1. Where is the all-stainless steel expansion joint "all"? Material Contrast and Applicable Scenarios
We've been working with stainless steel for half our lives. As soon as the customer came, he asked "all stainless steel expansion joint". What is "all"? Not all bellows sections are called all stainless steel using. 304. True all stainless steel refers to bellows, end tubes, guide tubes, tie rods, nuts-all parts in contact with the medium are austenitic stainless steel. Carbon steel brushed with silver powder paint for end tubes? That's called "stainless steel bellows + carbon steel end pipe", don't mix it up.
Then what occasion must all stainless steel be used? Corrosive media are hard indicators. For example, when the chemical industry transports sulfuric acid, hydrochloric acid, lye solution, or hot and humid SO₂ gas in the flue gas desulfurization system, the carbon steel end pipe will rust through in half a year. For example, medicine and food pipelines, iron ions are not allowed to pollute the medium. At this time, all stainless steel is just needed. There are also high-temperature steam pipes. Although the steam itself is not very corrosive, carbon steel will oxidize and peel at high temperature, and 304 or 316L can bear it more.
The material also has a doorway. The most common 304 is suitable for-196℃ ~700℃, and the high-temperature intensity attenuation problem should be considered when it exceeds 400℃. 316L has molybdenum added, which is more resistant to chloride corrosion. Choose it more for the pipeline behind the desulfurization flue gas baffle door. If the medium contains a high concentration of chloride ions, such as seawater heat exchange pipelines, then super austenite or even duplex stainless steel must be used. Doubles the price, but lifespan can be extended several times.
Second, general type, high-temperature axial type, pressure balance type... how to choose different structures?
Structural selection is a technical job, and we can't just look at "whether there are goods". Judging from the product catalog on our station, the general-purpose corrugated expansion joint is the most common, with applicable temperature of-20℃ ~450℃ and pressure of 0.25~2.5MPa, which can be used in almost all standard working conditions. However, if you run into the main steam pipe of the power plant, the temperature rushes to 540℃ and the pressure is above 10MPa, the general-purpose type will not work-the bellows material has to be upgraded to Inconel 625, and the structure has to be replaced with a high-temperature axial expansion joint.
Straight pipe pressure balance type and curved pipe pressure balance type. They have built-in balancing rings that can internally offset the blind plate force generated by the medium pressure, so that the pipe support does not have to bear huge thrust. Think about it, when the pressure of large-diameter pipelines is high, the blind plate force can easily be tens of tons. If there is no balanced expansion joint, the bracket has to be made into a steel bunker. Therefore, in the case of limited space and strict thrust requirements, pressure balance type is the first choice.
There are also double hinge transverse expansion joints that absorb transverse displacements, such as thermal expansion at the corners of L-shaped pipes. The external pressure single axial type is suitable for long straight pipe sections. The external pressure structure makes the bellows more stable under external pressure and is less prone to instability. Don't underestimate these structural differences. If you choose the wrong one, you will have abnormal vibration or rupture of bellows.
3. Selection and pit avoidance: in addition to pressure, temperature and compensation amount, don't forget the guide tube and tie rod
Many young engineers choose expansion joints, fill in the working condition parameter table: pressure 2.5MPa, temperature 350℃, compensation amount 30mm axially, and then find a model to match. Are you done? Does the deflector fit? Tie rod strap or not? There were quite a few people who stepped on these two invisible pits.
The function of the deflector is specifically talked about in our Q&A. It is installed on the inner wall of the bellows to guide the high-speed fluid away and avoid the air flow directly washing the bottom of the corrugated valley. For example, the corrugated expansion joint used in the power station industry has a flue gas flow rate of more than 20m/s. Without the guide tube, the bellows will wear out in a few months. At the same time, the guide tube can also reduce the fluid resistance and prevent eddy current excitation. But note: the material of the guide tube must be the same as the end tube, all stainless steel, and the guide tube must be stainless steel.
