I have been engaged in the profile bending industry for many years, and many things can be encountered. For example, a friend asked me to do stress analysis for angle iron flanges.
A friend sent me a picture,
“Urgent, this flange structure needs stress analysis.”
When I saw it, it turned out to be an angle iron flange.
The angle iron ring is equipped with a flat gasket and a flat plate, and two reinforcing ribs are made on the flat plate.
Temperature and pressure are not high, but why do you need to do stress analysis? Analysis is the master key?
I asked: Why not use conventional flange and flat cover calculation?
The friend replied: The owner did not want to change the angle iron used in the engineering drawing, saying that it was just a stress analysis. Anyway, he could do anything.
I looked at the structure again, and then said to my friend: This is essentially a sealing problem, and it is recommended not to use stress analysis to do it.
My friend asks: What if the owner insists on using this structure? What if the owner insists on using stress analysis to check this unconventional structure?
I replied: The owner should be persuaded not to use stress analysis and directly modify the structure.
Why shouldn’t stress analysis be the first choice for this structure?
The reason is as follows
Angle iron method flanges are generally used in situations where there is no requirement for sealing, such as the equipment is under normal pressure or the pressure is very very low. The main reason is that angle iron sealing is very difficult.
According to Table 6-4 of China’s industry standard NB/T 47003.1, the flat cover structure diagram has a similar structure.
The applicable range of this structure is 0.002MPa, which is 0.02 Barg, and the pressure of our equipment to be tested is 3.5 Barg, and the hydraulic test is 4.4 Barg, which far exceeds the allowable specification.
The sealing problem is complicated
The sealing problem is very complicated, especially the design of flange gasket bolt system requires contact analysis, too many assumptions are used, and many data are just guesses and not accurate. Whether the calculated result is credible is really a question.
This is not a question of strength, just calculate the stress. The seal must not only have sufficient strength, but also maintain sufficient rigidity. The angle iron is too weak.
Angle iron manufacturing is more troublesome
In terms of processing, the flange sealing surface must be machined to ensure the sealing performance.
With angle iron, it seems that the angle steel can be processed by enclosing the angle steel in a circle, which is convenient and trouble-free.
However, the angle iron is a structural part with pits and pits on the surface. The roundness and flatness after processing cannot be guaranteed. If it is pressurized, it must be leaked.
If the angle iron is processed and then machined, why not directly process the plate? Also save the step of simmering.
Routine calculations are perfectly suitable
According to the calculation of conventional flat welding flange, this equipment is completely feasible, mature and stable, and highly reliable.
Why use analysis to prove that angle iron is OK?
Why not directly use the conventional flat flange calculation?
It can be predicted from the pressure and the My value of the gasket that the flange will not be very thick. The cost will not be much more expensive than the angle steel.
Why create difficulty without difficulty?
If the owner or manufacturer wants to prove that the angle iron can withstand pressure, as a test, then I support it. Although the structure is very poor, it is a pleasure to know the pressure limit of the angle steel.
If it is a project, then I strongly do not recommend this risky structure.
If the water pressure is not enough, all aspects will be very uncomfortable.
The fuzzy area of contact analysis
This structure is a bolt flange gasket system, if it is really to be analyzed, it is a contact analysis.
Contact analysis is a nonlinear problem of boundary conditions. The contact area and pressure distribution of the contact body change with the external load.
For non-contact statics problems, most people’s calculation results are similar. The difference is often only the difference in the magnitude of the stress caused by the density of the grid, which does not affect the overall accuracy and accuracy.
Non-linear problems are more complicated to calculate and have a lot to do with human operations. Different people may have very different calculation results.
There are a lot of parameters to be set for non-linearity, and there are many choices for non-linear algorithms. Different people set different values, and the subsequent results will naturally vary. By adjusting the parameters, even unqualified ones can be made qualified.
Angle iron flange
For angle iron flanges, there are many uncertain data, such as the elastic modulus of gaskets, the bolt pre-tightening force of thin angle steel, my value. (Of course you can refer to GB150 to select, but you have calculated according to 150, so why not directly use GB150 to calculate the full set of flanges?)
Not to mention that nonlinear convergence is very troublesome, and is related to a series of factors such as grid accuracy, boundary conditions, and load steps.
It is an atmospheric equipment that uses such a complicated and low reliability analysis. It needs to be It is not necessary to use such a complex, low reliability, and machining analysis after calculation for an atmospheric equipment in itself.