-metallic expansion joints? What's the difference with metal expansion joints?
To put it bluntly, the exhaust non-metallic expansion joint is a soft connector used in the exhaust pipes such as flue and air duct to absorb thermal expansion and contraction and compensate for displacement. Structurally, it is different from the common metal bellows: the main body is fabric fiber (such as silicone cloth, fluorine tape, glass fiber cloth) plus multi-layer sealing layer, and a layer of stainless steel wire mesh or sheath may be wrapped outside. The metal expansion joint, whether it is a general-purpose corrugated expansion joint or a high-temperature axial expansion joint, is a corrugated pipe pressed out of stainless steel sheet, and the elastic deformation of the corrugation compensates for the displacement.
What is the biggest difference between the two? Two words: temperature resistance and compensation ability. The metal expansion joint can withstand the high temperature of seven or eight hundred degrees, but its compensation direction is single (you have to choose axial and transverse directions), and the thrust (elastic reaction force) on the pipe is relatively large. What about exhaust non-metallic expansion joints? The material determines that it is usually used below 600℃ (special coatings can reach 1000℃), but it can absorb axial, transverse and angular displacements at the same time in three-dimensional space, and the elastic reaction force is extremely low-almost no "push" force on pipelines and equipment. Let's just say that the metal expansion joint is like a stubborn spring, while the non-metal expansion joint is like a soft rubber cloth, but don't underestimate this cloth, which has a fine laminated structure behind it.
Why are non-metallic expansion joints always used in exhaust pipes-three hard indicators
Engineers who have worked in power plant desulfurization and cement kiln tail dust removal know that the most headache in the exhaust pipeline is not high temperature, but three problems: large diameter, low pressure and complicated pipeline direction.
First, the caliber is so large that metal can't be made.The diameter of the exhaust flue is often two or three meters, or even five or six meters. The metal expansion joint is so big that the mold cost alone is frightening, and the transportation and installation are even more nightmares. Non-metallic expansion joints are spliced in sections, and the on-site installation is flexible. Rectangular non-metallic expansion joints are used for rectangular flues and non-metallic expansion joints (fabric fiber expansion joints) are used for round flues, and the cost is only 1/3 to 1/2 of that of metal expansion joints with the same diameter.
Second, corrosion and wear resistance.There are often sulfide, acid and alkali steam and dust particles in the exhaust system. I've seen too many cases of corroded and perforated metal bellows. The fabric layer of non-metallic expansion joints is flexible protection in essence. Materials such as fluororubber and polytetrafluoroethylene compensator are naturally corrosion-resistant. Some multilayer structures also add wear-resistant layers, so they are not afraid of dust erosion.
Third, vibration isolation and noise reduction.As soon as the fan and supercharger were started, the whole pipeline was shaking. The metal expansion joint will transmit the vibration intact, while the non-metal expansion joint can effectively absorb the vibration due to the damping characteristics of the material, and the noise can be reduced by 5-10 decibels. This is especially obvious in air-cooled islands and desulfurization systems.
How many of the three easiest pits to step on when selecting models?
There are too many pits. I'll list three of the worst.
Pit 1: Only focus on temperature resistance parameters, not pressure.Many purchasers choose conventional silicone cloth non-metallic expansion joints when they see the working temperature of 350℃. As a result, there is a slight positive pressure in the pipeline (for example, 0.05MPa), which bulges and tears directly. The pressure bearing capacity of non-metallic expansion joints depends on the skeleton material (clamping wire, Kevlar) and the number of layers. Even if the pressure is only 0.1MPa, the structure with reinforcing ring must be used. Before choosing, turn over the JB/T 12235-2015 standard, which has a clear pressure classification.
Pit 2: Ignore the guide tube.The straight pipe section is okay. If there are elbows or reduced diameter in the pipe, the smoke directly washes the fabric layer of the expansion joint, and it will wear out in a few months. A guide tube (or lining tube) must be installed to allow the airflow to travel along the guide tube without directly contacting the fabric. The specific function of the expansion joint guide tube we wrote in the question and answer, that is, it is used to save life.
Pit 3: Set the compensation amount at will.Many people are too lazy to calculate the thermal displacement of the exhaust pipe, so they directly pat their heads to set ±50mm. During the actual installation, it was found that the thermal expansion direction of the pipeline was inconsistent with the layout direction of the expansion joint, and the sealing layer was torn by hard pulling. The compensation amount must be calculated in combination with the direction of the pipeline and the position of the fixed bracket, so don't save this step.
Installation and routine maintenance: Don't let a small detail ruin the entire system
Two days ago, a customer called and said that the non-metallic expansion joint exported by their desulfurization tower leaked after half a year. I looked at the photos-I forgot to remove the transport screw (mounting tie rod) when installing it. This thing is temporarily fixed to prevent deformation in transit and must be loosened or removed after installation. Does the screw of the expansion joint need to be removed? The answer is yes, unless it is a limit type product with a tie rod. If it is not disassembled, the screw will press when the pipe expands, and the compensator will be directly scrapped.
-Check whether the flange bolt is loose every three months (it is easy to loosen due to large temperature fluctuations, so twist it again)
-See if there is any local bulging, wear and whitening on the surface of the fabric (whitening means overburning)
-Pay attention to draining condensed water-there is water at the bottom of many exhaust pipes, and the bottom edge of non-metallic expansion joints soaked for a long time will corrode the flange sealing surface.
If you are using a non-metallic expansion joint with insulation, also check the insulation for moisture, once the insulation fails and the surface temperature exceeds the material limit, the fabric layer will age rapidly.
Real Cases Down: Lessons from a Failure in Desulfurization System
The year before last, I went to a thermal power plant to deal with the fault, and the non-metallic expansion joint of the inlet flue of the desulfurization tower was damaged in a large area in less than a year. The owner has always suspected that it is a product quality problem. When we removed it, we saw that the inner layer of fluororubber had been powdered, and the middle glass fiber cloth was all carbonized. Asking about the working conditions again, the flue gas temperature is designed to be 180℃, but in actual operation, it often rushes to 280℃ instantaneously-because the GGH (flue gas heat exchanger) in front of the desulfurization system is often bypassed. The maximum temperature resistance of the expansion joint they selected is only 200℃.
What does this case mean? When choosing the expansion joint, you can't only look at the design temperature, but also the peak temperature. There is a safety margin for the temperature resistance of non-metallic expansion joints, but if you have a high frequency of overtemperature in your working condition, either upgrade the material (such as perfluorororubber) or add water spray to the pipeline to reduce the temperature. I later replaced this plant with a non-metal expansion joint with a ceramic fiber liner and suggested they install an overtemperature alarm interlock. It has been two years now, and there have been no problems again.
You see, the failure of the exhaust pipe is most likely not that the product itself is not good, but that the working conditions and the selection are not matched. So my suggestion is just one sentence: tell the manufacturer your real working conditions (temperature, pressure, medium, displacement, operating cycle) completely, and don't hide them. Manufacturers have the opportunity to recommend the most suitable non-metallic expansion joint to you-whether it is fabric fiber type or composite type, even if the price is more expensive, it is better than changing it once every six months.