Introduction
When working with pipes, one has to deal with certain dimensional specifications. Over 60 pipe sizes are in use across different industries. Laypersons and technicians alike need to know about pipe size standards. Commonly used parameters in pipe sizes are outside diameter, inside diameter, and nominal pipe size. An understanding of these three measurements is crucial when working with pipes.
In this article, we will not only discuss these terms but also provide you with a comprehensive explanation of how to understand pipe reference charts. We will also discuss the Nominal Pipe Size standard and explain its application in the industry.
Pipes vs Tubes
Pipes are round cylindrical objects primarily meant for transporting liquid or gaseous materials. Tubes, on the other hand, come in a variety of shapes. They can be oval, circular, cubical, or cuboidal shapes. Tubes primarily support structures or frame components.
Both pipes and tubes are made from steel, copper, PVC, or PEX. While pipes are classified by OD, ID, and nominal size, tubes are recognized primarily by their OD. In terms of construction, tubes are more rigid than pipes and are costlier to produce.
What does OD (Outside Diameter) mean?
As the name suggests, the outside diameter of a pipe or tube is the length of its diameter measured from one external surface to the other. To measure the outside diameter, read the distance between the cross-section of the pipe as a whole.
The outside diameter is a critical component in pipe selection because it determines the volume occupied by the pipe. It is important in indoor plumbing where space restriction is a concern. The outside diameter of a pipe is a crucial factor in several industries, such as household piping, chemical plants, oil and gas, and irrigation.
What is ID (Inside Diameter)?
The inside diameter of a pipe is the diametrical distance between its inside surface. Unlike the outside diameter, the pipe thickness is not taken into account. Subtracting the outside diameter from the inside diameter gives the width of the pipe wall.
The inside diameter is a significant parameter that determines the pipe’s capacity. It represents the cross-sectional area available for the flow of liquid or gas. A pipe’s ID is used for its water-flow rate calculations. Consequently, it is a critical parameter for pipe capacity in the manufacturing industry.
What is Nominal Pipe Size (NPS)?
The Nominal Pipe Size is a piping standard that originated in North America. It provides a method of denoting the size of different pipes. It is often mistakenly called National Pipe Size or National Pipe Thread, but these are distinct concepts. It is a dimensionless number that corresponds to a pipe’s OD. It does not directly equate to the OD of the pipe – for example, a pipe with NPS 2” has 2.375” OD, and a pipe with NPS 6” has 6.625” OD. This disparity occurred due to the evolution of different types of pipes. As the availability of discrete pipe dimensions increased, it became necessary to distinguish the NPS number from the inside diameter according to the wall thickness standard.
The outside diameter of a pipe can be determined by looking up its NPS value and schedule on the reference table. The NPS values between 1/8 and 12 do not directly correspond to the outside diameter. From NPS 14 onwards, they begin to align correctly. For example, an NPS 16 pipe will have an outside diameter of 16” or 406mm.
The NPS system standardizes pipe dimensions across the industry. Manufacturers worldwide use NPS as a reference point for making pipes. Structural engineers, civil engineers, and technicians use NPS to convey information and design pipelines. NPS also plays a crucial role in standardizing the manufacturing of piping components like valves, flanges, and other fittings.
What is the pipe schedule?
Like the NPS, the pipe schedule is a dimensionless quantity that denotes the thickness of a pipe’s walls. Pipe reference charts use NPS and schedule numbers as the two parameters to determine the dimensions of pipes. A higher schedule value corresponds to a higher wall thickness and greater pressure rating. At the same time, the schedule value is inversely proportional to the pipe’s capacity since a thinner pipe can carry more material than a thick pipe for the same OD.
Schedules 30, 40, 80, and 120 are some typical pipe schedules. The thickness of pipe walls does not remain constant within a particular schedule. For instance, a schedule 30 pipe with NPS 1 will have a wall thickness of 2.86mm, but a schedule 30 pipe with NPS 2 will have a wall thickness of 3.17mm. As the schedule increases, the wall thickness also increases. At schedule 40, an NPS 6 pipe will have a wall thickness of 7.112mm, while an NPS 9 pipe will have a wall thickness of 8.687mm.
How to choose the correct pipe size for your application
Some crucial aspects of pipe size evaluation are as follows:
- Consider how much water will pass through the pipe, also called the flow rate.
- The cross-sectional area is a vital parameter in pipe flow calculations. Ensure that you have ample data on pipe diameter.
- In smaller pipes, friction plays a significant role in retarding the flow of water, a key consideration in domestic setups.
- The length of the pipeline is another critical factor, not only because it requires more resources but because water pressure reduces as it travels.
Based on the above considerations, you can plan out the piping technique using a few methods. The Fixture Unit method is suitable for residential buildings but not apt for complex applications. The Water Velocity method requires more calculations but is more versatile. The Pressure Drop method requires significant preparation. It is implemented in industrial pipe networks.
Conclusion
Piping solutions today have become somewhat complex, but they are a fundamental part of many industries. A thorough understanding of pipe sizes can enable laymen, technicians, and engineers to contribute effectively to the project. As one of the fundamental components of construction and fluid transportation, one cannot overstate the importance of OD, ID, and NPS. We invite you to contact our team of experts for guidance on your preferred piping solution.