1. How "difficult" is the boiler powder discharge pipe? — — High temperature, wear, corrosion, all of them
Anyone who has worked in a power station boiler knows that the powder pipe, which is said to be a "pipe", is actually more difficult to serve than a sewer. The temperature of pulverized coal is 120℃ ~150℃ all the year round. When the outlet temperature of inferior coal or coal mill is high, it is not unusual to rush to 180℃. The air-powder mixture washes the pipe wall at a speed of 20~30m/s, and there are still unground coal particles sandwiched inside, just like handfuls of sandpaper being grinded. Coupled with the acidic corrosion caused by sulfur and moisture, even ordinary metal pipes can't bear the triple critical attack, let alone the expansion joints at the pipeline connections.
Their factory changed three metal expansion joints in a month. The welds cracked, the bellows wore out, and the pulverized coal leaked all over the ground. The environmental protection department fined it twice. He slapped his thigh and said, "If I had known how much money would I have saved if I had gone directly to non-metallic ones!"
You see, the problem lies here-the working condition of the powder discharge tube determines that it can't choose a compensator according to the conventional idea. Either frequent shutdown to change parts, or gritting your teeth to plug leaks, which will compromise the efficiency of the production line.
Second, why is the metal expansion joint easy to "reduce life" on the powder discharge tube? Breaking the situation of non-metallic expansion joint
Don't rush to deny metal expansion joints yet. General-purpose corrugated expansion joints are used well in steam pipes and hot water pipes, but that is because the medium is clean and the temperature is stable. When we got to the powder outlet pipe, the situation completely changed:
- Fast wear: Pulverized coal particles hit the bellows crest at high speed, and the hole can be ground out in a few months;
- Severe powder accumulation: Powder is easy to hide inside the bellows, which causes additional weight and affects expansion and contraction once it is accumulated;
- Stress concentration: The fatigue life of metal bellows is limited by the number of displacements, and the powder discharge tube vibrates frequently, so the life of metal bellows decreases linearly.
To put it bluntly, using metal ones to deal with coal powder is equivalent to letting people in suits move bricks. It's weird if they don't rotten.
At this time,Non-metallic expansion jointThe advantages of (also called fabric fiber expansion joint) come out. It does not have the rigid structure of metal bellows, but absorbs displacement by soft connections. The main body is made of fabric fibers, rubber and insulation materials. The pulverized coal touches a soft surface, and the impact energy is dispersed, so the wear is naturally much less. Moreover, its interior is flat, and the powder is not easy to accumulate-this is precisely the most needed characteristic of the powder outlet tube.
Of course, non-metals are not a panacea, and if you select the type incorrectly, you will be finished. Let me break it apart and talk about it.
III. Structure and material selection of non-metallic expansion joints: how to build fabric fiber layer, rubber layer and thermal insulation layer?
According to the provisions of JB/T 12235-2015 "Non-metallic Expansion Joints", qualified non-metallic expansion joints for powder discharge tubes must have at least three layers:
- Fabric fiber layer(innermost layer): direct contact with pulverized coal. Commonly used materials are glass fiber cloth, polytetrafluoroethylene (PTFE) coated cloth, and even high silicon oxide fiber. If the temperature exceeds 200℃, you have to add ceramic fiber cloth. The benefit of PTFE coating is that it does not stick to powder and prevents dust accumulation.
- Thermal insulation(Intermediate Layer): Blocks heat conduction. Aluminum silicate fiber felt is the mainstream, and its thickness depends on the design temperature. Generally, 20~30mm is enough for the powder tube.
- rubber layer(Outer layer): Provides airtightness and mechanical strength. Nitrile rubber is oil and powder resistant, neoprene has good weather resistance, and fluororubber is high temperature resistant but expensive. Most manufacturers use nitrile rubber compound, which is cost-effective.
Some manufacturers also add a layer of stainless steel wire mesh or sheath on the outermost side to prevent foreign bodies from puncturing. Listen to me, this jacket is best added to the powder tube-whoever accidentally drops a wrench on it during maintenance, you'll know how important it is.
