Understanding the Key Difference Between Pipe and Tube

Pipes and tubes are fundamental components in various industries. They are used for the transportation of fluids like liquids and gases. Understanding their difference is important. In this article, we’ll explore the key difference between pipe and tube, including their construction, dimensions, and common applications.

What is Pipe?

A pipe is a cylindrical, hollow conduit used primarily to transport fluids (liquids or gases) from one location to another. Pipes are designed to handle pressure and flow. They are essential in industries such as oil and gas, water supply, chemical processing, and power generation.

Key Characteristics of a Pipe

1. Function:
   • Primarily used for carrying fluids like water, steam, oil, gas, and chemicals.
2. Measurement:
   • Measured by nominal pipe size (NPS) or nominal diameter (ND), which indicates the approximate inner diameter (ID).
   • The wall thickness is defined by schedule numbers (e.g., SCH 40, SCH 80).
3. Material:
   • Common materials include carbon steel, stainless steel, copper, PVC, and HDPE, chosen based on the fluid and operating conditions.
4. Pressure Rating:
   • Designed to withstand internal pressure depending on the application and material.
5. Shape:
   • Almost always round to ensure uniform distribution of stress and resistance to internal pressure.
6. Standards:
   • Manufactured according to standards like ASME B36.10 (welded and seamless steel pipes), ASTM specifications, or API for oil and gas pipelines.

Applications:

• Water distribution and plumbing.
• Oil and gas pipelines.
• Industrial chemical transport.
• HVAC systems (steam and chilled water lines).
• Structural uses in construction (scaffolding, casing).

What is tube?

A tube is a hollow cylindrical structure. It can also be square, rectangular, or oval. Tubes are designed for various purposes. These purposes include structural applications, precise fluid flow, and heat transfer. Tubes are known for their dimensional accuracy, versatility, and ability to handle stresses.

Key Characteristics of a Tube:

1. Function:
   • Used for structural support, mechanical applications, and specialized tasks like heat exchangers, medical devices, instrumentation, and hydraulic systems.
2. Measurement:
   • Measured by outside diameter (OD) and wall thickness, ensuring precise dimensions.
   • Internal diameter (ID) vary based on wall thickness.
3. Material:
   • Common materials include stainless steel, carbon steel, aluminum, copper, and alloys. The choice depends on the application’s mechanical, thermal, or corrosion resistance requirements.
4. Precision:
   • Manufactured with tight tolerances for applications requiring exact dimensions and smooth finishes.
5. Shape:
   • Available in various cross-sectional shapes: round, square, rectangular, or custom shapes.
6. Standards:
   • Manufactured according to standards like ASTM A213 (stainless steel tubes), ASTM B88 (copper tubes), and ISO/DIN standards.

Applications:

• Heat Exchangers: Tubes are critical for transferring heat in boilers, condensers, and radiators.
• Medical Equipment: Precision tubes are used in catheters, syringes, and surgical tools.
• Hydraulic and Pneumatic Systems: For precise fluid control.
• Structural Components: Used in frames, scaffolding, and aerospace structures.
• Instrumentation: Flow meters and gauges often require tubing for accuracy.

Key Difference Between Pipe and Tube

Material of Construction (MOC) for Pipes and Tubes

Pipes and tubes can be constructed from a wide range of materials, including metals, alloys, wood, ceramics, glass, and plastics. The choice of material depends on the application. For instance, PVC (Poly Vinyl Chloride) is commonly used for water pipes, while process plants often use low-carbon steel pipes. In special cases, wrought-iron and cast-iron pipes find their place in process industries.

Size of Pipe and Tubing

Pipe and tubing are specified in terms of their diameter and their wall thickness.

NPS – Nominal Pipe Size

In the United States, pipe sizes are specified using NPS, which stands for Nominal Pipe Size(NPS). This system shows the pipe’s diameter in inches. The standard sizes for pipes can be found in the ASME (American Society of Mechanical Engineers) Code B36.10 for wrought steel pipes and other steel pipes. For stainless steel pipes, refer to ASME Code 36.19 for the standard pipe sizes.

When it comes to steel pipes, NPS ranges from 1/8 inches to 30 inches. For standard pipe sizes larger than 12 inches in diameter, NPS matches the pipe’s outside diameter (OD). Yet, this relationship becomes complex for standard pipe sizes smaller than or equal to 12 inches. The connection between Nominal Pipe Size (NPS) and Outer Diameter OD is not straightforward. The relationship with Inside Diameter (ID) is also not straightforward. In such cases, NPS falls between OD and ID.

It’s important to note this. NPS closely corresponds to the actual Inside Diameter (ID) for pipes ranging from 3 to 12 inches. But for very small pipes, this correlation doesn’t hold true.

Examples: NPS 2” Steel pipe, NPS 12” pipe, etc.

DN – Diameter Nominal or Nominal Diameter

In European and international contexts, pipe sizing is represented using DN, which stands for Diameter Nominal or Nominal Diameter. DN is a dimensionless representation of pipe size, but it is approximately close to the actual diameter in millimeters (mm).

The relationship between NPS and DN is straightforward:

DN = 25 * NPS

To convert NPS into DN, simply multiply the NPS value by 25. For example, NPS 14″ is equivalent to DN 350.

Wall Thickness of Pipe

When dealing with pipes, it’s essential to grasp the concept of wall thickness. The wall thickness of the pipe is indicated by Schedule Number (SCH)

Schedule Number (SCH)

The wall thickness of a pipe is denoted by its Schedule Number (SCH). This number is a critical factor in determining a pipe’s characteristics. The general rule is simple: the higher the Schedule Number, the thicker the wall of the pipe. For instance, an SCH 80 pipe will have a thicker wall compared to an SCH 40 pipe.

