What exactly is a double flange metal expansion joint? — — Structural disassembly and working principle
To put it bluntly, the double-flange metal expansion joint is a telescopic metal bellows, with a flange welded at each end. The bellows itself is a thin-walled corrugated structure made of stainless steel (or alloy), which absorbs the displacement caused by thermal expansion and contraction of the pipe by elastic deformation of wave peaks and troughs. You put it on a steam pipe, and as soon as the temperature goes up, the pipe becomes longer, and the bellows retracts itself; The temperature drops and it bounces back. It's a simple truth.
The bellows body, the termination pipe (the straight section connecting the flange to the bellows), and the flange. Some models also come with a guide tube (anti-scouring), a tie rod (for limiting) or an outer sheath (to prevent mechanical damage). Note that when we say double flange, we mean that both ends are flanged, not loose flange or butt welding. Flange standards are usually implemented according to HG/T 20592 or GB/T 9119, with pressure classes ranging from PN6 to PN25. For higher pressures, tapered flanges or butt welded flanges are used-this will be described later.
How it works? Elastomer compensation. The corrugated pipe works under the combined action of pressure, temperature and displacement, and the design should ensure that it does not fail within the fatigue life. The failure mode is generally that the stress concentration at the wave peak leads to cracks, or the guide tube falls off and sticks the bellows. And guess what? Many people buy it back and just install it, regardless of the flow direction of the medium at all-the arrow on the expansion joint tells you to install it against the flow direction of the medium. If the deflector is installed backwards, it becomes a jacket, and it will be perforated in less than a year.
The three most easily overlooked parameters in model selection: pressure class, compensation amount and fatigue life
After asking a circle of customers, nine of the ten openings are "Give me an expansion joint of DN200". And then what? It's gone. The pressure level is not reported, the compensation amount is not mentioned, and the fatigue life is not mentioned. As a result, it cracked after three months of installation, and the product quality was not good when I turned back. Alas, the pot can't be carried by the manufacturer.
When the bellows is subjected to internal pressure, a blind plate force will be generated, which will pull and bend the pipe support. If the pressure is not checked during the selection, for example, the actual working pressure is 1.6MPa, and you choose a PN6, the wall thickness of the bellows is too thin, so it will bulge directly. On the other hand, it is useless to choose a high pressure level. The cost goes up, the stiffness is also greater, and the compensation ability decreases. Generally, the pressure resistance of steel bellows is related to wave height, wave pitch and number of layers. When selecting the type, you should look at the manufacturer's pressure-wave height curve table, so don't pat your head.
The compensation amount refers to the maximum axial displacement (in mm) absorbed by the expansion energy saving. The larger this value is not the better, because the larger the displacement, the larger the amount of single-wave deformation of the bellows, and the shorter the fatigue life. We have encountered customers requiring a compensation amount of 100mm, and as a result, the bellows only has four waves and a single wave deformation amount of 25mm-according to EJMA standards, the fatigue life of this condition may be less than 500 times. The correct way is to calculate the required compensation amount according to the thermal elongation of the pipeline, leave 10% ~15% margin, and then inverse the wave number.
Fatigue life, which is the most overlooked but deadliest parameter. The design life of double flange metal expansion joints is usually 1000~5000 cycles (see the standard). However, there are often vibrations and pressure fluctuations in actual working conditions, and each fluctuation is counted as a cycle. If your pipeline starts and stops once a day, 365 times a year, and has a designed life of 1,000 times, it will last less than three years. Many factories leaked after two and a half years, because the number of fatigue times was not clearly required at the beginning. We suggest that for systems with frequent start-and-stop (such as thermal pipe network), directly choose compound structures-such as compound hinge transverse expansion joints or compound straight pipe bypass pressure balance expansion joints, which can distribute a large displacement to multiple bellows, with small single wave deformation and long life.
Installation site rollover case: the guide bracket was not done correctly, and the expansion joint was installed for nothing
On a return visit to a chemical plant last month, I saw that their double flange metal expansion joint was pulled into a "twist". One question, it is installed at the horizontal 90° elbow, and there is neither a guide bracket nor a fixed bracket. As soon as the steam is opened, the pipe swings laterally, the expansion joint bears torque and lateral displacement, and the bellows is directly twisted and deformed. This thing is designed with only axial displacement in mind, and you give the lateral force, which is equivalent to letting the fist contestant go ballet-doomed to collapse.
