Comparison of pressure ratings for SCH 40 and SCH 80 steel pipes

Steel pipes of different wall thickness grades (SCH) are widely used in various piping systems, with SCH 40 and SCH 80 being two of the most common choices. Both conform to ASME/ANSI B36.10 standards, but their wall thickness and load-bearing capacity differ significantly.

Nominal pipe size (NPS) is a standard size designation used in the United States for steel and iron pipes. It's important to note that NPS only represents the approximate inner diameter of the pipe, not its actual inner diameter (ID) or outer diameter (OD). At the same nominal pipe size, the outer diameter remains constant, while the wall thickness varies depending on the pipe's wall thickness grade (Schedule Number).

Different wall thickness grades directly affect the pipe's inner diameter: a higher wall thickness grade means a thicker wall, a smaller inner diameter, and a higher pressure-bearing capacity.

For example, in 1-inch NPS pipe, SCH 40 has a thinner wall and a larger inner diameter than SCH 80, making it more suitable for medium-to-low pressure and cryogenic systems.

Among common wall thickness standards: SCH 40 is usually referred to as standard wall thickness (STD, Standard Weight); SCH 80 corresponds to extra strong wall thickness (XS, Extra Strong), which can withstand higher pressure.

SCH 40 vs SCH 80 Steel Pipe Pressure Rating

Nominal Pipe Size (NPS)	Outside Diameter (OD)	Schedule 40 Wall Thickness	Maximum Pressure (PSI)	Schedule 80 Wall Thickness	Maximum Pressure (PSI)
1/8"	0.405	0.068	810	0.095	1230
1/4	0.540	0.088	780	0.119	1130
3/8	0.675	0.091	620	0.126	920
1/2	0.840	0.109	600	0.147	850
3/4	1.050	0.113	480	0.154	690
1	1.315	0.133	450	0.179	630
1 1/4	1.660	0.140	370	0.191	520
1 1/2	1.900	0.145	330	0.200	470
2	2.375	0.154	280	0.218	400
2 1/2	2.875	0.203	300	0.276	420
3	3.500	0.216	260	0.300	370
3 1/2	4.000	0.226	240	0.318	350
4	4.500	0.237	220	0.337	320
5	5.563	0.258	190	0.375	290
6	6.625	0.280	180	0.432	280
8	8.625	0.322	160	0.500	250
10	10.750	0.365	140	0.593	230
12	12.750	0.406	130	0.687	230
14	14.000	0.437	130	0.750	220
16	16.000	0.500	130	0.843	220
18	18.000	0.562	130	0.937	220
20	20.000	0.593	120	1.031	220
24	24.000	0.687	120	1.218	210

What factors determine the rated pressure of a pipeline?

The rated pressure of a pipeline refers to the highest internal pressure that a pipeline can fully withstand at a specific temperature. Many factors influence pipeline pressure; the following are some of the most critical determining factors in engineering design:

1. Wall Thickness (Direct Factor)

The thicker the pipeline wall, the stronger its pressure-bearing capacity. Different grades such as SCH 40, SCH 80, SCH 160, and XXS correspond to different wall thicknesses. Thicker walls are less prone to deformation or rupture under pressure.

2. Pipe Material

Different materials have different strengths and allowable stresses. For example:
Carbon steel (ASTM A53, A106) → Commonly used for medium and high pressure
Alloy steel (ASTM A335) → High temperature and high pressure environments
Stainless steel (ASTM A312) → Corrosion resistant and temperature difference resistant
Duplex stainless steel, nickel-based alloys → Used in extreme conditions

The yield strength and tensile strength of the material directly affect the pressure rating.

3. Temperature
As temperature increases, the allowable stress of a material decreases. Engineering standards typically specify allowable stress values at different temperatures. Proper temperature control can significantly improve the material's compressive strength.

4. Manufacturing Method
Steel pipes are manufactured in two ways: seamless manufacturing and welded manufacturing.

Seamless steel pipes: Without weld seams, they have higher overall strength and are suitable for high-pressure applications.
Welded pipes: Have weld seams, resulting in lower overall strength. The weld seams are significant weak points, making them unsuitable for high-temperature and high-pressure applications.

The rated pressure of a pipeline is not determined by a single factor but by a combination of factors, including wall thickness, material, temperature, outer diameter, manufacturing process, design standards, and corrosion allowance.

Summary

SCH 40 and SCH 80 pipes have the same outer diameter but differ in wall thickness, weight, and pressure rating. SCH 80 offers higher pressure resistance and is ideal for harsh industrial environments, while SCH 40 is suitable for general applications.