The thermal displacement of the pipeline is not accurate, and the rest is all in vain
The first step of metal expansion joint pipeline compensation is not to pick the product, but to calculate the compensation amount. You don't even know where the pipeline runs and how far it runs. It's just a chance to choose what model to choose later.
How to calculate the amount of compensation? In the final analysis, it is thermal expansion and contraction: Δ L = α × L × Δ T. α is the linear expansion coefficient, about 12×10⁻⁶/℃ for carbon steel and slightly higher for stainless steel; L is the length of the straight pipe section; Δ T is the difference between the installation temperature and the maximum operating temperature. For example, a 30-meter carbon steel steam pipe rises from normal temperature of 20℃ to 200℃, and the expansion amount is calculated to be 12×10⁻⁶ ×30000mm ×180°C ≈ 64.8mm. This number doesn't look big, but if you don't count it in, the expansion joint is just a decoration.
After calculating the total amount, you have to split the displacement type. Axial displacement (pipe expanding and contracting along the axis)Universal corrugated expansion jointOrExternal pressure single type axial type expansion joint; Lateral displacement (pipe left and right deflection) depends onCompound hinge transverse expansion joint； The angular displacement (the change of the angle at the turn of the pipe) should be usedCurved tube pressure balance expansion joint。 If the direction is wrong, the bellows will be subjected to torque without tension, and there will be problems sooner or later.
Spacing of guide brackets. Many engineering teams are arranged according to the spacing of ordinary pipe brackets. The first guide bracket is too far from the expansion joint, and the pipe generates additional bending moment at the bellows. As a result, the expansion joint is twisted before the design life. The distance between the first group of guide brackets and the expansion joint is generally controlled within 4 times the nominal diameter, and the second group can be relaxed to about 14 times later. This is not a profound theory, but the scene is most likely falling on it.
There are so many products, you have to take your brain when you pick them
Power stations, cement and desulfurization, the working conditions of the three industries are surprisingly different. The temperature of the flue gas pipe fluctuates, and the dust is still large. If you take the metal expansion joint to carry it hard, the wear and corrosion will be enough for you to drink a pot. This kind of working condition usesNon-metallic expansion jointOrRectangular expansion jointOn the contrary, it is suitable-temperature resistance, corrosion resistance, large compensation, and cheap price. However, note that the pressure bearing capacity of non-metallic expansion joints is weak, so caution should be taken on positive pressure pipelines.
The same cannot be said for steam pipes. High temperature, high pressure, and possibly water hammer impact,High temperature axial expansion jointIs the main force,Straight pipe pressure balanced expansion jointIt is also commonly used in situations where the blind plate force needs to be eliminated. As forSleeve type pipe expansion jointIt relies on sliding seal to absorb displacement, has simple structure and large compensation, but the packing seal is easy to leak, so it is okay to use it on low-pressure and unimportant pipelines, but it is better not to touch high-pressure steam pipelines.
- Universal corrugated expansion joint: Wide applicability, mainly absorb axial displacement, affordable price, but can't bear large lateral displacement.
- Compound hinge transverse expansion joint: Two bellows plus hinge structure, specially absorbing transverse displacement, often used in L-shaped or Z-shaped pipe sections.
- Curved tube pressure balance expansion joint: By balancing the bellows to offset the pressure thrust, it is suitable for the situation where the pressure is high and the pipeline is not allowed to be stressed, with complicated structure and high cost.
- Sleeve type pipe expansion joint: The compensation amount is large, the flow resistance is small, but the sealing performance is average, and the packing needs to be maintained regularly.
Medium, temperature, pressure, displacement type, the four parameter boxes down, the range that can be selected is very small. For example, desulfurization flue gas pipeline, the medium is corrosive and the temperature fluctuates greatly, so non-metallic expansion joints are the first choice; The main steam pipeline of the power station, with a temperature above 550℃, can't be held by ordinary stainless steel bellows at all, so it has to be made of high-temperature alloy materials.
Installation details make or break
Installation is a high-incidence area for failure, especially tie rod nuts.
When the expansion joint leaves the factory, it is generally locked with transportation protection rods or nuts, so as to prevent damage to the bellows during transportation and hoisting. Once installed, these constraints must be loosened. Many sites directly remove the tie rod and throw it aside, or simply forget to loosen the nut-as a result, the bellows can't be displaced, and the thermal expansion and contraction of the pipeline is all held in the system, and the equipment suffers. In turn, some expansion joints with large tie rods (such asDirect buried (fully buried) type expansion joint), the tie rod itself is a part of the working structure and is used to bear the internal pressure thrust. At this time, a big thing will happen if the tie rod is dismantled.
