Two days ago, I met a customer who was doing a pipe gallery project, and asked, "How much can your expansion joint compensate? Give me the one with the largest expansion value." This sounds fine, but you will know after working in this business for a long time-it is the most common pit in model selection to regard the expansion value of metal expansion joint as a performance index of "the bigger the better". The expansion joint is not a spring, and the expansion value is not the longer it is pulled, the more fierce it is. It is a design value derived from the actual displacement requirements of your pipeline system. Set more and set less, almost, the scene will show you the color.
What exactly are scaling values saying?
To put it bluntly, the expansion value of the metal expansion joint is the amount of displacement that the bellows can absorb. There are three kinds of displacement: axial tension and compression, transverse misalignment and angular deflection. The "expansion value" we usually talk about should be strictly called "axial displacement", which measures how many millimeters the expansion joint can compress or stretch along its own axis.
Why do plumbing need this thing? Thermal expansion and cold contraction. A 50-meter-long carbon steel pipe, the medium temperature rises from normal temperature to 200℃, how much do you calculate it can be elongated? About 120mm. If this 120mm is not absorbed somewhere, the stress will be piled up in the weak links of the pipeline-flanges, valves and equipment nozzles, and accidents will occur sooner or later. The expansion value of the metal expansion joint is specially used to "eat" this displacement.
So you see, the expansion value is not as much as the expansion joint itself wants to set, it is determined by the thermal displacement of the pipe. The expansion joint has to absorb as much as the pipe expands. If you choose big, the bellows will have too much surplus, not enough stiffness, and the pressure and thrust will not hold up; If the choice is small, the displacement is not enough, the nozzle stress exceeds the standard, and the bellows will be fatigued and cracked early.
How is the scaling value calculated?
Δ L = α × L × Δ T
α is the linear expansion coefficient of the pipeline material, and carbon steel is about 12×10⁻⁶/℃; L is the length of the tube between the two fixed brackets, in meters; Δ T is the difference between the operating temperature and the installation temperature. For example, a 100-meter carbon steel pipe, with a temperature rise of 200 °C, calculates Δ L =12×10⁻⁶ ×100×200=240 mm. This is the total elongation of the pipe, which is the total expansion and contraction value that the expansion joint needs to absorb.
How do you match the expansion joint? Bellows have a "single wave displacement"-that is, the maximum amount of compression or tension that each wave peak can withstand, which is usually calculated by the fatigue life of the bellows. The single wave displacement multiplied by the wave number is the theoretical displacement capacity of this expansion joint. But note that theoretical and allowable values are not the same thing. There is a safety factor in the standard, such as the allowable displacement corresponding to the fatigue life of 1500 times, which may only be 60% ~70% of the theoretical limit. You choose according to the theoretical value, but in fact, you call for help when you go to half the working condition.
Fatigue life limits on stretch values. You let the expansion joint expand and contract at full capacity every moment, and the bellows won't last long. The design is generally checked according to the actual number of thermal cycles. 1500 times and 5000 times, the corresponding allowable single-wave displacement differs by a large distance. This is why in the same working condition, some people dare to choose 12 waves of bellows, while others insist on using 20 waves-the life requirements are different.
Temperature, pressure, direction, environment, four variables stuck expansion values
The calculated theoretical expansion and contraction value will be discounted in actual working conditions. Discounts mainly come from four aspects.
Medium temperature.The higher the temperature, the lower the permissible stress of the bellows material. The same stainless steel bellows can carry 10 MPa at 200℃ and may only carry 5 MPa at 500℃. The material and temperature determine the wall thickness and wave height of the bellows, thus affecting the single wave displacement. This is why ordinary general-purpose expansion joints are often insufficient under high-temperature working conditions, so high-temperature axial expansion joints have to be replaced. The material and heat treatment process of bellows are different.
Stress.Internal pressure creates a "column instability" threat to the bellows, and the higher the pressure, the less displacement the bellows can safely absorb. In high-pressure pipelines, if the displacement demand is large, it is useless to increase the wave number alone-the more wave number, the lower the critical pressure of column instability. What to do? Add pull rods and hinges, remove the pressure thrust, and let the bellows only work without labor. Or simply choose the external pressure single axial expansion joint, so that the bellows is in the external pressure state, and the stability is better.