Guide brackets must be installed at both ends of the expansion joint, and the number of guide brackets is determined according to the diameter of the pipe and the amount of displacement. Generally, the spacing between guide brackets and pipes of DN200 should not exceed 3 meters. Then set fixed brackets 2~3 meters at both ends to constrain the displacement of the pipeline in the section of the expansion joint. There's also a pit – the tie rod nut. The tie rod nut is pre-tightened before installation to prevent bellows from deforming during transportation. After installation in place, the nut must be loosened half a turn to one turn, so that the expansion energy saving can expand and contract freely. Many masters forget to loosen the nut, and as a result, the expansion joint is locked, which is equal to a straight pipe. The thermal expansion and contraction are all stuck on the weld joint, and the weld joint collapses. This matter has been repeatedly said in the FAQ: How to adjust the expansion joint tie rod nut? Loosen first and then adjust to the pre-stretching amount required by the manufacturer.
Double flange vs other flange connection forms (loose flange, butt welded flange), when and which to use?
Double flanges are not suitable for all occasions. The advantage of loose flange (also called looper flange) is that the flange can be rotated, and the installation of holes is convenient, which is suitable for the situation where the space is narrow and the bolt holes are not aligned. However, the sealing surface of the loose flange is flat, which is compressed by gaskets, and the pressure bearing capacity generally does not exceed PN10. The standard configuration of double-flange metal expansion joints is a plate flat welded flange, with a pressure capacity within PN16. If it exceeds PN16, it is recommended to use a butt welded flange-because the neck of the butt welded flange can effectively disperse stress and is suitable for high-pressure working conditions (above PN25).
So when should you use double flanges? Simple: large pipe diameter, high pressure, need regular disassembly and maintenance occasions. For example, the main steam pipeline of the power station, with a temperature of 540℃ and a pressure of 10MPa, must use butt welding flange + double flange expansion joint. Conversely, if the pipeline is low pressure (Sleeve type pipe expansion jointThere's a variation of the loose flange in it. However, it should be noted that the position of the bolt hole of the loose flange is easy to be misaligned, so spring gasket must be added to prevent loosening during installation.
"Three Dos and Three Don'ts" for maintenance: guide tube, tie rod nut, anti-corrosion coating
Regularly check whether the guide tube is loose (especially the high-temperature flue gas pipe, once the guide tube falls off, it will be blown away by the airflow and smash the bellows); It is necessary to check whether the anti-loosening gasket of the tie rod nut is intact every six months (it is particularly easy to loosen under vibration conditions); It is necessary to check the outer surface of stainless steel corrugated pipe for anti-corrosion coating (although stainless steel is corrosion resistant, chloride ion environment (such as coastal chemical plants) still pits, so it is recommended to apply a layer of epoxy zinc-rich paint).
Don't knock the flange surface with a hammer (it will deform the sealing surface and cause leakage); Do not clean the bellows trough with a wire brush (the wire brush will scratch the surface passivation film and accelerate corrosion); Do not weld the support on the expansion joint (welding the heat affected zone will change the metallographic structure of the stainless steel and reduce the fatigue strength. If you really need to add a bracket, use a mechanical clamp).
Asked 100 customers, their 5 most entangled questions (with real data)
We counted the customer consultation records in the past six months, picked out the five most frequent pain points, and directly went to dry goods.
- "Why did the expansion joint I bought leak after half a year?"Twenty failed parts were disassembled and analyzed, and 60% of them were because the medium flow rate was too high (> 15m/s) to scour the welding joint of the guide tube, resulting in peeling off. Solution: For pipes with high flow rate, choose thick guide tube or stainless steel + ceramic lining.
- "Does double flange expansion save energy instead of elbows?"Can't. The expansion joint only compensates for displacement and does not change the flow direction. Use the expansion joint as an elbow, and the lateral force directly gets scrapped. Don't do that.
- "Should the wall thickness of bellows be 0.6mm or 1.0mm?"Look at the pressure. DN200 or less, pressure
- "Do you need to press the expansion joint after it is installed?"A hydraulic test of 1.5 times the design pressure must be performed. However, note that after the hydraulic test, the water must be drained and blown dry, otherwise the residual water will freeze and swell the bellows. We've seen a power plant in the north break three expansion joints overnight without draining water in winter.
- "Can you use metal expansion joints instead of rubber compensators?"Where the temperature exceeds 100℃, do not use rubber. However, if it is a low-temperature and low-pressure circulating water system, the rubber compensator (non-metal expansion joint) has better vibration damping effect and low cost. The advantages of double flange metal expansion joints lie in high temperature resistance, corrosion resistance and long life, at the cost of strong rigidity and high requirements for pipeline supports.
In the final analysis, choosing a double-flange metal expansion joint is a balance question: pressure, displacement, life, cost, you have to give up one end. But at least don't roll over on the mounts and brackets – that's the most wronged.