What goes in the desulfurization flue is not a good thing-the wet flue gas carries sulfuric acid, sulfurous acid and chloride ions, the temperature fluctuates, and the negative pressure comes from time to time. When the ordinary carbon steel expansion joint is installed, it will perforate and leak in three or five months. You spend a lot of money to replace it, but as a result, it will have to be shut down for maintenance in less than a year. Tsk, this is the selection missing the root.
What about that? Don't worry, let's take it down step by step. When selecting the desulfurization metal expansion joint, the key focus is on several sets of data: temperature, pressure and corrosion allowance. If these three things are wrong, no matter how expensive the equipment is, it will be useless.
The "hard flaw" of desulfurization system: why can't ordinary expansion joints bear it?
The temperature of flue gas after wet desulfurization is generally 40~80℃, but it can soar to 150℃ instantly when starting and stopping. What's even worse is corrosion-SO₂ and SO₃ in flue gas form sulfuric acid when they meet water, and chloride ions cause pitting and stress corrosion. In this environment, ordinary 316L stainless steel can't last for a year before it cracks. Coupled with the dust accumulation in the flue and the washing water, the ordinary expansion joint can't bear it at all.
Therefore, the desulfurization metal expansion joint must be specially designed. The material is wrong and the structure is useless no matter how good it is.
Material and Structure: Core Technical Details of Desulfurized Metal Expansion Joints
Let's talk about the material first. Stainless steel 316L preferred for desulfurization conditions? Don't jump to conclusions. When the concentration of chloride ion exceeds 200ppm, 316L is prone to stress corrosion cracking. At this time, we have 254SMO or duplex steel (such as 2205), which has high pitting equivalent resistance and can withstand high chlorine environment. The price is a little more expensive, but it is worry-free.
Structurally, the bellows must have a deflector-a detail that many people ignore. The guide tube can avoid the medium directly washing the corrugation, and can also insulate heat and guide the flow. This site'sCorrugated expansion joint for power station industry、Universal corrugated expansion jointAll of them are equipped with guide tubes as standard, but in desulfurization applications, they have to thicken the lining, or even do PTFE lining. In addition, the number of bellows layers should not be too many, and single-layer or multi-layer thin-walled structures are prone to fatigue. It is recommended to use single-layer medium-thick wall.
Must-see for selection: How to determine temperature, pressure and corrosion allowance?
Temperature Don't just look at the design temperature, you have to consider the transient state. For example, the temperature of flue gas accident may reach 200℃. Is the high-temperature yield strength of the material enough? The pressure should not only be positive, but also check the instability of negative pressure. The design pressure of some projects is ±3kPa, but the actual operating negative pressure can reach-5kPa, and the bellows will deflate at once.
In the desulfurization flue of a power plant, the flue gas chloride ion is 500ppm, and the pH value is 3~4. According to experience, the corrosion rate of 316L corrugated pipe with a wall thickness of 1.5mm is about 0.3mm/year, and it will leak in less than 5 years. If the wall thickness is increased to 2.5mm, leaving a sufficient margin of 1mm, the life can be doubled. However, thickening affects the compensation ability, so it has to be calculated by finite element. It is recommended to do hanging test according to the actual smoke composition, and the data speaks for themselves.
Installation and Maintenance: Practical Points for Extending Life
The direction of the arrow on the expansion joint must coincide with the flow direction of the medium, with the opening of the guide tube facing downstream. The amount of pre-stretching/pre-compression is calculated according to the manufacturer, don't rely on feeling. The adjustment method of the nut of the expansion joint of the large tie rod can be seen in the Q&A of the station"Proper Installation Method of Large Tie Rod Expansion Joint"。
What about maintenance? Regularly inspect the bellows surface, especially the heat-affected zone of the weld-where microcracks are most likely to occur. If there is a chance to shut down the furnace, open the manhole to see the dust accumulation inside, and don't let hard particles get stuck in the trough. The water pressure should not be too high during flushing to prevent the water hammer from impacting the ripples.
Comparison of the same door: desulfurization metal expansion joint vs non-metal expansion joint vs desulfurization flue gas baffle door
Many people confuse these three things. Desulfurization metal expansion joint is specially used to compensate for thermal displacement, corrosion resistance, good pressure bearing capacity, is the mainstream of flue compensation. WhileNon-metallic expansion joints (fabric fiber expansion joints)It has stronger corrosion resistance and large compensation, but it has poor pressure and is not resistant to high temperature, so it is suitable for low pressure and large displacement positions. As forDesulfurization flue gas baffle door, which is used to cut off the smoke, not to compensate for displacement. For example, the baffle door should be used to partition it during maintenance, and it should be used together with the expansion joint.
The outlet flue of desulfurization tower in a steel mill was originally designed with non-metallic expansion joint, but it was broken by flue gas after half a year of operation. Replaced with desulfurized metal expansion joint (254SMO material, wall thickness 2.0mm, with guide tube), which has been running for 3 years without any problems. So don't try to save that little money, the right working conditions are the key.
Choose desulfurization metal expansion joint, don't just look at the price. After getting the design parameters, calculate the three sets of temperature, pressure and corrosion margin clearly, and then connect with the manufacturer's technology. Thickening and thickening, the upgraded material will be upgraded. Otherwise, what is waiting for you is frequent downtime and high maintenance, which is not worth the loss.