A Comprehensive Guide to Understanding Pipe Sizes, Schedules & Flexibility

Introduction

Whether you are selecting the right pipe size for a pipe system, considering the implications of thermal expansion on your system’s integrity, or specifying the intricacies of pipe schedules, pipe materials and pipe flexibility, this comprehensive guide is available to support designers and contractors with accurate information for piping infrastructure.

Common Pipe Sizes

The pipe size is specified with two numbers: the nominal pipe diameter (or bore) and the pipe schedule. This sets the outside pipe diameter, the wall thickness (and hence the internal diameter). Dimensions shown are to be ASME B36.10

Pipe Classification

There are four main pipe classifications that you can use to identify your pipe’s sizing quickly. These pipe size standards help you estimate the full size of your pipe, so you can safely install piping systems, pipe supports, and pipe layouts.

Nominal Pipe Size (NPS)

NPS is an abbreviation of Nominal Pipe Size, a term used as a guideline number defining the diameter of the pipe.

Nominal pipe size refers to only the outside diameter (OD) of a pipe, making it somewhat vague. For example, when we say pipe size is 2 NPS, it refers to all the pipes having 2.375-inch (or 60.3 mm) as outside diameter irrespective of wall thickness and inside diameter (ID).

Nominal Bore (NB)

Nominal Bore is the European designation equivalent to NPS, in which sizes are measured in millimetres. NB (nominal bore) is also frequently used interchangeably with NPS.   

Diameter Nominal (DN) 

This is the international and European metric method used to describe NPS (or NB).DN pipe sizes numbers are therefore different to NPS. For example, the metric equivalent of an NPS 6 pipe (6”) is DN 150. (To convert from NPS 6 to DIN simply multiply the 6 x 25=150. (150mm)

Outside Diameter (OD)

OD is the outside diameter of the pipe and is fixed for a given size.  The OD is one of the most important dimensions a pipe support manufacturer requires. This dimension is critical when forming a Ubolt or rolling a pipe clamp. The ID of the pipe support needs to suit the OD of the pipe. Refer to “Pipe Materials” section for further explanation of the varying pipe OD’s across pipe materials.

The NPS is very loosely related to the inside diameter in inches, but only for NPS 1/8 to NPS 12. For NPS 14 and larger, the NPS is equal to the outside diameter (OD) in inches. For a given NPS, the OD stays constant and the wall thickness increases with larger SCH. For a given SCH, the OD increases with increasing NPS while the wall thickness increases or stays constant.

Pipe Schedules

Pipe schedules refer to a standardised system used to specify the thickness of pipes. They indicate the thickness and pressure rating of pipes, helping ensure compatibility and safety in piping systems. The schedule number typically correlates with the wall thickness of the pipe, with higher schedule numbers indicating thicker walls and higher pressure ratings. Common pipe schedules include Schedule 40, Schedule 80, and Schedule 160, among others, each suitable for different applications and pressure requirements.

As the schedule number increases, the wall thickness of the pipe increases. The schedule number is roughly calculated as per formula below:

Schedule = 1000 x (P/S)
P = Internal service pressure of pipe (psig)
S = Ultimate tensile strength of the pipe material (psi)

Understanding the schedule of your pipes is crucial because the thickness directly affects how long your piping system will last. If your pipes are handling more weight than they can handle, they might break, burst, leak, or damage the entire system. Using standard measurements also helps you choose the right supports for your pipes, which improves the system’s performance. If you don’t match your pipes with the correct supports of the right size, it could cause damage, accidents, and project delays.

Pipe Materials

Across all sectors a diverse range of materials and pipe types are employed, each possessing distinct characteristics and often differing in sizing conventions. This variability can lead to confusion within the industry regarding the outside diameter of pipes and the selection of appropriate pipe supports. To address this, we’ve provided a chart below outlining the dimensions of ten commonly used pipe materials.

The material and type of pipe can significantly impact its outside diameter. Different materials, such as steel, copper, PVC, or HDPE, have varying physical properties, including density, elasticity, and thermal expansion coefficients. These properties affect how much the pipe expands or contracts in response to changes in temperature and pressure.

For instance, metal pipes like steel or copper tend to have a smaller outside diameter compared to plastic pipes like PVC or HDPE for a given nominal size. This is because metals generally have higher densities and less thermal expansion compared to plastics. As a result, metal pipes maintain their dimensions more consistently across different temperature and pressure conditions, leading to a smaller outside diameter.

