1. Design is not about patting the head: how to determine the parameters of bellows?
Many engineers come up and ask, "Give me an expansion joint for DN200", as if buying screws-IMHO, this is the wrong way. The expansion joint is designed to push down from the four dimensions of medium, temperature, pressure and displacement of the pipeline system. For example, the temperature of the steam pipe is always three or four hundred degrees, and the pressure is also high. For this kind of working condition, we have to use the steam pipe in our stationHigh temperature axial expansion joint； If you change to a flue gas system, such as the pipeline behind the desulfurization tower, which contains sulfur, chlorine and particles, you have to consider wear resistance and corrosion-the materials and structures of expansion joints used in the cement industry and the power station industry are completely different.
Do you want to add the deflector? Look at the flow rate and particle content. In our question and answer, we specifically talked about the function of the guide tube: high-speed fluid carries particles, and without the guide tube, the bellows wall will wear out in a few months. In turn, the clean medium runs at low speeds, and the guide tube increases local resistance instead. There is also a common rollover scene in engineering: the installation direction of the guide tube is reversed, and the arrow is pointed opposite to the flow direction of the medium, resulting in erosion and perforation. Alas, this job can't be based on feeling.
How to calculate stiffness? There is a question and answer in our product library that specifically talks about the calculation formula of bellows stiffness, which involves wave number, wave height, wall thickness and material elastic modulus. Designers have to substitute these parameters into the verification, not just set a model number. Take a real figure: the same DN200, 16 waves, wave height 40mm and wave height 50mm, the stiffness difference is more than 30%. The stiffness of the expansion joint you selected is too great to absorb the displacement, and the pipeline stress explodes; It's too small and can't hold internal pressure, so it's easy to become unstable. So, don't slap your head.
2. If you choose the right type, you will have fewer accidents than half
There are various expansion joints on the market, and there are more than 30 kinds listed in our station:Universal corrugated expansion joint、External pressure single axial expansion joint、Compound hinge transverse expansion joint、Straight pipe pressure balanced expansion joint、Curved tube pressure balance expansion joint... each corresponds to a different displacement compensation mode and pipe arrangement. And guess what? Choose the wrong type, from leakage to broken pipeline.
Two days ago, I met a customer. The steam pipeline used a general-purpose type. As a result, the axial thrust pushed the fixed bracket crooked-the general-purpose type can only absorb axial displacement, but your pipeline still has lateral displacement! Later replaced withCompound straight pipe bypass pressure balanced expansion jointTo solve the problem. There is also a large tie rod expansion joint, which is specialized in absorbing lateral displacement. If you use it as an axial direction, the tie rod is a display. Under high temperature conditions, should the bellows be insulated? It is necessary, otherwise the bellows will be directly exposed to radiant heat, and the austenitic stainless steel will be sensitized, and intergranular corrosion will come to your door sooner or later.
Another side door-directly buried pipe, you can't dig it up and inspect it, can you? Then you have to useDirect buried (fully buried) type expansion jointWith its own waterproof jacket and guide structure. There is also an air-cooled island vacuum pipe, the displacement is three-dimensional, ordinary expansion joints can't handle it at all, have to go upDouble hinge expansion joint for air-cooled island vacuum pipeline。 You see, it is not an exaggeration to say that if you choose the right type, there will be half the accidents.
3. Material and Manufacturing Process: Don't Save Money on Bellows
Austenitic stainless steel is usually selected as the material of corrugated pipe, such as 304 and 316L, but the environment containing chloride ions (such as desulfurization flue gas) has to be upgraded-with super austenite or nickel-based alloy. We have them in our stationPTFE-lined hoseTo deal with strong corrosive media, such as hydrochloric acid and hydrofluoric acid pipelines, a package of PTFE layer, everything will be fine. However, note that the pressure and temperature resistance of PTFE lining are limited. If the temperature exceeds 200℃ or the pressure fluctuates greatly, the PTFE layer may peel off.
In the manufacturing process, hydroforming and mechanical forming have their own advantages and disadvantages. Hydroformed corrugated pipe has uniform wall thickness and high fatigue life, but also high cost; Mechanical forming is cheap, but it is easy to thin at the trough. The thickness control of corrugated pipe is the key-the fatigue life of the same batch of materials, with a wall thickness difference of 0.1mm, may be twice as bad. The heat treatment process cannot be saved, and the solution treatment can eliminate the processing stress, otherwise the bellows will crack within a few days after being installed.