IV. Installation and maintenance: Should the expansion joint screw be disassembled? How to adjust the tie rod nut? Don't let the details ruin a good product
Good things have to be fitted and installed. The tragedy of many power plants is that they make low-level mistakes in installation.
First question: Should the screw (transport fixing screw) of the expansion joint be disassembled?— It must be dismantled! This thing is temporarily locked when it leaves the factory in order to prevent transportation deformation. If it is not disassembled, it is equivalent to forcibly fixing the expansion joint in the installation position. How can it compensate for the displacement? Direct tear when running. It is also clear in the FAQ that if the screw is not removed, the non-metallic layer will be damaged by excessive stretching.
Second question: How to adjust the tie rod nut?-Expansion joints are usually equipped with tie rods, which act to limit overstretching but allow compression. When the powder discharge pipe is hot, the pipe will extend. Generally, adjust the tie rod nut to the middle position during installation, and reserve the displacement in ± direction. The specific adjustment value depends on the design cold tightness, and there is no general value. But remember one thing: the nut cannot be locked, and there must be clearance for movement.
In addition, the flange connecting surface should be cleaned, the gasket should be temperature-resistant and oil-resistant (asbestos rubber pad or polytetrafluoroethylene pad), and the bolts should be tightened diagonally to achieve uniform torque. I've seen people trying to save trouble, only twisting two bolts, and as a result, the flange leaked powder within two days.
V. From the national standard JB/T 12235-2015 to the field application: how to select the type according to the working conditions to last for three years?
Medium temperature, working pressure, pipeline displacement。 The pressure of the powder discharge tube is usually not high, 0.1~0.5MPa, but the temperature fluctuates greatly. According to JB/T 12235-2015, the design temperature of non-metallic expansion joint should be 20℃ higher than the maximum operating temperature, leaving sufficient margin.
The amount of displacement depends on the tubing arrangement and the calculation of thermal expansion. Generally, the straight section of the powder outlet pipe is long, and the axial displacement is mainly 10~30mm, and the transverse displacement is several millimeters. When selecting, pay attention to the allowable compensation amount of the product, rather bigger than stuck. Suggested choiceNon-metallic expansion joint (fabric fiber expansion joint)The model with PTFE lining has better anti-abrasion effect.
There's another one that's easy to overlookguide tube。 A guide tube is added in the powder outlet pipe, which can guide the pulverized coal through the center of the pipe and avoid directly washing the fabric layer of the expansion joint. This thing looks simple, but it does a huge job of extending life. The question and answer also talks about the specific function of the guide tube, which is actually to protect the expansion joint body.
According to my experience, choose the right type and install it in place, and it will be used for three to five years without problems.
VI. Common faults and countermeasures: ash accumulation of pulverized coal, fiber tearing, flange leakage, disassembly one trick at a time
- Ash accumulation of pulverized coal: The inner PTFE coating is aged or peeled off resulting in sticky powder. Countermeasures: Stop the machine regularly to check the coating status, and replace the liner if necessary. If you find it troublesome to stop the furnace, you can chooseDouble-sealed single-axis circular baffle doorSuch isolation devices, shut off the pipeline during maintenance.
- Fiber tear: It is mostly caused by installation deflection or abnormal displacement exceeding the limit. Countermeasures: Check whether the tie rod is adjusted in place and whether the pipe bracket is loose. Also, don't let welding sparks splash on the fabric, a spark is a hole.
- Flange leakage: Aging spacers or loose bolts. Countermeasures: Replace with temperature-resistant gaskets and tighten the flange bolts once every six months.
To put it bluntly, the non-metallic expansion joint on the powder outlet tube is not just installed, but it has to be kept as a core component. Like when you buy a good pair of shoes, you have to rub oil regularly, don't you?
Alas, one last sentence: stop taking the metal expansion joint to eject the position of the powder tube, and change it early.