Most forged steel flanges correspond to the requirements of the American Standards Association (ANSI Standard B16.5), and the governing ASTM Specifications ASTM A-105 for Class 150 (PN 20) and Class 300 (PN 50) pressure ratings, as well as Class 400 (PN 64) through Class 2500 (PN 400) pressure ratings. The following types are stocked:
Welding Neck flanges, available in all pressure ratings and sizes, are butt-welded to the end of the pipe, and are usually specified when service conditions are severe and excellent workmanship is necessary. Since the inside diameter of the flange must match that of the pipe, the flange bore should be specified when ordering.
Slip-on flanges, are also available in most pressure ratings and sizes, and are a popular type due to their ease of application. This flange slips over the end of the pipe and is usually set so that the flange face is about .375" (9.5 mm) beyond the end of the pipe. This permits double-welding of the flange - one strength fillet weld to join the hub of the flange to the pipe, and a seal fillet weld inside the flange at the end of the pipe. Where operating conditions permit, the seal weld is omitted. Slip-on flanges are most frequently used at lower pressure - Class 150 or Class 300 primary service pressure ratings.
Screwed or Threaded flanges are attached to the pipe like any other screwed fitting, and may be back-welded to seal the joint between pipe and flange. Although still available in most sizes and pressure ratings, screwed fittings today are used almost exclusively in smaller pipe sizes and at low pressures.
Lap Joint flanges are used on piping equipped with lap joint stub ends or with lapped pipe. They may be used at all pressures and are available in a full size range. These flanges slip over the pipe, and are not welded or otherwise fastened to it; bolting pressure is transmitted to the gasket by the pressure of the flange against the back of the pipe lap. Lap Joint flanges have certain special advantages: (1) freedom to swivel around the pipe facilitates the lining up of opposing flange bolt holes; (2) lack of contact with the fluid in the pipe often permits the use of inexpensive carbon steel flanges with corrosion resistant pipe or tubing; (3) in systems which erode or corrode quickly, the flanges may be salvaged for re-use.
Socket-welding flanges contain a recess in the back of the flange to receive the end of the pipe, which is attached by a fillet weld around the hub of the flange. Since socket-welding connections are not as strong as butt-welded joints, the use of this type of flange is almost always confined to NPS 4 (DN 100) and smaller sizes, and to the lower pressure ratings. Its chief advantage lies in the ease of preparation and installation.
Blind flanges, available in all sizes and pressure ratings, are solid forgings used to close off the end of a piping system and to gain easy access to the interior of the line.
Unless otherwise specified, Class 150 (PN 20) and Class 300 (PN 50) flanges in all types except lap joint flanges are furnished with a .06" (1.6 mm) raised face (which is included in the flange thickness dimension). Heavier pressure ratings are machined with a .25" (6.4 mm) raised face, in addition to the designated flange thickness. Lap Joint flanges are machined with a flat face and a fillet radius to accommodate the stub end or pipe lap.
ASTM specifications are, basically, materials specifications. They regulate approved raw materials from which flanges can be made - ingots, or blooms, billets, slabs or bars. In addition, they govern the methods of manufacture, quality control procedures and marking of forged steel flanges. ASTM specifications are divided into four categories:
- A105 - Carbon grades for high temperature service
- A181 - Carbon grades for general service (Discontinued and no longer used)
- A182 - Alloy and stainless grades for high temperature service
- A350 - Carbon and alloy grades for low temperature service