Why do you need special expansion joints for pulverized coal pipelines?
Ordinary corrugated expansion joints are used on pulverized coal pipes and wear out in three months – this is not alarmist. Pulverized coal has fine particles, high hardness and fast flow rate. In addition, the temperature always soars above 200℃ during transportation, and there is a risk of explosion. The thin bellows of ordinary expansion joints simply can't handle it.
In the pulverizing system of power plant, the flow velocity of pulverized coal is usually 18~25m/s, and the scour of particles on the pipe wall is equivalent to continuous sandblasting operation. If only the general-purpose corrugated expansion joint is used, the inner wall will soon be perforated, and pulverized coal will leak out, which will cause environmental pollution in the slightest, and deflagration caused by fire source in the worst. This is why the industry has specifically developed the metal pulverized coal expansion joint-it is not an enhanced version of the ordinary expansion joint, but a special equipment completely redesigned from structure to material.
Understand this thoroughly, and the selection will not go wrong.
Core Structure Design of Metal Pulverized Coal Expansion Joint
A qualified metal pulverized coal expansion joint, the soul lies in triple protection: wear-resistant lining, guide tube, double seal structure.
Wear-resistant liner
The inner lining is usually made of high chromium cast iron or ceramic patch with a thickness of 5~10mm. It was directly facing the coal powder scour, like wearing a piece of armor into a bellows. We have seen an expansion joint with ceramic lining in a cement plant, which was removed and inspected after two years of continuous operation. The wear of the lining was less than 1mm-while ordinary products without lining were scrapped in half a year.
guide tube
Many people have a biased understanding of the role of deflector. It not only guides the airflow smoothly, but more importantly, establishes an "air barrier" between the bellows and the pulverized coal. There is a certain gap between the guide tube and the bellows, and a negative pressure area is formed when the air flow passes through, and the pulverized coal particles will be blocked by the guide tube and flow along the wall of the barrel, basically unable to touch the bellows. Moreover, the direction of the arrow on the guide tube must be consistent with the flow direction of the medium-if it is installed backwards, the wear speed will more than double.
Double seal structure
One of the conditions of pulverized coal explosion is that the concentration of combustible dust reaches the explosion limit, and the fire source is added. The double-seal structure avoids leakage if the single-layer seal fails, and at the same time prevents external air from penetrating into the interior to form an explosive mixture. This site'sRound Flap Door (Double Seal)AndDouble-sealed single-axis circular baffle doorThey are all the same design ideas, and they are equally reliable when used on expansion joints.
Four parameters that must be paid attention to when selecting a model
Don't ask the price as soon as you come up. Find out these four "guys" first: pressure, temperature, displacement, and pulverized coal flow rate.
Pressure:The design pressure of pulverized coal pipeline is generally 0.1~0.6MPa, but there may be instantaneous pressure fluctuation in the system. When choosing the metal pulverized coal expansion joint, the safety factor should be at least 1.5 times. If it is a positive pressure powder feeding system, the pressure resistance capacity of the bellows must be calculated to avoid bulging.
Temperature:The pulverized coal conveying temperature is usually 80~250℃, but the outlet of the coal mill may instantly rush to 350℃. Ordinary carbon steel bellows have a fast creep rate at high temperatures, so they must be made of heat-resistant alloys (such as Incoloy 800) or with heat insulation. This site'sHigh temperature axial expansion jointIt is a product for this type of working condition.
Displacement Amount:The axial, transverse and angular displacements caused by thermal expansion and contraction of pipelines should be calculated accurately. Many projects only give axial displacement and ignore transverse-as a result, the expansion joint is twisted and installed on the pipeline, and the life is directly discounted.
Pulverized coal flow rate:This is the easiest to overlook. The higher the flow rate, the worse the wear. The empirical value is: When the flow rate exceeds 20 m/s, the liner hardness is recommended to reach HRC ≥55. If the flow rate exceeds 30m/s? Don't think about it, it has to be lined with ceramic, and even metal liners can't handle it.
How to get these four parameters? They are all included in the pipeline stress analysis report given by the design institute. If not, find a craft professional to supplement the data, and never pat your head.
Common Installation Misunderstandings and Maintenance Points
I met a customer two days ago, saying that the expansion joint in their factory leaked after less than a year of use. When I went over, I found that the nut on the tie rod was tightly screwed, and the expansion joint was completely unable to retract-it turned out that the transport retainer was used as a permanent fixture. Tsk, that's a wrongful spend.
Tie Rod Adjustment:The function of the tie rod is to restrain the expansion joint from being over-stretched or compressed during installation, and at the same time bear the internal pressure thrust. After installation, the tie rod nut must be loosened, and the adjustment margin required by the design must be kept. How much looser exactly? Refer to this site'sCorrect installation method of expansion joint of large tie rodAnd the answer is there.
Direction of guide tube:Again, the arrows on the deflector must point to the direction of the media flow. How to judge? Usually an arrow is engraved on the housing. If not, look at the bell mouth of the guide tube: the small mouth is facing the incoming flow direction, and the large mouth is facing the outlet. Install backwards, the pulverized coal is directly poured into the gap of the bellows, and the wear speed soars.
Maintenance cycle:It is recommended to do a thickness measurement with each overhaul, focusing on liner wear and bellows surface corrosion. If you find that the liner is partially peeling, don't hesitate to replace it immediately. Save a little money often costs a lot of money.
Application scenario: Powder milling system in power plant and pulverized coal preparation in cement plant
The power plant milling system is the most typical scenario. The pulverized coal pipeline between the coal mill and the burner has many elbows, large temperature difference and frequent vibration. A 2×300MW unit has adopted a metal pulverized coal expansion joint with ceramic lining, which has been operated continuously for 4 years without leakage problem, and the life of the original common metal expansion joint has been increased by three times.
The pulverized coal preparation system of cement plant is not far behind. The pipeline between raw mill and coal mill, in addition to pulverized coal, has a certain humidity and corrosive gas. At this time, not only wear resistance, but also corrosion resistance is required. This site'sMetal Corrugated Expansion Joints in Cement IndustrySpecially optimized for this kind of working condition, with the double seal structure, the effect is very stable.
The selection parameters vary greatly under different working conditions. For example, the pressure fluctuation of pulverized coal pipeline in power plants is small but the temperature is high, while the cement plant is the opposite. Don't take a parameter table everywhere, you must discuss one item at a time.
Finally, back to the question at the beginning: Why is the metal pulverized coal expansion joint just needed? Because it does not solve the ordinary displacement compensation problem, but the industry pain point of "wear resistance + explosion-proof + high temperature compensation". Choose the right, install the right, and maintain the right, and it can keep you for ten or eight years under bad working conditions.