Flange Class 150 vs 300

Class 150 and Class 300 are the two most common flange pressure ratings in industrial piping. The class number is not a direct pressure value—it represents a pressure-temperature envelope defined in ASME B16.5. A Class 300 flange handles roughly twice the pressure of a Class 150 at any given temperature, but costs more, weighs more, and requires heavier bolting.

Pressure-Temperature Comparison

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

Temperature	Class 150 Max Pressure	Class 300 Max Pressure
-20 to 100degF (-29 to 38degC)	285 psig (19.6 barg)	740 psig (51.0 barg)
200degF (93degC)	260 psig (17.9 barg)	675 psig (46.5 barg)
400degF (204degC)	230 psig (15.9 barg)	600 psig (41.4 barg)
600degF (316degC)	140 psig (9.7 barg)	535 psig (36.9 barg)
750degF (399degC)	105 psig (7.2 barg)	425 psig (29.3 barg)

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

Pro Tip

At ambient temperature, Class 300 handles 2.6x the pressure of Class 150. But at 750degF, the ratio increases to 4x. Class 300 provides significantly more margin at elevated temperatures—an important consideration for steam and high-temperature process lines.

Dimensional Differences (NPS 6 WN Flange)

Parameter	Class 150	Class 300
OD	279.4 mm (11.00 in)	317.5 mm (12.50 in)
Thickness	22.4 mm (0.88 in)	31.8 mm (1.25 in)
Bolt circle	241.3 mm (9.50 in)	269.9 mm (10.63 in)
Number of bolts	8	12
Bolt size	3/4 in	3/4 in
Weight (approx.)	8 kg (18 lb)	14 kg (31 lb)
Approximate cost	$45-65	$80-120

Dimensions per ASME B16.5, A105 carbon steel.

When to Use Each

Application	Recommended Class
Cooling water, utility air	150
Low-pressure steam (<150 psig)	150
Medium-pressure steam (150-600 psig)	300
Process piping, moderate P/T	300
Hydrocarbon lines (project-specific)	300 (minimum per most specs)
Fire water	150
Instrument air	150

Common Mistake

Selecting flange class based on design pressure alone without checking the design temperature. A system at 200 psig and 600degF requires Class 300 flanges—Class 150 is only rated to 140 psig at that temperature.

Many EPC project specifications default to Class 300 for all hydrocarbon service as a safety margin, even when Class 150 would satisfy the pressure-temperature envelope. This increases cost but simplifies procurement and reduces the risk of mixing up pressure classes on site.

Read the full guide to flanges