Metal expansion joint instability? To put it bluntly, the bellows can't carry it
In the dry plumbing industry, there was no one who hadn't seen that thing before. It was originally a bellows, but suddenly a bulge, or twisted into a twist, or even burst. This is typicalMetal expansion joint instability。 The bellows cannot hold up under working stress, manifesting as local bulging (internal pressure instability), overall twisting (columnar instability) or plane instability (bulge on one side). On the surface, it looks like a minor problem, but once the pipe bursts, leakage, shutdown and even safety accidents follow one after another. And guess what? Many factories have suffered a big loss, and when they look back, they find that there are only a few root causes.
The three main culprits of instability, one than the other pit
First, the internal pressure is too high-the pressure level is not accurate when selecting the model
This is most prevalent. Some project drawings save trouble, just take themUniversal corrugated expansion jointGoing up, I didn't calculate the actual working pressure of the system at all. And the result? The internal pressure exceeds the permissible value of the bellows, and the bellows bulge like a balloon. Two days ago, I met a buddy from a chemical plant. Their steam pipeline was nominal to 1.0MPa, but in actual operation, it occasionally rushed to 1.3MPa. In less than half a year, the side wall of the expansion joint bulged. The criterion for judging internal pressure instability is very simple: visually inspect whether the ripples are obviously deformed. Once you find it, don't hesitate to change it immediately. Don't wait until it collapses before regretting it.
Second, the temperature exceeds the limit-the high temperature strength of the material breaks the cliff
Stainless steel, once the temperature rises, the yield strength drops faster than the stock market. The steam pipeline of a thermal power plant has a design temperature of 350℃, and a common expansion joint is used. As a result, the actual operating temperature soars to 420℃. The yield strength of the material has dropped by more than 30%, and the direct internal pressure of the bellows is unstable. You think this is a small probability? In fact, in many technical transformation projects, the temperature goes up when the pipeline insulation is not done well or the medium working conditions change. In this case, theHigh temperature axial expansion jointYou have to be willing to spend money, don't make do with the ordinary type. What about that? When selecting, leave sufficient margin. When the temperature exceeds 400℃, high-temperature-resistant alloy or special structure must be used.
Third, the amount of compensation is less during installation-the hard-pulled and hard-pulled bellows is twisted
The installer drawing is fast, the pre-stretching amount is not adjusted according to the drawing, or the spacing between the bracket and the guide frame exceeds the standard, which leads to the bellows not being able to absorb so much displacement, and it is directly pulled crooked or compressed. This kind of columnar instability is the most dangerous because the bellows has insufficient axial stiffness and will be wasted when twisted. Some engineering useCompound hinge transverse expansion jointOrThe straight pipe pressure balance expansion joint could have solved the problem of large displacement, but as a result, the tie rod nut was not adjusted in place during installation, and the spacing between the guide frames exceeded the design value, so there was still a problem. TuneExpansion joint tie rod nutDon't be lazy in this matter. Adjust each nut according to the instructions, and control the deviation within ±1mm.
Real case: A thermal power plant scrapped a batch of expansion joints in half a year
Last year, there was a main steam pipeline of a thermal power plant, with a total length of more than 500 meters, and more than a dozen general-purpose corrugated expansion joints were used. Design pressure 1.6MPa, temperature 350℃. After six months of operation, the inspection found that the side walls of three bellows appeared obvious bulging-typicalInternal pressure instability。 Reason: The outlet temperature of the boiler superheater actually reaches 420℃, and the temperature reduction and pressure reduction device is occasionally out of control. The permissible stress of the material at 420°C is nearly one-third lower than that at 350°C. The worst thing is that these expansion joints can't be repaired yet, so they can only be disassembled and replaced with high-temperature axial types. Once the account is settled, the direct loss plus the discontinuation cost is enough to buy five times the good goods.
How to avoid this pit? Selection, installation and maintenance are all indispensable
Do not make do with the model selection, working conditions determine the model
- Large displacement conditionPreferred selectionCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointThey can absorb multi-directional displacement and do not produce blind plate forces on the pipe.
- High temperature and high pressure occasions: onHigh temperature axial expansion jointThe material is Inconel or heat-resistant stainless steel, and the structure is thickened.
- Desulfurization flue gas system: must be used withDesulfurization flue gas baffle doorAnd corrosion-resistant bellows, ordinary expansion joints cannot bear sulfide corrosion.
- Power station, cement industry: ReferenceCorrugated expansion joint for power station industryAndMetal Corrugated Expansion Joints in Cement IndustryThese products have been optimized for industry working conditions.
- Other special scenarios: For example, for vacuum pipesSpecial hose for vacuumFor buried pipelinesDirect buried (fully buried) type expansion jointDon't use the general-purpose type.
Focus on three key points during installation
First, the direction of the guide tube must be consistent with the flow direction of the medium, and the direct erosion bellows is installed backwards. Second, the tie rod nut is adjusted according to the design value, and the function of the expansion joint tie rod is to prevent the bellows from over-stretching. If the adjustment is not accurate, it is equivalent to white installation. Third, the spacing of the guide frame cannot exceed the standard, which will lead to columnar instability if it exceeds the design value.Correct installation method of expansion joint of large tie rodIt is clearly stated in that the first corrugation of the guide frame from the expansion joint shall not be greater than 4 times the pipe diameter.
Don't be lazy in routine maintenance, small money saves big money
Inspect the bellows regularly and visually inspect the appearance of the bellows for any dents, bulges, wear or abnormal corrosion. Touch the surface temperature with your hand. Local temperature anomalies may suggest internal media flow problems. Hear a strange noise? There may be cracks in the bellows. If you find a bulge early, change the accessories for hundreds of dollars; When the pipe bursts, it will lose six figures for one day of shutdown. Is that the case?
Metal expansion joint instabilityThis matter, in the final analysis, is a serial pit of selection, installation and maintenance. Don't expect to be fooled by luck. The rules are in place in one step, which is much less worry-free than repairing them afterwards.