Although non-metallic expansion joints are cheap, they have limited temperature and pressure resistance. We have them in our stationNon-metallic expansion joint (fabric fiber expansion joint)AndRectangular non-metallic expansion jointIt is particularly good for flue gas duct compensation, but don't take it as a substitute for metal-if the temperature exceeds 400℃ or the pressure exceeds 0.1MPa, non-metals basically can't bear it. I'm really unsure. Reference standards: JB/T 12235-2015 for non-metallic ones, and national standards for metallic ones. But then again, the standard is only the bottom line. When the working conditions are severe, materials must be selected beyond the standard.
4. Core principle of selection: pipe layout and compensation ability should be matched
Expansion joint is not a single part, it is a whole system with pipe bracket, guide bracket and fixed bracket. When designing, calculate the amount of displacement absorbed by each expansion joint-axial, transverse and angular displacement, not just the total displacement. For example: a section of L-shaped pipe has a thermal elongation of 20mm between the fixed brackets on both sides, but there is a lateral offset of 5mm at the corner of the pipe. You only plug an axial expansion joint, who will eat the lateral displacement? It can only be carried by the pipe itself, and as a result, the flange bolts are bent.
What about that? Draw a sketch of the pipe system, mark all thermal displacements, and then select the minimum and most reasonable number of expansion joints. For the multi-directional displacement of air-cooled island vacuum pipeline, it is right to use double hinge expansion joint. Small tie rod, large tie rod, duplex, bypass…the more complex the structure, the higher the price, but also the greater the safety redundancy. Don't stress out to save two bucks-we have a customer, oneRotary compensatorThe problem that can be solved is that there are three general types of non-fortress. As a result, the fixed bracket is pushed down twice, and the maintenance cost is three times more expensive than the equipment.
Expansion joints of complex structure such asCurved tube pressure balance typeAndStraight pipe pressure balance typeThey come with their own balanced bellows, which can offset the internal pressure thrust, and the fixed bracket can be made much lighter. However, accordingly, the installation space should be reserved enough, and the spacing of the guide brackets is strictly limited-almost, the pressure balancing effect is greatly reduced. How to choose? There is no universal formula, but there is one principle: the compensation ability should be 20% ~30% greater than the actual displacement, leave a margin, and don't stick with the boundary.
5. Installation and Operation and Maintenance: Real Cases of Detail Rollover
Expansion joint installation, first thing to look at the direction of the arrow. We specifically said in our question and answer that the direction of the arrow of the expansion joint must point to the direction of the medium flow. Install backwards? The guide tube holds the fluid, directly forming vortex erosion, and it will inevitably perforate within three months. There is also the problem of the tie rod nut-the screw used for transportation and fixation, should it be removed after installation? Look at the type. Some expansion joints (such as the general type) have tie rods that are transport fixings and must be removed after installation, otherwise they limit displacement. But those working tie rods, like the large tie rod expansion joints, must be retained, they are responsible for withstanding internal pressure thrust. We specifically talked about whether the expansion joint screw needs to be removed in our Q&A, so don't get confused.
The deflector was installed in the wrong direction. As I said before, I'll emphasize it again here: there is more than one case, all of which are low-level errors. There are also rubber compensators and PTFE compensators, which have requirements for installation torque-use a torque wrench and twist according to the torque given by the manufacturer. Screw too tightly will damage the flange surface, and screw too loosely and leak. From our stationrubber compensatorAndRubber PTFE compensatorAll have recommended installation torque values, just follow them.
Finally, talk about operation and maintenance. Check the bellows regularly every year for cracks, corrosion and thinning. Shine the trough with a flashlight, and replace it immediately if there is a crack. Don't wait for the pipe to burst before you remember maintenance-there is a petrochemical plant, and the expansion joint has not been checked for five years. One day, in the middle of the night, there was a "bang", and the steam leakage burned two people. Tsk, do you think you can save this money? The service life can't just look at the nominal name of the manufacturer, but also the working conditions: the chlorine-containing environment may be scrapped in two or three years, and clean steam can be used for more than ten years. Therefore, regular inspection is more important than any type selection.