Talk clearly about what is flue stainless steel expansion joint? What is the difference between it and ordinary corrugated expansion joint?
When it comes to expansion joints, what pops up in many people's minds is the bellows used in steam pipes or hot water pipes. Yes, it is called universal corrugated expansion joints. But the stainless steel expansion joint of the flue, to put it bluntly, is specially designed to serve the flue gas system. Smoke is a thing with high temperature, strong corrosion, and a bunch of dust particles. If you go up with ordinary carbon steel bellows, it will rust through you in three months.
So what is the difference between it and ordinary corrugated expansion joint? The first difference is the material. The flue may be desulfurized wet flue gas, which contains corrosive media such as sulfuric acid and sulfurous acid, and must be made of austenitic stainless steel, such as 316L or even 254SMO. If you take 304 to do it, pitting and stress corrosion cracking will occur in less than half a year-I have seen a power plant project, and I chose 304, but it leaked after eight months. When I removed it, I saw that the bottom of the corrugated valley was full of holes as big as needle holes.
The second difference is the structural design. Most ordinary corrugated expansion joints only bear axial displacement, and the displacement direction in flue system is complicated. Thermal expansion and contraction, wind force and foundation settlement often require compound hinge transverse expansion joints or large tie rod structure to absorb multi-directional displacement. Moreover, the flue diameter is large, the pressure is low, and more of them are rectangular expansion joints-such as the metal rectangular expansion joints of this station, which are specially suitable for flue square tubes. So you have to find out: the flue stainless steel expansion joint is not simply replacing the bellows with stainless steel, it is tailor-made for the flue gas working condition.
The two things that are easiest to overlook when selecting the model: the material and the guide tube, which will leak after half a year of mistake
When it comes to model selection, many people ask, "What is the caliber? What is the pressure?" Then they place an order directly. If you ask me, that's laying a mine for yourself.
First the material. As mentioned earlier, 316L is the start, but if there are chloride ions in your flue gas (such as wet flue gas after garbage incineration and coal-fired desulfurization), 316L can't bear it, so you have to go up and use duplex stainless steel or corrosion-resistant alloy. Also, don't just look at the material of the corrugated pipe, and the accessories such as end pipe, tie rod and nut must also be anti-corrosion. Some manufacturers cut corners, and the end pipe is galvanized with carbon steel. After thermal expansion and contraction, the coating falls off, and the rust water flows along to the bellows, which corrodes faster. We often encounter customers saying, "Why is the flange leaking first when the bellows is not broken?" Nine times out of ten, the attachment material is not selected correctly.
Secondly, the guide tube. This stuff is so critical. What is the specific function of the expansion joint guide tube? To put it bluntly, it is to direct the airflow, reduce vortex scour, and at the same time prevent particulate matter from accumulating on the bottom of the corrugated valley. If you install a flue gas system with high flow rate, dust, and no guide tube, the bellows will be worn out in three months. The direction of the guide tube is also easy to reverse-the arrow is in the direction of smoke flow, and if it is installed backwards, it is equivalent to not installed, which also aggravates the wear. Remember a formula: the outlet of the guide tube points downstream, and the inlet faces the incoming flow.
In addition, don't forget the wall thickness. The guide tube of flue stainless steel expansion joint is usually thicker than the bellows, starting at 3mm, and some high-speed dusty conditions require 5-6mm. Don't choose a thin wall to save money. The labor cost of changing it later is much more expensive than the material cost.
Mistakes made during installation: the tie rod was not disassembled and the direction was reversed. How many have you been recruited?
Two days ago, an old customer called and said that the newly installed expansion joint had bulged only half a month after it was put into production. I asked him to send a picture to see, boy, the transport tie rod is still welded on it and not disassembled! The expansion joint is stuck in the installation position, and the bellows has nowhere to deform when the temperature rises, and it is directly unstable.
Such low-level errors are not uncommon. When the expansion joint is sent to you, it will have a transportation rod (also called an adjustment rod) on it, which is used to prevent the bellows from being damaged during transportation and hoisting. After installation in place, the tie rod nut must be loosened or even removed, so that the expansion energy saving can expand and contract freely. Some people are afraid that the displacement will be too large after installation, so they twist the tie rod-then why do you install the expansion joint? It is better to weld it directly as a straight pipe.