There are ten standard schedule numbers in use. The schedule numbers are SCH 10, SCH 20, SCH 30, and SCH 40. They also include SCH 60, SCH 80, SCH 100, SCH 120, SCH 140, and SCH 160. Yet, for pipes with a diameter of less than 8 inches, you’ll typically find SCH 40 and SCH 80. SCH 120 and SCH 160 are also common.

Examples:

• NPS 12’’ SCH 40 or DN 300 SCH 40
  • Precise Outside Diameter (OD) = 12.75 in (323.85 mm)
  • Wall thickness = 0.406 in (10.312 mm)
  • Inside Diameter (ID) = 12.75 – 0.406 = 11.938 in (303.2252 mm)
• NPS 14’’ SCH 40 or DN 350 SCH 40
  • Precise Outside Diameter (OD) = 14 in (355.6 mm)
  • Wall thickness = 0.437 in (11.1 mm)
  • Inside Diameter (ID) = 14 – 0.437 = 13.126 in (333.4004 mm)

IPS – Iron Pipe Size

Iron Pipe Size (IPS) is a historical system based on the inside diameter (ID) of the pipe. Although it was primarily used from the 19^th century to the mid-20^th century, it still finds applications in some industries. It’s worth noting that IPS is conceptually akin to the modern NPS system.

Pipe Thickness for IPS – Iron Pipe Size

Initially, IPS had three pipe wall thickness classifications:

• Standard (STD): Equivalent to SCH 40 for 1/8 to 10-inch pipes and indicates a 0.375-inch wall thickness for NPS 12” or larger pipes.
• Extra Strong (XS): Equivalent to SCH 80 for 1/8 to 8-inch pipes and indicates a 0.500-inch wall thickness for pipes greater than 8 inches.
• Double Extra Strong (XXS): Generally thicker than SCH 160.

To find the wall thickness, ASME B31.3 provides the next equation based on hoop stress:

t = PD/2S

Where:

• t = Thickness
• D = Outside Diameter (OD)
• P = Internal Pressure
• S = Hoop Stress

This equation offers a practical means to calculate the required wall thickness for various pipe applications.

Tube Size

Tube size is commonly described in terms of its actual outside diameters (OD). Additionally, wall thickness is typically indicated using the Birmingham wire gauge (BWG) number.

BWG numbers range from 24 to 7. BWG 24 denotes the thinnest tube wall. BWG 7 indicates the thickest.

In essence, as the BWG number increases, the wall thickness of the tube decreases.

Joint and fittings of Pipe and Tube

The techniques used to join pieces of pipe or tubing mainly depend on the thickness of the wall. They also partly depend on the properties of the material. Let’s explore the common techniques for different scenarios:

Thick-Walled Tubular Products

For thick-walled tubular products, several techniques are commonly used:

Screwed Fittings:

These involve externally threading the ends of the pipe, creating a tapered thread. The imperfect threads towards the end of the pipe make a tight joint when connected to the fitting. Polytetrafluoroethylene tape is wrapped around the threaded end to guarantee a good seal. It’s essential to note that screw fittings weaken the pipe wall. This makes a higher schedule number necessary compared to other joint types. These fittings are standardized for pipe sizes up to 12 inches. Yet, they are rarely used for pipes larger than 3 inches because of threading and handling difficulties.

Flange Fittings:

Pipes larger than about 2 inches are often connected using flanges or welding. Flanges are matching metal disks or rings bolted together, with a gasket compressed between them. Flanges can be attached to the pipe by screwing, welding, or brazing. A flange with no opening seals off a pipe. It’s known as a blind flange or blank flange.

Thin-Walled Tubing

For thin-walled tubing, different techniques are preferred:

Soldering: This method involves melting solder to join the ends of thin-walled tubing.

Compression or Flare Fittings: These fittings create a secure connection for thin-walled tubing.

Brittle Materials

Pipes made of brittle materials like glass, carbon, or cast iron are often connected using flanges or bell-and-spigot joints.

Welding Joints

Welding has become the standard method for joining large steel pipes in process piping, especially for high-pressure applications. It creates stronger joints than screw fittings. It doesn’t compromise the pipe wall’s integrity. This allows for the use of lighter pipes to handle the same pressure. Properly made welded joints are leakproof, which can be crucial in many applications. Still, it’s important to note that once a welded joint is made, it can’t be easily reopened without destroying it.

It’s worth noting that in the context of environmental protection legislation, flanged joints can emit volatile materials. Screwed joints are also considered potential sources of these emissions.

Allowing for Thermal Expansion in Piping Systems

Pipes in industrial applications often experience temperature fluctuations, and in certain scenarios, these temperature changes can be quite significant. Here is how thermal expansion impacts pipes and outlines the strategies employed to mitigate potential issues.

These temperature fluctuations result in the expansion and contraction of pipes. Rigidly securing the pipe to its supports can lead to it tearing loose, bending, or even breaking. In larger piping systems, engineers typically avoid fixed supports. Instead, engineers allow pipes to rest on rollers or suspend them from above using chains or rods. They use various techniques to prevent undue stress on fittings and valves in high-temperature lines. These approaches include incorporating bends or loops in the pipe. They also use packed expansion joints. Engineers use bellows or packless joints. In some cases, they use flexible metal hoses.

Conclusion

In conclusion, understanding the differences between pipes and tubes is crucial for selecting the appropriate material for various industrial applications. Pipes have thicker walls and larger diameters. They are typically used for transporting fluids under pressure. Tubes have thinner walls and precise dimensions. They are preferred for structural or fluid systems that need more specific tolerances. Industries make informed choices by considering factors like material, size, and joining ways. These choices guarantee efficiency, safety, and durability in their operations.

FAQ’s

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