How to determine the design temperature of the expansion joint? Three Key Points Engineers Must Understand
Nine times out of ten, people who select expansion joints have stumbled at the design temperature. Either I patted my head to get the medium temperature, or I missed the external heat source. This article is not false, and it directly dismantles the three decisive links.
1. Working temperature and design temperature: Don't regard "highest" as "design"
The maximum operating temperature of pipeline medium is 400℃, which is directly filled in the selection sheet when the design temperature is used. When the corrugated pipe material couldn't bear it, it was discovered that the allowable stress had long failed. The design temperature is not the operating temperature, which is clearly written in the standard-GB/T 12777 and ASME B31.3 both require a safety margin on the operating temperature. But this margin is not a fixed value, depending on the working conditions: continuous operation or intermittent operation? Is there a risk of local overheating? For example, the corrugated expansion joint used in the power station industry (such as the "corrugated expansion joint used in the power station industry" in our station), the temperature of the steam pipeline fluctuates greatly, and the peak value may rush to 550℃, while the average value is only 450℃. If you choose according to the average value, the bellows directly deforms plastically at the peak value. Therefore, the design temperature has to be calculated according to the extreme peak value, and the thermal shock margin at start-up has to be superimposed, generally from +20℃ to +50℃.
2. Internal media vs external environment: who has the final say?
This question is different from the occasion. If it is a high-temperature axial expansion joint, the internal medium temperature is naturally dominant. However, don't ignore the temperature gradient at the flange connection and near the guide tube-where local stress concentrations occur and lead to early cracking of the bellows. On the other hand, like directly buried (fully buried) expansion joints, the external soil temperature and the performance of insulation layer are the keys. Two days ago, a customer who made metal corrugated expansion joints in the cement industry had a medium temperature of only 450℃, but the site was close to the rotary kiln, and the radiant heat made the external temperature soar to 600℃. As a result, the outer wall of the bellows failed first, but the inner wall was still fine. Which do you say the design temperature should be taken? It must be the maximum of the two heat sources, and then the material creep strength is checked. Don't be lazy only measure the medium temperature, the ambient temperature is often the fatal blow.
3. Material selection: When the temperature is high, money and life have to be counted
When the temperature rises, the allowable stress of ordinary austenitic stainless steel (such as 304) cliffs down. Over 550℃, you have to change to a heat-resistant alloy, like Inconel 625. But heat-resistant alloys are expensive, several hundred yuan per kilogram, and the cost doubles. What about that? In some cases, the multi-layer structure of high-temperature axial expansion joint can be used-the inner layer is heat-resistant, the outer layer is pressure-bearing, and the cost is reduced. Or simply put on non-metallic expansion joints (fabric fiber expansion joints), and use temperature-resistant fibers and silica gel coating, which can carry above 1000℃, and the price is lower than that of heat-resistant alloys. Note that the design temperature also directly affects the fatigue life: for every 100°C increase in temperature, the cycle life of the bellows may be reduced by half. Therefore, when selecting the model, the design temperature must be calculated together with the pressure and displacement, so as to determine whether the general-purpose corrugated expansion joint or the large-diameter thick-wall expansion joint should be used. The former is cheap but weak in fatigue resistance, while the latter has large wall thickness and rigidity, which is suitable for high temperature and high cycle.
Here's a common "pitfall": the regulations of temperature cycling and thermal shock in the standard. For example, desulfurization flue gas baffle door and flue gas baffle door, the medium is acidic wet flue gas, and the temperature is not high but fluctuates violently-jumping back and forth from 100℃ to 180℃. If the design temperature is selected only according to the highest value of 180℃, and the thermal fatigue is ignored, the weld will crack after hundreds of cycles under the working condition of high start-stop frequency. National standards JB/T 12235-2015 (non-metal expansion joint) and JB/T 6171 (metal bellows) have detailed regulations on temperature, but in actual projects, I suggest leaving an extra margin of 10% ~15%, especially in working conditions with high start-stop frequency. Don't ask why, I've seen too many live lessons.
4. Actual estimation: What if there is no data?
In the absence of detailed process data, the saturated steam temperature corresponding to the design pressure of the pipeline can be inverted. For example, the pressure is 1.0MPa and the saturated steam temperature is about 180℃, so the design temperature starts at 200℃. Or referring to similar project cases, we have the product pages of "Double Hinge Expansion Joint for Air-cooled Island Vacuum Pipeline" and "Double Hinge Transverse Expansion Joint" on our station, which list the design temperature range for conventional working conditions. Generally, the design temperature of steam pipeline is the medium temperature +20℃, but it depends on the pipeline material and insulation thickness. Finally, a reminder: after the design temperature is determined, don't forget to link with the "guide tube" design. The guide tube can reduce the direct scouring of the bellows by high-temperature medium, which is equivalent to reducing the effective working temperature by 30~50℃. This amount can save a lot of cost when calculating the fatigue life.