Things to note when calculating hole reinforcement using the equal area method

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With the widespread application of computing software, the entry requirements for pressure vessel design work are getting lower and lower. However, it is undeniable that it is precisely because of the convenience that computing software brings to us that many people tend to ignore some design details in the design. They ask "why" less and less, and it seems to be less and less important to the origin of calculations and the judgment of whether the results are right or wrong. Anyway, the calculation software will tell me whether it is qualified or not... Even many leaders think that "you can design if you know CAD"... Okay, the more you talk, the more you talk, stop it! After chattering for a few words, he got to the point. This post mainly focuses on the discussion of the calculation of the hole reinforcement calculation using the equal area method for a single hole (mainly focusing on the design input and result judgment of SW6-2011), as well as discussing several common problems with friends, and everyone can point out what is wrong and discuss them together. Personally, I think designers should master the correctness of design input when applying SW6 software. In SW6, the design input for the equal-area method opening reinforcement calculation is actually relatively simple. For most of us designers, we often have questions or easily ignore it. It is nothing more than a few welding joint coefficients and the inward (outward) height of the nozzle.: 1. Issues related to welding joint coefficients in calculation of opening reinforcement ； 2. The value of the inward and outward extension height of the nozzle ； Issues related to the welding joint coefficient in the calculation of opening reinforcement. When the welding joint coefficient of the equipment body is 0.85, how to determine the values ​​of these two coefficients has become a point of concern for most designers. There are many opinions, which can be summarized as follows: a) Generally, it is 1.0 (SW6 default is 1.0) b) According to the welding joint coefficient of the shell, if the welding joint coefficient of the shell is 0.85, the welding joint coefficient of the shell should be 0.85 when calculating the opening reinforcement. c) When there are no A or B type welds in the reinforcement area, 1.0 can be taken. If there are welds, it should be based on the shell. (The design cannot know the assembly status of the cylinder. How to determine whether there is a weld at the opening? ) d) When there is no weld in the opening reinforcement area, the joint coefficient for calculating the thickness of the shell there can be taken as 1.0. As the name suggests, only when there are joints will there be a joint coefficient and the strength will be weakened. Why can't it be taken as 1.0 if there are no joints? This makes full use of the excess thickness of the casing. …… 1. Shell welding joint coefficient at the opening (for the convenience of distinction, we mark it as φ1 for the time being) ； 2. The welding joint coefficient of the nozzle (for the convenience of distinction, we mark it as φ2 for the time being) ； Before we look for the answer, we should first discuss what role or impact these two welding joint coefficients have on the calculation of opening reinforcement? As we all know, the qualifying conditions for equal-area opening reinforcement calculations are: A1+A2+A3(+A4)>AA——Reinforcing cross-sectional area required for opening weakening A1——Excess reinforcing area of ​​shell A2——Excess reinforcing area of ​​nozzle A3——Weld metal cross-sectional area A4——Additional reinforcing area (reinforcing ring, etc.) How is the excess reinforcing area obtained? Simply put, it is length × width: Length - the value of B-dop (effective reinforcement range) Width - excess thickness (effective thickness minus calculated thickness) Why is the standard such a long list of formulas? It is just the strength weakening coefficient introduced by taking into account the difference between the nozzle material and the shell. Then we can know that the values that affect the excess reinforcement area are actually only the B in the length (chang) and the thickness in the width. Then, our φ1 directly affects the value of A1, and φ2 directly affects the value of A2. Because most of the pipes on the equipment are seamless steel pipes or forgings (welded steel pipes are rarely seen), so there is no dispute that everyone takes the value of φ2 to 1.0 (note that it is not necessarily 1.0). So for φ1, it is the calculated thickness value that affects the shell wall thickness (the margin A1 that affects the shell thickness). For example, the cylinder is actually the calculated thickness of the cylinder. To put it bluntly, the welded joint coefficient of the cylinder should be taken when calculating the opening reinforcement. You can't always take 0.85 when calculating the shell. When calculating the opening reinforcement, the margin will become larger, right? Someone pointed out that "it is stipulated in GB/T 150.3-2011 6.1.4: The openings on the container should be away from the welded joints of the container. When the opening passes through or is adjacent to the welded joint of the vessel, it should be ensured that there are no excessive defects in the joint within 2dop of the center of the opening. ” So even if it is opened at the shell with welded joints, I have done 100% inspection, so when calculating the opening reinforcement, φ1 should be taken as 1.0. No. We can understand this article like this: This 100% detection is for "local parts" and falls within the 20% range. For example, if a certain weld of the shell is inspected, the weld is actually 100%, but only a part of the shell is inspected, and the final result is still 20%. Some comrades also focus on calculating the thickness of the shell at the "opening", which leads to the view that "only if there is a weld, the welding joint coefficient will be, if not, it will be 1". In fact, it is a deviation in understanding. In GB/T 150.3-2011 6.3.3.2 In the calculation formula for strip opening reinforcement, there is no distinction between whether there is a weld on the shell at the opening. Problems with the value of the inward and outward extension heights of the nozzles. For the calculation of opening reinforcement, Figure 6-2 of GB/T 150.3-2011 provides a diagram. However, many people have raised questions about the values ​​of the inward and outward extension heights of the nozzles. It should be noted that: 1) For axial openings in the head and oblique openings in the cylinder, the overhang height refers to the shortest length measured along the circumference of the nozzle. ； 2) The inward extension height of the nozzle refers to the longitudinal section of the cylinder rather than the transverse section. The understanding of words is always so awkward. Don't worry, let's take the picture. 1) For axial openings in the head and oblique openings in the cylinder, the overhang height refers to the shortest length measured along the circumference of the nozzle. ； The extended length of the pipes shown in the figure should all be L1 ； 2) The inward extension height of the nozzle refers to the longitudinal section of the cylinder rather than the transverse section. After knowing that the opening reinforcement calculation is a longitudinal section, you should be able to draw conclusions from the following figure: The calculation results for the hole reinforcement in Figure A and Figure B are the same. Case of discrepancy between opening reinforcement calculation and actual drawings 1. Discrepancy between the overhanging height of the nozzle and the actual drawing ； question: Are the results of the takeover reinforcement calculation in the picture correct? (Dopδnt)^0.5=58.5, so the calculation program design input is wrong, but it has no impact on the calculation results. Recommended practices: Roughly take the value when calculating. Preliminarily calculate the overhanging height of the nozzle when drawing. Return to check the calculation after drawing. Case 2: The actual structure of the takeover is inconsistent with the reinforcement calculation ； Notice: For the opening reinforcement of N1 and N4 pipes in the figure, attention should be paid to the actual input of the B value when calculating SW6. Finally, for the calculation of opening reinforcement, we should leave an appropriate margin on the premise that it meets the requirements. Taking into account the pipeline load, temperature difference stress and some inevitable mechanical loads, etc., it is recommended that the margin for the opening reinforcement calculation of the process nozzle should not be less than 20%. For manholes, hand holes, etc. that will not bear the pipe stress, it can be appropriately reduced, but I personally think it should not be less than 10%. I have seen some design drawings before, which may be to reduce costs. The margin for the opening reinforcement calculation is controlled at 3% to 5%. I personally think that the margin is indeed too small. Oh, finally done, call it a day.