Where do the cracks come from? Let's start with the most common "stress corrosion cracking"
When many field engineers see cracks in metal expansion joints, their first reaction is "it's over, you have to replace it with a new one". Oh, don't worry yet. Cracks in metal expansion joints are not necessarily a material or manufacturing problem, and they are not necessarily scrapped immediately. We have to figure out where the crack came from before deciding whether to fix it or replace it.
The most common type of crack, calledStress corrosion cracking。 To put it bluntly, the expansion joint is subjected to tensile stress (whether it is the circumferential stress caused by internal pressure or the installation residual stress) and is exposed to specific corrosive media-such as water vapor containing chloride ions, hydrogen sulfide, and alkaline solution. If these two things are put together, the grain boundary of the metal will be unable to hold up first, and cracks will quietly spread along the intergranular or transgranular lines. The characteristics of this type of crack are obvious: many branches, like tree roots, and often occur in bellows troughs or weld heat-affected zones, because of the concentration of stress in these places.
Take a real case. The desulfurization system of a power plant usedCorrugated expansion joint for power station industry, operated for less than two years, and a large number of fine cracks appeared in the trough. Check the medium, the concentration of chloride ions in the flue gas is high, and the additional stress caused by pipeline vibration is typical stress corrosion. Solution? Later replaced withPTFE-lined hoseThe tetrafluorine layer separates the metal from the corrosive medium, and the problem is solved.
Another high incidence case: fatigue crack, cumulative damage under cyclic loading
Unlike stress corrosion, fatigue cracks are purely "tired". The frequent temperature rise and fall of the pipeline system, or the pressure fluctuation caused by the start and stop of the equipment, each expansion joint has to expand and contract with it. Bellows can withstand a certain number of cycles within the design life, but if the actual working conditions exceed the design value-for example, the design temperature cycles are 5,000 times, but the actual runs are 3,000 times a year-then fatigue cracks will be reported in advance.
What Do Fatigue Cracks Look Like? It is usually a main crack, without branches, and obvious "shell lines" or "beach lines" can be seen at the fracture, which is the trace left by the crack expanding again and again. Common inUniversal corrugated expansion jointOrHigh temperature axial expansion joints, especially those mounted on steam ducts or hot air ducts.
The expansion joint has been used for seven or eight years, and it looks okay on the surface. Suddenly, a patrol inspection found that it leaked. In all likelihood, fatigue has accumulated to a critical point. At this time, don't rush to scold the manufacturer. First, flip through the running records to see if the actual number of starts and stops is twice that written in the bidding documents. If so, it is that the model selection did not take into account enough safety factors.
Pits in selection and deviation in installation-human factors tend to come first
I have seen more than half of the "cracks in metal expansion joints" accidents, and the root cause is the selection and installation. For example, the thermal displacement of the pipeline is the composite displacement of axial and transverse direction, and the thermal displacement that can only absorb axial direction is selectedStraight pipe pressure balance type expansion joint, isn't that equivalent to holding all the transverse force on the bellows? Can the crack not come?
In order to make the flange bolt holes, the expansion joints were fastened with a crowbar. The tie rod nut is also not adjusted properly, and the preload force is either too large or too small. These installation residual stresses, superimposed with operational stresses, directly allow the actual stress level of the expansion joint to skyrocket. There was a chemical factory a while agoPTFE compensatorThe corrosion resistance itself is fine, but the center line of the pipe was deviated by 3mm during installation. As a result, the corrugation on one side of the compensator was elongated, and the other side was compressed, which cracked in two months.
Therefore, when you check the cause of the crack by yourself, first take out the selection drawings and check them-displacement, pressure, temperature and media composition, each item should be checked with the actual working conditions. Check the installation record again to see if there is any forced matching and whether the tie rod nut is adjusted according to the instructions (How to adjust the tie rod nut of expansion joint, refer to in-station Q&A). Don't underestimate this step, 80% of your questions can be answered here.
How to judge the crack type on site? Just add a few simple tools to the naked eye
No high-tech equipment is needed, and the field engineer can make a preliminary judgment by adding a few tools to the naked eye.
First of all,Look at the crack morphology。 If the crack is dendritic and branched, the high probability is stress corrosion; If it is a straight main crack with fatigue lines at the fracture, it is fatigue. If it is cracked along the heat-affected zone of the weld with some oxidized color, it may be material deterioration or weld defect in the heat-affected zone.
Secondly,With the help of penetration detection。 Buy a can of coloring penetrant (imaging agent), spray it on and wait for more than ten minutes, and the crack will show red lines. This method is very sensitive to surface opening cracks, and is especially suitable for rapid screening on site.
Finally,Measuring crack depth。 Using ultrasonic thickness gauge or eddy current flaw detector, you can know how deep the crack is. If the crack depth is less than 10% of the wall thickness and the length is less than 10mm, it can be repaired in many cases. But be aware: if the crack penetrates the bellows wall, or the depth exceeds 50% wall thickness, then don't hesitate to change it directly.
By the way, don't forget to look at the media and environment. If the pipeline is filled with acid gas or chloride-containing liquid, and the crack is at the trough position, it can basically lock in stress corrosion.
Repair or replace? Disposal recommendations for different crack depths and locations
Good question, that's what everyone is most concerned about. My suggestions fall into three situations:
Case 1: Shallow crack, depth ≤20% wall thickness, length ≤10mm, and not at the crest or trough.These kinds can be patched. Use argon arc welding or laser welding, grind the cracks off, repair welding, and then polish them flat. However, it should be noted that after repair welding, local heat treatment must be done to relieve stress, otherwise the same position will soon crack again. This approach works forMetal rectangular expansion joint、Large diameter thick wall expansion jointThis kind of product with relatively thick wall thickness.
Case 2: Medium crack, depth 20% ~50% wall thickness, or located in trough.Direct replacement is recommended. Because the trough is the area with the highest stress, it may not be able to hold the subsequent cycle after repair. EspeciallyExternal pressure single axial expansion jointOrCompound hinge transverse expansion jointThey themselves have complex structures, and local welding will destroy the overall stress. Don't put the safety of the entire plumbing system into it just to save thousands of dollars.
Case 3: The depth exceeds 50% wall thickness, or it has been penetrated.No need to discuss, change. At this time, it is necessary not only to change the expansion joint, but also to check whether the upstream and downstream pipelines have secondary damage caused by leakage. For example, if the medium is high-temperature steam, the leak may have brought nearbyNon-metallic expansion joints (fabric fiber expansion joints) are baked, or corrosive gasesFlue gas baffle doorThe seal was destroyed.
In fact, the best strategy is prevention. Consider the corrosion allowance and fatigue life in the selection stage, and strictly follow theCorrect installation method of expansion joint of large tie rodOperation, use penetration detection for random inspection during regular inspection. In this way, the probability of cracking in the metal expansion joint can be reduced by more than half.