An expansion seam isn't as simple as leaving a seam – what is it digesting anyway?
Many people think that the expansion joint is a knife cut in the middle of the pipe, leaving a few millimeters of gap to finish. If you do this, something will go wrong in three months. The essence of expansion joint treatment of metal expansion joint is to leave buffer space for "unsolicited displacement" such as thermal expansion and contraction of pipeline, mechanical vibration and foundation settlement. Think about it, the operating temperature of a steam pipeline soars from normal temperature to 500 to 600 degrees, and every meter of steel pipe can be extended by seven or eight millimeters. The accumulated displacement of hundreds of meters of pipelines is tens of centimeters. If the remaining seam is small, the bellows will be forcibly compressed to the limit, and the fatigue crack will start to spread from the trough; It is only a matter of time before the bellows is over-stretched, the weld cracks, the guide tube falls off, and the medium leaks.
Therefore, the expansion joint is not to "leave a seam", but to accurately calculate the displacement, predict the medium temperature and find out the direction of the pipeline, and then set an initial installation length for the expansion joint. The error is controlled within ±2mm, which is a basic skill, not a bonus.
The three principles of leaving seams: displacement, medium temperature and pipe direction are all indispensable
If one of these three parameters is missing, the treatment of expansion joints is blind.
- Displacement calculation: Axial displacement, lateral displacement and angular displacement. TakeUniversal corrugated expansion jointThe nominal compensation amount usually refers to the axial compression amount, and the value of pre-tension or pre-compression during installation must match the design displacement. For example, the total thermal elongation of the pipeline is 100mm. If you choose an expansion joint with a compensation amount of 150mm, it should be pre-stretched by 50mm during installation to ensure that both cold and hot states are within the safe range.
- Medium temperature: Determine that material and wall thickness of the bellows. For high temperature pipelinesHigh temperature axial expansion jointStainless steel material must consider creep and stress relaxation, and the reserved margin of expansion joint should be 10% ~15% more than that of normal temperature pipeline. Whilerubber compensatorOrPTFE compensatorIt can only be used in low temperature and low pressure applications, and the calculation method of expansion joint is completely different.
- Pipeline Strike: Lateral thrust generated by L-shaped and Z-shaped pipes, which will add additional lateral displacement. At this time, one axial expansion joint alone can't do it, so it has to be usedCompound hinge transverse expansion jointOrCurved tube pressure balance expansion jointThe treatment of expansion joints should take into account multi-directional degrees of freedom, and should not only focus on the axial direction.
I met a customer two days ago. The steam pipe clearly had three 90-degree elbows, but only two were installedUniversal corrugated expansion jointAs a result, the expansion joint tie rod was bent on the third day of production. When I remove it, I see that the top of the guide tube is deformed-a typical account that does not count the direction of the pipeline.
High Temperature Pipeline vs Normal Temperature Pipeline: Completely Different Thinking of Expansion Joint Treatment
Let's talk about high-temperature pipelines (such as the main steam pipeline of the power station, the temperature is> 400℃). In this case, the corrugated pipe with metal expansion joint must choose heat-resistant alloy (such as Incoloy 800H), and the fatigue life of the corrugated pipe should be considered more than 1000 times when designing. During installation, the amount of pre-stretching is not simply equal to half of the thermal elongation, but the change of elastic stiffness of bellows during the process of cold state to hot state should be deducted. More importantly, there must be reliable guide brackets on both sides of the expansion joint of the high-temperature pipe to prevent column instability of the bellows. We had a batchHigh temperature axial expansion jointFor a power plant, during on-site installation, the workers welded the guide bracket by 5mm. As a result, the bellows made a harsh screaming sound during trial operation, which was a precursor to instability. Stop the machine immediately and readjust the bracket to avoid pipe bursting.
Normal temperature pipelines (such as circulating water pipelines, Universal corrugated expansion joint orrubber compensatorEither way, the reserved amount of expansion joint is directly given according to the thermal elongation amount, without considering creep. But be aware: the initial seam width is not the same for winter installation and summer installation. At the site of minus 20℃ in the north, you reserve the expansion joint according to the working condition of 40℃ in summer. As soon as the pipeline heats up, the bellows directly changes from the pre-stretched state to the compressed state, and it may explode at once. So it has to be corrected according to the measured ambient temperature.
