Old masters in dry power plants and cement plants all know that what high-pressure pipelines are most afraid of is not high temperature, but thermal expansion and contraction. When the pipe system is stiff, the flange leaks steam, the bracket deforms, and even the weld cracks, it will cost hundreds of thousands to stop for maintenance. At this time, we have to rely on metal high-pressure corrugated expansion joints to play. To put it bluntly, this thing is a flexible joint that can be stretched and retracted, which absorbs the displacement of the pipe due to temperature changes, so that the pipe system is not subjected to extra force. But it is really different from ordinary corrugated expansion joints. Today, let me talk about the door inside.
What is a metal high pressure corrugated expansion joint? What is that difference between it and ordinary corrugated expansion joint
You may have seen ordinary corrugated expansion joints, which are used in air conditioning water pipes and low-pressure air pipes. The pipe wall is thin, the corrugation is shallow, and it can absorb point displacement. But metal high-pressure corrugated expansion joints are another species altogether. It has to carry the working conditions of steam pipeline, high-pressure gas pipeline and main steam pipeline of power station, which are often 4.0MPa, 6.4MPa or even more than 10MPa, and the temperature may rush above 500℃.
What's the difference? Just look at a few hard indicators. The wall thickness of ordinary expansion joints may only be 0.5mm to 1mm, and the high-pressure joints must be 2mm, 3mm or even thicker; One layer of corrugation is enough for ordinary expansion joints, while high-pressure joints often have multiple layers-two or three layers stacked, which not only ensures flexibility but also withstands pressure. Look at the port again. Ordinary expansion joints can be directly welded with steel pipes. High-pressure ones have to be equipped with thick-walled joints, reinforcing rings and guide tubes. The weight difference of a set is several times. When you tap the ordinary expansion joint with your hand, it is a crisp thin plate sound, while when you tap the high pressure, it is a muffled sound. The density and stiffness are completely not at the same level.
Another point is that the material of ordinary expansion joints is generally 304 stainless steel. Under high pressure conditions, 304 can't stand it, and it has to be 316L, 321, or even superalloy Incoloy 800H. Why? High pressure is accompanied by high temperature, intergranular corrosion and high temperature creep are the obstacles that ordinary materials cannot bypass. Once the material is chosen wrong, the bellows will be like a rubber band that is slowly stretched under high temperature and pressure, and finally it will crack through fatigue. It is no joke that the pipe collapses.
What problems should be solved in the design of corrugated expansion joints under high pressure conditions
What is the difficulty in designing a corrugated expansion joint that can carry high pressure? The first is the stress distribution. Under the action of pressure, the stress concentration of wave peaks and valleys is particularly severe. The higher the pressure, the greater the film stress and bending stress. If the calculation is slightly unaccurate, the bellows will "retire" early. Therefore, the design and calculation of high-pressure expansion joints must be checked item by item according to EJMA (American Expansion Joint Manufacturers Association Standard), not just pat your head and set the wall thickness.
The second conundrum is stability. Think about it, bellows have a natural "bulging effect" under high pressure-pressure will try to straighten and round it. If the length and diameter ratio of the bellows is not properly designed, or the position of the reinforcing ring is not correctly calculated, it will cause column instability or plane instability, corrugation distortion and deformation, and the seal will fail directly. This is why high-pressure expansion joints often have reinforcing rings or multi-layer thin-walled structures-each layer shares a part of the pressure, and the whole is not easily deflated.
The third conundrum is fatigue. Although the displacement cycle of high-pressure pipeline is infrequent, every start and stop is a test for the bellows. When pressure cycle and displacement cycle are superimposed, can the low cycle fatigue life of bellows reach the design requirements of 1000 times and 2000 times? This has to be verified by the fatigue life curve. In the actual project, in order to save money, some manufacturers pressed the fatigue life to the lowest standard. As a result, the bellows cracked after two or three years. Finally, customers spent a lot of money to change the pipe, which was not worth the loss. Alas, we have seen too many cases of this kind.
Type selection does not only look at pressure: displacement, stiffness and fatigue life are indispensable
"I want a 10MPa expansion joint, how much does it cost?" This question is actually unprofessional. Pressure is only the threshold. What really determines that expansion energy saving can't be used is the match of displacement, stiffness and fatigue life.
The displacement is easy to understand-which way and how many millimeters will the thermal expansion and contraction of the pipe run, and the ripples of the expansion joint must be able to eat. However, the displacement is not just a number. It is divided into axial, transverse and angular directions. The influence of displacement in different directions on the stress of bellows is very different. If you take the lateral displacement as the axial displacement, the bellows will break.
The parameter stiffness is also interesting. The stiffness is too great, the bellows can't be pushed, and the force of the pipe system can't be removed; The stiffness is too small, the bellows is too soft, and it is easy to become unstable. Because of the wall thickness and the number of layers, the high-pressure expansion joint is naturally more rigid than the low-pressure joint, which is a physical law. However, the design can be adjusted by wave number and wave height to match the stiffness and displacement requirements.
