From Fabric to Multilayer Composite: Material Evolution and Core Structure Dismantling for Nonmetallic Compensators
To talk about the non-metallic composite layer of the compensator, we have to find out how it came from first. In the early days, the compensation of industrial pipelines all relied on metal bellows, but when it encountered high temperature and corrosive media, the metal was like entering a meat grinder-fatigue, intergranular corrosion and frequent replacement. Later, someone put their mind on the fabric, and the prototype of the non-metallic compensator came into being. To put it bluntly, it is to replace the rigid displacement of metal with flexible materials.
Nowadays, the structure of non-metallic expansion joints (fabric fiber expansion joints) on the market is not a layer of cloth. The standard configuration is: the outermost layer is silicone coated glass fiber cloth, which is responsible for temperature resistance and oxygen insulation; The intermediate layer is a polytetrafluoroethylene film or rubber sheet to prevent leakage; The inner layer is a ceramic fiber blanket reinforced by stainless steel wire mesh, which can carry high temperature; Further inside there may also be a PTFE membrane specifically designed to deal with corrosive gases. Only after this combination of punches can the non-metallic composite layer of the compensator be considered complete.
How to bond between composite layers? Instead of simply pressing together, high-temperature vulcanization process is used to make the layers blend into one. Otherwise, after three months of running, it will be bulging in layers. Who do you want to cry? I met a customer two days ago. After buying it back and using it for two months, it began to leak. When I opened it, I found that the layers were not stuck firmly at all, which was a typical cutting corners.
The game between metal and non-metal: Why do high temperature and corrosion conditions rely more on non-metal composite layers?
The metal expansion joint performs well below 300℃. If it is higher, the stainless steel will start to creep, oxidize and peel. And guess what? The non-metallic composite layer has become the main force at this time. Ceramic fiber can withstand above 1000℃, and with silica gel coating, the long-term operating temperature can cover 600℃-800℃. Isn't this just needed for power plant flue and cement kiln tail pipe?
Not to mention corrosive operating conditions. There are sulfuric acid and sulfurous acid in the desulfurization flue gas, and the metal bellows will be perforated in one year. However, the PTFE layer in the non-metallic composite layer of the compensator is almost indifferent to all acids and bases. In addition, the fabric fiber itself is not conductive and does not produce electrochemical corrosion, and its life is directly thrown away from metal by a few streets. But there is one thing: non-metals can't stand high pressure, and if it exceeds 0.1MPa, metal reinforcement layer or limiting structure must be used, which is the bottom line of design.
Therefore, not all working conditions are non-metallic. Low pressure, high temperature, corrosion, these three together two, it is worth using it. Otherwise, general corrugated expansion joints or metal rectangular expansion joints are more hassle-free.
Rectangular versus circular, rubber versus PTFE: Selection logic and misunderstanding of different composite layer designs
Mix rectangular non-metallic expansion joints with round ones. The flue with rectangular cross section is usually used with flue gas baffle door and circular baffle door (double seal). The non-metallic composite layer of its compensator must be made into a rectangular frame with flanged edges, and the wrinkle absorption amount should be reserved at the four corners. Circular pipes are easy to handle, direct sleeve type or butt type. Some design drawings use circular compensators for rectangular flues, but they have to add reducer pipes. Who carries the pressure loss and installation trouble?
Let's talk about rubber compensator vs rubber PTFE compensator vs PTFE compensator. The simple rubber compensator is suitable for water, weak acid and alkali, and the temperature is below 70℃. Once the medium has a strong solvent or the temperature is above 150℃, the rubber can't bear it, and it must be replaced with PTFE composite layer or all PTFE. Many people chose rubber cheaply. After a few months, the surface cracked and leaked. Finally, they found that the saved money was not enough to change the labor cost three times. Tsk, why bother.
It is believed that the thicker the thickness of PTFE, the better. In fact, too thick PTFE layer will affect the elasticity and reduce the ability to compensate for displacement. Generally, 0.5mm-1.5mm is enough, but only thickened to 2mm under special corrosion conditions. The key is to fit firmly with the fabric layer, otherwise it will delaminate.
