The metal expansion joint in the desulfurization system, with short life, slurry leakage and cracking, is a pit that almost every power plant and desulfurization project has stepped on. Two days ago, a customer told me that the expansion joint at the entrance of their flue gas desulfurization tower began to leak just after a maintenance cycle. After removing it, the inner wall of the bellows was full of corrosion pits and cracks. What's the problem? Instead of buying fake goods, the general-purpose corrugated expansion joint was directly moved to the desulfurization condition.
What is special about the desulfurization condition
Many people think that the temperature of desulfurization flue gas is not high, so just choose stainless steel. In fact, there are three attacks in the desulfurization system simultaneously: slurry scour corrosion, chloride ion stress corrosion cracking, and low pH condensate corrosion. The slurry contains gypsum particles, and when the flow rate is high, the bellows is like being sanded by sandpaper. Chloride ions in flue gas are enriched in wet state, which is especially fatal for corrosion of austenitic stainless steel, especially pitting and stress corrosion cracking. In addition, when the flue gas temperature is below the acid dew point, dilute sulfuric acid will condense from the pipe wall, and the pH value is as low as 2~3, which ordinary 304 can't bear at all.
Therefore, the material threshold of metal expansion joints for desulfurization has never been as simple as the word "stainless steel".
Selection Logic from Failure Cases
How to choose the material? At least it should be determined according to the medium temperature and chloride ion concentration. In low-temperature and high-humidity positions such as the rear of the flue gas heat exchanger and the clean flue, 316L is only regarded as the entry-level, and a large number of pitting corrosion occur in actual use. If the chloride ion concentration exceeds 500mg/L, it is recommended to go directly to 254SMO, that is, S31254. If the working condition contains extremely high chlorine, or there are frequent start-stops and alternating wet and dry, then you have to consider nickel-based alloys, such as C-276 or 625. Don't worry about money. An unplanned downtime loss is enough to buy several expansion joints. Is this the truth?
Wall thickness and wave number design are also particular. The increase of wall thickness can improve the corrosion resistance margin, but the stiffness also goes up. If the wave number is not enough, the compensation amount will not be enough, and the system stress will be fully pressed on the nozzle and equipment. On the contrary, too many wavenumbers, the axial stability of the bellows deteriorates, and it is easy to become unstable. A good design is calculated by the fatigue life curve, not by patting the head.
Structural details make or break
Installing the guide tube in the reverse direction is the most common low-level error of metal expansion joint for desulfurization. The guide tube must face the flow direction of the medium, otherwise the airflow will directly scour the root of the corrugation, and it will be a matter of time before it wears out. Wear-resistant lining is not standard, but where the slurry has high solid content, adding a layer of wear-resistant lining can last for two or three years.
And the fit clearance. Sufficient space must be reserved between the expansion joint and the flue gas baffle door and the desulfurization flue gas baffle door, otherwise the bellows will be squeezed and deformed when the valve is opened and closed. The position of the drainage hole should not be opened indiscriminately. It should be opened at the lowest point of the trough, facing the hydrophobic side, otherwise the accumulated liquid will accumulate in it, chloride ions will be concentrated, and the corrosion rate will double.
Things that are easily overlooked in installation and operation
Do you want to do it coldly? Depends on the design temperature of the pipe. Desulfurization system usually belongs to medium and low temperature, and cold tightening is not necessary. However, if the pipeline is a high-temperature section (such as the original flue gas flue), cold tightening according to the specification can reduce the initial stress during operation.
The trap setup is critical. There must be a drainage pipeline at the lowest point of the bellows to discharge the condensate in time. At the start-up stage, the temperature shock and liquid accumulation appear at the same time: the flue gas temperature rises suddenly, the bellows is not heated through, the liquid has accumulated in the trough, and thermal stress and corrosion lead to cracks. The correct thing to do is to check the hydrophobicity before starting up and let the tube wall heat up slowly to avoid large temperature steps.
How to divide labor between metal expansion joint and non-metal expansion joint
Not all locations in the desulfurization system are suitable for metals. Large-diameter flues, places that need large displacement compensation, such as the inlet and outlet of absorption tower, often use metal rectangular expansion joints or corrugated expansion joints for power station industry, which have high pressure resistance and long life. However, for positions such as fan inlet and outlet and flue connection section with large vibration, complicated displacement and vibration isolation, it is more cost-effective to use non-metallic expansion joints (fabric fiber expansion joints), which have low cost, light weight, no thrust, and are convenient for maintenance and replacement.
How to decide? Look at the medium temperature, pressure, and displacement type. High-temperature dusty flue gas is used for metal resistance to scour; Non-metal for low-temperature wet flue gas and heterogeneous pipe connection is more flexible.