Why is the flue system inseparable from the expansion joint? Temperature, corrosion and negative pressure are all necessary
The flue is running with high-temperature flue gas, and the temperature soars from normal temperature to five or six hundred degrees. Have you calculated the thermal expansion and contraction of the pipe? Taking a 10-meter-long steel flue as an example, the axial elongation is close to 50 mm when the temperature rises at 400℃. Hard connection? Either the tube bulges or the flange bursts. The expansion joint is used to swallow this displacement, and the professional point is called compensation.
But this is only one of them. There are also corrosive gases in the flue-sulfur dioxide and nitrogen oxides, which become dilute sulfuric acid when the humidity is high. Do you choose regular carbon steel expansion joints? Piercing in a few months. Corrosive environment must be lined with stainless steel or PTFE. There is also the problem of negative pressure: once the induced draft fan pumps, the negative pressure in the flue may reach several kilopascals. If the stiffness of the expansion joint is not enough, it will be directly deflated. Therefore, the expansion joint selection, temperature, corrosion, negative pressure three parameters can not be less.
Does the flange connection work well in the flue? Seals, bolts, gaskets, whatever step is not in place is useless
Flange connection is the most commonly used installation method of flue. The advantage is that it is convenient to disassemble and does not need to move the pipe during maintenance. However, many people think that flange butt tightening bolts is enough, and eight out of ten projects are planted in details.
Let's start with the sealing surface. Flue flanges are usually flat or convex and are used in conjunction with gaskets. The material of gasket is very particular: asbestos rubber plate for low temperature and low pressure, graphite composite gasket for high temperature, polytetrafluoroethylene coating pad for corrosive environment. If you use it indiscriminately, it will be a matter of time before the air leaks. Let's talk about bolt pre-tightening. Flue flange bolts are usually made of M16-M20, and the tighter the preloading force is not the better-too much force will deform the flange, but leak; If the force is too small, the gasket will not be compressed firmly. The specification requires two to three symmetrical tightenings according to the torque value, this step cannot be omitted.
Between the flange connection and the expansion joint, a straight pipe section transition is often required to avoid the expansion joint from bearing additional bending moment. If you weld the expansion joint directly behind the flange elbow, it will deflect at a large angle, and the bellows will quickly fatigue and crack.
Metal expansion joint, non-metal expansion joint, which one should be used in the flue? How to pick between rectangle and circle?
Flue section shape, medium temperature, pressure, corrosivity, displacement direction. Metal expansion joints (e.g.Corrugated expansion joint for power station industry、High temperature axial expansion joint) High temperature resistance, strong pressure bearing capacity, suitable for round pipeline, high temperature and high pressure occasions. Non-metallic expansion joint (Non-metallic expansion joint (fabric fiber expansion joint)、Rectangular non-metallic expansion joint) Corrosion resistance, large amount of compensation, easy installation, especially suitable for rectangular flue and low pressure flue gas system.
The flue at the inlet of the desulfurization tower, the flue gas temperature is 120-180℃, containing a large amount of acidic condensate water, with metal bellows? Soon corrosion penetrates. Replace with non-metallic expansion joints, lined with PTFE or silicone cloth, and the service life is directly doubled. Conversely, the temperature of the main flue at the boiler outlet is 600℃, and the non-metallic fabric is burned into ash, which must be usedHigh temperature axial expansion jointThe material is Inconel 625 or 316L.
How to pick a rectangular flue? PreferredMetal rectangular expansion jointOr rectangular non-metallic expansion joints. Circular pipes use circular corrugated expansion joints. Note: The stress at the corner of the rectangular expansion joint is concentrated, so the fatigue life should be checked during design; Nonmetallic expansion joints should pay attention to the frame stiffness to prevent deformation under negative pressure.
When installing, is the pull rod adjusted or not, and which way does the guide tube face? These details determine how long it lasts
Many construction site expansion joints have problems in less than half a year after being installed. It is not that the product is not good, but that the installation is not in place. Let's start with the pull rod. Expansion joints with tie rods (e.g.Large tie rod expansion joint), the tie rod nut is locked when it leaves the factory to prevent over-stretching of the bellows during transportation and hoisting. Once installed in place, the locking nut must be loosened and the tie rod adjusted so that the expansion joint is at working length. No adjustments? The bellows is restricted by the tie rod, does not work, and the pipe displacement has nowhere to go. Finally, all the force is eaten on the flange bolt, and the bolt breaks.
Let's talk about the deflector. The function of the guide tube is to guide the flue gas smoothly through the bellows and avoid vortex erosion. The question and answer of "The Specific Function of the Expansion Joint Guide Tube" makes it very clear: it is installed on the medium flow side. How to judge the direction? The expansion joint cylinder is usually marked with arrows pointing to the direction of media flow. When installed backwards, the windward side of the guide tube is washed by high-speed smoke, and it is worn out for several months. Then the bellows is directly exposed to the smoke, and the corrosion is aggravated.
The expansion joint cannot be installed to bear the weight of the pipe. Independent supports and hangers must be provided. You let the expansion joint be the support? The bellows was squashed and the compensation function failed.
Three pits of model selection rollover: random specifications, blind material selection, and compensation amount depending on patting the head
After more than ten years of sales, I can write a book about the rollover cases I have seen. In summary, the three pits are the worst.
Pit 1: Specifications are messy
The size of the flue is 800×500 rectangle. Do you take a universal corrugated expansion joint and push it up hard? The flange hole spacing is not matched, and the sealing surface is damaged by reaming and changing the hole on the spot. The right thing to do: Customize to actual sizeMetal rectangular expansion jointOr rectangular non-metallic expansion joints, flange bolt holes with drilling.
Pit 2: Blind Material Selection
The flue gas contains sulfur, the temperature is 150 ℃, and 304 stainless steel is used? 304 is not resistant to pitting in chlorine-containing environments. 316L or duplex steel is used. The same is true for non-metallic expansion joints: ordinary silicone cloth has a temperature resistance of 200℃, and fluororubber cloth has a temperature resistance of 250℃. If it exceeds, polytetrafluoroethylene or composite ceramic fiber must be used.
Pit 3: The amount of compensation depends on slapping the head
I think it is "almost enough", but as a result, the actual displacement exceeds the design value, and the bellows is over-compressed to produce plastic deformation. The amount of compensation must be calculated based on the thermal expansion of the pipeline system, and the installation temperature deviation must also be taken into account.
The flange connects the flue and the expansion joint. If it is matched well, it is a golden partner, but if it is not matched well, it is a time bomb. Before selection, fill in the working condition parameter table honestly, and sign the process card for confirmation during installation. Don't be too troubled. If something goes wrong, the rework cost is enough to buy ten sets of expansion joints.