How to Choose Air Separation Metal Expansion Joint? Understand these key issues first
Pipeline compensation in air separation unit is not the same as ordinary chemical pipeline. Low-temperature liquid oxygen and liquid nitrogen are more than minus 100 degrees at every turn, and the pressure can reach several MPa, not to mention the cleanliness requirement-if oil and particulate matter slightly exceed the standard, the whole system has to be stopped and purged. Under this working condition, we have seen too many accidents such as low-temperature brittleness of bellows, erosion of guide tube and even media leakage. To put it bluntly, the air separation metal expansion joint can't be topped by just taking a general-purpose corrugated expansion joint.
Then how do you choose? You have to understand the following key points first.
Structural Design: Applicable Scenarios of Three Mainstream Forms
According to the displacement absorption mode and installation position, there are three most commonly used categories:External pressure single axial expansion joint、Curved tube pressure balance type expansion joint andCompound hinge transverse expansion joint。
Let's talk about external pressure single axial type first. Its bellows are on the outside, and the medium can't touch the inner wall of the bellows at all, which eliminates the root of high-speed fluid erosion. It is suitable for absorbing the axial thermal expansion and cold contraction of pipelines, especially under low temperature working conditions, when the amount of cold contraction of pipelines is large, this structure can distribute the displacement more evenly. The disadvantage is that the absorption capacity of lateral displacement and angular displacement is weak, so it must be equipped withguide bracketTo constrain the lateral sloshing of the pipeline. After you calculate the thermal expansion of the pipe, installation error, and equipment vibration displacement, if it is mainly axial displacement, use this first.
Look at the curved tube pressure balance type again. This expansion joint has its own balance ring and tie rod, which can eat the internal pressure thrust by itself, and will not transmit the thrust to sensitive equipment such as valves and air separation towers. On the outlet pipeline of the air separation tower, the interface between the valve and the equipment is the weak link, and this is the most stable. The unit price is a little higher, but it saves the cost of reinforcing brackets and maintenance in the later period, and the account is not a loss.
The lateral type of compound hinge is suitable for the bent pipe section with limited space. It can absorb the lateral displacement in the plane, and the cooperation of two hinges can also absorb the angular displacement. However, the angle should be aligned during installation, otherwise the hinge will get stuck, which requires high installation accuracy.
Material: Low temperature brittleness is a big pit
If you choose the wrong material, no matter how good the structure is, it will be useless. Can the impact toughness of stainless steel 304L pass the test in the liquid oxygen environment of-196℃? Have you checked the standards? Austenitic stainless steel will undergo martensite transformation at low temperature. Once the ductile-brittle transition temperature is lower than the working temperature, the crack propagation speed is extremely fast. Many manufacturers' label of "low temperature resistance" actually only reaches-40℃, which is used in air-separated cars. Guess what? Three months later, micro-cracks appeared in the bellows, leakage alarm, and the whole cold box had to be opened and replaced.
So for serious short metallic expansion joints, either use 316L plus low temperature impact test, or use more expensive INCONEL 625 or Hastelloy. Don't lay a mine just to save some material costs. The material of the guide tube also has to keep up-the airflow velocity in the air separation system often exceeds 30m/s, and the thickness of the guide tube is not enough or the material is not resistant to erosion, so it can be worn out in two years. We recommend 304L or 316L for the guide tube, with a thickness of not less than 3mm, and sufficient gap between it and the bellows to avoid getting stuck during cold shrinkage at low temperature.
Installation: Install against the arrow, do not use the tie rod as a bracket
Two days ago, I met a customer. The air separation metal expansion joint was installed backwards, and the guide tube became a throttle plate. The pressure loss soared to 1.5 times of the design value, and the flow rate was insufficient, so I was forced to stop for inspection. The direction of the arrow must point to the direction of the media flow-this is ignored in many scenes.
In addition, how to adjust the tie rod nut? When cold, it should be pre-adjusted in place, but the tie rod is not a fixed support and cannot replace the pipe support. The expansion joint absorbs the displacement by deformation. If you screw the nut to death so that it can't move, the bellows will become a rigid part, and the compensation function will be completely wasted. The transport screw tied at the time of leaving the factory must be removed before it is put into use. An old master asked: Can you not dismantle it? If it is not dismantled, the pipe will expand thermally, the bellows will be held, and the possibility of weld cracking will be extremely high.
Practical Suggestions: Different Choices in Different Positions
Air separation tower outlet pipeline, high pressure, large vibration, and equipment interface, recommended first choiceCurved tube pressure balance type expansion joint orCompound hinge transverse expansion joint。 The former has good thrust control, while the latter occupies a small area. If it is a large diameter pipeline, such as DN600 or above, axial displacement is the main one, it is recommended to useExternal pressure single axial expansion jointWith the use of the guide bracket, it is the most cost-effective.
Incidentally, air separation systems sometimes involve isolation doors for flue gas switching. For example, although the desulfurization flue gas baffle door is not in the body of the air separation unit, a reliable double seal structure is required when the system is switched.Double-seal single-axis circular baffle door sealing in place, leakage rate is less than 0.1%, you can refer to this type of product to select.
In the final analysis, the selection of air separation metal expansion joint is a systematic project. Calculation data, material certification, installation inspection, one link is missed, followed by the fault sheet. In the project at hand, what kind of selection and rollover cases have you encountered? Welcome to chat.