The horizontal flue expansion joint of the boiler is not selected or installed properly, which can affect the efficiency from air leakage to cracking the pipeline or even cause accidents. Two days ago, a customer spat with me, saying that the flue vibration of his boiler was particularly strong, and it didn't work to change two or three expansion joints. When I asked, I found out that the model was not selected correctly at all-the displacement was miscalculated, and the tie rod was not adjusted during installation. So today, let's disassemble it step by step, from selection to installation to maintenance, and explain every step to you.
Step 1: Find out the bottom details of the horizontal flue of the boiler-the temperature, pressure and displacement must be missed
Don't be in a hurry to place an order, first get the three key parameters of your flue:Maximum operating temperature、Design pressure、Amount of displacement in each direction。 The temperature depends on the flue gas temperature at the boiler outlet. Generally, medium and low pressure boilers are 200~400℃, and high pressure boilers may be above 600℃. Most of the pressures are slightly positive or negative, but there are positive pressure boilers that can reach tens of kPa. The amount of displacement is the easiest to overlook-you have to consider axial elongation due to thermal expansion, lateral displacement due to pipe vibration, and angular displacement due to installation errors.
How to measure? Check the calculated value of thermal expansion on the boiler design drawings, or directly use the field measured data. When measuring the displacement, don't only measure one direction. Horizontal flue usually has three displacements (X, Y, Z), especially the lateral displacement is often larger than the axial displacement. Remember: with less displacement, the expansion joint will be pulled out; More, and waste of cost.
Tip: If the drawing is lost, it can be estimated by empirical formula: carbon steel pipe expands about 1.2mm per meter of temperature rise of 100℃. For example, the 10-meter-long flue rises from normal temperature to 400℃, and the axial expansion is about 48mm. Of course, this is only for reference, and it is best to let the manufacturer calculate according to the actual working conditions.
Step 2: Metal corrugated expansion joint vs non-metallic expansion joint, which one to choose?
Now that you know the parameters, select the type next. There are two mainstream categories in the market:Metal corrugated expansion jointAndNon-metallic expansion joint(also called fabric fiber expansion joint). The metal has high strength, high temperature resistance and strong pressure bearing capacity, like this station'sHigh temperature axial expansion jointAndUniversal corrugated expansion jointIt is suitable for working conditions where the temperature exceeds 400 ℃ and the pressure is high. However, the disadvantage is that it cannot absorb too much lateral displacement, and the cost is relatively high.
What about non-metallic expansion joints? It is made of fabric fiber and rubber/fluoroplastic composite layer, which is soft and elastic, can absorb large multi-directional displacement, and can also isolate vibration and silence noise. This site'sNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion jointIs a typical representative. It is suitable for horizontal flue with low temperature (generally below 300℃), normal pressure or micro pressure and large displacement. The disadvantages are that it is not resistant to high pressure and has a relatively short life.
How to choose? Think about it: if the flue temperature exceeds 400℃, the pressure is greater than 10kPa, and the displacement is mainly axial, decisively on metal. If the temperature is not high and the pressure is low, but the flue vibration is large and the lateral displacement is large, or you are particularly concerned about vibration and noise reduction, then choose non-metal. Of course, there are also hybrid schemes-metal for the front section of the flue and non-metal for the rear section, each taking its strengths.
Tsk, to tell the truth, most of the boiler horizontal flue is non-standard rectangular section, I recommend you to see this siteMetal rectangular expansion jointAndRectangular non-metallic expansion joint, these two series are specifically designed for flues, and the interface size and flange match well.
Step 3: Rectangle or Circle? Scheme according to flue size and installation space
The shape of the flue section determines the shape of the expansion joint. The vast majority of boiler horizontal flues are rectangular because it is convenient to connect the boiler outlet with equipment such as dust collectors and fans. Then choose the rectangular expansion joint. Circular expansion joints are mainly used on circular pipes, such as the tail flue of power station boilers or desulfurization systems. If your flue is round, this siteCorrugated expansion joint for power station industryAndDouble-sealed single-axis circular baffle doorMatching compensators can be considered.
But rectangle and circle are not random choices, you also have to look at the installation space. Rectangular expansion joints typically require larger flange widths, and if the site space is narrow, such as close to a wall, it may be necessary to custom-shape or choose a single-side bolt-mounted model. In addition, the corner stress of the rectangular expansion joint is concentrated, and the structure needs to be strengthened.Large diameter thick wall expansion jointIt is suitable for large size and high strength needs.
