Raised face (RF) and flat face (FF) flanges appear identical on specification sheets, sharing identical overall dimensions and pressure ratings. However, their sealing mechanisms differ significantly. For raised face flanges, the gasket is confined to a narrow raised ring area. For flat face flanges, the gasket covers the entire flange face surface.
Improper mixing of these two flange types on-site or incorrect application will eventually lead to leakage or flange cracking. Such failures are frequently misattributed to faulty gaskets or incorrect bolt torque. The following guide explains the differences between these two flange styles and their optimal applications.
What Flange Facing Means and Why It Affects Sealing
Facing is the machined surface where two flanges meet and bolt up. It’s what the gasket sits on, and it controls how the gasket compresses once the bolts are tightened. Get the facing wrong and torque won’t save you.
Most piping jobs use one of two facing types: raised face (RF) or flat face (FF). There’s a third, ring type joint (RTJ), which uses a metal ring instead of a flat gasket. That’s a different topic. For almost everything you’ll spec day to day, it comes down to RF or FF.
Raised Face Flanges
What a Raised Face Is and How It Is Machined
A raised face falnge consists of a machined annular projection on the flange, located inside the bolt circle. The gasket is placed on this raised ring.
The height and surface finish are specified by ASME dimensional standards rather than arbitrary manufacturing choices. This ensures that raised face flanges manufactured by different factories achieve consistent mating performance.
Sealing Mechanism and Gasket Compression
Bolt two raised faces together, and the raised rings squeeze the gasket into a small area instead of the whole face. Less area, same bolt load, means more pressure on the gasket. That’s what makes the seal hold. It’s a solid design for moderate to high pressure work.
Pressure Classes Where Raised Face Is Standard
Raised face is the default facing type for nearly all ASME B16.5 pressure classes, from Class 150 through Class 2500. If you walk through any refinery or process plant, it is what you will see on the majority of piping systems.
Typical Use with Weld Neck Flanges
Raised face weld neck flanges are the standard for high-pressure, high-temperature lines. The weld neck gives you a smooth bore and a strong weld joint. The raised face does the sealing work. That combination is why you’ll find it all through refineries, chemical plants, and power piping. Our weld neck options are in the flange product category.
Flat Face Flanges
What a Flat Face Is and How It Differs from Raised Face
Flat face flanges have no raised sealing ring. The entire contact surface is flat, extending from the bore out to the bolt circle. When assembled, the two flanges make contact across the full face, rather than only along a narrow annular strip
Sealing Mechanism with Full Face Gaskets
Flat face flanges use full-face gaskets, which cover the whole face, not just the bolt circle area. Bolt load spreads out across that entire surface instead of squeezing into one spot. That means less stress on whatever’s bolted to the other side.
Why Flat Face Is Specified with Cast Iron or Brittle Mating Equipment
Cast iron usually cracks under concentrated load. If you bolt a raised face flange to a cast iron pump or valve, it will break during installation. Flat face spreads the load out and sidesteps that problem entirely.
Typical Use with Slip-On Flanges
Flat face shows up most on slip-on flanges going onto cast iron equipment or lower pressure lines. Carbon steel slip-on flanges with a flat face are widely used in water treatment, HVAC, and general utility piping, wherever pressure isn’t extreme. Our slip-on options, flat face included, are in the flange product category.
Raised Face vs Flat Face: Direct Comparison
The core difference is contact area. Raised face squeezes the gasket into a small ring while Flat face spreads it across the whole face. Raised face means higher pressure on a small gasket area. Flat face means the same bolt load is spread more thinly, so lower pressure per square inch but a more even stress pattern across the joint.
Equipment compatibility follows the same logic. Raised-face suits steel valves and pumps that can withstand a concentrated load. Flat face suits cast iron and anything else that can’t.
The pressure range splits the same way, too. Raised face covers the full ASME B16.5 range; on the other hand, flat face sticks mostly to lower-pressure work and specific equipment pairings.
| Feature | Raised Face | Flat Face |
| Gasket Contact Area | Small, concentrated | Full face |
| Unit Pressure on Gasket | Higher | Lower |
| Common Mating Equipment | Steel valves and pumps | Cast iron equipment |
| Typical Pressure Range | Full ASME B16.5 range | Lower pressure applications |
| Common Pairing | Weld neck flanges | Slip-on flanges |
Choosing Between Raised Face and Flat Face for Your Application
Look at what you are bolting to first. Cast iron pump or valve on the other side? Then go flat face. Steel equipment rated for higher pressure? Then raised face is usually the better call.
Check your project’s governing standard too for a better choice. ASME B16.5 defaults to a raised face in most cases, but some specs or regional standards, such as DIN and GB, call for a flat face in specific cases. Confirm this before you order to avoid any inconvenience.
Two mistakes happen a lot on site. First, mismatched facing between mating flanges. Even a perfectly rated gasket can’t fix that. Second, wrong gasket for the facing. Full face gaskets go with flat face. Ring gaskets go with raised face. Mix those up, and you’ll get a leak no matter how well everything else was done.
Material and Facing Combinations Available from Hebei Pengrun
We maintain stock of flat face carbon steel slip-on flanges and raised face weld neck flanges. Alternative combinations can be manufactured to meet your project requirements. All processes including forging, machining and inspection are completed within our facility. This ensures the facing height and surface finish comply with ASME B16.5, DIN and GB standards. For projects requiring both facing types, we can supply fully matched assemblies, eliminating the need to split orders between multiple suppliers. Submit your specifications via our free quotation form, and we will provide pricing and lead time information. Hebei Pengrun offers one-stop supply for both flange facing types.
FAQs
What gasket type suits a flat face slip –on flange?
Full face gaskets. They cover the whole face and spread the bolt load out, which is exactly how flat face flanges are meant to seal.
Does facing type change the pressure rating of a flange?
No, not the rating itself. But it changes how well the joint holds up at that rating. Wrong facing for the job, and you can get leaks even inside the rated pressure range.
Is raised face the default for ASME B16.5 flanges?
Yes. Across most classes, 150 through 2500, unless the project spec calls for something else.
