Guide
Flat face vs raised face flanges in data center plant rooms
A data center plant room mixes steel pipe with cast iron chillers, pumps and valves, and sometimes fibreglass cooling tower piping. The face of each flange decides whether the joint seals cleanly or cracks a casting.
Use raised face flanges between steel flanges and flat face flanges with full-face gaskets wherever a steel pipe meets cast iron, ductile iron, fibreglass, plastic or AWWA flanges.
The difference
| Feature | Flat face (FF) | Raised face (RF) |
|---|---|---|
| Sealing surface | Whole face | Raised ring inside the bolt circle |
| Gasket | Full face, with bolt holes | Ring gasket inside the bolts |
| Gasket stress | Lower, spread out | Higher, concentrated |
| Mates with | Cast iron, ductile iron, FRP, plastic, AWWA | Steel raised face flanges |
| Class 150 face height | 0 | 1.6 mm (0.06 in) |
Why raised face cracks cast iron
When a raised face flange is bolted to a flat cast iron flange, the gasket and raised face act as a fulcrum inside the bolt circle. Tightening the bolts bends the outer rim of the cast iron flange toward the steel flange. Cast iron cannot take that bending, and the flange can crack, sometimes only when the system is pressurised. A flat face steel flange with a full-face gasket supports the cast iron across its whole face. ASME B31.1 sets this as the rule for steel flanges bolted to Class 125 cast iron, and the same practice is followed in building services piping.
Chiller water boxes, end suction and split case pumps, strainers, check valves and some butterfly valves. Fibreglass and plastic cooling tower connections need the same treatment.
Gaskets
For water and glycol, full-face gaskets are typically EPDM rubber or compressed non-asbestos fibre. EPDM is the usual elastomer for water and propylene glycol; confirm compatibility with the coolant's inhibitor package. Ring gaskets for raised face joints are compressed fibre, PTFE-based or spiral wound depending on the specification.
Ordering
- State the face on every line: WN FF NPS 10 Class 150 A105 B16.5.
- Flag every equipment connection on the flange schedule with the equipment's flange standard and face.
- A raised face can be machined off to make a flat face if the remaining thickness meets the standard, but ordering FF from the start is cleaner.
Frequently asked questions
What height is the raised face on a Class 150 flange?
1.6 mm (0.06 in) for ASME B16.5 Class 150 and 300. Classes 400 and above have a 6.4 mm (0.25 in) raised face.
Can I use a ring gasket on a flat face flange?
A ring gasket inside the bolt circle can be used between two steel flat face flanges, but against cast iron, fibreglass or plastic flanges use a full-face gasket so the load is spread over the whole face.
Which is better for chilled water, FF or RF?
RF for steel-to-steel joints, FF wherever the mating flange is cast iron, ductile iron, fibreglass, plastic or an AWWA flat face flange.