The reverse direction installation is also the hardest hit area. Many expansion joints are marked with flow direction arrows pointing to the direction in which the medium flows. For example, the external pressure single axial expansion joint, one end of the guide tube is long and the other end is short, and the long end must face the incoming flow direction. If the guide tube is installed backwards for a ride, the bellows will directly deflate. There are also expansion joints with liner barrels, such as straight tube pressure balanced expansion joints, with the clearance of the liner barrel also facing the outlet side. Remember: Read the instructions before installation, look for arrows, don't just feel.
Don't just focus on the appearance in routine maintenance. The life of expansion joints has something to do with these details
Many people's maintenance concept is "don't care if you don't leak". That doesn't work on the flue. Flue stainless steel expansion joint failure tends to be progressive, and by the time you see a leak, the bellows may have cracked a few cuts.
Check the bellows surface regularly for corrosion pits, oxide scales or cracks. Especially at the bottom of the corrugated valley, if there is a slight pitting corrosion, it means that the material selection is weak, so you have to quickly consider replacing a higher-grade alloy. Second detail: See how worn the deflector is. Take a flashlight from the pipeline to see if the leading edge of the guide tube is worn and worn out? If the guide tube is found to have local perforations, it can be welded and repaired separately without changing the expansion joint as a whole.
The third detail is to care about insulation. Many flue expansion joints are covered with insulation layer on the outside, but water vapor easily seeps in through the gaps, causing external corrosion of stainless steel. This kind of "chloride ion stress corrosion cracking" tends to spread from the outside to the inside. It can't be found by routine visual inspection, so penetration detection or ultrasonic thickness measurement is needed. It is recommended to open the insulation layer once every six months, especially in places with high environmental humidity and chloride ions.
And the tightness of the tie rods and bolts. Although how to adjust the expansion joint tie rod nut depends on the model you buy, it is generally necessary to ensure that the tie rod is in a free state, not a stressed state, after installation. If it is found that the tie rod is bent or the thread slips, it means that the displacement of the pipeline exceeds the design value, and the pipeline stress must be re-checked and the limiting device must be installed.
How to use it with baffle doors and non-metal expansion joints? Coordination skills in flue systems
A complete flue system that won't have just one flue stainless steel expansion joint. It is often used in conjunction with smoke baffle doors and non-metal expansion joints. Here are a few matching pits.
For example, at the inlet and outlet of the desulfurization tower, there is high-temperature raw flue gas (about 150℃) on one side and wet saturated flue gas (about 50℃) on the other side, and the temperature changes drastically. If only metal expansion joints are used, repeated thermal expansion and contraction will cause fatigue cracking. Many people here simultaneously install non-metallic expansion joints (fabric fiber expansion joints) to absorb multi-directional displacement and shock absorption. However, the non-metallic expansion joint is afraid of negative pressure. If the negative pressure of the flue system is large, it is necessary to choose the reinforced type with steel wire skeleton, otherwise it will be deflated.
The same goes for flapper doors. For example, when the double-sealed single-axis circular baffle door is used to cut off the flue gas, the expansion joints in the front and rear sections should be paid attention to: after the baffle door is closed, the upstream may have to bear thermal expansion pressure, and the expansion joint should not be too weak. Moreover, the spacing between the baffle door and the expansion joint should be left sufficient to facilitate maintenance and replacement. I have seen a factory put the baffle door directly on the end pipe of the expansion joint, with a distance of less than 100mm. When the baffle door is removed, the expansion joint can't be removed at all, and in the end, a cutting machine can only be used.
At the expansion joint downstream of the baffle door, it is best to choose one with sewage discharge hole or drainage structure, especially in the wet flue gas system. Condensed water is easy to accumulate at the bottom of the bellows, which accelerates corrosion. The desulfurization flue gas baffle door of this station and the corrugated expansion joint used in the power station industry have matching drainage design, which is very important.
Flue stainless steel expansion joint is not isolated, it forms a system with baffle door, non-metal expansion joint and pipe support. When selecting the model, find out the whole working condition clearly-temperature, pressure, media composition, displacement direction, installation space and maintenance convenience, all of which are indispensable. Alas, in the final analysis, this job is a delicate job. Don't try to save trouble. All the money saved in trouble will be made up in the maintenance expenses later.