Then what exactly should and shouldn't be demolished? There is one criterion to judge: whether the tie rod is a transportation constraint or a working part. Working parts will be clearly marked in the drawings and instructions, and must be compared and confirmed before on-site installation. If you are not sure, ask the manufacturer and don't pat your head.
Let's talk about the direction. The direction of the arrow is marked on the expansion joint cylinder, pointing to the direction of the medium flow. This arrow is not random-the position of the deflector has something to do with it. Generally, the direction of the guide tube is required to be consistent with the flow direction of the medium, in order to reduce the direct erosion of the bellows by the medium. If you install it backwards, the deflector will not only fail to protect it, but it will also jam the bellows. As forDirect buried (fully buried) type expansion jointIt is directly buried underground, which requires higher anti-corrosion and waterproofing. Before installation, check whether the outer sheath is intact and seal the interface.
Pre-stretched or pre-compressed. When the pipeline installation temperature is lower than the design temperature, the expansion joint needs to be pre-stretched, so that the bellows is in a tensile stress state at low temperature. When the temperature rises, the displacement just returns to the middle position. Conversely, the installation temperature is higher than the design temperature and pre-compressed. This matter is not a formality, it directly determines whether the bellows can eat the design displacement in the working state. The pre-stretching amount is usually 50% of the compensation amount, and the specific value is subject to the design document.
Do you still have the pre-stretch records from your site? Or can you not find it after installing it?
Just put it on? More pits during the running period
For many projects, buying expansion joints is like buying groceries. After installing them, they don't care until the inspection finds that they leak. In fact, the problems in the running period are all harbinger.
Bellows stiffnessIt directly affects the force on the pipe. The stiffness is too large, and the force on the pipe support and the fixed point exceeds the standard; If the stiffness is too small, the bellows will easily become unstable. This parameter selection stage should be calculated clearly, not the thicker and stronger the equipment, the better.
One of the high-incidence failures during operation is the deflector tube wear. The function of the guide tube is to protect the inner wall of the bellows and prevent direct erosion of the medium. If the medium contains solid particles (such as raw powder in the cement industry), the guide tube wears out particularly quickly, and it is directly exposed to the medium after wearing out the bellows. At this time, the countermeasure is to check more, thicken the guide tube or change the wear-resistant material when necessary.
The other is bellows fatigue crack. Each time a displacement is absorbed, the bellows undergoes a stress cycle. The fatigue life of pipes with frequent pressure fluctuation and large vibration will be greatly shortened. The design life is calculated according to 5000 cycles. In fact, if the working conditions fluctuate drastically, cracks may occur in three to five years. Therefore, in the design stage, it is necessary to find out the number of cycles of working conditions, and don't choose according to the idea of static pipeline.
There are two things to do in daily inspection: look at the displacement indicator, record the actual compensation amount, and compare the design value. If it is biased, it means that there is a problem with the pipeline constraint; By the way, it means that the system is running properly. By the way, see if there are any corrosion spots and cracking marks on the surface of the bellows. These can be seen with a flashlight. They don't need an expert, but they need people to be diligent.
When choosing a model, calculate the price clearly
Finally, let's talk about money. Metal expansion joints look cheap, but when the working conditions are complicated, it is normal to double the cost with tie rods, hinges and pressure balance structures. Such asStraight pipe pressure balanced expansion jointIt is one or two times more expensive than the general-purpose type, but it can eliminate the blind plate force on high-pressure pipelines and save the cost of large fixed brackets, which is cost-effective as a whole.
The initial investment of non-metallic expansion joints is low, but the replacement cycle is short, which generally needs to be replaced once every 3-5 years. The metal expansion joint will last 10 or 8 years without any problem. How to calculate the full lifecycle cost? Add the purchase price, installation fee, downtime loss, maintenance fee, and replacement fee all over, and you'll find that bargains are often the most expensive.
Initial procurement cost + installation cost + annual maintenance cost × design life + midway replacement cost. Several types of products are put in, and the data will tell you the answer.
In the final analysis, the compensation of metal expansion joint pipeline has two cores: calculating the displacement and installing the right position. You can save a lot of time by leaving the rest of the details to qualified manufacturers and responsible construction teams.