The pipeline goes.Straight pipe sections require only axial compensation; However, the L-shaped and Z-shaped pipe sections will produce lateral displacement and angular displacement. The lateral displacement and angular displacement will generate additional bending moments on the bellows, and the actual allowable axial expansion and contraction values must be reduced accordingly. You just take an axial expansion joint to absorb the lateral displacement, and the bellows will be forced laterally. Short life is a sure thing.
Installation environment.Open-air pipe gallery, underground pipe trench, high-temperature workshop, the ambient temperature difference is large, which means that the same expansion joint is installed in winter and installed in summer, and the amount of pre-stretching is completely different. This detail will be detailed later.
Do not focus on the maximum scaling value when selecting a model
"Give me one with a telescopic value of 200 mm." But you think back, 200 mm is the total displacement, and it could be 10 waves ×20 mm, or 20 waves ×10 mm. The former has large axial stiffness and low price, but short fatigue life and large bellows stress; The latter has strong flexibility and long life, but its axial stiffness is small, and it is easy to become unstable under the action of internal pressure and thrust.
Determine the required total expansion and contraction value according to the compensation amount of pipeline, and then deduce the wave number and wave height inversely according to the requirements of pressure, temperature and life. For example, the main steam pipeline of a power plant has high temperature, high pressure and tens of thousands of life requirements. At this time, it is necessary to choose the corrugated expansion joint for the power station industry, which has a large wave number, multi-layer corrugation and a guide tube. Instead, the expansion value is conservative-enough, leaving a margin for safety.
The universal corrugated expansion joint is suitable for ordinary low-temperature flexible occasions. When the displacement demand is large, it should be matched with wave number, but attention should be paid to the matching of pressure and displacement. In the scenario of high temperature, high pressure and large displacement, high-temperature axial expansion joint or compound hinge transverse expansion joint with tie rod is more reliable-the tie rod eats the internal pressure thrust, and the bellows concentrates on absorbing displacement, so that the expansion value can not be discounted.
Pre-stretching/pre-compression。 The difference between the temperature and the working temperature of the pipe installation determines whether the expansion joint should be pre-stretched or not. For example, the working temperature of the pipeline is higher than the installation temperature, and the expansion joint should be pre-stretched by half of the expansion value when it leaves the factory; After installation in place, the thermal expansion of the pipe just slowly "eats" the pre-stretched amount, and the expansion joint always works near the zero position. If the cold tightness is not in place, the actual available expansion value will shrink by half, and the bellows will reach its limit not long after operation.
How to verify scaling values during installation and acceptance?
No matter how accurate the design value is, it is useless if it is not honored on the spot. How to confirm the expansion and contraction value of the metal expansion joint according to the design during acceptance? Look at three things.
One isCold tight recording。 Whether the expansion joints are pre-stretched/pre-compressed according to the design values during pipeline installation, and whether the position of the tie rod nut is recorded. Many projects are not recorded after cold tightening, and they don't know what state the bellows is in at the time of trial operation.
Second,Spacing and setting of guide brackets。 The expansion energy saving can't absorb the displacement according to the design direction, and it all depends on the guide bracket to "rush" the displacement to the expansion joint. The spacing of the brackets is wrong, the pipeline is transversely unstable, the expansion joint bears the bending moment outside the design, and the expansion and contraction value shrinks immediately. Especially in vertical pipe sections and bends, the bracket setting directly determines the actual displacement shape of the expansion joint.
Third,displacement indicator。 A reliable expansion joint manufacturer will weld the displacement indicator scale on the pipe, and compare the pointer position before and after operation to verify whether the actual displacement is within the allowable range. Take a photo of each cold state and hot state, and the data is clear at a glance.
The field believes that the expansion and contraction value of the expansion joint is the change amount of flange spacing. Hey, flange spacing only reflects axial compression/tension, lateral displacement and angular displacement are not visible at all. The bellows may have been crooked, but the flange spacing still looks normal. It is necessary to combine the data of multiple measuring points to judge the actual displacement state of the expansion joint. If you focus only on the axial direction, you ignore all the risks of angular and lateral displacement.
In the final analysis, the expansion and contraction value of metal expansion joint is an engineering parameter that is "matter-of-fact", and the bigger it is, the more respectable it is. The principle of selection is always: calculate the heat clearing displacement, leave enough safety margin, match the pressure and thrust well, and explain the installation environment well. After understanding these numbers, the expansion joint selection is more than half successful.