First, it is not unnecessary to install expansion joints in the flue-let's talk about thermal expansion and cold contraction first
Have you ever seen the flue of a thermal power plant, cement plant or chemical plant? Steel pipes of tens or even hundreds of meters are filled with hundreds of degrees of high-temperature smoke. It is cold in winter and hot in summer, and the temperature difference is just two or three hundred degrees Celsius. The coefficient of thermal expansion and contraction of steel is fixed. It is calculated that a 50-meter-long flue can be elongated by more than 120 mm when the temperature rises by 200℃. If this displacement is hardly carried and indigested, the welds, bolts and flanges at the pipe connections will be cracked or deformed in minutes. So here comes the role of the flue internal expansion joint-it is not an unnecessary part, but an elastic element specifically designed to eat these thermal displacements.
Two days ago, I met a customer who was doing a desulfurization project, and told me that their factory used to think that the expansion joint was dispensable. As a result, less than half a year after it was put into operation, there were sparks at the flue interface, and the maintenance window couldn't be opened. Later, we changed the corrugated expansion joint used in our power station industry, and there was no problem again. Therefore, to put it bluntly, the function of the expansion joint inside the flue is to make the pipe "breathe". When the temperature changes, it can expand and contract without transmitting stress to the equipment.
2. Three hard-core functions: displacement absorption, vibration isolation and noise reduction, sealing and leakage prevention, none of which can be missed
What exactly do the expansion joints do in the flue? Broke open your fingers and counted, there were only three cores:
- Absorption displacement。 Whether it is axial expansion or contraction, lateral offset or angular displacement, the expansion joint is absorbed by the deformation of the corrugation. For example, our commonly used general-purpose corrugated expansion joint has a single wave compensation of 10~30mm, and multiple waves are used in series, so it is no problem to deal with the displacement of several hundred millimeters. If you think about it, without it, the thrust generated by the thermal expansion of the pipe can push the flue support askew, or even make the fan misalign.
- Vibration isolation and noise reduction。 The flue flow in the flue and the rotation of the fan will generate vibration, and the vibration spread throughout the whole pipeline system will accelerate fatigue damage. The corrugated structure of the expansion joint is equivalent to a flexible joint, which can cut off the vibration transmission path. Especially the multi-hinge structure such as the double hinge expansion joint of air-cooled island vacuum pipeline can also absorb the compound displacement and have a more fierce vibration reduction effect.
- Sealed and leakproof。 There are often SO2, NOx and even dust particles in the flue gas, which leak out to either pollute the environment or corrode outdoor equipment. Non-metallic expansion joint (fabric fiber expansion joint) is made of multi-layer composite fabric + sealing layer, which is temperature-resistant and corrosion-resistant; The metal rectangular expansion joint is pressurized by stainless steel bellows and guide tube. Once the seal fails, the consequence is shutdown for emergency repair.
None of these three functions can be missing-less displacement absorption, pipeline stress accumulation; Without vibration isolation and noise reduction, the equipment life is discounted; Without sealing and leakproof, environmental fines can't run. Do you say that the function of the expansion joint inside the flue is important?
3. What are the most commonly used expansion joints in flue? Metal or non-metal, depends on the working conditions
There are many types of expansion joints on the market, but there are only two most common types in flue systems: metal corrugated expansion joints and non-metal expansion joints.
Metals such as high-temperature axial expansion joints, compound hinge transverse expansion joints and curved tube pressure balance expansion joints are mainly used in occasions with high temperature and high pressure. The boiler flue of power station and the exhaust gas pipeline at the tail of cement kiln have a temperature of five or six hundred degrees Celsius and a pressure of several MPa. They must be made of metal. Our corrugated expansion joints for the power station industry are made of multi-layered stainless steel, resistant to high temperature fatigue, and also equipped with a guide tube (Specific Function of Expansion Joint Guide TubeIs to prevent high-speed airflow from washing the inside of the corrugation).
Non-metallic, such as rectangular non-metallic expansion joints, rubber compensators and PTFE compensators, have the advantages of large compensation, corrosion resistance and easy installation. In the desulfurization and denitrification system, the flue gas contains acid, humidity and low temperature (100~200℃), so non-metallic expansion joints are particularly suitable. In some places, rubber PTFE compensator is also used, which can be corrosion-proof and flexible. In one sentence: high temperature and high pressure selection of metals, low temperature corrosion selection of non-metals.
