They really go all out to save costs!

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This post was last edited by Douwan on 2024-5-31 09:06. A few days ago, I was at a factory for a safety inspection and came across such a device: Nitrogen/Air buffer tank. Operating pressure: 0.8 MPa; Design pressure: 0.84 MPa. Temperature: 100°C; Design temperature: 110°C. Material: Q345R; Corrosion allowance C = 1 mm. Dimensions (diameter/height): φ1200*2000. 20% partial non-destructive testing； Welding joint coefficient: 0.85/0.8. Structure: standard elliptical head + supported bearings; maximum pipe diameter is DN80. The equipment is equipped with a safety valve, with a set pressure of 0.84 Mpa. Oh, right, I forgot to mention the wall thickness – it’s 4.5 mm for the cylinder ； Head 4.5 (4.05) __________________________________________ In fact, based on an analysis of the data, the manufacturer of this device has set all parameters at their minimum values in order to save costs: 1) The design pressure is set at 1.05 times the minimum value ； 2) The value of the corrosion allowance shall be 1 mm, in accordance with the minimum requirement specified in GB150 ； 3) The setting pressure of the safety valve shall, in accordance with GB150, not be greater than the design pressure; that is, the design pressure shall be used ； From a standard specification perspective, although all the values mentioned above represent the minimum values, they are still within legal and regulatory limits. What surprised me was that the drawings indicated a cylinder thickness of 4.5 mm, while the minimum thickness of the head after molding was 4.05 mm. By doing the calculations manually, the thickness of the cylinder body is 3.15 + 1.3 = 4.45 mm; 0.05 mm is really not much as a margin. As for the thickness of the head, based on the welding joint coefficient specified in the drawings, 4.05 mm is not sufficient to meet the strength requirements. It is assumed that the manufacturer formed the head from a single sheet of metal, and while they intended to use a welding joint coefficient of 1.0, the drawings specify 0.8 instead. From a structural design perspective, cost savings have led to the omission of inspection holes such as manholes or access holes. However, according to regulatory requirements, inspection holes should be provided when internal inspections are necessary (i.e., surface inspections). In this case, since internal surface inspections are not required per regulations, and there are no internal components that need cleaning during operation, it’s understandable why the manufacturer did not include such holes. In the manufacturer’s non-destructive testing report, local RT testing was carried out using the “double-wall single-shadow method”: 2 tests were conducted on Class A welds and 6 on Class B welds. According to standard practices, this method is usually used for the last circumferential weld on cylinders with smaller diameters, as it’s not possible to use the single-wall projection method in those cases. Since the diameter of this equipment is 1200 mm, shouldn’t all inspections have been carried out using the double-wall single-shadow method? Shouldn’t local RT testing have been done first on the welded joints, followed by RT testing on the circumferential welds? After thinking about it, this would require 2 times, making the cost high! The manufacturer uses double-wall single-shadow for everything, solving it all in one step. Thus, costs are saved again. Finally, regarding safety valves, their setting pressure should not be higher than the design pressure; the design pressure itself is used as the reference value. It should be noted that there is a certain deviation in the setting pressure of safety valves – according to the current GB150 standard, this deviation is 3%, while the new draft standards have expanded the range between the setting pressure and the operating pressure of safety valves. The minimum design pressure value + the minimum corrosion margin + a wall thickness with almost no design margin left + the lowest inspection standards + the least stringent inspection methods – is this equipment really safe? It’s like with every car accident: if one left home a few seconds later, perhaps the accident could have been avoided ； If I hadn’t run the yellow light just now, I would be safe now ； If that aunt pushing the stroller hadn’t rushed over in such a hurry, I wouldn’t have had to brake suddenly and turn the steering wheel. Even if I had braked suddenly, if that food delivery worker had stayed in the non-motorized vehicle lane as required, I wouldn’t have hit him... All those random factors, combined together, lead to an inevitable outcome