What is a skin? Where is the "life gate" of non-metallic compensator
Anyone who has worked on flue gas pipes knows that once the non-metallic compensator leaks, the whole system has to stop. And the most accidental part is the skin. What do you mean skin? To put it bluntly, it is the layer of flexible material in non-metallic expansion joints (fabric fiber expansion joints) that is responsible for sealing and absorbing displacement. It is usually made of fluororubber, silicone, glass fiber cloth, polytetrafluoroethylene film, etc., laminated one layer after another, like a thousand-layer cake. But don't underestimate this layer of "cloth", which carries the pulling of thermal expansion and contraction of pipelines, the erosion of corrosive media, and the tossing of positive and negative pressures-this is the "life gate" of non-metallic compensators. As soon as the skin breaks, the compensator is crippled.
JB/T 12235-2015 What are the hard indicators stipulated in this standard
The national standard for non-metallic expansion joints in China is JB/T 12235-2015. This standard is not written casually. It details several barriers that the skin must pass:
- Temperature Resistance Grade:It is divided into three categories: A, B and C. The maximum operating temperature of Class A is 250℃, Class B is 350℃, and Class C is 450℃. Beyond this range, the skin material has to be re-selected.
- Pressure resistance:The design pressure is generally 0.1~0.5MPa, but the standard requires that the skin should be kept under 1.5 times the design pressure for 5 minutes without leakage.
- Compensation amount:The axial, transverse and angular displacements must be clearly marked, and the actual tensile and compressive capacity of the skin should match the design value.
- Corrosion Resistance:The tolerance time of different media (acids, bases, oils) is specified, such as soaking in 10% sulfuric acid for 72 hours without swelling.
When choosing skin material, you can't just look at the temperature: the medium, pressure and displacement must all be less
"My working temperature is only 200℃, can I cover my skin with silicone cloth?" I asked him back, "Is there any sulfur-containing flue gas in your pipeline? What is the pressure? What is the displacement of the pipeline?" He was stunned for a moment. You see, temperature is just an entry condition. If the medium contains acidic gases such as SO₂ and HF, ordinary silica gel cloth will be embrittled in half a year, SO polytetrafluoroethylene or fluororubber composite layer has to be used. When the pressure exceeds 0.3MPa, the skin must be reinforced with steel mesh. In cases where the displacement is large, such as the flue of a power station, the number of skin layers should start with at least 5 layers, and a margin must be reserved.
To put it bluntly, skin selection is a systematic project. You only stare at the temperature, which is equivalent to driving only by stepping on the accelerator and not looking at the road, and sooner or later the car will roll over.
On-site cases of skin failure: Why do some manufacturers' products leak in one year, while others can last for eight years
Also at the exit of the desulfurization tower, it is also a rectangular non-metallic expansion joint. The products of manufacturer A began to seep water after 13 months, but those of manufacturer B lasted for more than 8 years before they were replaced for the first time. After dismantling and comparing, the gap is clear at a glance: Manufacturer A's skin has only three layers-one layer of fluororubber cloth, one layer of glass fiber cloth and one layer of non-woven cloth, and the warp and weft density of fiber cloth is only 8×8/cm. Manufacturer B uses a 5-layer structure: external fluorine rubber, medium polytetrafluoroethylene membrane, medium and high silicon fiber cloth, medium polytetrafluoroethylene membrane and internal acid-resistant rubber. The density of fiber cloth is 12×12 pieces/cm, and additional anti-penetration coating is made.
And guess what? The skin cost of manufacturer A is only 60% of that of manufacturer B. However, Party A calculated the accounts. In eight years, Manufacturer A changed it three times, plus the downtime loss, and the total cost was twice that of Manufacturer B. So don't just look at the initial quote, the cost of life is real money.
The easiest pits to step on during installation and maintenance-the influence of tie rods, deflectors and sealing surfaces
No matter how good the skin is, it will be useless if it is not installed correctly. I have seen too many people screw the tie rod nut to death and ask "how to adjust the tie rod nut of the expansion joint"-the role of the tie rod is to limit overstretching, not to lock the compensation amount. After installation, the tie rod nut must be returned 3~5mm to leave free displacement space for the compensator. There is also a guide tube, which functions to guide the flow direction of the medium and avoid direct washing of the skin. If the guide tube is installed backwards or there are burrs in the welding, the high-speed dusty smoke will break the inner layer of the skin, and it will be perforated in three months.
The sealing surface is the hardest hit area. Water seepage from the threaded hole, uneven flange surface, uneven bolt torque... If any detail is not in place, the skin will change from "life door" to "dead door". Don't forget to check whether the sealing gasket is aging during maintenance. By the way, the specific function of the expansion joint guide tube is to protect the skin. Don't dismantle it and think it is redundant parts.
Differential requirements for skin standards under different working conditions (desulfurization, cement, power station)
As you can see the product classification in our station, there are several kinds of light baffle doors, such as desulfurization flue gas baffle doors and circular baffle doors (double seals). The same is true for non-metallic compensators. Different industries have to "watch the dishes and eat".
Desulfurization condition:The flue gas is wet, acid-containing, and the temperature fluctuates greatly (50~180℃). The skin must be acid and alkali resistant. It is recommended that fluororubber + polytetrafluoroethylene composite layer, the number of layers ≥4 layers. The pressure is generally low (
Cement industry:High temperature (200~400℃), containing hard particle dust, negative pressure operation. The skin should be resistant to wear and high temperature. High silicon fiber cloth + ceramic fiber felt is standard, and the outer layer is reinforced by stainless steel wire mesh. Although metal corrugated expansion joints in cement industry are also commonly used, non-metal compensators at pipe corners are more flexible.
Power station industry:Large diameter flue, large temperature gradient, difficult to control installation error. There are many corrugated expansion joints in power station industry, but non-metallic compensators on hot and cold air ducts are also inseparable. The skin needs high elasticity, mostly using silicone cloth + glass fiber cloth, the number of layers is ≥5, and a reasonable tie rod limit structure should be designed to prevent over-stretching.
At the end of the day, there is no universal standard, only matching selection. You're willing to spend three more days doing your homework, and the equipment will last three more years.