1. What exactly is it? — — Structure and Principle of High Temperature Resistant Nonmetallic Flexible Compensator
To put it bluntly, this thing is a "flexible joint" used in high-temperature flues and air ducts. Imagine a metal flue dozens of meters long. It expands as soon as it is heated, shrinks as soon as it cools, and it will definitely crack if it is carried hard. Install a high-temperature resistant non-metallic flexible compensator, so that the pipeline can expand and contract freely without air leakage.
The outermost layer is a high-temperature resistant fabric fiber layer (we call it non-metallic expansion joint, or fabric fiber expansion joint), with insulation cotton and sealing layer sandwiched in the middle, and the inner layer can be lined with PTFE or stainless steel wire mesh according to the working conditions. The principle is to absorb the axial, radial and angular displacement of the pipe through the elastic deformation of the multilayer composite material. No matter how high the temperature is, it is not afraid-because non-metallic materials are naturally non-conductive, and once the insulation layer is wrapped, the surface temperature can drop by 70 or 80 degrees.
Second, the shortcomings of metals are precisely the strengths of non-metals: temperature resistance, corrosion resistance and multi-directional displacement
The metal expansion joint is not bad, but the shortcomings are revealed in the face of high-temperature flue gas. When the temperature exceeds 600℃, ordinary stainless steel starts to oxidize, and nickel-based alloys are ridiculously expensive. When a corrosive medium (such as wet flue gas after desulfurization) touches, the metal bellows will perforate for months. Who can stand it?
How high temperature can a non-metallic flexible compensator hold? Look at the materials: silicone coated glass fiber cloth can withstand about 300℃, polytetrafluoroethylene film can withstand 250℃, and ceramic fiber cotton can withstand 1260℃. Our commonly used rectangular non-metallic expansion joint is used in the tail flue of cement kiln. The flue gas temperature is 800-1000℃, so it will not be changed for three years. Moreover, it can also absorb three-way displacement at the same time-axial stretching, lateral misalignment and angular deflection. The metal bellows can only eat in one direction, which can't be compared.
Let's talk about embalming. The wet flue gas in the desulfurization system contains SO₂ and SO₃, which is extremely corrosive. Stainless steel? Can't handle it. The non-metallic fabric layer itself is inert, lined with a layer of polytetrafluoroethylene (that is, our material lined with PTFE hose), which takes both acid and alkali.
Third, power station, cement, desulfurization... What occasions must it be?
Two days ago, I met a customer who was doing power station boiler renovation and asked if I could use metal. I said that the smoke temperature at the outlet of your boiler is 550℃. If it is made of metal, you have to choose super austenitic stainless steel. A pipe can cover the price of five non-metals. Moreover, the installation space of metal is high, you can't come crooked, and the soft connection of non-metal can be stuffed in.
- Power station industry: The flue between the boiler and the dust collector has a large amount of thermal expansion and must be made of non-metal. The corrugated expansion joint we do for the power station industry (although the name is metal, the actual non-metal version is more common) is typical.
- Cement industry: The pipeline from the outlet of the preheater to the humidification tower has large temperature fluctuations and lots of dust. Rectangular non-metallic expansion joint, flanged connection, easy replacement.
- Desulfurization system: The inlet and outlet of the absorption tower, the flue gas contains acid and humidity. Desulfurization flue gas baffle door with non-metal compensator, good sealing performance, long life.
- Iron and steel metallurgy: Blast furnace hot blast furnace pipeline, high vibration, high temperature, non-metal flexible compensator can reduce vibration and noise.
Are there places where you can't use non-metals? Yes. When the pressure exceeds 50kPa, you have to be cautious, after all, the strength of the fabric layer is limited. Also, if the medium in the pipe contains a large amount of particle erosion (such as pneumatic conveying), wear-resistant lining must be added, otherwise the fabric will wear out quickly.