4. Power station, cement, desulfurization and denitrification... Different flue scenarios, the selection difference is bigger than you think
- Power station: Between the tail flue of the boiler and the desulfurization tower, the pipe diameter is large, the temperature is high, and the axial displacement is the main one. Usually, external pressure single axial expansion joint or straight tube pressure balanced expansion joint are used. The former can withstand high internal pressure, and the latter can balance the blind plate force and reduce the thrust on the bracket. Flue gas baffle doors and double-sealed single-axis circular baffle doors are also often used in conjunction with expansion joints to achieve maintenance and isolation.
- Cement: From the kiln tail preheater to the humidification tower, the temperature fluctuates greatly (from 700℃ to 300℃), and there is a lot of dust. The metal corrugated expansion joint in cement industry must be lined with wear-resistant lining, and the guide tube is designed as a bell mouth to prevent dust accumulation. Sometimes it is also cooperated with manual plug-in plate insulation door or electric plug-in plate insulation door to cut off the flue when the kiln is shut down.
- Desulfurization and denitrification: The original flue gas contains sulfur and acid, and the net flue gas has high humidity. Here the non-metallic expansion joint is the main force, the rectangular non-metallic expansion energy saving adapts to the rectangular flue, and the installation is flexible; The desulfurization flue gas baffle door is used to switch the bypass. If the temperature increases occasionally, also use a PTFE-lined hose to transition a section.
You see, it is also called the flue internal expansion joint. The selection of power plants is completely different from that of cement plants. What happens if you choose the wrong one? Here are a few real lessons.
5. Don't take it lightly on installation and daily maintenance: guide tube, pull rod and direction arrow are all the keys
Many factories buy the expansion joint back, and just screw a few screws when installing it. It broke down within a few months. In fact, there are many ways to install and maintain:
The direction of the guide tube cannot be reversed.The specific function of the expansion joint guide tube is to guide the airflow and avoid the high-speed flue gas directly washing the ripples. During installation, the opening direction of the guide tube must be facing the direction of air flow, and the arrow should point in the same direction as the medium flow direction (The direction of the arrow of the expansion joint refers toThis). If it is reversed, the guide tube will be blown off by the airflow, and the ripples will quickly wear down.
The tie rod nut must be adjusted in place.Large tie rod expansion joint or general-purpose corrugated expansion joint with tie rod, the tie rod nut is pre-tightened when leaving the factory, and there is no displacement during transportation. After installation in place, loosen the nut to allow the corrugation to expand and contract freely. Some people don't dismantle the tie rod, but directly use the expansion joint as a rigid part, so what is the compensation? The role of the expansion joint tie rod is to limit excessive elongation or compression, but the working stroke must be allowed.
Look at three points in daily inspection:See whether there are cracks and corrosion spots on the corrugated surface; Second, see whether the non-metallic expansion joint fabric layer is aged and delaminated; Third, see if the bracket is loose. The service life of the expansion joint depends on the working condition. Generally, the metal can be used for 5~10 years, and the non-metal need to be replaced for 2~5 years. Check it regularly and don't wait for an air leak before repairing it.
6. What happens if the wrong expansion joint is selected? Talk about the live lessons we've seen in those years
I have been in this business for more than ten years, and the rollover cases I have seen are worse than the last. Let's talk about three representative ones:
In the desulfurization flue of a chemical plant, the rubber compensator was originally designed. As a result, the temperature of the inlet flue gas occasionally exceeded the standard to 250℃, and the rubber was directly carbonized and cracked, and white smoke billowed on the spot. Later, it was replaced with a polytetrafluoroethylene compensator, and the temperature resistance can reach 280℃, which is considered stable.
The outlet of induced draft fan of a thermal power plant uses ordinary general-purpose corrugated expansion joint, and the lateral displacement is not counted. As soon as the fan was turned on, the pipe swung too much laterally, and the bellows were pulled out of wrinkles, leaking air in less than three months. After replacing the double hinge transverse expansion joint, it can absorb the transverse and angular displacement, and there is no problem again.
There is also a cement factory. The metal rectangular expansion joint is selected in the kiln tail flue but no guide tube is added. The flue gas washes the inner wall of the corrugation with clinker particles, and the corrugation is worn out in half a year. Later, the guide tube was added, and it was replaced with a thickened large-diameter thick-walled expansion joint, which tripled its life.
In order to really play the role of the internal expansion joint of the flue, one link of selection, installation and maintenance cannot be saved. Don't buy the wrong model cheaply, and don't be lazy and save the installation steps.
Okay, enough talk for today. If you are doing a flue design or modification, it is recommended to re-check the expansion joint you used against the above-is it the right choice? Are you playing right? Have you checked regularly?