Class 3000 vs. Class 6000 Socket Weld Fittings

Class 3000 and Class 6000 are two main pressure ratings of forged socket weld fittings defined in ASME B16.11. Their main differences lie in pressure capacity, compatible pipe wall thickness (SCH), physical dimensions, and applicable scenarios.

Class 3000 vs. Class 6000 Forged Socket Weld Fittings:

Comparison Dimensions	Class 3000	Class 6000
Pressure Rating (Normal Temperature, A105 Carbon Steel)	3,000 psi (approx. 20.7 MPa)	6,000 psi (approx. 41.4 MPa)
Compatible Pipe Wall Thickness	Sch 80 / XS	Sch 160 / XXS (Class 9000)
Size Range	1/8″–4″	1/8″–2″ (Larger sizes are generally not available in Class 6000 due to manufacturing and strength limitations)
Weight (for a 1" elbow)	~0.22 kg	~0.40 kg
Wall Thickness (1" elbow)	~4.5 mm	~7.5 mm
Socket Depth	Standard Depth	Deeper (due to thicker body)
Applications	For medium to high pressure (e.g., 300–600 psi steam, oil)	Higher pressure or critical sealing systems (e.g., high-pressure hydraulics, chemical, refining)
Relative Cost	1.0 times	1.5-2.0 times
Inventory Status	Wide inventory	Low inventory, longer lead time

Note: Neither is considered "low pressure," but Class 6000 is a high-strength upgrade of Class 3000 and they are not interchangeable; selection must strictly match the design pressure, temperature, and pipe wall thickness.

Fitting Body Dimension Differences:

Class 6000 fittings, due to their need to withstand higher pressures, have significantly thicker body walls than Class 3000 fittings. Both are limited to small diameters of DN50 (2 inches) and below.

Nominal Size (NPS)	Class 3000 Body Diameter (mm)	Class 6000 Body Diameter (mm)
1/2"	38	47
3/4"	47	53
1"	56	63
1-1/4"	66	74
1-1/2"	75	83
2"	90	104

Selection Recommendations and Precautions:

Pipes and fittings must have matching wall thicknesses. Mixing Class 6000 fittings with Sch 80 pipes (or Class 3000 with Sch 160 pipes) will cause a mismatch between the fitting's inner diameter and the pipe's inner diameter, creating a step at the connection and resulting in:

● Fluid turbulence and erosion
● Stress concentration
● Potential fatigue cracking

Selection Principles:

Scenarios for choosing Class 3000:
Design pressure ≤ 3,000 psi (207 bar);
The default choice for most petrochemical and utility piping;
Requires Sch 80 or XS piping;
Diameter may exceed 2".

Scenarios for choosing Class 6000:
Design pressure > 3,000 psi (up to 6,000 psi / 414 bar);
Specifications require Sch 160 piping;
High-pressure instrument lines or hydraulic systems;

Requires additional corrosion allowance (thicker pipe walls provide more margin).

Read more: Thick-Walled Elbows or ASME B16.5 & B16.47 Steel Flange Dimensions