In the majority of industrial projects, the applicable design code is not selected arbitrarily: it is defined by the nature of the system, the owner’s requirements, and the project basis documents. However, understanding the differences between ASME B31.1 and ASME B31.3 remains essential, as key aspects of piping design, fabrication, inspection, and commissioning depend on them.

Although both codes share general design principles and pursue the same goal of ensuring the mechanical integrity of piping systems, their requirements differ in fundamental aspects such as the scope of application, calculation criteria, fluid classification, inspection requirements, and verification procedures. These differences have direct implications for the design, construction, and final cost of a facility.

Scope and Field of Application

ASME B31.1 applies to piping in power generation plants: steam, boiler feedwater, condensate, and blows in power generation facilities, thermal power plants, and industrial cogeneration systems.
ASME B31.3 applies to process piping in refineries, petrochemical plants, and chemical plants, covering a much wider variety of fluids: hydrocarbons, corrosive chemicals, cryogenic fluids, and toxic gases.

A frequent error is to assume that the applicable code is determined solely by the physical boundaries of the facility. In reality, the assignment between B31.1 and B31.3 depends on the function of the system and the contractual/regulatory definition of the project. In facilities that combine steam generation and chemical processing, both codes may coexist: high-pressure steam generation and distribution piping under B31.1, and process piping under B31.3. This definition must be explicitly documented in the project Basis of Design.

Design Equation and Calculation Factors

Both codes use equivalent formulations based on stress equilibrium to calculate the required wall thickness for internal pressure, although their equations, nomenclature, and correction factors are not identical. The general form in ASME B31.3 (paragraph 304.1.2) is:

t=(P⋅D)/(2(SEW+PY))

Where:
P: design pressure
D: outside diameter
S: allowable stress of the material
E: longitudinal weld joint quality factor
W: weld joint strength reduction factor
Y: coefficient dependent on material and temperature

The W factor (weld joint strength reduction factor) is exclusive to B31.3. It recognizes that, under high-temperature creep regimes, the welded joint can degrade faster than the base metal. For most process services, W=1.0 and does not affect the calculation; it gains relevance in low-alloy Cr-Mo steels at elevated temperatures.
B31.1 does not use a weld joint strength reduction factor equivalent to the W of B31.3 and establishes its design criteria through other regulatory mechanisms, including tabulated allowable values.

The E factor is the efficiency of the longitudinal joint of the pipe and depends on the product form (seamless, welded), joint quality, manufacturing method, and material specification. Many seamless pipes have E=1.0 without requiring any additional testing. It is important not to confuse this factor with the scope of field circumferential NDE, which is an independent requirement determined by the fluid category and project requirements.

The values of the Y coefficient are presented in specific tables for each material type and temperature range, and they differ between both codes.

Fluid Service Categories in B31.3

One of the most relevant contributions of B31.3, which has no direct equivalent in B31.1, is its system of fluid service categories, which determines the examination, testing, and qualification requirements for each system:

Category D Fluid Service: non-flammable, non-toxic fluids with a temperature between -29°C and 186°C and a pressure under 1035 kPa (150 psi). The minimum required examination is visual.
Normal Fluid Service: the default category for most process fluids. It does not generally require a mandatory minimum percentage of radiography or ultrasonics; visual examination is mandatory, and the scope of additional NDE depends on the requirements applicable to the system.
Category M Fluid Service: toxic fluids where exposure, even in trace amounts, poses a severe risk to personnel. It imposes significantly more stringent examination and leak-tightness verification requirements than Normal Fluid Service.
High Pressure Fluid Service (Chapter IX): service subject to the specific design criteria of Chapter IX of B31.3, applicable to certain pressure and size conditions outside the typical design scope of the code.

This system allows the level of examination to be adjusted to the actual criticality of the service, optimizing resources without compromising integrity.

Post-Weld Heat Treatment

PWHT requirements in both codes depend on the material, thickness, welding process, and joint geometry, meaning there is no universally applicable simplified rule. In general terms, B31.1 establishes broader PWHT requirements for certain materials and thicknesses than B31.3, which responds to the nature of the services it addresses: severe thermal cycling and prolonged operation at high temperatures.

It is important to note that PWHT requirements for special services, such as high-pressure hydrogen, H2S , or amines, do not originate directly from B31.3, but rather from complementary standards such as API 934, API RP 945, or NACE MR0175/ISO 15156. Applying only the general criteria of the piping code in these services, without consulting the specific applicable standards, is an error with potentially severe consequences.

Pressure Testing

Both codes use the hydrostatic test as the primary method for integrity verification and apply equivalent criteria to determine the test pressure, which in general terms equals 1.5 times the design pressure corrected based on the allowable stresses of the material at test temperature and design temperature. Both codes also establish an upper limit to prevent the test pressure from inducing excessive stresses in the components. The differences between B31.1 and B31.3 in this section reside primarily in the test duration requirements, the alternative methods accepted, and the acceptance criteria.

Pneumatic testing is permitted in both codes when hydrostatic testing is not feasible, but it requires additional safety measures given the higher stored energy in systems pressurized with gas.

Frequent Errors

From experience in detail engineering projects, we highlight the most recurrent errors:

Determining the applicable code by the physical location of the piping instead of by the function of the system.
Confusing the longitudinal joint quality factor E with the scope of field circumferential NDE, incorrectly linking them during design.
Assuming that B31.3 imposes a mandatory minimum percentage of radiography in Normal Fluid Service, when that requirement usually originates from the owner or project specifications.
Failing to consult complementary standards (API, NACE) in services with aggressive fluids such as H2S , hydrogen, or amines.

Conclusion

ASME B31.1 and ASME B31.3 are not alternative codes: each responds to a specific type of facility and a well-defined risk profile. Their differences in the design equation, the fluid service category system, PWHT requirements, and testing criteria have direct consequences on wall thickness, fabrication costs, and the system’s level of integrity.
Knowing precisely when and how each code applies, and being aware of their limitations and the complementary standards that accompany them, is a fundamental competency for the piping engineer. Standards are not a documentary formality: they are the primary engineering tool to ensure that a piping system is safe, sound, and optimized throughout its entire service life.