The

Slip-on flange

has a low hub because the pipe slips into the flange prior to welding. It is welded both inside and out to provide suffcient strength and prevent leakage.Slip-on flanges are all bored slightly larger than the O.D. of the pipe. They are preferred over welding neck flanges bu many users due to their lower intial cost, but final intallation cost is probably not much less than that of the welding neck flange because of the additional welding involved.
Specifications:

 

Slip-On Flanges, SO Flange
Size: 1/2” to 60”, DN 12mm to DN 1500mm view...
Pressure class: Class 150 to Class 2,500, PN 2.5 to PN 250
Facing: RF / RTJ
Material:view...
Cabon steel: A105, A350 LF2
Stainless steel: A182 F304/F304L/F316/F316L/F321
Alloy steel: F11, F12
Packing: plywood case and plywood pallet
 
Advantages of slip on flanges:
Low cost installation
Less time needed to spent on ensuring the accuracy of the cut pipe
They are somewhat easier to align
The slip-on flanges have low hub because the pipe slips into the flange before welding
The flange is welded both inside and outside to provide sufficient strength
They prevent leakage
Standards, Dimensions & Weight
ASME/ANSI B16.5 Class 150 SO
ASME/ANSI B16.5 Class 300 SO
ASME/ANSI B16.5 Class 600 SO
ASME/ANSI B16.5 Class 900 SO
BS 3293 Class 150 SO
BS 3293 Class 300 SO
BS 3293 Class 600 SO