How serious is this problem when the pressure drops fast and the medium sprays out?
I ran into a customer two days ago and said that one of their steam linesMetal expansion joint air leakageIt took less than half an hour for the pressure to drop from 1.2 MPa to 0.6 MPa. When the maintenance worker arrived at the scene, he heard a "hissing" sound from a distance. When he looked closer, he saw a crack on the side of the bellows, and the steam rushed out like a fountain. Do you think this situation can last for another two days? Of course not. Media leakage is not just a waste of energy-once the temperature and pressure get out of control, the whole system may shut down, or maybe hurt people.
In fact, in the industrial site,Metal expansion joint air leakageThere are two most common signals: First, the system pressure is unstable and drops faster than usual; Second, you can hear the hissing sound of gas leakage, or directly see the medium (water, steam, smoke) coming out. The severity depends on the nature of the medium-if it is just room temperature air, it can still hold up for the time being; If it is high-temperature steam or toxic gas, it must be stopped immediately. Don't take chances. Once the bellows cracks, the cracks will only expand and expand.
Find out the culprit: Is it cracked bellows or failed flange seal?
Check the air leak. Don't dismantle it when you come up. Let's first look at these five positions:
- Bellows body: This is the most vulnerable area. Take a flashlight in a circle, and pay attention to whether there are cracks and trachoma visible to the naked eye in the peaks and troughs. If it isUniversal corrugated expansion jointOrHigh temperature axial expansion jointLong-term high-temperature fatigue or corrosion fatigue is most likely to crack at the trough.
- Flange connecting surface: Where the flange is connected with the pipeline, the gasket will leak when the gasket is aged and the bolt is loose. A lot of "air leakage" is actually not that the expansion joint is broken at all, but that the flange is not tightened. Spray it with soapy water to see if any bubbles pop up.
- The overlap between the guide tube and the bellowsReferring to the "Specific Function of Expansion Joint Guide Tube" we talked about before, if the weld of the guide tube is cracked, the medium will rush into the bellows gap from the weld, causing leakage.
- Tie rod/nut locking structure: likeCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointIf the tie rod nut is loose and the pre-tension fails, the bellows may produce plastic deformation and lead to leakage.
- Pipe and bellows circumferential weld: This place is the easiest to overlook. If the welding quality is not up to standard or the stress is excessive during operation, pinhole leakage will occur in the weld.
Then how do you tell which one? It is very simple: after shutting down and relieving pressure, check it in sections with air tightness test. First, use a blind plate to isolate the expansion joint, pump low pressure air into the pipeline, and spray leak detection liquid one by one. Don't bother, this step is ten times cheaper than replacing a new expansion joint.
On-site first aid: temporary blocking and tightness adjustment, are these operations reliable?
Some old masters encounterMetal expansion joint air leakageThe first reaction is to wrap the crack with a clamp or iron sheet, and then apply sealant. This method can last for a while for non-critical pipelines at low pressure and normal temperature, but it can last for a week at most. Why? Because the pipeline is still expanding and contracting, every time the bellows moves, the crack is expanded once, and the sealant fails quickly. And guess what? When I went to the scene the next day, the leak was even worse.
Another operation is to tighten the tie rod nut-note that this trick is only applicable when the pre-tension is not in place during the installation phase. If it has started to leak, adjusting the nut will only subject the bellows to additional force, accelerating the destruction. So my suggestion is:On-site first aid can only be temporary pressure reduction operation, or with repair welding (only slight trachoma).Real air leaks must be replaced or returned to the factory for repair.
When it comes to repair welding, it has to be mentioned in particular: the material of corrugated pipe is usually stainless steel (such as 304 and 316L), and the weld should be cleaned before welding. During welding, argon arc welding should be used for rapid welding with small current, and pickling and passivation should be done after welding. Improper operation will burn the bellows through instead.
Radical solution: select the right model, standardize the installation, and avoid secondary rework
The air leakage is repaired and leaked again. Nine times out of ten, the original model was chosen wrong or the installation was not standardized. For example, a power plant uses ordinary pulverized coal pipelinesUniversal corrugated expansion jointAs a result, the medium temperature in the tube was 700 DEG C, and the bellows was directly burned through. Later replaced withHigh temperature axial expansion joint(with temperature-resistant lining), the problem is solved. You have to calculate clearly when selecting the model:
- Temperature, pressure, medium corrosivity— Contrast UsCorrugated expansion joint for power station industryOrMetal Corrugated Expansion Joints in Cement IndustrySelection table, see the applicable working conditions.
- Compensation amount— — How much is the heat expansion of the tube, it must be usedDirect buried (fully buried) type expansion jointStillExternal pressure single axial expansion joint？ If the calculation is wrong, the bellows will be over-stretched, and the life will be directly discounted by 50%.
- Direction of guide tube-The direction of the arrow on the expansion joint must coincide with the flow direction of the medium, otherwise the guide tube will be washed and perforated.
Even more serious when installing. Such asCorrect installation method of expansion joint of large tie rodWhat is emphasized in: It must be cold tight (pre-tensioned), and after the tie rod nut is adjusted in place, lock it to prevent loosening. The flange end faces should be parallel, and the bolts should be tightened symmetrically according to the moment. If these details can't be done, they will leak after half a year-do you think it's wrong or not?
Extending Life: Daily Inspection and Maintenance Checklist
- Appearance inspection once a month: See whether there are rust spots, indentations and scratches on the surface of the bellows. EspeciallyLarge diameter thick wall expansion jointHas heavy weight and is easy to be deformed due to improper support.
- Quarterly displacement check: Make marks on both sides of the bellows and measure the actual expansion and contraction with a ruler. If it exceeds 10% of the design value, it is necessary to check whether there is a problem with the pipe support and hanger.
- Bolt tightening semi-annually: Flange bolts and tie rod nuts are tightened one by one. Be careful to use a torque wrench, don't rely on the feel.
- Annual airtightness test: For key pipelines (steam, gas, toxic media), be sure to arrange shutdown and pressure for leak detection.
Metal hoseAndrubber compensatorIt should also be included in the scope of inspection. While they are not expansion joints, whichever leaks on the same line will affect the overall system.
Choose products from regular manufacturers, and don't buy non-standard parts cheaply. The 30 product categories we have listed above, fromSpecial hose for vacuumToDesulfurization flue gas baffle doorEach has a corresponding working condition parameter. When you are not sure, provide the manufacturer with the working condition parameters and let them help you calculate it-it is much more reliable than being blind yourself.