Find out first: What is the fatigue life of expansion joints?
The fatigue life of the expansion joint, to put it bluntly, is how many times the bellows can carry the expansion and contraction. With each expansion and contraction, an alternating stress cycle will occur at the trough and peak of the corrugation-the material is constantly pulled over and pressed over, and when it accumulates to a certain extent, cracks appear.
Fatigue life is never determined unilaterally by material grade. It is common for the same SUS304 to have different corrugation geometries and a difference in life by two or three times. Wave height, wave pitch, wall thickness, number of layers, every parameter changes the stress distribution. The displacement form is also a variable-pure axial tension and compression, which is very different from the transverse shear superimposed on the axial direction, and the calculated stress is very different. What about pressure fluctuations? Every time the pressure in the tube fluctuates, there is an additional layer of circumferential stress cycle on the corrugated wall. So you see, although the general-purpose corrugated expansion joint and the external pressure single axial expansion joint are both called expansion joints, one corrugation is swollen by internal pressure and the other corrugation is compressed by external pressure. The stress state is completely different, and the path of calculating fatigue life is naturally different.
What is the reasonable safety factor?
The recommended practice of GB/T 12777 and EJMA is essentially to push down the theoretical fatigue life with a large multiple of the safety factor. The design fatigue curve given by EJMA itself considers about 10 times the safety factor, and GB/T 12777 also follows this set of logic. Why 10x? Because the fatigue test data of corrugated pipe is very dispersed, the life of the same design and the same batch of materials may jump from 800 to 3,000 times after ten tests. The safety factor is used to cover this spread.
However, the safety factor cannot be one-size-fits-all in different working conditions. After the directly buried (fully buried) expansion joint is buried, it is impossible to repair and replace the pipe at all. The failure is an accident, and it is reasonable for the safety factor to be 15 or even 20. Once the high-temperature axial expansion joint exceeds the creep temperature range of the material-for example, 304 stainless steel is above 425℃-the failure mode changes from pure fatigue to creep-fatigue interaction. At this time, it is gambling on your life to take the safety factor of 10 times of normal temperature.
How to take the number of fatigue life? Conversion from design displacement to actual working condition
Many people step on pits at this step. The "fatigue life 1000 times" written on the sample or drawing is calculated based on a specific cyclic displacement. For example: the design displacement is ±20mm in the axial direction, and the life span is 1000 times, which means that it can run 1000 times under ±20mm pure axial displacement. But in the actual pipeline system, where does pure axial direction come from? Thermal expansion and contraction drive the pipe system to swing, and the axial displacement always comes with the lateral displacement.
The transverse displacement is converted into the equivalent axial displacement according to the geometric characteristics of the bellows, and the equivalent axial displacement is obtained after the two are superimposed, and then substituted into the fatigue life curve to check the number of times. Note that the relationship between fatigue life and displacement is not linear-a 10% magnification of displacement may result in a 30% drop in life. This is why the full sample life cannot be used. The corrugated expansion joint in power station industry has to be discounted by 30% or 20%. Because the transverse expansion joint of compound hinge bears the combination of angular displacement, the conversion is more complicated, and the discount range is often larger.
The most overlooked pits in engineering: temperature correction and corrosion margin
If you take the fatigue curve at room temperature to set the flue gas pipe at 400℃, it is basically a gamble with your life. As soon as the material is at high temperature, the yield strength drops, the creep begins to accelerate, the plastic deformation produced by the bellows every cycle is greater, and the crack initiation is much faster than the room temperature.
A general-purpose corrugated expansion joint used in a cement plant has a design temperature of 350℃. The manufacturer has checked the life of 2000 times according to the normal temperature curve. As a result, the bellows cracked in less than 500 cycles after the actual operation reached 420℃. Some people also blame the manufacturer's poor quality-in fact, the problem lies in the user's negation of the temperature correction coefficient. Corrode the environment harder. The flue gas baffle door and the desulfurization flue gas baffle door are sulfur-containing flue gas, and chloride ions and sulfite make pitting pits on the corrugated surface, which are the natural starting points of fatigue cracks. Corrosion and fatigue are superimposed, not 1+1=2, but 1+1=5. In this environment, the safety factor is at least doubled, and with measures such as the heat insulation of the guide tube and the external heat insulation of the corrugated pipe, it can be played.
What is the appropriate number of fatigue life times?
Landing recommendations are in two gears. For conventional pipeline systems, such as thermal pipelines and general chemical pipelines, 70% ~80% of the design life is taken as the allowable cycle number. The sample is marked 1000 times, and you will set the maintenance and replacement cycle according to 700~800 times, leaving a safety margin to cope with the fluctuation of working conditions.
In critical situations or parts where maintenance is difficult, the control line should be pressed lower. The double hinge expansion joint of air-cooled island vacuum pipeline is on a pipe frame tens of meters high, and it is necessary to build a full hall of scaffolding to change it; Once the straight pipe pressure balance expansion joint fails, the blind plate force of the whole pipe system loses balance, and the consequence is chain. Such products are controlled at 50 percent or less of the design life-and replacement plans are required for less than half the life. The reason is simple: the fatigue life itself is a statistical value. Some people are still running when they use 120%, while others leak 60%. You lower the line of control, not conservatively, but to leave uncertainty alive.
How to communicate fatigue life requirements with manufacturers during model selection?
Don't just drop "Give me something that lives longer" and be done. The design engineer of the manufacturer is not a fortune teller. You have to give him at least these things: medium temperature, cyclic displacement (how many millimeters in the axial direction and how many millimeters in the transverse direction, if the measurement is not accurate, give the stress analysis report of the pipe system), cycle frequency (how many times a day to start and stop, how many thermal cycles a year), whether there is pressure fluctuation, and the type and concentration of corrosive medium. In this corrosive environment, sulfide or chloride content should be declared separately.
Only when the parameters are in place can the manufacturer select the safety factor to a reasonable gear, and then deduce the fatigue life times. If you don't say anything, the other party can only match you with a universal corrugated expansion joint according to the general working conditions of "normal temperature, pure axial direction and no corrosion". It looks quite cheap when you get it, and it will be revealed after running two laps on the scene. How to take the safety factor of fatigue life of expansion joint and the number of times of fatigue life of expansion joint? The answer is never in the sample, but in your own working condition data.