What are the differences between direct buried expansion joint and general corrugated expansion joint and expansion joint treatment?
This question is particularly asked by many people. On the surface, they are all expansion joints, but the working conditions are very different.
Direct buried (fully buried) type expansion jointBuried directly in the soil, it can't be inspected at any time like ground pipelines. Once the expansion joint treatment goes wrong, the cost of digging it out and reworking is enough to buy several new expansion joints. The exterior of the directly buried expansion joint should have an integral insulation layer, and a waterproof and anti-corrosion sleeve should be installed. The reserved amount of expansion joints must be magnified by 5% to 10% more than the calculated value, because soil friction will consume an additional part of the compensation capacity. Moreover, the guide tube of the directly buried expansion joint must be designed as a complete through-type to prevent the medium from washing the bellows-we have seen a case where the weld of the guide tube fell off, and the high-temperature hot water directly washed the bellows, which perforated and leaked within half a year.
WhileUniversal corrugated expansion jointMounted on trenches or overhead lines, the installation position is flexible, and the expansion seam width can even be fine-tuned at any time through the tie rod nut. For example, when the manufacturer leaves the factory, adjust the expansion joint to the working length, remove the transportation screw after installation, and then adjust the tie rod nut according to the measured displacement. This operation seems simple, but many people have gone in the opposite direction-tightening the nut is to compress the bellows, loosening it is to stretch, and if you remember it wrongly, you will crush the expansion seam directly to death.
Expansion Joint Treatment 3 Common Rollover Cases, and the Right Remedies
Case 1: The expansion joint is left small, and the bellows is pressed into a "discus"
Of a chemical plantExternal pressure single axial expansion jointThe design temperature is 300℃, and the workers save trouble during on-site installation, so they are welded to death directly without pre-stretching. After putting into production, the pipeline heats up, and the bellows is forcibly compressed until the wave pitch returns to zero, and the corrugations are closely fitted, resulting in elastic failure. Remedy: Stop the furnace for cooling, cut the weld, remove the expansion joint and readjust the installation length. If the bellows has not cracked yet, it can be annealed to restore elasticity; If cracks have appeared, they can only be scrapped and replaced with new ones. The direct loss of this accident is not large, but the indirect loss of one week of shutdown is enough to buy 20 expansion joints.
Case 2: The pipes on both sides of the expansion joint are not fixed, bulldozer effect
Used in a cement plantMetal Corrugated Expansion Joints in Cement IndustryThe displacement of the kiln tail pipe was compensated. As a result, the expansion joint was crooked by the transverse thrust after two months of operation, and a large bulge bulged on one side of the bellows. Cha root cause, no limit brackets are installed on both sides of the expansion joint, and a huge bending moment is generated when the pipe is thermally expanded, which twists the bellows into a twist. The correct way is to set a fixed bracket on both sides of the expansion joint, and the thrust calculation of the fixed bracket must include the blind plate force of the pipe. Remedy: After installing the bracket, reset the expansion joint with a jack and replace the bellows with severe bulging.
Case 3: Direct buried expansion joint insulation water enters, cold bridge leads to local supercooling of bellows
If the insulation layer of directly buried expansion joint is not waterproof, groundwater will seep in, freezing in winter will crack the insulation layer, and water evaporation in summer will accelerate corrosion. Bellows produce temperature differential stresses in local low temperature regions, which are more lethal than overall thermal stresses. Prevention is better than remedy: Always wrap at least two waterproofing layers around the outside of the expansion joint and set up a moisture drain pipe when installing. If water has entered, you can only dig out the expansion joint, remove the insulation material, and check the corrosion of the corrugated pipe. Local corrosion can be polished and welded, and large-area corrosion must be replaced.
Metal expansion joint expansion joint treatment this matter, to put it bluntly, is three words: calculation, pre-determination and defence. Calculate the displacement, preset the margin, and keep the installation specification. Don't be greedy to save that half an hour of work, the price to pay later is often ten times a hundred times.