How "damaged" is the ash pipeline?
A while ago, the person in charge of the maintenance of a thermal power plant complained to me, saying that the longest expansion joint on the ash pipeline of their plant began to leak ash in less than eight months. When I removed it, I saw that the bellows was ground as if it had been beaten with sandpaper, and several holes were directly pierced through the guide tube. This is not an isolated case. Many power plants and cement plants are having a headache for the same thing-why is the ash-transporting metal expansion joint so "short-lived"?
To put it bluntly, the working conditions of ash pipelines are much worse than those of ordinary steam and hot water pipelines. Think about it. In the pneumatic ash transportation system, fly ash flies in the pipeline at a speed of more than ten or even twenty meters per second, and the particles are highly abrasive. When it meets elbows, reducing diameter and expansion joints, the flow direction changes, and the ash particles directly hit the bellows, which is the same as sandblasting. In addition, the flue gas temperature is generally above 150℃, and some working conditions can reach more than 300℃. The acidic medium produced by the combustion of high-sulfur coal will still adhere to the metal surface, and the triple overlap of high temperature + corrosion + erosion. If the metal expansion joint is selected according to the standard of ordinary thermal pipelines, it will really not be used.
The selection parameters are not filled in by patting the head
When choosing ash-conveying metal expansion joints, the most afraid thing is that the designer only gives a nominal diameter and design pressure. Let me tell you, the medium temperature, the design pressure, the compensation amount, the fatigue life and the material of the guide tube must be clearly picked out one by one.
The medium temperature directly affects the selection of corrugated pipe material. Below 350 DEG C, ordinary austenitic stainless steel 316L is basically sufficient; If it exceeds 400℃, you have to consider Incoloy 825 or higher superalloy, otherwise intergranular corrosion and high-temperature oxidation will quickly "bite" through the bellows. In terms of pressure, don't just look at the normal operating pressure of the system. Sometimes, because of ash plugging and blowing, the instantaneous pressure of ash pipelines will soar much higher than the design value, and enough margin should be left for the pressure resistance of bellows.
Let's talk about the amount of compensation, which many people tend to overlook. Ash pipelines are generally relatively long, and the thermal displacement is not simple axial expansion and contraction, but also lateral displacement and angular displacement. You have to figure out the direction of the whole pipe system, the position of the fixed bracket, and the calculation of thermal expansion before deciding whether to use the universal corrugated expansion joint or the double hinge transverse expansion joint. The compensation amount is small, the bellows is hard pulled, and the fatigue life plummets; Give it big, and the cost goes up again, so it's not necessary.
There is also the deflector, which is the role of "scapegoat" in ash transportation conditions. The function of the guide tube is to separate the ash particles with high flow rate, so that the medium does not directly flush the bellows. But the deflector itself is also being ground, so the material is critical. Ordinary carbon steel deflectors are a joke in front of abrasive fly ash. At least wear-resistant steel with 16Mn or higher hardness must be used. Some manufacturers will use carburizing treatment or lining ceramics, which is really designed for ash transportation conditions.
Power station and cement industry, the emphasis of type selection is really different
Don't think that it's all ash, and the expansion joint can take it all. The working conditions of power stations and cement plants are very different.
Generally, the ash conveying system of power station has long pneumatic conveying distance, high flow rate and fine ash fineness. Moreover, the boiler of power station runs relatively stably, and the fluctuation amplitude of temperature and pressure is small. However, the power station has a characteristic-high safety level requirements and large loss when the furnace is shut down. Therefore, the working conditions of power stations pay more attention to the fatigue life and reliability of bellows, and corrugated expansion joints for power station industry are usually selected. The structure emphasizes multi-layer bellows with reinforcing rings, which has stronger pressure resistance, and the fatigue life is designed according to a higher number of cycles.
