Flange Class 1500 vs 2500

Quick Answer

Class 1500 and Class 2500 are the two highest pressure ratings in ASME B16.5. At ambient temperature in A105 carbon steel, Class 1500 handles 3,705 psig (255.5 barg) and Class 2500 reaches 6,170 psig (425.4 barg). Both classes almost exclusively use RTJ (Ring Type Joint) facing for metal-to-metal sealing. Class 2500 flanges are massive, expensive, and reserved for the most demanding services: high-pressure wellheads, lethal gas systems, and ultra-high-pressure process equipment.

Pressure-Temperature Comparison

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

Temperature	Class 1500 Max Pressure	Class 2500 Max Pressure
-20 to 100degF (-29 to 38degC)	3,705 psig (255.5 barg)	6,170 psig (425.4 barg)
200degF (93degC)	3,375 psig (232.7 barg)	5,625 psig (387.8 barg)
400degF (204degC)	3,000 psig (206.8 barg)	5,000 psig (344.7 barg)
600degF (316degC)	2,685 psig (185.1 barg)	4,475 psig (308.5 barg)
750degF (399degC)	2,135 psig (147.2 barg)	3,560 psig (245.5 barg)

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

Dimensional Differences (NPS 4 WN Flange)

Parameter	Class 1500	Class 2500
OD	317.5 mm (12.50 in)	368.3 mm (14.50 in)
Thickness	69.9 mm (2.75 in)	101.6 mm (4.00 in)
Bolt circle	266.7 mm (10.50 in)	304.8 mm (12.00 in)
Number of bolts	12	12
Bolt size	1-1/4 in	1-1/2 in
Flange face	RTJ	RTJ
Weight (approx.)	52 kg (115 lb)	95 kg (210 lb)

Dimensions per ASME B16.5, A105 carbon steel.

Common Mistake

Class 2500 flanges are available only up to NPS 12 in ASME B16.5. For larger sizes at extreme pressures, engineers must use special-class flanges designed per ASME Section VIII or proprietary standards. Do not assume B16.5 covers all size-class combinations.

Application Comparison

Application	Typical Class
HP steam (boiler superheater piping)	1500
Hydrocracker reactor piping	1500
HP gas injection (offshore)	1500 or 2500
Wellhead and christmas tree connections	2500 (or API 6A)
Lethal service (HF alkylation)	1500
Ultra-HP test manifolds	2500
Ammonia synthesis loops	1500

Gasket and Bolting

Both classes use RTJ gaskets—typically R-type or RX-type metal rings. BX-type rings are common on API 6A wellhead connections. Stud bolts for Class 2500 flanges can exceed 2 inches in diameter, requiring hydraulic bolt tensioning equipment rather than manual torquing.

Pro Tip

For HP/HT applications above 800degF, consider upgrading from A105 to alloy steel flanges (A182 F11, F22) or stainless steel (A182 F316). The pressure-temperature ratings for these material groups may allow a lower class at the same operating conditions—potentially reducing flange size, weight, and cost.

Read the full guide to flanges