How "bad" are catalytic cracking flue systems? High temperature, dust and large displacement are quite a few
Anyone engaged in catalytic cracking knows that the environment of the flue system is simply the "hell mode" of equipment. The temperature soars above 650℃ at every turn, and when the regenerator runs the agent, it is also common to have seven or eight hundred degrees in an instant. Coupled with the catalyst dust, the particles were finer than flour. Under high-speed washing, ordinary steel could be worn through in a few days. What's even more headache is that the pipeline expands from cold to hot, and the displacement direction and amplitude are still changing when driving, parking and reverse switching. Think about it, under such working conditions, if the flue expansion joint is not selected correctly, it can't hold it at all.
What exactly does the flue expansion joint carry? Not just thermal expansion and cold contraction
Many people think that the expansion joint is to absorb heat expansion and contraction, wrong! The expansion joint in the catalytic flue has to carry at least three things:Axial displacement, lateral displacement, angular displacement。 Not only the length change, but also the blind plate force generated by the gravity, wind load, earthquake load and internal pressure of the pipeline itself have to be digested by it. One more thing – vibration. The inlet of the hood, the outlet of the cyclone separator, the low-frequency vibration brought by the pulsation of the air flow, if the expansion section is selected into a rigid structure, the weld will crack sooner or later. To put it bluntly, it is the "flexible joint" in the pipeline system, which must be able to move and air-proof.
For example, the flue from the regenerator outlet to the triple rotary inlet often needs to compensate for both horizontal and vertical displacements. At this time, if you only have aUniversal corrugated expansion jointNo guide tube was added, the fatigue life was not calculated, and it was scrapped in half a year. A truly reliable scheme has to be usedHigh temperature axial expansion jointWith a guide tube, or for large displacementCompound hinge transverse expansion joint。
Why do your expansion joints always break? Three common ways to die
I have seen too many cases at the scene, and I can sum it up in three ways to die:
- Death Way 1: Cracking from fatigue.If the wall thickness of the bellows is thin, the wavenumber is not enough, the displacement stress exceeds the limit, and it will crack after hundreds of thermal cycles. This kind of death is the most injustice, obviously choose oneLarge diameter thick wall expansion jointOr you can solve it by adding multiple layers of corrugation, but you have to save that little money.
- Death Way Two: Corrosion Perforation.Catalytic flue gas contains SOx and NOx, and when it encounters condensed water, it becomes sulfurous acid and nitric acid, plus chloride ions, and 316L can't bear it. What's terrible is that many people don't know that there is a condensation zone hidden in the lining or insulation, which can't be seen from the outside. This kind of occasion must beNon-metallic expansion joint (fabric fiber expansion joint)OrPTFE-lined hoseBut the cost is that the pressure resistance and wear resistance should be accounted for separately.
- Death 3: Instability and deformation.If the internal pressure is too large, the bellows will become columnar instable or plane instable, and the whole bulge. The reason is simple: the design doesn't count for stabilizing pressure, or the tie rod nut is locked during installation. Remember,How to adjust the tie rod nut of expansion jointIt is particular-it must be loosened to the designed pre-stretch amount before driving, otherwise it is equivalent to letting it carry hard.
Alas, in the final analysis, the expansion joint can't speak itself. If it is broken, it will directly emit black smoke, channel the catalyst, and even stop the car in an emergency. One stop, hundreds of thousands are gone in a day.
Don't be blind in type selection: material, structure and seal can't be missed
Then how to choose? Don't report the model number with the drawing, you have to dismantle it from three dimensions.
Let's startMaterials。 If the temperature is lower than 400℃, 304 or 316L is enough, but the main path of catalytic flue is basically above 550℃. At this time, heat-resistant alloys, such as Inconel 625 and Hastelloy C-276, must be applied. If you use ordinary stainless steel for cheap, high-temperature creep will directly scrap it. In addition, the material of the guide tube should also be matched, otherwise it will be the first to rot.
AgainStructure。 Do you choose metal or non-metal? The metal has good pressure bearing and long life, but the displacement ability is limited; Non-metallic (fabric fiber) can absorb large displacement and dampen vibration, but it has poor pressure and wear resistance. Therefore, the typical practice is: the main flueHigh temperature axial expansion jointFor (metal bellows), bypass or cold flueNon-metallic expansion jointEven on some rectangular fluesMetal rectangular expansion joint。 One more detail – do you want to be lined with insulation? A must! Otherwise, the bellows is directly exposed to high-temperature flue gas, and the oxidation rate is extremely fast.
And finallySeal。 Although the catalytic flue pressure is not high (0.1~0.3MPa), it cannot leak, which will affect the energy recovery and environmental protection index. The metal bellows itself is a rigid seal, but the welding quality of the trough and the lap clearance of the guide tube are the key points. For non-metallic expansion joints, we should pay attention to the number of fabric layers and the structure of beading. The common failure point is flange surface leakage. Oh, yeah,Flue gas baffle doorWhen used in conjunction with the expansion joint, the sealing property of the baffle door directly affects the force of the expansion joint, so don't underestimate this point.
Right installation can be used: doorways in installation and maintenance
Choosing the right doesn't mean it is once and for all, and pretending to be wrong is useless. Let's talk about the installation direction first-the expansion joint has directions!The direction of the arrow of the expansion joint refers toThe medium flow direction, the small opening of the guide tube faces upstream, otherwise the airflow directly washes the bellows. Once there was a project that loaded the direction backwards and wore out in three months.
Pre-tension/Pre-compression: If the installation temperature of the pipe is lower than the operating temperature, the pre-tension value is calculated based on the expansion amount, which is achieved by adjusting the tie rod nut. How exactly to adjust? Read the manufacturer's installation manual, but the core principle is-reserve the hot displacement in the cold state, and don't pull it to death.
Supports and hangers can't be ignored either. Guide brackets must be set near the expansion joint to prevent lateral instability of the pipeline. Otherwise, when the expansion joint is pushed during operation, it will be crooked by itself, and it will not be far from damage.
What about routine maintenance? You check the appearance at least once every six months to see if there are bulges, corrosion, cracks, and listen to any abnormal noises by the way. If possible, do an airtightness test when parking. In addition, if you find that the guide tube falls off or is seriously worn, change it quickly, otherwise the consequence will be that the bellows "streaks naked" directly.
To put it bluntly, the catalytic device flue expansion joint is not something that can be dealt with by picking a general-purpose part. You have to check one by one according to the actual temperature, pressure, displacement and media composition. It is best to ask the supplier to provide a fatigue life calculation sheet. I've seen a refinery becauseCorrugated expansion joint for power station industryIt was used directly on the catalytic flue, but it was changed three times in less than half a year. Finally, I honestly customized itCurved tube pressure balance expansion jointThen he stopped.
Therefore, the secret to fewer failures is four words:Prescribe the right medicine。 Next time, choose the flue expansion joint of the catalytic device. Go through these points, which can help you save hundreds of thousands of maintenance costs.