Pipe Fitting Standards Clarified: Class 150 vs 150 Pound Fittings

Understanding critical differences in pipe standards and descriptions to ensure optimal performance and safety

As with most piping products, the little details matter when picking the ideal pipe fittings for your piping process.

One commonly seen issue is the difference between Class 150 (CL150) fittings and 150 Pound (150# or 150LB) fittings.

While these products will share similarities, failing to choose the right product can result in safety risks and premature product failure or cause systems to fail to meet regulatory and engineering requirements.

Not sure what the differences are? Keep reading as we look at both labels and what they mean.

Stainless Steel Pipe Fitting Categories Explained

Stainless steel threaded pipe fittings come in various sizes and designs suited for a particular use case, environment, and budget.

Organizations have worked hard to develop standards for pipe fittings to ensure comparable designs and specifications between manufacturers and help create a consistent, predictable, and safe experience when implementing these popular products in a range of applications.

Before we dive into the specific standards relevant to categorizing stainless steel threaded pipe fittings, let’s take a moment to look at where these standards come from.

These standards are based mainly on the work of three organizations:

• ASTM - American Society for Testing and Materials International
• ANSI - American National Standards Institute
• ISO - International Organization for Standardization

These professional bodies are widely trusted across the globe for standards related to stainless steel pipe products, pipe system engineering, and a range of other topics.

Taking the standards of these three groups, the Manufacturers Standardization Society (MSS) took things further by filling out gaps in standards and further clarifying definitions and requirements.

The American Society of Mechanical Engineers (ASME) standards are also derived and compiled from the standards released by all four organizations.

The most relevant stainless steel pipe fittings standards are ANSI B1.20.1 and ASTM A351.

ANSI B1.20.1 provides the considerations when categorizing pipe fittings by dimension, while ASTM A351 defines the materials and castings used within pipe fittings.

These categorical standards ensure consistency and allow engineers to select pipe fitting products with confidence that they’ll provide long-lasting and safe performance for their intended application.

By understanding these standards and the specifications they define for different pipe fitting products, you can help to avoid miscommunications while reliably sourcing comparable products across a range of manufacturers or providers.

Three Considerations for Choosing the Proper Pipe Fitting

Stainless steel threaded pipe fittings should provide information about each of the following:

• Connection types
• Pressure classes
• Material types

Let’s look at each.

Confirming a Solid Seal with Standardized Pipe Threading

Connection types regarding threaded pipe fittings typically focus on the thread pattern.

Thread patterns are most often defined by the size and taper type.

In North America, it is common to find threaded fittings with a National Pipe Thread (NPT) taper. Standard sizes range between 1/8-inch and 4-inches NPT.

Ensuring a solid fit with your threaded components is critical for minimizing leaks and maintenance requirements at pipe connection points.

Differing threading configurations can cause serious issues, including damage to connecting components, leaky connections, cross-threading, or incompatibility with other parts in your piping system.

For a look at other international thread standards, consult this guide.

We also discuss how threaded pipe ends, such as those used in pipe fittings, compare to other end types in our guide, Pipe Ends: Explained.

A Quick Look at Pressure Classes

With the connection and seal type considered, we’ll now look at more functional classification elements.

The pressure class defines the maximum pressure a pipe, fitting, or valve should be capable of withstanding at a given temperature under typical operating conditions.

It’s important to realize that these classes can provide different pressure tolerances based on the component type.

For example, class 150 (CL150) threaded fittings have different limits than CL150 flanges.

As the definition highlights, pressure classes are also defined at specific temperatures.

This means that you need to understand any external and internal temperatures throughout the piping process to ensure that all threaded pipe fittings are operating within their tolerance limits.

In most cases, pressure tolerances–also referred to as maximum working pressure measurements–will decrease as temperatures increase.

Casting and Material Standards to Consider

Pressure ratings are only one facet of a component. You must also look at dimensional and casting standards as well.

Threaded stainless steel pipe fittings typically fall under one of four differing standards.

