Why do you need special metal expansion joints for roller press systems?
Ordinary expansion joints are used on roller presses, and they leak in three months. This is not an exaggeration-the temperature of the inlet and outlet pipeline of the roller press in cement plant always rushes to 350℃, the pressure is 0.8MPa, the material impact plus continuous vibration, the axial, transverse and angular displacement are trinity, and ordinary general-purpose bellows can't bear it at all. From our stationMetal Corrugated Expansion Joints in Cement Industry、High temperature axial expansion jointIs specially designed for this type of working condition. The corrugated pipe is made of high-temperature resistant stainless steel, the thickness of the guide tube is thickened, and the flange sealing surface is equipped with high-strength bolts, so that the whole set can cope with high dust erosion and complicated displacement.
Then why do you have to use special parts? For example: When the roller press is running, the material hits the pipe elbow at a speed of tens of meters per second, generating a strong impact load. Ordinary expansion joint bellows have thin walls, missing or too short guide tubes, particles directly wear the bellows, and the fatigue life is cut in half. Coupled with the thermal expansion and contraction caused by the start and stop of the equipment, every cycle of the bellows is a fatigue damage. Without targeted design, it is normal to leak in half a year.
Common Types and Structural Characteristics of Metal Expansion Joints of Roller Press
Don't just focus on the bellows when selecting the model-the guide tube, tie rod and flange sealing surfaces must all match. According to the actual working conditions, let's go through the common types:
- High temperature section:chooseExternal pressure single axial expansion joint。 This structure corrugated pipe is subjected to external pressure, has good stability, and is suitable for pipelines with temperatures exceeding 400 ℃. With high-temperature resistant stainless steel (such as 254SMO), it can hold high-temperature steam and clinker dust at the outlet of the roller press.
- Large displacement occasions:For example, the connecting pipeline between the roller press and the hoist has a large axial displacement and a lateral offset. At this time, useCompound hinge transverse expansion jointThe two bellows are hinged in the middle, which can absorb large lateral displacement and bear the pressure thrust of the pipeline at the same time.
- Rectangular pipe:Some old line modifications or special interfaces adopt rectangular cross-sections, so you have to goMetal rectangular expansion joint。 This structure is specially matched with non-standard pipelines, the corrugated plate is molded, and the guide tube is made into a rectangular shape, so the sealing effect is no worse than that of a circular shape.
What problem does each structure solve? For example, a customer used a general-purpose expansion joint at the import of a roller press. As a result, the bellows was pulled and twisted because the lateral displacement was not absorbed. After changing to the double hinge type, the problem is solved. Therefore, you can't pat your head when selecting a model, you have to look at the actual displacement data.
Selection core parameters: temperature, pressure, displacement and fatigue life
How to determine the design parameters? Take the experience of cement industry to say: the temperature of the outlet pipeline of roller press usually reaches 350℃, the pressure is 0.8MPa, and the axial displacement is 30mm. According to EJMA standards, the fatigue life is not less than 1000 cycles-this number is not written casually, corresponding to the equipment overhaul cycle of 3-5 years. High-temperature resistant stainless steel, such as 310S or Incoloy 800, must be used when the temperature exceeds 400℃. The creep strength of ordinary 304 drops sharply at high temperature.
What if there are frequent pressure fluctuations? Check bellows stiffness. The stiffness is low, the bellows deforms greatly and the stress is concentrated under the action of pressure; When the stiffness is higher, the displacement compensation ability decreases. Compromise: Match pressure and displacement by adjusting the number of bellows layers and wave height. In addition, the displacement amount should not only look at the design value-installation deviation and foundation settlement should be considered. For example, the actual level of the pipeline has a deviation of 5mm, the designed axial displacement is 30mm, and the actual comprehensive displacement may reach 35mm. If the effective length of the bellows is not enough, it is easy to crack.
Installation and maintenance: common myths and correct practices
Two days ago, I met a customer who installed the arrow of the expansion joint in the reverse direction. And guess what? The guide tube was directly washed away by the materials, and the bellows was exposed, which was worn out in one month. Remember:The direction of the arrow of the expansion joint refers to the direction of the medium flowWith the arrow pointing in the direction of the exit. The function of the guide tube is to divert the flow and protect the bellows from particle erosion-once this thing fails, the life of the bellows will be cut in half (refer to the question and answer "Specific function of the expansion joint guide tube" on this site).
How to adjust the tie rod nut? After installation, the positioning nut should be loosened. The screw cannot be removed but the pre-compression amount can be adjusted. In the cold state, the expansion joint is pre-compressed by 10-15mm to compensate for thermal elongation. Some customers did not do pre-compression, and the bellows was overstretched and directly cracked when it was hot. So installation details: nitrogen displacement (to prevent internal hydrogen embrittlement), cold displacement compensation, symmetrical loosening of nuts on both sides of the tie rod-none can be saved. For the correct installation method, please refer to "Correct Installation Method of Large Tie Rod Expansion Joint" on this site.
Fault Cases and Solutions
Tell me two true stories.
In the roller press system of a cement plant, the expansion joint leaked after half a year. When disassembled, it was found that the inner wall of the bellows was ground out of pits by materials. The reason? No deflector! They use a general-purpose expansion joint without a guide tube. Change this siteMetal Corrugated Expansion Joints in Cement Industry(with thickened deflector), used for two years without problems. Therefore, in high dust conditions, the guide tube is standard, not optional.
Another factory did not do enough cold pre-compression during installation. During hot operation, the bellows is overstretched, the wave distance is pulled away, and the wave trough is directly torn. The design fatigue life is 1000 times, but it is actually scrapped after only 200 times. Solution: Re-install, set the pre-compression amount according to the calculated value, and add a tie rod limit device to prevent accidental over-stretching. Now this factory has not changed the expansion joint for three years.
Another time, the flange sealing surface was not cleaned, and the bolts were loose during operation after installation, resulting in air leakage. Therefore, the maintenance inspection should pay attention to three points: whether the guide tube is worn, whether the tie rod nut is loose, and whether the flange bolt torque is attenuated. These problems seem small, but if you don't pay attention, the cost of replacing a set of expansion joints is worth ten inspections.
Alas, after doing this for a long time, I found that most faults are not design problems, but the details of installation and maintenance are not in place. So the last long-winded sentence: find a professional in the selection, install according to the standard, and don't be lazy in the inspection-only the metal expansion joint of the roller press can be used.