What is the key to the longevity of non-metallic expansion joints (fabric fiber expansion joints) vs. rubber compensators?
The lifetime depends on the anti-aging ability and interlayer bond strength of the composite layer. What is the outer silicone of fabric fiber expansion joints most afraid of? Ozone, UV. If it is installed in the open air without shading, it will crack in two years. The inner ceramic fiber is afraid of moisture, and once the condensed water seeps in, the fiber becomes mud. Therefore, regular products will add a moisture-proof insulation layer between the inner and outer layers.
The life of the rubber compensator mainly depends on the rubber formula. Natural rubber is not resistant to acids and alkali, butyl rubber is heat resistant but has poor elasticity, and fluororubber is expensive for everything. When selecting the type, it depends on the medium temperature and chemical composition. Don't expect one kind of rubber to dominate the world. For example, in the desulfurization system, the pH value of acidic condensate is as low as 1, and ordinary rubber swells in one month, so it has to be lined with chlorosulfonated polyethylene rubber or PTFE.
In addition, pre-deformation during installation is critical. The non-metallic composite layer of the compensator is generally in a relaxed state when it leaves the factory. During installation, it should be pre-stretched or pre-compressed by 20%-30% according to the design displacement, otherwise it will be broken or wrinkled and stuck during operation. In this respect, JB/T 12235-2015 has clear requirements, but many sites don't look at them at all.
Project Example: Common Failure and Countermeasures of Non-metal Composite Compensator for Flue Gas Baffle Door of Desulfurization
Tell me a practical case. In the desulfurization system of a power plant, the flue gas baffle door is double-sealed single-axis circular baffle door, and a non-metallic expansion joint (fabric fiber expansion joint) is equipped at the inlet and outlet. After 18 months of operation, one of the compensator nonmetallic composite layers experienced a lateral tear, resulting in a flue gas leak. When I removed it, I saw that the ceramic fiber in the inner layer had been broken into slag.
In the start-stop stage, the temperature of desulfurization flue gas fluctuates greatly (from the ambient temperature to 180℃), and the water vapor inside the composite layer has no time to discharge, and the vapor pressure spreads the interlayer. The countermeasure is very simple: add a layer of stainless steel wire mesh air guide layer on the inner surface of the composite layer, or open small exhaust holes. In addition, when the baffle door is closed, the axial compression of the compensator exceeds 150% of the design value, which is because there is no sufficient clearance during installation. Therefore, the compensator equipped with the desulfurization flue gas baffle door must be clearly marked with the maximum compression amount and temperature gradient range.
Another failure mode is flange surface corrosion. At the connection between non-metallic composite layer and metal flange, due to frequent moisture, crevice corrosion leads to bolt failure. Treatment method: Epoxy resin sealant on flange surface, stainless steel for bolts, and anti-loosening washer.
Installation, maintenance and standards: How to inspect and accept non-metallic composite products under JB/T 12235-2015
No matter how good the material is, it is useless if the installation is not standardized. JB/T 12235-2015 is the current national standard of non-metallic expansion joints. During acceptance, several points should be died: first, see whether the composite layer has bubbles, delamination and mechanical damage, and check with naked eye and hand feel; Secondly, measure the flatness of flange surface, and the error does not exceed 1.5mm; Then check the displacement, temperature and pressure parameters on the design drawings to match the product nameplate.
During installation, it is prohibited to tap the surface of the composite layer with hard objects. The bolts connecting the flange should be tightened evenly and diagonally, and the torque should be according to the manufacturer's requirements (generally, the torque of M16 bolts is 80-100N·m). The maintenance cycle is recommended once a year, mainly to check whether there is bulging, degumming, and loose flange bolts between layers. If there is a suspicious area, pierce the detection depth with a probe, and replace the stratification immediately if it is found.
By the way, many sites treat compensators as disposable consumables, don't inspect them, and wait until they leak before replacing them. In fact, regular maintenance can double the life span. For example, coating a layer of protective wax on the outer surface of non-metallic composite layer can significantly delay UV aging. It doesn't cost much, and the effect is obvious.