1. Why does the non-metallic compensator change the diameter? -Practical requirements from pipe layout to stress relief
It is impossible for a pipe to go one diameter from beginning to end. Fan outlet, desulfurization tower inlet, chimney interface... these places have to change diameter nine times out of ten. Why does the non-metallic compensator change? To put it bluntly, just one word —"Match"。
Non-metallic compensator (also called fabric fiber expansion joint) itself has good flexibility and low pressure. After diameter reduction, it can connect equipment or pipelines with different diameters at both ends, and absorb thermal displacement and vibration at the same time. For example, in the cement industry, the diameter of the pipe from the kiln tail to the dust collector is shrunk from 3 meters to 2 meters, and there is no reducing diameter compensator in the middle. If it is hard connected, thermal expansion can crack the flange.
There are also desulfurization baffle door interfaces – it is common for double-sealed single-axis circular baffle doors to have a mismatch between the caliber and the flue. At this time, the non-metallic compensator with variable diameter is the most hassle-free scheme, which can both seal and compensate, saving the need to add a separate reducer.
2. Core limitations of diameter reduction: allowable diameter reduction ratio, pressure level and medium temperature
How much does the non-metallic compensator change? Let's give a bottom line:The diameter reduction ratio (large end diameter ÷ small end diameter) is generally controlled within 1.6。 Beyond this number, the fabric layer will wrinkle due to excessive taper, resulting in stress concentration, and the life span will be directly discounted.
Import DN2500 and export DN1500, with a diameter reduction ratio of 1.67, which looks only a little more, but after half a year's actual operation, the skin on the small end side is cracked. Later, it was changed to two-stage diameter reduction, with a transition section in the middle, and the problem was solved.
The pressure rating is also a hard lever. Non-metallic compensators are commonly used in negative pressure or low pressure systems (
Therefore, don't just look at the caliber when selecting the model. The pressure level and temperature must be looked at together. Yesterday, another customer asked: Can the flue gas temperature be 350℃, the pressure is 0.05MPa, and the diameter can be reduced by 1.5 times? The answer is: OK, but you should choose high-temperature resistant silicone cloth + stainless steel wire mesh reinforcement layer, which can't be handled by conventional PTFE compensator.
3. How to calculate the diameter reduction design? — — Empirical formula and key points of structure selection
The core parameter of variable diameter compensator design isCone angle α。 Empirical formula: α ≤15° (unilateral), corresponding to length L ≥ (D-d) / (2× tan7.5°). D is the large end diameter and D is the small end diameter. For example, from 2000 to 1200, Δ D =800 mm, L needs to be at least 800/ (2×0.1317) ≈ 3037 mm. Not long enough? Then you can only do multi-wave non-metallic or divide into two levels.
Integral taperAndSegmented straight tube + cone。 The overall tapered shape saves material, but when the diameter ratio is> 1.4, the skin is difficult to fit and easy to wrinkle; The segmented type has many welds, but the ability to compensate displacement is stronger, and the angle can be adjusted flexibly. Our rectangular non-metallic expansion joint commonly uses segmented type, because the rectangular diameter reduction needs to consider the stress concentration of four corners.
Add a guide plate or guide tube on the inner wall of the cone section (refer to the function of the guide tube of the expansion joint) to prevent high-speed dusty air flow from directly washing the fabric layer. Working conditions in the cement industry especially require that dust particles can wear through two layers of fabric.
4. Variable diameter selection cases in different industries: cement industry, power station flue gas system, desulfurization baffle door interface
Cement industry: The diameter of the pipeline from the kiln head/kiln tail to the humidification tower is reduced, the diameter is mostly 2000~4000mm, the temperature is about 400℃, and the negative pressure operation is carried out. Non-metallic expansion joints (fabric fiber expansion joints) are mostly used in the selection, and the material is high silicon fiber + ceramic fiber cloth, with stainless steel wire mesh in the middle. The diameter reduction ratio is recommended to be ≤1.4, and the overall taper shape must be designed because the space is limited and it cannot be lengthened.
Flue gas system of power station: The diameter of the pipe from the boiler outlet to the denitrification device and then to the air preheater changes from large to small and then becomes large. The reducing diameter compensator is often used in conjunction with the metal rectangular expansion joint. Once, a reducing diameter compensator was adopted at the outlet of the induced draft fan of a power plant, with a reducing diameter ratio of 1.3, but the vibration of the fan was not considered, and the fabric wore out three months later. Later, the compound hinge transverse expansion joint was added to absorb the lateral displacement, and the problem was solved.
Desulfurization baffle door interface: It is often necessary to reduce the diameter between the desulfurization flue gas baffle door and the flue. At this time, the double-sealed single-axis circular baffle door and the compensator are directly flanged, and anti-corrosion must be considered in the reduced diameter cone section-the smoke temperature is low and the humidity is high, and the lining with PTFE compensator or PTFE-lined metal hose can withstand the acid dew point corrosion. The diameter reduction ratio can be 1.6, but the length is enough, otherwise the sealing performance is poor.
5. Common pitfalls and countermeasures of variable diameter compensator in installation and maintenance
Brutal Installation。 The skin of the non-metallic compensator is very brittle, and some people take a crowbar to hardtop the cone section. As a result, the internal wire mesh is deformed, and the smoke leaks within a few days. Countermeasures: Hoisting should use a sling, no holes should be punched, and the nut adjustment of the threaded rod (refer to how to adjust the expansion joint tie rod nut) should leave enough pre-cooling and heat margin.
Forget about cold tightness。 If the reducing diameter compensator is not pre-stretched or pre-compressed according to the design during installation, it will either be cracked or crushed to death after operation. Especially in winter construction in the north, the cold gap of the pipeline is large, and the hot state shift exceeds the stroke after the compensator is installed-does the screw need to be disassembled? Don't disassemble it. Calculate the displacement first and then adjust it.
Ignore the deflector。 The flow velocity changes greatly at the variable diameter. Without the deflector, the airflow will directly impact the inside of the cone section and wear quickly. There was a case before: the reducing diameter compensator at the inlet of the desulfurization tower was worn out by the fabric layer for 9 months. When I disassembled it, I saw that only 1/3 of the deflector was welded. Later, it was fully welded as required, and it has not been changed for 3 years now.
Reducing diameter compensator, design accounts for 60%, installation accounts for 30% and maintenance accounts for 10%. Choose it right, install it, once and for all; If you choose the wrong picture or the picture is cheap, you will be repaired to death later.