On the other hand, plastic pipes often have a larger outside diameter for the same nominal size due to their lower density and greater thermal expansion. Plastic materials tend to expand more when exposed to heat, causing the pipe to increase in size. Additionally, plastic pipes may have thicker walls to compensate for their lower strength compared to metal pipes, further contributing to a larger outside diameter.

In summary, the material and type of pipe influence its outside diameter due to differences in density, thermal expansion characteristics, and wall thickness requirements. Understanding these factors is essential for proper pipe selection and sizing in various applications.

As a manufacturer of pipe supports, we understand the importance of accommodating various sizes and materials. Whether it’s steel, copper, PVC, or any other material, our products are designed to cater to the needs of different piping systems, ensuring reliable support regardless of the pipe type or size.

Pipe Thermal Expansion

When a pipe is subjected to temperature variations, it expands or contracts in response to the change in temperature. This expansion or contraction can lead to several notable impacts on piping systems:

Stress on Supports and Connections: Thermal expansion can exert significant forces on pipe supports, anchors, and connections. If the supports or connections are not designed to accommodate thermal movement, they may experience excessive stress, leading to deformation, cracking, or failure.

Buckling or Bowing: In long piping runs, thermal expansion can cause the pipe to bow or buckle if it’s constrained at both ends. This can result in misalignment, reduced flow efficiency, and potential damage to the pipe and surrounding structures.

Leakage: Fluctuations in temperature can cause joints and seals to weaken or loosen over time, increasing the risk of leakage. This is particularly critical in systems conveying fluids or gases where leakage can lead to environmental hazards, operational disruptions, and safety concerns.

System Integrity: Over time, repeated thermal cycling can weaken the structural integrity of pipes and components, leading to premature aging and eventual failure. This can result in costly repairs, downtime, and potential safety hazards.

Mitigating Issues Caused by Thermal Expansion

To mitigate the impacts of thermal expansion on piping systems, engineers employ various strategies to ensure flexibility in pipe systems, including:

Expansion Joints: Installing expansion joints at strategic locations allows the pipe to expand or contract without exerting excessive stress on the system.

Expansion loops: Pipe expansion loops are used to provide flexibility in piping systems to accommodate thermal expansion and contraction. These loops are added to the straight sections of pipes, allowing them to bend when the pipe expands. By introducing perpendicular sections to the pipe’s direction of expansion, the loop absorbs the stress before transmitting it to the anchor points. The longer the loop, the less force is created.

Flexible Supports: Using flexible pipe supports, height adjustable supports and pipe hangers that can absorb thermal movement helps prevent stress on the piping system. 

Proper Sizing and Layout: Designing piping systems with adequate space for thermal expansion and contraction reduces the likelihood of buckling or stress-induced damage.

Material Selection: Choosing materials with low coefficients of thermal expansion can help minimise the effects of temperature fluctuations on piping systems.

Overall, understanding and properly managing thermal expansion is essential for ensuring the long-term reliability, safety, and efficiency of piping systems in various industrial, commercial, and residential applications.

The cost of the project can be reduced substantially if the right amount of flexibility is built in the piping at the initial layout stage. Read more about pipe flexibility here.

Calculating Thermal Movement & Pipe Flexibility

The first step in accommodating thermal movement is to compute the exact change in the linear length of the piping system over the distance of interest, along with a suitable safety factor.

The actual expansion of 100-foot pipe lengths has been computed at different temperatures for the most common piping materials (carbon steel, stainless steel and copper tubing) and are shown in the table here bellow. These values should not be applied to pipe of alternate materials as they will vary. Expansion coefficients may vary 5% or more when obtained from different sources and should be taken into account.

See an example of calculating pipe flexibility here.

Conclusion

In conclusion, navigating the complexities of piping infrastructure demands a thorough understanding of various factors, from selecting the appropriate pipe size to managing thermal expansion and considering pipe schedules, materials, and flexibility. This comprehensive guide serves as a valuable resource for designers and contractors alike, providing accurate and insightful information to ensure the successful planning, implementation, and maintenance of piping systems.

If you’ve hit a wall when specifying standardised piping system supports or simply faster custom solutions, Swift Metal answers your call with custom pipe supports.

From tight spaces to heavy loads, our custom pipe supports evenly distribute weight, prevent stress points, and mitigate movement and vibration. All custom solutions are designed to suit projects with challenging site conditions, diverse climates, specification requirements and limited space.

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Our team of experienced technicians engineer tailor-made pipe supports that meet stress adherence standards, accommodate for thermal expansion, distribute weight evenly, survive rugged and corrosive conditions and mitigate all movement and vibrations. When it comes to precision engineering and custom fabrication, the Swift Metal team are the experts.

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