The pull rod is even more critical. The function of the tie rod is to bear the blind plate force of the pipeline, restrain the bellows to bear the pressure, and at the same time, it can adjust the pre-deformation. The diameter, quantity and length of the tie rod are determined by the pressure grade and caliber during model selection. How to adjust the nut of the tie rod? When the manufacturer leaves the factory, the nut is generally screwed to the positioning position, and then fine-tuned according to the actual pipeline cold tightness after on-site installation. This adjustment technique will be specifically discussed later.
4. Several details that are easy to overturn during installation: the direction of the arrow, whether the screw is removed or not, and how to adjust the tie rod nut
Installation errors are the number one cause of expansion joint failure, more than selection errors. Let's talk about it one by one.
First, the direction of the arrow. An arrow sprayed on the expansion joint housing indicates the flow direction of the medium. Why must it be right? Because the guide tube is directional-the small mouth end faces the medium coming, and the large mouth end faces downstream. Installed backwards, the guide tube becomes a flow choke, which produces eddy vibration and abnormal wear of the bellows. Remember: the arrow points to the direction of media flow, which is the large mouth end of the guide tube.
Second, is the screw disassembled or not? The screw mentioned here is a transport protection screw, also called a limit screw. Everyone at the site will ask, "Do you want to dismantle it?" The answer is: the transport screw must be dismantled after the pipeline is installed in place, the weld is cooled, and before the system is tested for pressure. If it is not disassembled, the expansion joint is equal to a rigid connection, and the compensation ability is completely lost. As soon as the temperature rises, the pipeline stress is fully pressed on the expansion joint, and it will leak within three days. There was once a buddy in a cement factory who forgot to demolish it, and it exploded on the first day of production. The lesson was profound.
Third, how to adjust the tie rod nut. The correct installation method of the expansion joint of the large tie rod: There are two nuts at each end of the tie rod, one locking nut and one adjusting nut. When installing, loosen the locking nut, adjust the adjustment nut so that the length conforms to the pre-stretch amount (given in general drawings), and then tighten the locking nut. Note: After adjustment, the two nuts must be tightened together to prevent loosening. When testing the pressure, check whether the pull rod is uniformly stressed, and it is no problem if the temperature difference is not obvious when touched by hand.
V. Power station, cement, chemical industry... Expansion joint selection cases in different industries
Let's talk about a few cases I personally handled.
Power station industry: Main steam pipeline of a 600MW unit, temperature 540℃, pressure 17.5MPa, diameter DN500. The high-temperature axial expansion joint was selected, the bellows material was Inconel 625, and the end tube and guide tube were both 316H. Note here: The creep resistance of materials at high temperatures is the primary consideration, and ordinary 304 cannot be used. It is also equipped with an inner lining insulation layer to prevent the bellows from directly radiating heat. It has been running for 8 years with no problems.
Cement industry: The air duct at the outlet of the decomposition furnace, the temperature instantly rushed to 900℃, but the smoke and dust were large and contained SO₂. In this case, the metal corrugated expansion joint can't bear it, so it is necessary to use non-metal expansion joint (fabric fiber expansion joint) or lining structure. But what about all stainless steel? We are equipped with double guide tube plus high-temperature ceramic fiber insulation, and 321 stainless steel for outer bellows. The lifespan is 3 times longer than that of carbon steel, but the cost is also high.
Chemical industry: Acetic acid slurry is transported by the pipeline of PTA plant, the temperature is 250℃, the pressure is 1.6MPa, and the medium is highly corrosive. The straight pipe pressure balance expansion joint was selected, all made of 316L material. Why not use the universal type? Because of the complicated pipeline direction and the large spacing of supports, the pressure balance type can reduce the thrust of supports. At the same time, it is equipped with a guide tube to prevent the slurry from depositing and fouling at the trough. The customer has used it for 5 years, and the interior of the bellows is still smooth and clean.
You see, the same all-stainless steel expansion joint has completely different selection logic in power stations, cement and chemical industries. Temperature, pressure, media corrosiveness, pipe direction, bracket conditions – every parameter is hammering the head. Don't be lazy, check our product information and Q&A step by step, so as to hand over a reliable plan.