Characteristics of Weld Neck Flanges
As a sort of pipe fitting, the weld neck flange
refers to a flange with a neck and a cylinder-shaped tube to make the transition. These kinds of flanges are connected with pipelines through welding method.
Weld neck flanges
do not readily transform. They have superior airtightness and are widely used in many fields. WN flanges have corresponding demands on rigidity and flexibility, as well as reasonable thinning transition. The distance between welding point and joint face make the latter get rid of transformation which may be caused by welding temperatures. The complex horn-type construction is available for pipelines that bearing a wide range of pressures or temperatures. It can also be applied to pipelines that have to suffer from extremely high pressures or low temperatures. In general, weld neck flanges are used to connect valves with pipelines that PN go beyond 2.5MPa. They are also used in pipelines that responsible for transmitting valuable, inflammable or explosive media.
There are many technical requirements for the manufacture of weld neck flanges.
First. The grades and technical requirements of WN flanges should accord with the homologous requirements listed in JB 4726-4728.
1.Carbon steel forgings or austenitic stainless steel forgings that nominal pressures range from 0.25MPa to 1.0MPa are permitted to use requirements on GradeⅠforgings.
2.Except for forgings that meet demands of the third point, all forgings that have 1.6MPa-6.3MPa nominal pressures should in accordance with requirements on GradeⅡforgings or beyond that.
3.Manufacture of following forgings should conform to requirements on Grade Ⅲ forgings.
(1) PN≥10.0MPa Flange forgings.
(2) PN>4.0MPa Chrome-molybdenum steel forgings.
(3) PN>1.6MPa ferrite steel forgings that working under temperatures below -20℃。
Second. Weld neck flanges are commonly made with forging process or forging and pressing process.
When manufactured of steel plates or structural steels, one should pay attention to the following points.
1.The weld neck flanges should undergo ultrasonic flaw detection to guarantee no existence of delamination defects.
2.Manufacturers should cut the steels into strips along their pressing direction. And then bend them and weld them together to produce circles. At the same time, annular cylinder should be formed on the surface of steels. It is forbidden to process steel plates directly into weld neck flanges.
3.The connection of circles should use full penetration weld.
4.Manufacturers should take post weld heat treatment, as well as 100% radiographic detection or ultrasonic flaw detection (Radiographic detection should in accordance with JB 4730 Grade II while ultrasonic flaw detection should conform to JB 4730 Grade I).
Third. The lateral angles of necks of weld neck flanges should be smaller than 7 degree.