Pipe Dimensions and Standards
When discussing pipe dimensions, it’s important to recognize that accurate measurement requires more than just a tape measure. Pipe dimensions are primarily defined by their size and schedule numbers. The American Society of Mechanical Engineers (ASME) has established standards, particularly ASME B36.10 and B36.19, which provide guidelines for these dimensions. A complete description of pipe dimensions typically includes the outer diameter (OD), wall thickness (WT), and pipe length.
Key Terminology in Pipe Sizing
Nominal Pipe Size (NPS): The North American standard for pipe sizes is defined by a non-dimensional number known as NPS. It’s important to understand that for NPS sizes ranging from 1/8 inch to 12 inches, there is a difference between the NPS value and the pipe’s actual outside diameter (OD). For example, a pipe with an NPS of 12 inches has an actual OD of 12.75 inches. This discrepancy exists because the NPS values were originally established to ensure a consistent inside diameter. However, for NPS sizes of 14 inches and larger, the NPS value corresponds directly to the actual outside diameter of the pipe.
Nominal Diameter (DN): This is the European and international equivalent of NPS, with sizes measured in millimeters. DN denotes the nominal diameter for various pipes and fittings, ensuring they can be interconnected.
Outer Diameter (OD): This refers to the actual external measurement of the pipe, which remains consistent for a given NPS size, irrespective of the pipe’s wall thickness.
Wall Thickness (WT): The pipe schedule (Sch.) is an important factor in determining pipe dimensions. As the schedule number increases, wall thickness also increases, thereby reducing the pipe’s inside diameter (ID). Thicker walls enhance the pipe’s strength and its ability to withstand internal pressures. Commonly used schedules include SCH 40 and SCH 80, which are popular across various industries for handling typical pressure levels. Pipe manufacturers are permitted a 12.5% fabrication mill tolerance on wall thickness.
Piping professionals may specify pipe dimensions in several formats, such as “pipe outside diameter × wall thickness” (for example, Φ 88.9 mm x 5.49 mm) or simply as “NPS x Schedule” (for example, NPS 3 inch x Sch 40).
Small-Bore vs. Large-Bore Pipes
Pipes are also categorized by their bore size:
Large-bore pipes: pipes with a nominal size greater than 2 inches.
Small-bore pipes: Generally defined as those with a nominal size of 2 inches or less. These pipes are typically used for applications such as instrument connections, drain lines, sampling points, and vents. While they are often considered non-critical and do not usually require detailed stress analysis, smaller pipes can be susceptible to vibration and fatigue failure if not adequately supported.
Standard Pipe Lengths
Pipes aren’t always manufactured in a single, fixed length. To accommodate varying construction needs, standard pipe lengths are categorized as:
Double Random Length (DRL): These are longer, usually measuring 38 to 40 feet (approximately 10.7 to 12 meters), with a minimum average length of 10.7 meters.
Single Random Length (SRL): These pipes typically range from 18 to 25 feet (approximately 4.8 to 6.7 meters).
Common Types of Pipe Ends
The preparation of a pipe’s end is crucial for proper connection to other components, significantly affecting the assembly process and the integrity of the joint. Here are the most common types of pipe ends:
The preparation of a pipe’s end is crucial for proper connection to other components, significantly affecting the assembly process and the integrity of the joint. Here are the most common types of pipe ends:
Plain Ends (PE):
The simplest type of pipe end is created by cutting the pipe at a 90-degree angle, perpendicular to its length. Plain end pipes are typically used for smaller sizes (less than 2 inches) and are connected with mechanical couplings, socket-weld fittings, or slip-on flanges, all of which require fillet welding. While these methods offer easier alignment and avoid weld metal intrusion, plain ends may trap liquid and are not recommended in situations where severe erosion or crevice corrosion could occur.
Beveled Ends (BW):
Often called ‘weld end’ or ‘butt weld end,’ these pipes are cut at an angle, typically a 37.5-degree bevel, to be welded. Beveled ends allow for a strong, reliable leak-proof butt weld, especially practical for larger diameter piping. However, weld intrusion can affect flow, and proper end preparation is necessary.
Threaded Ends (TE):
These pipes have threads cut into their walls, either on the inside or the outside. Threaded ends are commonly used for pipes 3 inches or smaller in diameter. They allow assembly without welding, which can be beneficial in situations where welding is prohibited due to fire hazards. In North America, the National Pipe Thread (NPT) standard is often used for these tapered threads, which are designed to create an effective seal. However, threaded joints are generally suitable only for lower-pressure and lower-temperature systems because their integrity is lower. Physical distortion can cause leaks, making them unsuitable for carrying hazardous or flammable fluids. Additionally, the presence of threads reduces the pipe’s wall thickness, thereby lowering its overall strength.
Grooved Mechanical Joints (Grooved Ends):
These pipes have a formed or machined groove at the end that accommodates a gasket. A housing is then tightened around the gasket to secure the connection and create a seal. This design makes disassembly easier and reduces the risk of damage to the piping components.
Socket & Spigot Ends:
These ends are primarily found in ductile iron and non-metallic pipelines, such as PVC. They can be easily fabricated on-site and accommodate joints with up to 10 degrees of misalignment. However, they are designed for low-pressure applications and require a special configuration at the pipe ends.
Flanged Ends:
These pipes are designed for situations where a bolted connection is required. They are convenient for on-site assembly and can be used in circumstances where welding is not possible due to material properties or fire hazards. Additionally, they allow for easy dismantling. However, a disadvantage of these pipes is their potential for leakage, and they may not be suitable for piping that experiences high bending moments.
Buttress Ends:
In glass piping, the ends are connected by bolting with backing flanges. However, this type of connection is not suitable for high-pressure applications.
It is essential to understand the various types of pipe ends to ensure proper connections and maintain the overall integrity of any piping system. Every element of a piping system, from the initial design and specifications to the careful selection of materials, dimensions, and end preparations, is meticulously planned to ensure safe, efficient, and reliable fluid transport.