Note: The aspect ratio of rectangular expansion joint should not be too exaggerated, and it is best not to exceed 2:1, otherwise the corrugation compensation will be uneven and it is easy to crack locally.
Step 4: 3 Things You Must Do Before Installation-Check the Deflector, Adjust the Tie Rod Nut, Remove the Transport Screw
When the goods arrive, don't rush to weld on the flue. Do three things first, none less.
- Check the guide tube orientation.The function of the guide tube is to guide the flow direction of the flue gas and reduce the erosion of the bellows by the airflow. The direction of its arrow must be consistent with the direction of flue gas flow (see the question and answer of this site "Specific Function of Expansion Joint Guide Tube"). Install it backwards, and the smoke directly impacts the bellows, which will wear out in three months. So look at the arrow mark on the deflector as soon as you unpack it.
- Adjust the tie rod nut.The role of the tie rod is to limit the expansion joint from excessive stretching or compression, and it is also a positioning tool during installation. When many expansion joints leave the factory, the tie rod nut is locked. You need to adjust the nut to the pre-tensioned or pre-compressed position according to the design displacement. How exactly to adjust? Look at the question and answer "How to adjust the tie rod nut of the expansion joint" on this site. Simply put, it means to loosen the locking nut with a wrench, adjust the movable nut to the specified scale, and then lock it.
- Remove the transport screw.This is often overlooked! The transport screw is temporarily fixed to prevent the expansion joint from deforming during transportation, and must be completely removed during installation. If it is not disassembled, the expansion joint is equal to being welded to the pipeline, which can't compensate for the displacement at all, and will soon be cracked. Remember: the removed screws and nuts are kept away, don't lose them, may be used for future maintenance.
Step 5: On-site installation practical operation-welding sequence, bolt tightening torque, seal inspection
The preparations are done and the installation begins. If it is welded, the welding sequence is very critical: first spot weld two fixed positions, then weld them in sections from the middle to both ends, and then weld the next section after each section is cooled to prevent local overheating and deformation. The welding current should not be too large to avoid burning through the guide tube or bellows body. It is recommended to use stainless steel welding electrode or argon arc welding, clean the welding slag after welding, and apply anti-rust paint.
If it is a flange bolt connection, use a torque wrench. The torque value refers to the manufacturer's instructions. Generally, the M16 bolt is 40~60N·m, and the M20 bolt is 80~100N·m. The tightening sequence should be alternately tightened diagonally and done in two times, the first time preloading to 50% and the second time to the end. Don't screw to death at once, otherwise the flange surface will deform and leak.
Spray soapy water at the junction of the weld and flange, introduce compressed air (or start the boiler to run at low load), and see if any bubbles pop up. If there are bubbles, mark the location, repair weld or replace the sealing gasket. Advanced points can detect the air leakage rate with a flue gas analyzer. And guess what? Many leaks are caused by unevenly tightened bolts, so torque control is a top priority.
Step 6: Routine Maintenance and Troubleshooting – When Should You Change? How to judge an air leak?
Get it installed, don't be a hands-off shopkeeper. Regular inspections are key. Check at least once a year: see if there are cracks, corrosion pits and deformations on the surface of the bellows; Non-metal expansion joints see whether the fabric layer is aged, brittle and delaminated; Whether the guide tube is worn or perforated. Listen for a hissing air leak and smell for smoke.
So when should I change it? Change decisively when the following situation occurs:
- Corrosion pits or cracks in metal bellows deeper than 30% of the wall thickness;
- The non-metallic fabric layer is partially damaged or severely aged and loses its elasticity;
- The expansion joint is permanently deformed and cannot be restored to its original size;
- The amount of leakage exceeds the design allowable value (e.g. air leakage rate is greater than 3%).
How to judge an air leak? The most earthy way: stick a paper towel to the weld or flange gap to see if it blows. Or use a smoke generator (fireworks sticks will work) to observe the direction of the smoke near the suspected leak point. In the stationFlue gas baffle doorAndDesulfurization flue gas baffle doorIt is often used in conjunction with expansion joints. If the pressure difference between the front and rear of the baffle door is found to be abnormal, the expansion joint may be leaking.
Generally speaking, it is not a problem to use a boiler horizontal flue expansion joint for five or six years if you choose it right, install it well and check it frequently. If you are still unsure, don't guess. It is much more reliable than patting your head directly with the three parameters of temperature, pressure and displacement to the manufacturer. That's it for today. I'll see you in the comment section if you have any questions.