Pipes and tubes are fundamental components in various industries, and understanding their difference is important. In this article, we’ll explore the key differences between pipes and tubes, including their construction, dimensions, and common applications.
Key Difference Between Pipe and Tube
| Aspect | Pipe | Tube |
| Wall Thickness | Heavier | Thinner |
| Diameter | Relatively larger then tube diameter | Relatively smaller than the pipe diameter |
| Length | Typically moderate length (20-40 feet) | Available in coils several hundred feet long. |
| Surface | Slightly rough | Very smooth |
| Threading | Metallic pipes can be threaded | Tubing usually cannot be threaded due to their thin wall |
| Joining Method | Screwed, flanged, or welded | Compression, flare, or soldered |
| Manufacturing | Welding, casting, or piercing | Extruded or cold-drawn |
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 represents 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. Notably, for standard pipe sizes larger than 12 inches in diameter, NPS is equal to the outside diameter (OD) of the pipe. However, for standard pipe sizes smaller than or equal to 12 inches, the relationship between Nominal Pipe Size (NPS), Outside Diameter (OD), and Inside Diameter (ID) is not straightforward. In such cases, NPS falls between OD and ID.
It’s important to note that 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. 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: SCH 10, SCH 20, SCH 30, SCH 40, SCH 60, SCH 80, SCH 100, SCH 120, SCH 140, and SCH 160. However, when it comes to pipes with a diameter of less than 8 inches, you’ll typically encounter only SCH 40, SCH 80, SCH 120, and SCH 160.
Examples:
- NPS 12’’ SCH 40 or DN 300 SCH 40
- 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
- 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 19th century to the mid-20th century, it still finds applications in some industries. It’s worth noting that IPS is conceptually similar 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 determine the wall thickness, ASME B31.3 provides the following 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 diameter (OD). Additionally, wall thickness is typically indicated using the Birmingham wire gauge (BWG) number.
BWG numbers range from 24 to 7, with BWG 24 denoting the thinnest tube wall, while 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 methods used to join pieces of pipe or tubing mainly depend on the thickness of the wall and 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 methods 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 ensure a tight joint when connected to the fitting. To guarantee a good seal, polytetrafluoroethylene tape is wrapped around the threaded end. It’s essential to note that screw fittings may weaken the pipe wall, making a higher schedule number necessary compared to other joint types. These fittings are standardized for pipe sizes up to 12 inches but are rarely used for pipes larger than 3 inches due to 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. When a flange has no opening and is used to seal off a pipe, it’s known as a blind flange or blank flange.
Thin-Walled Tubing
For thin-walled tubing, different methods 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 compared to screw fittings, and it doesn’t compromise the pipe wall’s integrity, allowing for the use of lighter pipes to handle the same pressure. Properly made welded joints are leakproof, which can be crucial in many applications. However, it’s important to note that once a welded joint is made, it cannot be easily reopened without destroying it.
It’s worth noting that in the context of environmental protection legislation, flanged and screwed joints are considered potential sources of volatile material 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 employ various methods to prevent undue stress on fittings and valves in high-temperature lines. These include incorporating bends or loops in the pipe, utilizing packed expansion joints, employing bellows or packless joints, and, in some cases, using flexible metal hoses.
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