• No standard: These components are typically manufactured using proprietary systems which do not ensure compliance with ASTM A351 standards. These are often called 150LB components and should not be confused with Class 150 components. Many of these parts result from early mass production designs and processes involving other metals and materials (brass, iron, or steel.) As such, they’re not always optimized to leverage the specific benefits of stainless steel in the component design.
• ISO 4144: While these components are at least standardized and offer cost savings over components further down this list, they are 150LB fittings and typically cast thinner, failing to comply with ASTM A351 standards.
• ANSI B16.3: Originally created to define standards for iron fittings before the rise in popularity of stainless steel fittings, some manufacturers still use this standard to determine CL150 fittings.
• MSS SP-114: Based on ANSI B16.3 dimensions, this standard is fully compliant with ASTM A351 standards. It is also the first explicitly defined for use with stainless steel fittings. As the most comprehensive standard in use today, it typically results in components with improved tolerances compared to lesser engineered 150LB part types.

Materials can also vary between products and significantly impact both cost and performance.

In most cases, cast stainless steel threaded pipe fittings are readily available in 304 and 316 grades.

Low-carbon grades are less available and might require custom ordering.

However, in many cases, these designations are used in place of the actual ASTM A351 compliant grades.

For full compliance, pipe fittings must be cast using CF8 grade (18% Chromium and 8% Nickel) or CF8M grade (18% Chromium, 10% Nickel, 2% Molybdenum.)

Bringing the Considerations Together

With the significant considerations covered, we can now bring together a clear definition of what to look for in Class 150 stainless steel threaded pipe fittings.

To earn classification, the fitting must:

• Meet MSS SP-114 standards for NPT sizes 1/8-inch through 4-inches.
• Have ANSI B1.20.1 compliant threading.
• Utilize materials which meet the ASTM A351 Standard Specification for Castings, Austenitic, for Pressure Containing Parts in grades CF8M or CF8.

Pipe fittings which do not meet all of these specifications might be classified instead at 150LB fittings.

In most cases, 150LB fittings are best suited for commercial, residential, marine, food and beverage, and industrial applications where ASME B31 codes do not apply. Although there would be utility and possibly process applications within an industrial plant where working pressures less than 225psi and temperatures less than 150F do exist—the fittings specified must still be compliant with the applicable code.

Higher temperatures or pressures will typically require ASTM A351 compliance and Class 150 fittings.

However, the best product for your needs will depend on a range of variables, including intended use, piping process design, operational and environment temperatures, pipe sizes, and many other factors.

If you’re unsure whether you need Class 150 fittings or if 150# fittings will do, consult an engineer.

Key Takeaways

• Confusing the requirements between Class 150 (CL150) and 150 Pound (150LB or 150#) fittings can create operational safety concerns, lead to premature product failure, or cause failures in regulatory compliance.
• With the myriad variables related to stainless steel threaded pipe fittings—and just as many parts manufacturers available—standards help to ensure reliability and create a consistent experience when implementing fittings in your piping process design.
• Standards are created by a variety of organizations. Relevant bodies related to stainless steel product standards include the ASTM, ANSI, ISO, MSS, and ASME organizations.
• Two major standards to keep in mind when choosing stainless steel threaded pipe fittings are ANSI B1.20.1—which deals with dimensions—and ASTM A351—which deals with materials used and castings quality.
• When determining the best pipe fitting for your needs, three primary concerns are connection types, pressure classes, and material types.
• When dealing with threaded fittings, connection types—and thread standards in particular—are critical to ensuring a tight seal and avoiding damage to fittings, piping, or other system components.
• In North America, most fittings follow the National Pipe Thread (NPT) taper standard with sizes ranging from 1/8-inch to 4-inches NPT.
• Pressure classes measure the maximum pressure a fitting should withstand at a given temperature.
• Fittings will typically handle more pressure at lower temperatures and less pressure as temperatures rise.
• Pressure classes can denote different exact pressures across product types, even at the same pressure class.
• 150LB rating does not guarantee ASTM A351 compliance.
• If you require ASTM A351 compliance, CL150 pipe fittings that meet ANSI B16.3 or MSS SP-114 standards will provide sufficient specifications.
• For full compliance, pipe fittings must be cast using CF8 grade (18% Chromium and 8% Nickel) or CF8M grade (18% Chromium, 10% Nickel, 2% Molybdenum.)
• In most cases, 150LB fittings are best suited for commercial, residential, marine, food and beverage, and industrial applications where ASME B31 codes do not apply. Although there would be utility and possibly process applications within an industrial plant where working pressures less than 225psi and temperatures less than 150F do exist—the fittings specified must still be compliant with the applicable code.

With decades of experience serving industries of all sizes throughout Canada and North America, Unified Alloys’ experienced sales analysts can help you to explore the options available to you for your next project. Contact us today to start the process!