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.
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Frequently Asked Questions
What is the difference between a slip-on and a weld neck flange?
A slip-on flange slides over the outside of the pipe and is secured with two fillet welds — one inside and one outside. It is easier to align and less expensive, making it suitable for moderate-pressure, non-cyclic service. A weld neck flange has a long tapered hub that is butt-welded to the pipe end, distributing stress more evenly. Weld neck is the stronger joint and is specified for high-pressure, high-temperature, or cyclic (fatigue) service.
What is a companion flange (threaded flange) and when is it used?
A companion flange — also called a threaded flange — has a female NPT thread that screws directly onto a male-threaded pipe end with no welding required. It is used in smaller pipe sizes where welding is impractical or prohibited (such as in certain hazardous-area or field-repair situations). Because the thread is the only mechanical connection, threaded flanges are limited to lower pressures and temperatures compared to welded types.
Can I bolt a Class 150 flange to a Class 300 flange?
No. Class 150 and Class 300 flanges have different bolt patterns (different bolt circle diameters and bolt counts at some sizes), so they are not directly interchangeable. More importantly, mixing pressure classes in a joint undermines the integrity of the connection. Both flanges in a bolted joint must be the same pressure class and follow the same drilling standard.
When should I use a flat face instead of a raised face flange?
Use a flat face (FF) flange when connecting to cast-iron equipment such as pumps, compressors, or valves with cast-iron flanges. The raised face on a steel flange creates a concentrated load that can crack brittle cast iron. With a flat face flange and a full-face gasket, the load distributes evenly across the entire flange face, protecting the cast-iron component.
What does flange pressure class actually mean — is it the maximum pressure?
The class number (150, 300, 600, etc.) is not a direct pressure value — it is a pressure-temperature rating from ASME B16.5. The actual maximum allowable pressure depends on the material and the service temperature. A Class 150 carbon steel flange is rated higher at ambient temperature than at elevated temperature. Always consult the B16.5 pressure-temperature tables for your material group and service temperature when selecting a class.
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Technical data disclaimer: Specifications, dimensions, pressure/temperature ratings, and compatibility information on this page are compiled from manufacturer publications and are provided for reference only. Always verify suitability for your application against the manufacturer’s current documentation before purchase or installation. Apex Flow Solutions assumes no liability for errors, omissions, or misapplication of this information.