Disassembly of Technical Parameters of Flue Expansion Joints: From Design Temperature to Fatigue Life, Hard Indicators Must Be Known Before Type Selection
Why do flue operating conditions make the technical parameters of expansion joints extra demanding? — — Let's talk about the "temper" of smoke first
People who do flue systems know that the "temper" of smoke is much greater than that of steam and hot water. High temperature, corrosion, dust, pressure fluctuations-all of which can make ordinary expansion joints "roll over". Two days ago, I met a customer of a power plant. The flue expansion joint leaked after less than a year of use. When I disassembled it, I found that the trough was full of sulfide crystals, which corroded the bellows like a sieve. What's the problem? The sulfur content of flue gas was not taken seriously when selecting the model.
Therefore, when talking about the technical parameters of flue expansion joint, we can't just look at the numbers, but first understand the working conditions: is the flue gas temperature continuous or intermittent? Is there acidic condensation? Does dust grind or not? These determine which hard indicators you should focus on.
Core parameters are disassembled item by item: temperature, pressure, compensation amount and fatigue life, which is the key to success or failure?
Four parameters, none of which can be less, but the priority is different. Let's start with the design temperature. The temperature in the flue system often deviates-the boiler starts and stops, the load changes, and the instantaneous temperature difference is tens of degrees. If you choose the material according to 350℃, if it is actually repeatedly rushed to 420℃, the corrosion-resistant layer of the bellows will not be able to bear it. At this time, you have to look at the high-temperature axial expansion joint, or use an alloy such as Incoloy 825. Don't try to save money and use 304.
The pressure is simple, and the flue system pressure is generally not high (within ± 0.05MPa), but do not ignore the negative pressure condition. I've seen several cases of bellows instability once the system sucks. What about the amount of compensation? The flue direction is complicated, and axial displacement, transverse displacement and angular displacement often exist simultaneously. It is more reliable to rely on a general-purpose corrugated expansion joint alone than to combine a double hinge transverse expansion joint or a straight tube pressure balance expansion joint.
Finally, fatigue life. The national standard requires that the fatigue life of bellows is not less than 1,000 times, but the actual number of flue starts and stops far exceeds this number. If you follow the minimum standards, it will be scrapped in two or three years. I recommend requiring at least 3000 times, and the material is austenitic stainless steel that is resistant to fatigue.
Deflectors, tie rods, number of corrugated layers-those accessory parameters that are easily overlooked but directly affect performance
In addition to the major items of temperature and pressure, there are several accessory parameters on the parameter table, which are easiest to fool when selecting. The first one is the deflector tube. Anyone who has read our question and answer "The Specific Function of Expansion Joint Guide Tube" knows that the guide tube not only diverts flow, but also protects the bellows from being washed by high-speed dust. Wall thickness of guide tube less than 3mm? Sorry for half a year's wear.
The second is the pull rod. The role of the expansion joint tie rod is to withstand the internal pressure thrust and prevent the bellows from over-stretching. But how to adjust the tie rod nut? Too loose has no protection, too tight limits displacement. Adjust according to the instructions during installation, and do not screw to death by feeling. In the correct installation method of large tie rod expansion joint, we emphasize the amount of pre-tension/pre-compression-this value is provided by the manufacturer and must be implemented on site.
The third is the number of corrugated layers. Multi-layer bellows can withstand higher pressure, but it also brings the problems of increased stiffness and decreased compensation ability. How many floors to choose? Calculate with pressure, temperature and compensation amount, and don't blindly pursue more layers.
Different industries (power stations, cement, desulfurization) have different requirements for flue expansion joint parameters. Never apply one selection standard
For corrugated expansion joints used in power station industry, the biggest headache is large-size circular flue and high-temperature flue gas. For example, the outlet of the induced draft fan of the power plant has a diameter of three or four meters, a temperature of about 180℃ and a small amount of SO2. In this case, the metal rectangular expansion joint or double-sealed single-axis circular baffle door are the first choice, focusing on the pressure resistance and sealing performance.
What about metal corrugated expansion joints in cement industry? Working conditions are worse: high flue gas temperature (up to 400℃), high dust concentration, and alkaline corrosion. We commonly use heat-resistant stainless steel plus wear-resistant guide tube in cement projects, and the wall thickness of corrugated pipe is at least 1.5mm, otherwise it will be finished as soon as the dust is washed.
The desulfurization industry is now the hardest hit. After wet desulfurization, the flue gas has low temperature and high humidity, and the acidic condensation is serious. Ordinary 304 bellows will perforate in two or three months. At this time, it must be lined with PTFE hose or PTFE compensator, and cooperate with desulfurization flue gas baffle door isolation. Temperature parameters are not the focus, but corrosion resistance is the first priority. Never apply the selection criteria of power stations to the desulfurization system-a bloody lesson.
Common Selection Myth: The higher the stack of parameters, the better? Is life expectancy not accurate? Finally, let's talk about two lessons from actual combat
The higher the parameter, the safer it is. Some people choose expansion joints, insisting on leaving three or four times the margin of temperature, pressure and compensation. That's it. The stiffness of the bellows rises with the temperature and pressure. The greater the margin, the greater the stiffness, and the greater the force on the pipe support. Maybe the equipment will be broken instead. Reasonable margin 1.2~1.5 times is enough, special working conditions consult the manufacturer.
Fatigue life is not accurate. Many design institutes use theoretical formulas to calculate the life, and as a result, the actual working conditions fluctuate greatly-such as frequent start-stop and vibration superposition, and the actual life is only one-third of the theoretical value. We have a cement customer who selected the bellows according to the life of 5,000 times, but it cracked in two years. Later, the reason was found to be fretting fatigue caused by pipeline vibration. Therefore, the life calculation cannot only be based on the number of designs, but also additional factors such as vibration and corrosion must be evaluated.
In the desulfurization flue of an iron and steel plant, the general corrugated expansion joint was selected, and the guide tube was not added. After half a year of operation, the high-speed dusty flue gas wore and leaked the bellows, and the production was stopped for three days. Later, it was changed to the model with thickened deflector, which stopped.
In a gas turbine power plant, six compound straight pipe bypass pressure balance expansion joints were used, and the tie rod was wrongly adjusted during installation-the transport bolts were not removed. As a result, the bellows cannot expand and contract freely, resulting in cracking of the weld. This matter shows that the installation specification is more important than the parameters, and the on-site disclosure must be in place.
After saying so much, just one sentence:Technical parameters of flue expansion jointThere is no universal formula, and every parameter has to fight with the working conditions. Do you have specific project parameters in hand? Welcome to throw over, we'll take your pulse.