1. What is the expansion margin? The Mystery of "Surplus" in Pipeline Design
Old masters in the dry pipe industry know that thermal expansion and cold contraction is an inevitable curse. As soon as the steam pipeline heats up, the pipeline of tens of meters can be extended by more than ten centimeters. At this time, the metal expansion joint is the "breathing port" of the pipeline. But here's the problem-the thermal displacement calculated on the design drawings is 100mm. Why is the rated compensation of expansion joints given by manufacturers often 120mm or more? This extra 20mm is the "expansion margin" we are going to talk about today.
To put it bluntly, the margin is the insurance money beyond the design value. For example, a general-purpose corrugated expansion joint, the nameplate is marked with "axial compensation ±80mm", but the actual thermal displacement of the pipeline is only 60mm, and 20mm is the margin. Some people think it is a waste, others think it is a life-saving talisman. Who is right? Don't worry, look down.
2. Accounting: No matter how accurate the thermal expansion and contraction is, why do you have to keep a hand? — — Actual displacement vs design margin
When the design institute does stress analysis, the theoretical displacement value is usually calculated according to the highest temperature of the medium, the lowest ambient temperature and the linear expansion coefficient of the pipeline material. But the reality is often much more complicated than the formula:
- Temperature fluctuation: The steam pipeline may overheat when it is started, the medium temperature is occasionally higher than the design value by 10℃-20℃, and the thermal displacement increases instantaneously.
- Installation error: On-site welding shrinkage and cold tightening operation are not in place, and the actual displacement and calculated value often deviate.
- Foundation settlement: Especially in the scenario of directly buried (fully buried) expansion joint, uneven soil settlement will add additional displacement to the pipeline.
- Fatigue life: Every time the metal expansion joint absorbs displacement, the bellows undergoes a stress cycle. The larger the margin, the lower the stress amplitude in a single cycle and the longer the fatigue life-this principle is the same as the bending radius of a metal hose, don't get stuck to the limit value.
Two days ago, I met a customer who was doing power plant renovation. The high-temperature axial expansion joint they originally used had a theoretical displacement of 200mm, but in fact it ran 30mm more because the pipeline support and hanger were stuck. Fortunately, 35mm margin was left at the beginning, so the bellows was not cracked. Do you think this balance is a life-saving medicine?
3. How to determine the margin of expansion joints with different "physiques"? From general-purpose type to high-temperature axial type, each has its own particular emphasis
It is not that all expansion joints have a margin in one proportion. Different structures, different working conditions, pay attention to world differences.
Universal corrugated expansion joint: The most common single axial type, usually the margin is considered according to 15%-20% of the design displacement. For example, the design displacement is 50mm, which is about 60mm. This kind of expansion joint bellows has thin walls, sensitive fatigue life to displacement, and is prone to problems if the margin is less.
High temperature axial expansion joint: The kind used for boiler outlets and high-temperature flue gas pipes. Because of the high temperature creep of the material, the actual compensation ability of the bellows will decrease with the temperature. The balance is usually enlarged to 25%-30%. Check the standard of "Corrugated Expansion Joints for Power Station Industry". Under high temperature conditions, the design displacement generally only uses 70% of the product rating.
Compound hinge transverse expansion jointAndCurved tube pressure balance expansion joint: Such products are usually used in large pipelines and high-pressure scenes, and the margin of 10%-15% is enough, because their structural rigidity is strong, the displacement direction is constrained by the guide, and it is not easy to overload. However, the balance of straight pipe pressure balance expansion joint should balance the pressure thrust, and the margin should not be too key, at least 15%.
Sleeve type pipe expansion joint: This thing is sealed by packing, and the margin is different from that of bellows-it needs to be manually adjusted after the seal gap is worn, so the margin is more reflected in the adjustable range. In design, it is generally given 1.2 times of the theoretical displacement, but after actual installation, it has to be adjusted by the tie rod nut to eat up the balance.
4. Installing it is just the beginning: the tie rod nut is adjusted and pre-stretched. How can the margin be "spent" on the blade
When the expansion joint arrives, it is welded directly, the tie rod nut does not move, and the pre-tension is not done. As a result, the balance was wasted in the cold position. When the pipeline heated up, the expansion joint was directly pushed to the hard limit position.
The right thing to do is- -"Advance" the balance to the thermal demand。 For example, the design displacement is 100mm, and the product is rated at 150mm, which is 50mm more. If the installation temperature of the pipeline is 20℃ and the operating temperature is 300℃, then the expansion joint is compressed by 25mm (pre-stretched) through the tie rod in the cold state, so that the bellows is in the "half-pressure state" in advance. In this way, the bellows is extended when running, and the extended end and the use end are just symmetrical, so that the bellows has the longest life.
aboutHow to adjust the tie rod nut of expansion jointWe wrote specifically before: The role of the tie rod is to limit the excessive stretching or compression of the bellows, but it is not to fix it dead. For cold adjustment, screw the nut to the designed pre-stretch position, and then lock the back nut. Note that the adjustment of the expansion joint of the large tie rod (such as the transverse type of the double hinge) is more complicated, and it is necessary to coordinate the synchronization of the tie rods on both sides, otherwise it is easy to cause the bellows to twist.
Another situation —Pre-stretching compensates for the amount of cold drawingWhile the margin is used to cover future settlements, shocks, earthquakes and other factors. Therefore, the margin is not for you to "keep it still", but for the installer to dynamically adjust the "reserve fund" according to the on-site measured displacement.
5. Don't fall into the pit: the larger the margin, the safer it is? Real cases tell you the cost of overdesigning
Since the margin has so many benefits, can I just order at twice the design displacement? Don't. The more margin is not the better, and overdesign will still lay mines.
The design displacement of the outlet pipe of the high-temperature fan of a production line is 80mm. The customer insists on buying the model with the rated compensation of 200mm for the high-temperature axial expansion joint. As a result, the wave pitch of the bellows was too large, the axial stiffness was extremely low, and the medium pressure in the pipe drummed the bellows into a "balloon". Coupled with the vibration of the pipe support and hanger, the bellows buckled transversely in the non-design direction, and it cracked in half a year.
Why? Because the larger the rated compensation amount, the more wave number and wave height of bellows, the relatively thinner wall thickness, and the pressure resistance and stability decrease instead. Especially forMetal Corrugated Expansion Joints in Cement IndustryIn this kind of dust environment, the rigidity of the bellows changes drastically after dust accumulation, and too much margin makes the bellows in a "soft" state, and its life is discounted.
Another pit isInstallation space。 Large margin means that the total length of the expansion joint is longer, and some site space is limited and can't be stuffed in, so the direction of the pipeline has to be changed. Or something likeDirect buried (fully buried) type expansion jointThe buried space is limited, and blindly increasing the margin leads to the size of the outer sheath exceeding the standard and doubling the civil construction cost.
Design displacement + (10% ~30%) as the rating value, but this percentage should be clearly communicated with the manufacturer at the inquiry stage. Like the one in our stationUniversal corrugated expansion jointThe product page will be marked with "Design Displacement Suggested Use Value", which directly tells you how safe it is to make a discount on the basis of the rated value.
The expansion margin of metal expansion joint is not a metaphysics, but a quantification of engineering experience. If you leave less, the pipe will crack and leak; If you leave too much, the equipment will become brittle. Spend money to buy sins. The real master is to deduct the most suitable "excess" amount after knowing the temperature distribution per inch of the pipeline and the constraint stiffness at each place. Have you learned it?