Why can't cooler pipes be separated from metal expansion joints? -Thermal expansion and contraction are not a minor problem
The cooler looks solid, but in fact, it is the stress of the pipe that is most afraid of. Think about it, when cold fluid goes in and hot fluid comes out, the temperature difference is always dozens of degrees or even hundreds of degrees. The force of thermal expansion and contraction of pipes is no joke-a DN200 steel pipe can be extended by 2 meters when the temperature rises by 100℃? Not that exaggeration, but the length of a 2-meter tube is 2.4 mm. But if the pipeline is stuck by the flange and bracket, this force will all be held on the equipment interface. The flange leaks, and the cooler housing cracks. Tsk, when the time comes to stop work and change equipment, the cost is enough to buy a hundred expansion joints.
Therefore, the metal expansion joint used in the cooler is essentially a "soft joint". It relies on the elastic deformation of the bellows to eat up the thermal displacement and prevent the stress from being transmitted to the equipment and pipes. Moreover, it can absorb vibration-when the air-cooled island's fans and pumps are turned on, the pipes shake like sieves, and there is no expansion joint to support them, and the welds crack early. This thing is not a matter of choosing or not, but a matter of how to choose without stepping on a pit.
Three parameters that must be clarified before selection: temperature, pressure and displacement
Two days ago, I met a customer, patted his chest and said, "I want a bellows. It's up to you how thick and long it is". I get a big head when I hear it. In the matter of type selection, asking one parameter less is to lay mines for the equipment. Three things you have to figure out first:
First, temperature.The cooler medium temperature determines the material and stainless steel grade of the expansion joint. 304 is enough for ordinary water cooling. If it is high-temperature steam or corrosive medium, you have to use 316L or even Inconel. Moreover, the temperature also affects the fatigue life of the bellows-for every 50℃ increase in temperature, the fatigue life may be discounted by half. Don't ask me how I know, just check the service life of the expansion joint that FAQ will know.
Second, stress.The cooler piping is usually low pressure, but it is also necessary to distinguish between design and test pressures. In some sections, the hydraulic pressure test reaches 1.5 times. If the bellows is selected according to the working pressure, it will burst during the pressure test. General-purpose corrugated expansion joint can generally withstand less than 1.0MPa, but if it is the vacuum working condition of air-cooled island, it is necessary to use air-cooled island vacuum pipeline double-hinged expansion joint, which has different wall thickness and structure.
Third, the amount of displacement.Many people only look at how many millimeters the pipe is elongated and ignore the lateral displacement and angular displacement. For example, there is often a turn in the inlet and outlet pipe of the cooler. When the heat expands, the pipe not only elongates, but also swings left and right. If you only buy an axial expansion joint, who will absorb the lateral displacement? The pipeline was still holding its strength. Therefore, before selecting the type, it is necessary to calculate the displacement of the pipe system in three directions, axial, transverse and angular, and one cannot be less.
Which Expansion Joint Corresponds to Different Cooler Conditions? - -From high temperature axial type to pressure balanced type
There are many types of coolers, such as air coolers, shell-and-tube coolers, plate coolers... The working conditions are different, and the selection of expansion joints is also very different.
High Temperature Conditions:For example, the temperature of the steam cooler has gone to 400℃. Don't think of using a rubber compensator at this time, the rubber is directly melted. High-temperature axial expansion joints with high-temperature-resistant ceramic fiber packing are required, and the bellows must be multi-layered. If the pipeline is complicated, you can also consider the external pressure single axial expansion joint. Its bellows is outside, which is not directly washed by the medium and has a longer life.
Large diameter, large caliber:The air duct and flue of the air-cooled island are always two or three meters in diameter. At this time, metal rectangular expansion joint or large diameter thick wall expansion joint is more suitable. Especially the air-cooled island, the pipeline is under negative pressure (vacuum), and the ordinary expansion joint will be deflated. It is necessary to use air-cooled island vacuum pipe double hinge expansion joint, which has two sets of hinges, which can both absorb axial displacement and resist vacuum instability.
Pressure Balancing Requirements:If there are no fixed brackets on the inlet and outlet pipes of the cooler, or the main equipment is not allowed to withstand thrust, then a straight tube pressure balanced expansion joint is required. It has a balancing cavity that counteracts the blind plate force generated by the pressure itself, so that the equipment is not subjected to any thrust. There are also curved pipe pressure balance expansion joints, which are suitable for bending pipe sections. These two things look expensive, but save the money of civil construction and brackets, and calculating the general ledger is cost-effective.
Details that are easiest to overturn during installation: direction of guide tube, adjustment of tie rod, and whether the bolts are removed or not
If you choose the right model, everything will be fine? You're so naive. Installation site rollover example I can tell you about three days and three nights. Let's talk about the three worst things:
First, the direction of the guide tube. The specific function of the expansion joint guide tube is to guide the medium flow and protect the bellows from being washed. However, many people install it backwards-the direction of the arrow must point to the direction of the medium flow, otherwise the guide tube will be overturned, the broken iron piece will be blocked at the nozzle, and the cooler will stop directly. Therefore, before on-site installation, be sure to see where the arrow direction of the expansion joint is pointing.
Second, pull rod adjustment. For those expansion joints with tie rods (such as general-purpose type and large tie rod type), the tie rod nut is locked when it appears, so as to prevent the bellows from stretching during transportation. Once installed in place, the nut must be adjusted to the design position-usually by loosening the nut and allowing the tie rod to slide freely, only as a limit. How to tune it? How to adjust the expansion joint tie rod nut is very clear in that question and answer: first center the pipe, then loosen the locking nut, and tighten the positioning nut according to the calculated pre-tension amount. Don't screw too much, or the bellows will stay tense and its life will be ruined.
Third, whether the bolts are removed or not. Some expansion joints leave the factory with transportation screws, and if they are not disassembled after installation, they are equivalent to welding the expansion joints to death, and they can't suck up any displacement. Does the screw of the expansion joint need to be removed? The answer is that it has to be dismantled. But some people forget that the pipe system collapses as soon as the pressure is tested. There is also the bolt on the flange of the equipment. Don't use the expansion joint as a pipeline compensation part to hard pull. Fix one end first, and then the other end to the flange. Don't tighten the bolts to deform the bellows.
Real case: How is the vibration of air-cooled island pipeline cured by an expansion joint?
Last year, the air-cooled island of a thermal power plant was put into operation for three months, and the pipeline vibrated so hard that it could shake people down. The owner changed the bracket three times and added four dampers, but it was useless. When I went over, the problem was that there was a 2-meter-long straight pipe section between the vacuum pipe and the air cooler. The temperature change and vacuum suction caused the pipe to produce low-frequency resonance. The original universal axial expansion joint can't bear the vacuum negative pressure at all, and the bellows are sucked together, losing the ability to compensate.
Later, it was replaced with what we matchedDouble hinge expansion joint for air-cooled island vacuum pipelineThe corrugated pipe is thickened, the support ring is added, and the hinge structure makes the pipe only expand and contract axially, and the lateral limit is dead. After installing and turning on, the vibration directly drops to less than 0.2mm. The owner's buddy said on the spot, "Why do you use this model earlier? Why do you have to toss for three months?"
So you see, the metal expansion joint for the cooler is really not just paying for one and installing it. Temperature, pressure, displacement, one parameter is incorrectly calculated; Guide tube, tie rod, bolt, one step wrong, the price is enough for you to drink a pot. Before selecting, find out the working conditions thoroughly, choose the right model, and stare at the details during installation-this is the responsible attitude towards the equipment.