4. When it comes to type selection, don't just look at temperature-pressure, medium and displacement are indispensable
Many people choose non-metallic compensators, and the first sentence asks "how much is it resistant?" Temperature is important, of course, but just looking at the temperature is bound to roll over. For example, the same flue gas is 600℃, one contains dust and the other does not. If it contains dust, you have to choose one with anti-wear lining; A pressure of 10kPa, a pressure of 30kPa, 30kPa has to strengthen the skeleton and encrypt the pressure plate bolts.
- Temperature T: Take the highest continuous temperature, not the instantaneous peak. The instantaneous peak is allowed to exceed 50℃, but not often.
- Pressure P: Non-metallic expansion joints are generally subjected to slight positive pressure or slight negative pressure. Standard JB/T 12235-2015 specifies that the test pressure is 1.5 times the design pressure. Caution over 20kPa, consider metal bellows over 50kPa.
- Media composition: Acid, alkali, oil, dust, determine the fabric material and lining layer. For example, polytetrafluoroethylene compensator must be used for fluorine-containing waste gas.
- Amount and direction of displacement: Axial, lateral, angular displacement, how much value to give in each direction. We generally design on a "gimbal compensation" basis, but the customer has to provide the maximum offset of the installation location.
Speaking of which, I have a long-winded sentence: in order to save money, some customers report the displacement amount to be small, and the result is that it is cracked when installed. It's not a province, it's a waste.
5. Installation and routine maintenance, pit avoidance guide
Installation is actually not complicated, but there are a few pits:
- Don't use forced alignment: The compensator is used to absorb displacement, not to forcibly align two sections of pipe. Installation should be a natural transition, do not firmly pull the bolts.
- Pay attention to the flow direction identification: Some non-metallic expansion joints have a guide tube in the inner layer (the same way as the guide tube of expansion joints), and the installation direction cannot be reversed, otherwise the airflow will scour the fabric layer.
- Leave allowance for tie rod nut: Refer to the FAQ, how to adjust the expansion joint tie rod nut, generally loosen 1-2mm after installation to avoid limiting displacement.
- The insulation layer is protected from rain: Fabric fiber is afraid of long-term water accumulation, so open-air installation should be made with waterproof cover. One of our customers installed a non-metallic compensator on the top of a blast furnace without a cover. As a result, rainwater seeped in, and the insulation cotton failed, which was scrapped in two years.
Routine maintenance mainly depends on whether there are bulges, wear or cracks on the fabric surface. Generally, power plants are inspected once a quarter, and the surface temperature is measured with an infrared thermometer. If the local temperature rises abnormally, it means that the insulation layer may be damaged. In addition, loose bolts are a common disease-if thermal expansion and contraction occurs more times, the nut will loosen and be tightened every six months.
6. Don't take it as a "low-end goods"-the technological evolution trend of non-metallic compensators
To be honest, when I entered the industry, many engineers thought that non-metallic expansion joints were "cloth bags", low-end and short-lived. Actually, this idea is long outdated. Nowadays, the high-temperature-resistant non-metallic flexible compensator has mature multilayer composite technology, and its life is generally 3-5 years. Some power plants are still running after seven or eight years.
First, the number of composite layers has increased. In the past, the three-layer structure (fabric + insulation + sealing) can now achieve five layers, plus radiation protection layer and antistatic layer. Second, the connection method is improved. In the past, only flange connections were available, but now there are plug-in and clamp types, which doubles the installation efficiency. In addition, our non-metallic expansion joints (fabric fiber expansion joints) are combined with digital monitoring, embedded with optical fiber temperature measurement or strain gauge, and real-time feedback on the aging degree of fabrics to achieve preventive maintenance.
Plus, standards are being upgraded. JB/T 12235-2015 National Standard for Non-metallic Expansion Joints has covered the material, test and inspection requirements. When you select the model, you should look for this standard, and the quality is guaranteed. Don't buy goods from small workshops in order to save thousands of dollars. The money saved is not enough to lose a shutdown for maintenance.