Why slip on flanges are preferred to welding neck flanges?
For many users, slip on flanges continue to be preferred to welding neck flanges because of the following reasons:

• On account of their initially lower cost.
• The reduced accuracy needed in cutting the pipe to length.
• The greater ease of alignment of the assembly.
• The calculated strength of slip-on flanges under internal pressure is approximately two-thirds that of welding neck flanges.

How to measure slip-on flanges?
Take the measurements of:

• OD: Outside Diameter
• ID: Inside Diameter
• BC: Bolt Circle
• HD: Hole diameter

Key Features of Slip-On Flanges : 
Some important features are as follows:
One size fits all pipe schedules.
Fabricators can more easily cut pipe to length for slip-on flanges.
The smaller thickness of this flange allows for easier alignment of bolting holes.
They are generally not preferred for high pressure temperature environments.

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

Product Range-
Size : 1/8″ NB TO 48″NB.
Class : 150#, 300#,400 #,600#,900#, 1500# & 2500 #.  
Stainless Steel Slip on Flanges: ASTM A 182, A 240
Grade : F 304, 304L, 304H, 316, 316L, 316Ti, 310, 310S, 321, 321H, 317, 347, 347H,904L, Duplex stainless steel UNS S31803, 2205, Super Duplex stainless steel UNS S32750 

Carbon Steel Slip on Flanges : ASTM A 105, ASTM A 181

Alloy Steel Slip on Flanges: ASTM A 182, GR F1, F11, F22, F5, F9, F91

Nickel Alloys Slip on Flanges : Monel 400 & 500, Inconel 600 & 625, Incolloy 800, 825, Hastelloy C22,C276

Copper Alloys Slip on Flanges : Copper, Brass & Gunmetal

Slip on Flanges Standard
ANSI : ANSI B16.5, ANSI B16.47, MSS SP44, ANSI B16.36, ANSI B16.48
DIN : DIN2527, DIN2566, DIN2573, DIN2576, DIN2641, DIN2642, DIN2655, DIN2656, DIN2627, DIN2628, DIN2629, DIN 2631, DIN2632, DIN2633, DIN2634, DIN2635, DIN2636, DIN2637, DIN2638, DIN2673

B.S : BS4504 , BS4504, BS1560, BS10