Fatigue life is another key. It has to do with pressure, displacement, material, corrugation shape. The same expansion joint, if it absorbs 10mm displacement, can carry 5,000 times, and if it absorbs 30mm displacement, it may crack 500 times. Therefore, when selecting the type, the actual operating conditions must be clearly stated-the pressure, the temperature, how many times a day to start and stop, and the displacement of the pipeline, so that the manufacturer can calculate the life. Don't just focus on the pressure level, or the money saved won't be enough to change the pipes once.
Installation and maintenance: What are expansion joints on high-pressure pipelines most afraid of
When installing high-pressure expansion joints, I am most afraid of three things.
The first thing, the cold tightening was not done properly. If cold tightness is required during design, the expansion joint must be pre-stretched or pre-compressed by a certain amount according to the drawing for on-site installation. Some construction teams save trouble and install it directly. As a result, as soon as the operating temperature of the pipeline goes up, the displacement directly exceeds the design value, and the bellows is forcibly cracked. This is not without precedent.
The second piece, the guide bracket is not in place. The expansion joint is not used in isolation, and it must be equipped with guide brackets upstream and downstream to make the displacement direction controllable. If the spacing of the brackets is too large or the position is wrong, the pipe will generate large bending stress at the expansion joint, and the bellows will soon be fatigued. Think about it, the pipe is off, and the expansion joint is working twisting there. Can it not break?
Third, fear of water hammer and overpressure. When the high-pressure steam pipeline is started, if the drainage of the warm pipe is not thorough, the condensed water will form a water hammer under the push of hot steam, and the instantaneous impact force can beat the bellows into a twist. Therefore, when selecting the expansion joint, it is necessary to consider whether there is a risk of water hammer, and install a trap and a buffer device if necessary.
The maintenance aspect is also not worry-free. The high-pressure expansion joint can't just be installed and ignored. It is necessary to regularly check the corrugated surface for corrosion points, cracks and pits. When parking for maintenance, people drill into the pipeline and look at the inside of the bellows with an endoscope. Those fine cracks can't be found with the naked eye. We met a cement factory customer. The outside of the bellows looked good, but there were already penetrating cracks on the inside. If it wasn't for regular inspection, the whole high-temperature fan pipe would have to be scrapped.
From power stations to cement, in which scenarios are metal high-pressure corrugated expansion joints carrying things
Let's talk about the power station first. The main steam pipeline, reheating hot section pipeline and water supply pipeline of power station boiler are all hard bones of high temperature and high pressure. The main steam temperature can reach above 540℃, and the pressure is more than ten MPa. Under this working condition, ordinary expansion joints can't be thought of at all. Specially designed high-temperature axial expansion joint or straight pipe pressure balance expansion joint should be used for corrugated expansion joint in power station industry. The latter can balance the internal pressure thrust while absorbing displacement, thus greatly reducing the force on the pipe frame. There are also special varieties of air-cooled island pipelines and vacuum pipelines, such as double-hinged expansion joints of air-cooled island vacuum pipelines, which are used to absorb the multi-dimensional displacement of pipelines under the joint action of vacuum and temperature.
Look at the cement industry again. On the cement production line, the kiln tail flue gas pipeline, the circulation air pipeline of the raw material mill and the transmission pipeline of the raw material homogenization warehouse are not as high in temperature as that of the power station, but the requirements for wear resistance and corrosion resistance are not low at all. Metal corrugated expansion joints in cement industry often have to deal with high-speed scour of dusty air flow, so the design of guide tube and lining plate is particularly special. The function of the expansion joint guide tube is to guide the airflow away and prevent the wear particles from directly hitting the corrugation-if this detail is saved, the bellows will be worn out in half a year. In addition, the temperature of the secondary air duct and the tertiary air duct of the cement kiln is also above 1000℃. Although the pressure is not high, the thermal displacement is huge, so it has to be absorbed by high-temperature expansion joint and hinge system.
There are also syngas pipelines in the chemical industry and high-pressure hydrogenation units in oil refineries, and the working conditions are no milder than those of power stations. In these situations, external pressure single axial expansion joint or double hinge transverse expansion joint are often used. The former can withstand higher external pressure, and the latter can absorb larger transverse displacement. Large-diameter thick-walled expansion joints are also indispensable in large-scale wind power and petrochemical projects. To put it bluntly, there are metal high-pressure corrugated expansion joints everywhere high-temperature and high-pressure pipelines need to compensate for displacement. This is not a coincidence, but it is clearly stipulated in the pipe design code that the pipe system must be flexible enough, and the expansion joint is the preferred element to achieve flexibility.
In the final analysis, the metal high-pressure corrugated expansion joint is a "one man is in charge" role. It holds the pressure for the pipe, swallows the displacement, and keeps it safe. Think more about the selection, collect more rules during installation, and give more snacks in later maintenance. It can serve safely and steadily on high-pressure pipelines for more than ten years. On the other hand, if you fool it, it dares to give you a big accident. In this business, safety is always the first priority.