1. The specifications of the flange are usually divided into nominal diameter. And flange type, generally is divided into the integral flange, threaded flange, butt welding flange, flat welding flange with neck,  hubbed socked welding flange, butt welding ring neck loose flange, flat welding flange, butt welding plate loose flange, flat welding ring plate loose flange, flange ring plate loose flange and flange cover. 

2. According to the chemical industry norm the flange types are divided into: Integral flange (IF), threaded flange (Th), plate flat welding flange(PL), butt welding flange with neck (WN), flat welding flange with neck (SO), socket welding flange (SW), butt welding ring loose flange (PJ/SE), flat welding ring loose flange (PJ/RJ), lining flange cover (BL (S)), flange cover (BL). 

3. According to the petrochemical industry standard, the flange type are divided into: Threaded flange (PT), butt welding flange (WN), flat welding flange (SO), socket welding flange (SW), loose flange (LJ), flange cover (not indicated). 

4. According to the mechanical industry norm, the flange types are divided into: Integral flange, butt welding flange, plate flat welding flange, butt welding plate loose flange rings, flat welding ring plate loose flange, flange ring plate loose flange and flange cover. 

(1) The flat welding steel flange is suitable for the connection of carbon steel pipe with nominal pressure not more than 2.5 MPa. The sealing surface of flat welding flange can be made into 3 kind of smooth type, concave and convex type and mortise type. The smooth and flat welding flange are used widely, it is mostly used for the condition with more moderate medium, such as low-pressure non purified compressed air and low-pressure circulation water. The advantage of it cheaper price. 

(2) Butt welding steel flange: it is used for butt welding of flange and pipes, its structure is reasonable, its strength and stiffness are bigger, it can withstand high temperature and high pressure and repeatedly curving and temperature fluctuation, its sealing is reliable. Its nominal pressure is 0.25 ~ 2.5 MPa . The butt welding flange is in concave and convex type sealing surface. 

(3) Socket welding flange: It is commonly used for the pipelines with PN ≤10.0MPa，DN≤40 PN. 

(4) Loose flanges: Loose flange is commonly known as the lap joint flange, it includes welding ring flange, flanging lap joint flange and butt welding flange. It is often used in the condition that the medium temperature and pressure are not high but the corrosively of medium is stronger. When the corrosivity of medium is stronger, the part of the flange contacting medium (flanging nipple) is the corrosion- resistance high grade materials, such as stainless steel materials, while the external of the flange is clamped with flange ring with low -grade materials, such as carbon steel material, in order to realize the seal. 

(5) The integral flange: The flange is often integrated with equipment, pipes, pipe fittings and valves etc, this type of integral flange is commonly used on the equipment and valves. 

(6) Form of sealing surface: Plane (FF), Raised face(RF), convex (M), convex (F), concave and convex surface (MF) , tongue face (T), tongue and groove face (TG) and ring connecting surface (RJ).