How should the allowable bending moment for the ear-type supports of vacuum equipment be determined? The moment calculation keeps failing
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As the title suggests, I’d appreciate it if everyone could help explain this. The standard values are only 0 MPa, 0.6 MPa, 1 MPa, and 1.6 MPa. The greater the internal pressure, the smaller the allowable bending moment; so should the allowable bending moment for vacuum equipment be higher than that at 0 MPa? There is another issue: in NB/T 47065-2018, the formula A.1 for the actual load has been doubled, to 2(ph+GeSe)/nD, whereas the old standard used a value of 4 times that, right?
Whether it is internal or external pressure, the primary stress generated is the hoop stress; the difference lies in whether it is a tensile stress or a compressive stress. In a three-dimensional stress state, the radial stress is relatively small. Regardless of external or internal pressure, the radial stress is negative, and according to the Lame formula, its order of magnitude is roughly the same. The ear seat imposes bending stress on the housing; this bending stress is combined with radial stress. As the radial stress increases, in this regard, whether it is internal pressure or external pressure, the local bending stress at the ear seat results from gravitational loads or other lateral forces acting on the same structure, and there is no essential difference in their magnitude. There is a slight difference in radial stress between internal and external pressures, and the degree of increase resulting from their combination is not significant; this is not the main issue. For externally pressurized equipment, the impact of the lugs on the shell is primarily represented by the uneven loads that cause the cylindrical structure to deviate from a perfect geometric shape. If the equipment is very heavy and the wall thickness of the cylinder does not provide much margin against instability, then local slight deformations of the cylinder caused by the lugs can lead to a significant reduction in the cylinder’s overall stability, and this cannot be addressed by the standard allowable bending moments specified for the lugs. Therefore, I believe that vacuum equipment cannot be evaluated using the allowable bending moment for internal pressure. The principle is the same for supported bearings.
The original poster is a genius. So, how can this be solved?
For the design of vacuum equipment, we generally use rigid ring supports; support-type supports are rarely used. Depending on the equipment’s diameter, wall thickness, load, and engineering experience, stress analysis may sometimes be conducted for verification
We rarely do calculations; generally, we choose standard ear-type supports based on diameter and weight.
Recently, while studying NB/T47065.5-2018, I noticed that the height-to-diameter ratio of these devices is quite high; the dimensions of the rigid rings specified are also large. However, the allowable vertical load and allowable bending moment are not very high. Devices that are so large and tall have a considerable weight due to their own mass. Therefore, I would like to ask: 1. Does “here” refer to each individual rigid ring bracket? Looking at the calculations below, W and M0 are not divided by the number of ear mounts. 2. The standard specifies W≤, but in the SW6 calculation it is verified that Fb≤ – is the algorithm for SW6 incorrect?
Our company’s internal design regulations stipulate that for standard components used in vacuum equipment, the minimum pressure rating required is 0.6 MPa; this applies to things such as pipe flanges, vessel flanges, viewports, and so on
It is used for the preliminary selection of supports. Since the supports must withstand not only the force W but also the bending moment caused by horizontal forces, it is necessary to calculate the reaction force Fb at the support ears; therefore, the SW6 algorithm is correct.