Flat Faced Flanges

Written by Matt Milbury on . Posted in Pipe Flanges

A flat faced flange is a flange that is machined flat and does not have a ridge like a raised face or ring type joint flange. The flat surface allows for the gasket to have full contact with the entire mating surface. The purpose of the flat face is to eliminate the bending moment put on the flange when the bolts are torqued. When using cast iron or fiberglass flanges, this bending moment can easily break the flange.

It is possible but uncommon for flat face flange gaskets to be metallic gaskets. Usually gaskets are made from non-metallic materials such as EPDM or Viton. It should go without saying that only flat face gaskets should be used on flat face flanges.

In general, flat faced flanges are used in low pressure and temperature environments such as water treatment or pump suction flanges.   Flat face flanges are usually common when the piping or piping components are brittle such as plastic, fiberglass or cast iron. However, it should be noted that these flanges come in all different types of materials, including steel.

These flanges typically are available in weld neck, slip-on, socket weld, or threaded. Typically, flat face flanges are found in ANSI 150 & 300 ratings.

Design Considerations

Flat faced flanges should not be installed to flanges that are not flat faced. While some codes specify that mating surfaces should both be flat (ASME B31.1), it is good design practice to not mix flange facings whenever possible. The reasoning for this is that the flat faced flange may not be designed to withstand the forces put on the outer most edge when the bolts and nuts are tightened.

The dimensions for a flat faced flange are the same as the dimenisions for a raised face flange but without the raised face. 

Datasheets and Dimensional Information

The datasheets we have on this site are shown below. For simplicity sake, only datasheets that adhere to B16.5 are shown. ASME B16.5 covers flange dimensions from ½” to 24”. For sizes larger than this, please visit our flange datasheets page. 

 Blind FlangeLap Joint FlangesSlip-On FlangeSocket Weld FlangesThreaded FlangeWeld Neck Flange
ANSI 150 Blind, ANSI 150 (in) Lap Joint, ANSI 150 (in) Slip-on, ANSI 150 (in) Socket, ANSI 150 (in) Threaded, ANSI 150 (in) Weld Neck, ANSI 150 (in)
ANSI 300 Blind, ANSI 300 (in) Lap Joint, ANSI 300 (in) Slip-on, ANSI 300 (in) Socket, ANSI 300 (in) Threaded, ANSI 300 (in) Weld Neck, ANSI 300 (in)
ANSI 400 Blind, ANSI 400 (in) Lap Joint, ANSI 400 (in) Slip-on, ANSI 400 (in) Socket, ANSI 400 (in) Threaded, ANSI 400 (in) Weld Neck, ANSI 400 (in)
ANSI 600 Blind, ANSI 600 (in) Lap Joint, ANSI 600 (in) Slip-on, ANSI 600 (in) Socket, ANSI 600 (in) Threaded, ANSI 600 (in) Weld Neck, ANSI 600 (in)
ANSI 900 Blind, ANSI 900 (in) Lap Joint, ANSI 900 (in) Slip-on, ANSI 900 (in) Socket, ANSI 900 (in) Threaded, ANSI 900 (in) Weld Neck, ANSI 900 (in)
ANSI 1500 Blind, ANSI 1500 (in) Lap Joint, ANSI 1500 (in) Slip-on, ANSI 1500 (in) Socket, ANSI 1500 (in) Threaded, ANSI 1500 (in) Weld Neck, ANSI 1500 (in)
ANSI 2500 Blind, ANSI 2500 (in) Lap Joint, ANSI 2500 (in) Slip-on, ANSI 2500 (in) Socket, ANSI 2500 (in) Threaded, ANSI 2500 (in) Weld Neck, ANSI 2500 (in)