Flange Class 600 vs 900

Class 600 and Class 900 flanges occupy the middle-to-upper range of the ASME B16.5 pressure rating system. Both are used in high-pressure process piping, but Class 900 handles approximately 50% more pressure than Class 600 at any given temperature. The jump from 600 to 900 also brings significantly larger bolt diameters, thicker flanges, and higher weight—factors that affect structural support, installation logistics, and cost.

Pressure-Temperature Comparison

For ASTM A105 carbon steel flanges (Material Group 1.1):

Temperature	Class 600 Max Pressure	Class 900 Max Pressure
-20 to 100degF (-29 to 38degC)	1,480 psig (102.1 barg)	2,220 psig (153.1 barg)
200degF (93degC)	1,350 psig (93.1 barg)	2,025 psig (139.6 barg)
400degF (204degC)	1,200 psig (82.7 barg)	1,800 psig (124.1 barg)
600degF (316degC)	1,075 psig (74.1 barg)	1,610 psig (111.0 barg)
750degF (399degC)	855 psig (58.9 barg)	1,280 psig (88.3 barg)

Values from ASME B16.5-2020, Table 2-1.1.

Pro Tip

The Class 900 rating is exactly 1.5x the Class 600 rating at every temperature point for the same material group. This consistent ratio makes quick engineering estimates straightforward when evaluating whether to step up from 600 to 900.

Dimensional Differences (NPS 6 WN Flange)

Parameter	Class 600	Class 900
OD	355.6 mm (14.00 in)	381.0 mm (15.00 in)
Thickness	47.8 mm (1.88 in)	57.2 mm (2.25 in)
Bolt circle	317.5 mm (12.50 in)	317.5 mm (12.50 in)
Number of bolts	12	12
Bolt size	1 in	1-1/8 in
Flange face	RF or RTJ	RF or RTJ
Weight (approx.)	28 kg (62 lb)	38 kg (84 lb)

Dimensions per ASME B16.5, A105 carbon steel.

When to Use Each

Application	Recommended Class
High-pressure steam (600-1,200 psig)	600
HP/HT process piping (refineries)	600 or 900
Hydrocracker, reformer circuits	900
High-pressure gas pipelines	600 (typical)
Subsea flowlines	900 (or higher)
Lethal service / critical safety	900 (added safety margin)

Common Mistake

Specifying Class 900 when Class 600 is sufficient “just for safety.” The cost premium is 30-50%, bolting torques are significantly higher, and the heavier flanges require stronger pipe supports. Always select the class based on the actual pressure-temperature envelope, not guesswork.

Bolting and Gasket Considerations

Class 900 flanges use larger stud bolts and require higher bolt torque. RTJ facing becomes more common at Class 600 and above. Spiral wound gaskets are still used on RF faces, but many project specifications mandate RTJ flanges for Class 600+ in hydrocarbon service to minimize leak risk.

Both classes are covered under the same ASME B16.5 standard for sizes NPS 1/2 through NPS 24, and ASME B16.47 for NPS 26-60.

Read the full guide to flanges