Flange Selection Guide: Slip-On vs. Weld Neck vs. Companion (Class 150/300)
A flange is a bolted pipe connection that can be taken apart for service. Choosing one comes down to three questions: how it attaches to the pipe, the pressure class, and the face type.
Flange types
| Type | How it attaches | Best for |
|---|---|---|
| Slip-On | Slides over the pipe, fillet-welded inside and out | Easy alignment, lower cost, moderate pressure |
| Weld Neck | Butt-welded to the pipe via a tapered hub | High pressure/temperature and cyclic service — strongest joint |
| Threaded / Companion | Screws onto threaded pipe — no welding | Small lines and where welding isn't allowed |
| Blind | Solid disc — bolts on to cap a flanged opening | Closing off a line or vessel for isolation/future tie-in |
Pressure class: 150 vs. 300
The class (150#, 300#, etc.) is the pressure-temperature rating, not a literal pressure. Class 300 flanges are heavier, have more/larger bolts, and handle higher pressure and temperature than Class 150. Both flanges in a joint must be the same class and drilling — you can't bolt a 150 to a 300.
Face type
Raised Face (RF) is the most common; Flat Face (FF) is used with cast-iron equipment to avoid cracking the flange. Match faces and use the correct gasket.
Standards & drilling
Steel/stainless flanges follow ASME B16.5; cast/ductile iron flanges follow ASME B16.1 (Class 125/250). For bolt circle, bolt count, and bolt size by line size, see the flange bolt-pattern & dimension chart.
Shop flanges
Browse flanges or contact us with your pipe size, class, and material for help selecting the right one.