Let's start with the standards: What red lines does JB/T 12235-2015 draw?
Non-metallic compensator (also called non-metallic expansion joint or fabric fiber expansion joint) is different from metal bellows. It has no unified stiffness calculation formula, and the assessment indicators mostly fall on materials, temperature resistance, pressure resistance and fatigue life. Standard JB/T 12235-2015 clearly specifies the combination of tape layer, insulation layer and sealing layer. Think about it, if you only use a single-layer fiberglass cloth for cheap, the high-temperature smoke will leak in a few days. The standard requires that the loop belt is made of at least three parts: temperature resistance layer, heat insulation layer and sealing layer. Each layer of material has the minimum thickness and temperature resistance grade. For example, the insulation layer must be made of ceramic fiber cotton with a thickness of not less than 20mm; If the sealing layer is a fluororubber coating, the thickness should not be less than 0.8mm. These red lines are not decorations-I have seen a cement factory use a single layer of silicone cloth to make rectangular non-metallic expansion joints in order to save 500 yuan. As a result, it began to smoke on the third day of production, and burned through directly a week later.
Temperature and medium are two variables that determine success or failure
The most common use scenarios for flexible non-metallic compensators are power stations, cement, and desulfurization flue gas systems. When the temperature exceeds 400℃, high-temperature resistant ceramic fiber cotton insulation layer must be installed, which is not negotiable. When encountering acidic medium (such as wet flue gas after desulfurization), the thickness of the outer fluororubber coating or PTFE coating should not be less than 0.8mm, otherwise it will corrode and perforate in half a year. Two days ago, a customer asked if a rubber compensator could be used instead of a non-metal expansion joint. The upper limit of rubber temperature resistance is only 120℃, and the flue gas temperature is 200℃. If you install it, you are looking for death. The composition of the medium is also critical-SO₂ and HCl in the wet flue gas will chemically corrode the ordinary coating, which must be carried by double-sealed single-axis circular baffle door and PTFE lining. When selecting, you must write the medium temperature, composition and pH value clearly. Don't just say "flue gas", but specifically "wet flue gas after desulfurization, temperature 180℃, sulfur content 3%".
How to calculate displacement compensation capacity? Don't just look at axial compression
It can be absorbed in axial, transverse and angular directions. However, note that the four corners of rectangular non-metallic expansion joint are stress concentration zones, and reinforcing ribs or circular arc transition must be added at the corners during design, otherwise the weld will crack after two months of operation. In addition, the orientation of the guide tube must be consistent with the flow direction of the medium during installation, and the arrows on the expansion joints are for this purpose. How to calculate the amount of compensation? The standard recommends calculation according to the thermal expansion length of the pipe: Δ L = α × L × Δ T, α is the linear expansion coefficient (carbon steel ≈ 0.012mm/m·℃), L is the length of the pipe, and Δ T is the temperature difference. The lateral and angular displacements are vectored according to the actual pipe arrangement. For example: a 30 m long flue gas duct with temperature rising from 20°C to 250°C, axial expansion ≈ 0.012×30×230=82.8 mm. When you choose a compensator, the rated compensation amount should leave at least 1.2 times margin, that is, more than 100mm. Don't ask me why-in the hot state, there may be installation errors and vibration offsets in the pipeline, and the ring belt will break directly if the margin is not enough.
Installation and pre-stretching is an easy pit to step on
Flexible non-metallic compensators are generally in a free state when they leave the factory, and they need to be pre-compressed or pre-stretched according to the ambient temperature for on-site installation. For example, the compression amount of 10-15mm is reserved for winter installation and the stretching amount of 5-10mm is reserved for summer installation, so as to ensure that the loop belt does not slack under the working conditions throughout the year. If you install it directly, the circle belt will break directly as soon as the expansion amount goes up in summer. Specific operation: First screw the bolt to the scale position corresponding to the pre-tension/compression amount, fix the flange, and then lock the nut. Don't forget that the arrow on the expansion joint must point to the direction of the media flow-this direction determines the direction of installation of the guide tube. If installed backwards, it will directly flush the inner wall of the ring belt, and the life of the tube will be halved. Also, the two ends of the expansion joint should leave enough free distance to avoid being restricted by the pipe support and hanger after installation.
Don't ignore the fittings: linkage design of baffle door and metal hose
In many pipeline systems, circular baffle doors or double-sealed single-axis circular baffle doors are provided next to non-metallic expansion joints for inspection and isolation. At this time, the bolt hole distance and the flange surface must be matched, and the thickness of the gasket must be controlled within 3mm as much as possible, otherwise the air leakage rate exceeds the standard. I have seen a project where the flange hole spacing of the baffle door is 4 holes φ 18, but the expansion joint flange is 8 holes φ 22, and the hole is hard grinded and reamed on site. As a result, the sealing surface is damaged and the air leakage rate soars from 1% to 5%. In addition, if the system requires high-frequency vibration compensation (such as the fan outlet), it is recommended to install metal hoses at both ends of the expansion joint to release residual stress. The metal hose can absorb high-frequency vibration and prevent the non-metal ring belt from bearing repeated shear. Especially for the desulfurization system, flue gas erosion and vibration can't be carried by the expansion joint alone, and it can last for two years with a PTFE-lined hose.
How long is the lifespan?
Under normal working conditions, the replacement cycle of non-metallic expansion joints is 3-5 years, but the key depends on the aging speed of the ring belt. Regularly check the surface for cracks, bulges or exposed fibers. If a crack of more than 2mm is found, it has to be replaced. Don't wait until it leaks to remember the shutdown, the loss of one day of shutdown is enough for you to buy ten sets of compensators. How to extend life? Control flue gas temperature and acid dew point well-install flue gas baffle door at the outlet of desulfurization tower to reduce cold air backflow during shutdown and avoid condensation corrosion of ring belt. In addition, the multi-layer composite structure is given priority when selecting the type: the inner layer of aluminum silicate fiber cotton is temperature-resistant at 1260℃, the middle layer of ceramic fiber is heat-insulated, and the outer layer of fluororubber is anti-corrosion. This combination can last for 5 years in desulfurization flue gas at 200℃. If you use a low-end product (such as a single-layer EPDM coating), it will be screwed in a year. Remember, the requirements of flexible non-metallic compensators are not a set of fixed parameters, but comprehensive matching for temperature, medium, displacement, installation, and accessories.