Schedule 40 Pipe Pressure Rating Chart & Guide

In the field of pipelines, Schedule 40 steel pipe is widely used, especially in water, gas, and industrial piping applications. These pipes are known for their mechanical strength, wide temperature range and strong pressure resistance capabilities. Understanding the intricacies of pressure ratings is critical to ensuring optimal performance of Schedule 40 pipe. 

This article provides pressure rating chart for both carbon steel (ASME B36.10) and stainless steel (ASME B36.19) Schedule 40 pipes, along with calculation methods and key influencing factors.

Pressure Rating Estimation Chart for Carbon Steel SCH 40 Pipes:

The following table presents the Maximum Allowable Pressure (also known as pressure rating) for Schedule 40 steel pipes across a range of nominal pipe sizes (NPS). The pressure values are shown in psi (pounds per square inch) and kPa (kilopascal), reflecting common units used globally. For carbon steel (typical ASTM A106 Gr.B) at room temperature, these are standard reference values.

1 in (inch) = 25.4 mm

1 psi (lb/in2) = 6,894.8 Pa (N/m2) = 6.895x10-2 bar

Note: The above pressure ratings are based on carbon steel (ASTM A106 Gr.B) at room temperature. For stainless steel Schedule 40S pipes (ASME B36.19), pressure ratings may differ due to different allowable stress values. 

Pressure Rating Estimation Chart for Stainless Steel SCH 40S (ASME B31.3)

1. Diverse Steel Pipe Schedules

Within the spectrum of steel pipes, there exist 14 different schedules, each designed to meet specific requirements. Schedule 40, being the standard and widely used option, holds a special place in the construction and plumbing industries. The schedule number plays a pivotal role, influencing both wall thickness and pressure ratings, and is standardized by the American Society of Mechanical Engineers (ASME).

2. Dimensions and Specifications

For clarity and consistency in construction, ASME B36.10 standardizes dimensions for carbon and alloy steel pipes, while ASME B36.19 does the same for stainless steel pipes (SCH 40S). Detailed tables provide insight into the dimensions and weights of Schedule 40 pipes, aiding in precise installations. It is important to specify the correct standard when ordering, as for NPS 10 and above, the wall thickness of stainless steel Sch 40S is thinner than carbon steel Sch 40.

3. Pressure Rating Formula
The Schedule number is a key factor in determining a pipe's stress-bearing capacity and its ability to withstand pressure. The pressure rating formula, expressed as SCH * P = S, where SCH represents the schedule number, P is the maximum internal pressure, and S is the allowable stress, provides a systematic approach to assessing pressure capabilities.

4. Advanced Pressure Rating Calculation
Delving into the intricacies of pressure rating calculation involves a modified Barlow’s formula. Rearranged to consider factors such as pipe wall thickness, outside diameter, allowable pressure, stress, longitudinal-joint quality, and wall thickness correction, this advanced approach ensures a thorough understanding of a pipe's pressure-bearing capabilities.

5. Stainless Steel Pressure Rating Dynamics
Schedule 40 stainless steel pipes introduce additional complexities due to the material's unique properties. Barlow’s formula, when applied to stainless steel, considers the impact of design temperature on allowable stress. The article systematically calculates and tabulates pressure ratings at various temperatures for a comprehensive understanding.

In general, for the same NPS at room temperature, carbon steel Sch 40 may have a slightly higher pressure rating than stainless steel Sch 40S due to the higher allowable stress of carbon steel. However, at elevated temperatures (typically above 500°F / 260°C), the pressure rating of stainless steel may surpass that of carbon steel, as stainless steel retains its strength better at high temperatures.

How to Calculate Pressure Rating for Schedule 40 Pipes?

The pressure rating can be estimated using the Barlow's formula:

Where:
P = Maximum allowable pressure (psi)
S= Material allowable stress (psi)
t = Wall thickness (inch)
D = Outside diameter (inch)

For example, for a 2-inch NPS Sch 40 carbon steel pipe (D=2.375 in, t=0.154 in, S≈20,000 psi at room temperature):
P = (2 × 20,000 × 0.154) / 2.375 ≈ 2,595 psi

This calculation demonstrates why thicker pipes and stronger materials have higher pressure ratings.

Factors Influencing Pressure Ratings:

(1) Material Composition
The material composition of Schedule 40 pipes significantly influences their pressure ratings. For steel pipes, common materials include carbon steel (e.g., ASTM A106 Gr.B) and stainless steel (e.g., 304/316). Each material has distinct allowable stress values that affect how well the pipe can withstand pressure.

(2) Temperature Considerations
Temperature plays a crucial role in determining the pressure a Schedule 40 pipe can handle. As temperature increases, the allowable stress of both carbon steel and stainless steel decreases, which reduces the pressure rating. Different materials exhibit varying temperature resistance, and exceeding the recommended temperature limits can compromise the pipe's integrity.

(3) Pipe Diameter
The diameter of the pipe is another critical factor affecting pressure ratings. For the same schedule, larger-diameter pipes have lower pressure ratings because the stress is distributed over a larger circumference. This is why the pressure rating decreases as NPS increases, as shown in the table above.

Mastering the pressure ratings of Schedule 40 pipes is a fundamental step in ensuring the success and safety of your plumbing projects. By delving into the intricacies of steel pipe schedules, dimensions, pressure rating formulas, and advanced calculations, this article equips you with the knowledge needed to make informed decisions and construct reliable plumbing systems that stand the test of time.

For precise pressure ratings based on your specific material grade, temperature, and application, always consult the relevant ASME codes and material data sheets.

Read more: Schedule 40 vs Schedule 80 pipe or Schedule 80 Steel Pipe Pressure Rating