Steel Pipe Pressure Ratings Guide

Pipe pressure rating is a key parameter in piping design, defining the maximum permissible internal pressure a pipe can withstand under specific conditions. In carbon steel piping systems, the pressure rating is closely related to pipe wall thickness, material properties, and applicable design specifications.

This article primarily discusses pipe pressure ratings based on the ASME B31.3 standard, explaining the method for determining pressure ratings and listing typical pressure ratings for Schedule 40 and Schedule 80 seamless carbon steel pipes.

What is a Pipe Pressure Rating?

A pipe pressure rating refers to the permissible working pressure a pipe can withstand during operation without exceeding the stress limits specified in the design specifications. It is not a fixed value but a limit calculated based on the following factors:

Pipe dimensions
Material allowable stress
Design temperature
Manufacturing method
Applicable piping specifications

In process piping systems, pressure ratings serve as a design reference, ensuring sufficient safety margins under normal operating conditions.

How to Calculate a Pipe Pressure Rating?

For carbon steel pipes designed according to ASME B31.3 (process piping) standard, their allowable pressure is typically calculated using the following formula: P = (2 × S × t) / D

Where:

P = Allowable internal pressure
S = Allowable stress of the material
t = Nominal wall thickness
E = Longitudinal welded joint efficiency
D = Pipe outer diameter

For seamless steel pipes, the joint efficiency E = 1.0, therefore, under the same conditions, seamless steel pipes have a higher pressure resistance than welded steel pipes.

Key Factors Affecting Pipe Pressure Rating

The following factors affect the pressure rating of a pipe:

Plate thickness rating (Pchedule) – Higher wall thickness ratings result in stronger pressure resistance.
Outer diameter (OD) – Larger outer diameters result in lower allowable pressure resistance.
Steel grade – Different steel grades have different allowable stresses.
Manufacturing process – Seamless steel pipes typically have higher pressure limits.
Design temperature – Allowable stress decreases as temperature increases.
Corrosion Allowance – The corrosion allowance decreases the effective wall thickness over time.

Among these, pipe wall thickness rating is one of the most commonly used reference indicators when selecting pressure piping.

Pressure Rating Table Design Basis

Pipe Code: ASME B31.3 – Process Piping
Pipe Type: Seamless Carbon Steel Pipe
Material Reference: ASTM A106 Class B
Design Temperature: 20°C / 68°F (Ambient Temperature)
Allowable Stress (S): 20,000 psi
Longitudinal Joint Efficiency (E): 1.0

The pressure values shown are typical reference values only and are not applicable to detailed engineering designs.

40 Pipe Pressure Rating

40 pipe is widely used in low to medium pressure applications, including general process piping, water supply piping, and industrial systems.

NPS	OD (inch)	Wall (inch)	Typical Allowable Pressure* (psi)
1/2"	0.84	0.109	5,000
3/4"	1.05	0.113	4,300
1"	1.315	0.133	4,000
1.5"	1.90	0.145	3,000
2"	2.375	0.154	2,600
3"	3.50	0.216	2,400
4"	4.50	0.237	2,100
6"	6.625	0.280	1,700
8"	8.625	0.322	1,500
10"	10.75	0.365	1,350
12"	12.75	0.375	1,150

Pressure Rating of 80 Pipe

80 pipe has a thicker wall, making it suitable for higher pressures and more demanding operating environments, such as chemical processing and high-pressure fluid systems.

Schedule 40 vs Schedule 80: Pressure Comparison

Schedule 40 and Schedule 80 pipes have the same outer diameter for the same nominal diameter (NPS), but their wall thicknesses differ significantly, resulting in different pressure ratings.
Schedule 80 pipes typically have 60% to 80% higher pressure rating than Schedule 40 pipes.
Schedule 80 pipes are also heavier and more expensive; the larger wall thickness increases both weight and cost.

Conclusion

Pipe pressure rating is a fundamental consideration in the design and selection of carbon steel piping systems. Under ASME B31.3, allowable pressure is determined by pipe dimensions, wall thickness, material allowable stress, and service conditions, rather than by pipe schedule alone.