Bellows cracking and guide tube falling off- -the most common methods of death of metal expansion joints
After more than a decade of pipe compensation, I've seen too many sites where metal expansion joints are damaged. Some leaked just after installing it in the first half of the year, and some were finished with a loud noise after three or five years of use. What is the most common? Bellows cracking – a lateral crack along a trough or a penetrating crack at the crest. Then, the guide tube falls off, falls into the pipeline with a bang, and directly blocks the medium flow channel in severe cases. Others are more insidious: a loose tie rod nut causes the bellows to overstretch, or a stuck guide bracket causes the expansion joint to twist. Don't be in a hurry to shout "the quality is not good", the root of many problems is not in the manufacturing link.
Don't throw away the broken piece you removed. Take a wrench and knock it, and you can judge 70% to 80% by listening to the sound-the crisp metal sound means that the material is fine, and the muffled sound may have been delaminated or corroded inside. Alas, two days ago, a power plant customer sent a photo. The metal corrugated expansion joint in their cement industry cracked after less than a year. When I looked at the photo, the wave pitch was uneven, which was obviously not the right selection. This is not an isolated case.
Pit dug by design selection: temperature, pressure and displacement, if one is not accurate, it will be useless
When it comes to model selection, I am most afraid of "Mr. Almost". The temperature is estimated to be 200℃ lower, and the bellows material directly exceeds the allowable stress range-the strength of 304 stainless steel drops rapidly above 400℃, let alone 600℃. Pressure Choice Less? It's a matter of time before the bellows bulges. Displacement is even more a pit: many purchases only look at axial compensation, ignoring lateral and angular displacements. For example, a general-purpose corrugated expansion joint is installed on the steam pipeline. As a result, after thermal expansion, the pipeline not only elongates axially, but also displaces laterally by 20mm. The one-sided wave peak of the expansion joint is directly squeezed to death, and the fatigue life drops sharply from 10,000 times to 500 times.
How to avoid it?High-temperature axial expansion joints are preferred for high-temperature working conditions (above 600℃), or designs with external insulation sleeves. Large diameter pipe considerationLarge diameter thick wall expansion jointThickened wall thickness can withstand internal pressure. If there's a lateral displacement, don't save that money, go onCompound hinge transverse type expansion joint orCurved tube pressure balance expansion joint。 For systems with frequent pressure fluctuations, selectStraight pipe pressure balanced expansion jointCan balance the blind plate force and protect the fixed bracket. When selecting the model, honestly report the temperature, pressure and displacement to the manufacturer. Don't pat your head to reduce one gear. The purchase fee saved back is not enough to shut down for emergency repair.
By the way, if the medium is corrosive, such as desulfurization flue gas, you must usePTFE-lined hoseOrPTFE compensator。 Ordinary stainless steel bellows encounter chloride ions, intergranular corrosion for a few days before perforation. You got it?
Those unremarkable details in installation and maintenance: guide brackets, cold tightening, tie rod nuts, if you make one mistake, you will scrap it
Choosing the right type is only the first step. The installation site is the high accident incidence area. Let's talk about the guide bracket first-many installers want to save trouble, so they can casually weld a few limit blocks to the pipes at both ends of the expansion joint as guides. Fucked. If the spacing of the guide brackets is wrong, the pipeline will have lateral displacement at the expansion joint, and the bellows will be broken and stressed, thus halving its life. How to judge? See if the pipe around the expansion joint is abnormally bent, or if the bracket has any signs of being crooked.
Cold tightness is also an overlooked detail. When the steam pipeline is installed in cold state, if it is not pre-stretched (cold tightened) according to the design value, the displacement of the expansion joint directly exceeds the standard during hot operation. Many sites use tie rods to forcibly pull the expansion joint to the specified length-this is wrong. The correct adjustment method of the tie rod nut: loosen the nut before installation, so that the natural length of the bellows corresponds to the cold state; In the hot state, the expansion amount is absorbed by the bellows, and the pull rod is only a limit, not an adjustment tool. Many workers don't know that if they screw the tie rod to death directly, the expansion joint will become a rigid section, and the bellows will hold the stress and crack sooner or later.
There is also the mounting direction of the guide tube. The direction of the guide tube arrow must be consistent with the flow direction of the medium. If it is reversed, the medium will directly flush the bellows and wear it out in several months. This problem is particularly acute in flue gas ducts and particulate-containing media. If you findGuide tube falling offEighty percent is because the medium flow rate is too high or the installation direction is reversed.
On-site rapid diagnosis: naked eye, tapping, stress test, three steps to find the cause
If the expansion joint is broken, don't rush to dismantle it. Do a three-step quick diagnosis first.
Step 1: See with the naked eye.Observe whether there are cracks, bulges and corrosion spots on the surface of the bellows. Pay attention to the trough – the crack usually starts here. Are there any signs of displacement or detachment of the deflector? Is the tie rod nut loose? Are the pipe supports distorted? These can be spotted at a glance. Take a flashlight from one end of the expansion joint to see if there is any light leakage at the other end-if there is light leakage, it means that there is a penetrating crack.
Step 2: Tap and listen.Use a small hammer to gently tap different parts of the bellows. The normal sound is crisp and the lingering sound long. If the sound is dull and short somewhere, it indicates that there may be delamination, internal corrosion, or fluid accumulation in the area. Then tap the tie rod nut and the loose nut will make a "click" sound.
Step 3: Stress test.If conditions permit, do air tightness test or hydraulic pressure test. Hit the pressure to 1.5 times the design pressure, hold the pressure for 5 minutes, and see the pressure drop. If the pressure drops significantly, apply soapy water to the bellows surface to find leakage points. If there is no condition, just do a simple static pressure observation-close the valve and see if the pressure gauge pointer is slowly dropping.
After these three steps, we can basically judge whether it is a design selection problem, an installation problem or a material problem. Then prescribe the right medicine: if it is a selection problem, change the model; If it is an installation problem, readjust the bracket, cold tightening, and pull rod; If it is a material problem, change to a corrosion-resistant or high-temperature resistant model. Remember, don't blindly replace the same model. If the root cause is not eliminated, it will still be bad if it is changed a hundred times.
To be honest, in many cases of metal expansion joint damage, 70% are buried mines in early selection or installation, and 30% are sudden changes in working conditions (such as abnormal increase of medium temperature and water hammer impact). Next time you encounter damage on site, follow this method to walk through, and make sure you have a confidence in your heart.