Ash transportation in the cement industry, such as kiln tail, raw material homogenization warehouse and cement storage, in addition to powder, sometimes some clinker particles with relatively large particles are mixed in the conveying medium, which is more abrasive than fly ash. The structure of metal corrugated expansion joints in cement industry should pay more attention to wear-resistant design. For example, the guide tube is thickened, wear-resistant protective sleeves may be added at the trough of the bellows, and some even make double guide tube structures. In addition, the temperature fluctuation of flue gas at the tail of cement kiln is also large, and the machine is started and stopped frequently, so the expansion joint has to bear frequent thermal shock. Therefore, the expansion joints used in the cement industry often have a thicker wall thickness of corrugated pipes, and the fatigue design margin of materials is also greater.
Installation details make or break
If you choose the right type, the installation is a mess, and it is still useless. The most common low-level mistake is to install the guide tube in the reverse direction. The air flow in the ash pipe is directional, and the bell mouth of the guide tube must face the flow direction of the medium, so that the ash particles slip out along the guide tube. If you install it backwards, it is equivalent to directing the ash particles into the gap between the bellows and the guide tube, and the wear rate, alas, can wear it out for you in a month.
Then there's the tie rod nut. Many expansion joints are shipped with tie rods and nuts, which are used to adjust pre-deformation and limit displacement, not for fixed transport. After installation, how should the tie rod nut be adjusted? If the pipeline has been installed in the cold state, when the cold drawing amount needs to be preset, the nut should be adjusted to the designated position according to the calculated value, so that the expansion joint can expand and contract freely in the hot state. If you don't adjust it or adjust it wrong, the expansion joint is equivalent to being locked, the compensation function is completely wasted, and the bellows will be torn by thermal stress sooner or later. Here is a practical detail. Before installation, you must remove the transportation fixings, that is, those positioning screws and locking nuts, otherwise the expansion joint can't move at all.
There is another one that everyone tends to overlook-pre-deformation. If the operating temperature of the pipe is much higher than the installation temperature, the bellows should be pre-stretched in the opposite direction for a section when cold to offset the thermal elongation during operation. The specific pre-deformation amount will be written on the manufacturer's drawings, so just follow it. However, in reality, many installation teams find it troublesome, so they directly finish the welding. As a result, the bellows exceeds the design compensation amount as soon as it heats up, and the fatigue life is greatly reduced.
It leaked when you used it? Knowledge of common failure modes
Even if the selection and installation are standardized, ash-conveying metal expansion joints can't be done once and for all. You need to know where it is most likely to break down, so that daily inspections can be targeted.
The first and most common is bellows wear. It is manifested as local thinning, shining, and even pinholes in the peaks or troughs. This is mostly caused by the failure of the guide tube or unreasonable gap design. During the inspection, take a flashlight to shine on the surface of the bellows. If you see obvious metal wear marks, you should plan to replace them.
Secondly, intergranular corrosion. The flue gas in the ash pipeline contains corrosive media such as sulfur and chlorine, which is easy to cause intergranular corrosion cracks near the welds of stainless steel at high temperatures, looking like dense lines like turtle shells. Once this crack appears, the bellows is basically wasted and can't be repaired. During the inspection, focus on whether there are any areas with redness, peeling and fine cracks on the surface of the bellows.
Then there are fatigue cracks. This kind of crack usually appears at the trough, the direction is perpendicular to the bellows axial direction, and propagates from the inside to the outside. If you are an ash transportation system that starts and stops every day, the bellows have to expand and contract every day, and the number of cycles will soon be saved to hundreds of thousands. The fatigue crack is very fine at the beginning, and when the ash seepage can be seen by the naked eye, it has actually cracked through. Therefore, if conditions permit, do regular flaw detection on the expansion joint during the overhaul cycle, even with the simplest penetration detection, it can be found in advance.
In the final analysis, the ash-transporting metal expansion joint can't be used. 80% of it is because the working conditions are not taken seriously when selecting the model, and 20% of it is because the installation and inspection can't keep up. If you go through the above points, you will find that many "accidents" are actually man-made disasters. Don't be too troublesome. The expansion joint looks inconspicuous. Once it explodes, the price of shutting down the furnace and cleaning the dust is much more expensive than buying a good expansion joint.