Materials for flat welding flanges with neck
Thread Content
This post was last edited by marke7 on 2018-7-11 09:54. Can the material for standard-sized flat-weld flanges with necks be made from sheet metal (through direct cutting, rather than being formed from coils)? 1. It is clearly stated in HG20592 that flanges with necks shall not be made of sheet metal. 2. GB150 specifies many requirements for necked flanges made by rolling sheet metal. But it seems to be aimed at the equipment flange ； 3. For all other manholes, handholes, etc., the flat-welded flanges with necks required to be made of forgings. Question: For flanges with custom designs, having a pressure rating of PN16 and a diameter ranging from DN100 to DN400, and a neck height of only 3–8 mm, is it necessary to use forged parts? Due to adherence to the original American design, it is difficult to modify the structure to use plate-type butt weld flanges (the welding quality is poor, deformation is likely to occur during welding, and leaks can happen). Moreover, this is used on a large scale; if the cost of such forgings increases significantly (by about 4-5 times), that would be a problem.
It would be better to change it to a flat welding flange during design, wouldn’t it? Now, to save costs, it has been modified after negotiating with the design firm.
We are the design firm; the original design came from the United States and used a flange with a neck for butt welding. Now it has been changed to a butt-welded flange, and we are concerned that the stiffness of this flange may not be sufficient, leading to welding deformation and subsequent leaks – which would make it difficult to explain why a flange with a neck was replaced by a butt-welded one. However, if no change is made, then according to the standards, the flange should be made of forged material, and the cost of such flanges would increase by about 5 times compared to those fabricated from sheet metal.
There should be confidence in domestic standards. Flange stiffness can be fully ensured as long as the maximum allowable working pressure of the selected material and pressure rating at the design temperature is not lower than the design pressure of the equipment.
I have confidence in the standards, but not in the welders in our workshop (if there are leaks after the structure is modified, it will be blamed on the structure rather than the welding)； Moreover, the medium contains lubricating oil, resulting in severe leakage ；
Generally, after pipeline welding is completed, corresponding inspections are carried out. As long as the weld can pass the inspection, this problem will be easily solved.
What I’m saying is that plate-type butt weld flanges have relatively lower stiffness compared to necked butt weld flanges. On one hand, they are prone to welding deformation; it’s not a problem with the welds themselves, but rather the deformation of the flange surface leads to poor sealing and thus leaks. On the other hand, due to the tensile forces acting on the pipeline, the insufficient stiffness of the flanges results in leaks at the sealing surfaces； Welding issues can be improved through multiple passes of welding and the use of deformation prevention plates, but this is time-consuming. The problem that needs to be solved now is ； Are there any standards or standard definitions that specify that flanges with necks can be manufactured from sheet metal? This would indicate that such changes are based on established standards.
Without corresponding standards and criteria for modification, forged parts must be used for necked flanges under both HG/T20592 and ASME B16.5.
In terms of mechanical properties, forgings are definitely better. The most conservative. The technical requirements for steel plate are high; the plates themselves are used in small quantities, and it’s difficult for anyone to guarantee the quality from a technical standpoint.
We should use plate-type flat welding flanges whenever possible.