Many customers ask for an expansion joint quotation with the pipeline parameters, and the opening is "Give me a DN300 corrugated compensator". But if you ask him what the temperature of the medium is, whether it is corrosive, and whether it is a dusty airflow, he is often stunned. In fact, this is not the user's fault-the expansion joint looks simple, and there are many doors inside. Today, let's talk about a part that is easy to overlook:Metal expansion joint end tube。
What is an end tube? What kind of work does it carry in the metal expansion joint
The end pipe, to put it bluntly, is the straight pipe section where the two ends of the expansion joint connect the pipe. The bellows is responsible for deformation and absorption of displacement, and the end pipe is responsible for bordering with the pipeline system. The location is inconspicuous, but there are many jobs.
First, the connection-you have to weld it to the pipe, right? Second, force transmission-the pressure thrust, bending moment and torque pushed by the pipe are all passed to the bellows and intermediate structural members by the end pipe. Third, protection-the bellows has thin walls and low stiffness, so it is most afraid of bumping outside and washing inside. The end pipe is equivalent to holding the portal at both ends of the bellows.
If the end pipe is short, thin, and the material is wrong, the bellows will be in vain. Two days ago, I encountered a case in a chemical plant. The bellows itself was not broken, but the weld of the end pipe cracked first, and the whole pipeline was stopped for maintenance for two days. Whose is this loss?
Material selection and size design of end pipe: how to determine wall thickness, material and length
Let's talk about the material first. The end pipe is not just taking a 20#steel and welding it to it. The medium contains chloride ions. You use 304 and wait for stress corrosion to crack. When the medium is above 450℃, you use 304L, and the high temperature intensity is severely attenuated. You have to consider 321 or 316L at this time. If the medium contains sulfur and acid, it has to go to the direction of high alloy, or even be lined with corrosion-resistant alloy.
For ordinary water steam pipes, the material of the end pipe is generally the same as the pipe body or one grade higher; Because the flue gas pipeline contains sulfur and dust, the requirements of wear resistance and corrosion resistance of the end pipe are much higher than those of the pipeline body, and the wall thickness usually needs to be added 2~3mm as the corrosion margin.
Let's talk about wall thickness. The wall thickness of the end tube is not determined by patting the head. Pressure, temperature, caliber, corrosiveness of the medium, and whether there is any external load, all have to be counted. Many people only check the wall thickness according to the design pressure, and forget about the axial thrust and torque. The load of the end pipe is often greater than that of the pipe body-because the elastic reaction force and pressure thrust of the bellows have to be transmitted through the end pipe. Wall thickness allowance is not a waste, it is to buy insurance.
Finally, talk about length. The length of the end tube directly affects the stability of the bellows. If it is short, it will be welded to the root of the bellows. The heat affected zone is too close to the bellows, which may ruin the tissue of the bellows. It is a little longer, and it can leave space for the flow guide tube to be arranged. Generally, it is not less than 1/3 of the pipe diameter, and the length of the end pipe of the small-diameter pipeline must be at least 100mm. When the installation space is tight, it is better to think of a way in the design of corrugated pipe than to compress the length of the end pipe.
How to Match Bellows and End Pipe: Welding, Guide Tube and Pressure Balance Expansion Joint
The most common way of connecting bellows and end pipes is butt welding. When the multi-layer bellows is welded to the end pipe, the misalignment between layers and the infiltration of welding nodules into the corrugated root of the bellows are all prone to problems in actual production. The good practice is to reserve the fusion zone when the end of the end pipe is grooved, and after welding, the bellows side is pickled and passivated to clean the welding slag and oxide scale.
The installation of the guide tube is also related to the end tube. The fixed end of the head of the guide tube generally sits on the inner wall of the end tube. There is a link that is easy to overlook here-the gap between the guide tube and the inner wall of the bellows. If the gap is small, the expansion section is compressed, and the guide tube may push against the bellows; When left large, the air flow forms a vortex in the gap, which produces erosion and vibration to the thin-walled bellows. Generally, the displacement of bellows is controlled by adding 10~15mm. The flow direction of the medium must flow from the fixed end of the guide tube to the free end. If the direction is reversed, the guide tube will become a throttle orifice plate, and the pressure loss will increase sharply.
In the straight pipe pressure balancing expansion joint, the end pipe has a special task: it is connected to the middle pipe and the balancing bellows, and the pressure thrust generated by the bellows is digested inside the expansion joint. In this structure, the end pipe is subjected to the combined load of tension and compression, and the wall thickness and welding quality are higher. If something goes wrong, it is not as simple as an air leak, which may directly lead to the instability of the bellows.
What are the common places for end pipe problems
There are many types of end tube failure. The most common is intergranular corrosion-the heat-affected zone stays in the sensitization temperature range for too long during welding, and chromium carbide precipitates at the grain boundary, resulting in a sharp drop in local corrosion resistance. After welding, no solution treatment or stabilization treatment is done, and knife-like corrosion will appear around the weld in a short time.
Chloride ion stress corrosion cracking is also a big player. 304 In an environment above 60℃ and containing chloride ions, tensile stress + sensitive material + corrosive medium are in line, and the crack develops from the inner wall to the outer wall of the end pipe, sometimes it can't be seen by the naked eye until it penetrates and leaks. This problem can be alleviated but not eradicated with 304L. When the chloride ion concentration is high, it is stable to directly apply duplex steel or nickel-based alloy.
The cracking of the weld is a problem of force. If there is vibration in the pipeline system, or the bracket is in place, the weld of the end pipe is under alternating stress, and the crack initiates and propagates from the weld toe, and finally penetrates the wall thickness. This is related to the welding quality, and more related to the system design. Don't save the place where the support and hanger should be set during installation, and don't let the expansion joint go to "part-time" pipe support.
There is also an easily overlooked occasion-deflector wear-out. When the dust-containing airflow speed is high, the head of the guide tube is ground into a blade shape, and the dust continues to wash the bellows, so it is only a matter of time before the bellows wear out. Therefore, in the design, the head of the guide tube should be thickened or surfaced with wear-resistant layer, and the inner wall of the end tube should also be wear-resistant.
How to raise end pipe requirements during type selection
When placing an order with the manufacturer, don't just write "Universal Corrugated Expansion Joint DN500 PN16" and be done. You have to write the relevant parameters of the end pipe clearly: the material (preferably the grade, such as 06Cr19Ni10 or 022Cr17Ni12Mo2), the wall thickness (specify whether it needs to be thickened on the pipe body), the length of the end pipe (or indicate "configuration on demand"), the groove form (whether it is flat butt or grooved), and the surface treatment requirements (pickling passivation, sandblasting, or inner wall polishing).
If conditions permit, it is best to throw the pipeline layout diagram to the manufacturer. Many problems can be directly seen on the drawings-the installation position of the expansion joint is too close to the elbow, the distance between the support and the hanger is too large, and the medium flow direction is reversed. These problems can be avoided in advance with drawings.
Metal expansion joint end tubeLike bellows, it is a pressure-bearing element. Is it too much to ask the manufacturer to provide material certificate and welding process evaluation record when leaving the factory? Don't be too troubled. These documents are life-saving in the inspection report of the Special Inspection Institute and the subsequent accident tracing.
Selecting the end tube is essentially selecting the safety margin. The expansion joint is the elastic weak link in the pipeline system, and the end pipe is the anchor point of this weak link. If the anchor point is not firm, no matter how good the performance of the bellows is, it can't be played. The next time you select a model, it takes two minutes to write the end pipe requirements into the inquiry form, which is much more cost-effective than removing and replacing the pipe afterwards.