Can a elbow be directly welded to a flange?
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Previously in the petrochemical industry, equipment was welded directly. For the low-pressure steam being produced now, the factory wants shorter pipes to be used; however, it seems that it’s not necessary to use them, as it doesn’t affect usage or installation. Could someone expert please explain why it needs to be added? What is that standard?
This post was last edited by ylb913 on 2018-10-17 at 20:08. This issue was discussed a few years ago; I can’t remember which standard states that such things are not allowed. Exceptions apply to cases where a vent valve, flow meter, or other devices requiring a straight pipe section need to be installed at the bottom.
Regarding elbows with flanges, it is recommended to use pipes; the flange connection is a weak point. Corrosion caused by impact on such elbows is greater than in other areas. Moreover, when the bolts are removed later, it sometimes happens that they cannot be pulled out.
No, a straight pipe needs to be added. It is specified in the design standards, and ease of maintenance also needs to be taken into account.
It depends on the specific installation location, I guess? If the bolt can be easily pulled out, then it’s possible ； If it cannot be pulled out, a straight pipe must be added, such as an elbow + flange + equipment flange. It’s just my personal experience; I’m not sure what the standard is.
Under normal circumstances: for pipes with small diameters (DN150 and below), a short section needs to be added after the elbow; this is not necessary for pipes with larger diameters. The specifics should be determined based on the on-site space and the nature of the pipelines.
I think regulations do not allow it. But it actually did happen
The specifications clearly state that such construction is not allowed; a straight pipe section with a diameter 1.5 times that of the pipe is required. Due to the different thicknesses of the flange and the elbow, large differences exist between the inner and outer surfaces of the elbow when temperatures change, resulting in significant variations in expansion amounts. If the flange is welded directly, its strength is much higher than that of the elbow, which makes it easy for stress to build up at the weld site; this can lead to cracking of the weld, especially in media such as steam where temperature changes are substantial.