Flange Rating vs Pipe Schedule Selection

Flange rating vs pipe schedule selection is a common source of confusion in piping design. The flange pressure class (150, 300, 600, etc.) and the pipe schedule (Sch 10, 40, 80, 160, etc.) are determined independently—the flange class from the ASME B16.5 PT rating tables and the pipe schedule from pressure-thickness calculations per ASME B31.3 or B31.1. However, they must be compatible: the pipe wall thickness at the flange connection must be sufficient for the selected flange class.

Flange Rating vs Pipe Schedule Comparison

Flange Class	Typical Pipe Schedule (CS, NPS 2-12)	Pipe Wall Basis	Max Pressure at Ambient (A105)
Class 150	Sch 40 (Std)	Code minimum + corrosion allowance	285 psig
Class 300	Sch 40 or Sch 80	Moderate pressure service	740 psig
Class 600	Sch 80 or Sch 120	High pressure service	1,480 psig
Class 900	Sch 120 or Sch 160	Very high pressure	2,220 psig
Class 1500	Sch 160 or XXS	Ultra-high pressure	3,705 psig
Class 2500	XXS	Maximum pressure	6,170 psig

These are common combinations for carbon steel pipe. Actual pipe schedule is determined by pressure-thickness calculations, not by the flange class alone.

How They Are Determined

Flange class selection: Based on the design pressure and design temperature using the ASME B16.5 PT rating tables. The flange class must provide a pressure rating equal to or greater than the design pressure at the design temperature.

Pipe schedule selection: Based on the minimum wall thickness calculated from the internal pressure formula in ASME B31.3 (t = PD / 2(SE + PY) + CA), where P = design pressure, D = outside diameter, S = allowable stress, E = weld joint factor, Y = temperature coefficient, and CA = corrosion allowance. The selected schedule must provide at least this minimum wall thickness.

Common Class-Schedule Combinations

Application	Flange Class	Pipe Schedule	Pipe Spec Example
Utility water, air	150	Sch 40 (Std)	A106 Gr. B
Low-pressure steam	150	Sch 40 or 80	A106 Gr. B
Process hydrocarbon (moderate P/T)	300	Sch 40	A106 Gr. B
High-pressure steam	600	Sch 80	A335 P11
High-pressure gas	900	Sch 120	A106 Gr. B
HP/HT well testing	1500	Sch 160	A106 Gr. B
Ultra-high pressure	2500	XXS	A106 Gr. B

Matching Rules

The weld neck flange bore must match the pipe inside diameter. ASME B16.5 specifies standard bore dimensions for each flange class. When ordering flanges, the pipe schedule must be stated so the manufacturer machines the correct bore. A mismatch between the flange bore and pipe ID creates a step at the weld joint, which:

• Creates turbulence and potential erosion
• Complicates radiographic weld inspection
• May require internal tapering per ASME B31.3
• Can trap debris in sanitary or clean service applications

Pro Tip

The flange class is often more conservative than the pipe schedule because the flange must accommodate the full PT envelope including upset conditions, while the pipe schedule is sized for normal design conditions. This is intentional. Never reduce the flange class to match a thinner pipe schedule without verifying the full PT rating.

For pipe schedule wall thickness data, see ASME pipe wall thickness charts. For flange type selection (weld neck vs. slip-on), see what is a flange.

Read the full guide to flange ratings