What is a Pipe?
A pipe refers to a tubular component used for the conveyance of fluids or gases. It is a fundamental part of a piping system and plays a crucial role in various industries, including oil and gas, petrochemical, power generation, and others.
In piping engineering, pipes are designed, selected, and installed to transport fluids or gases from one location to another within a facility or between different facilities. These pipes are typically made of materials such as steel, stainless steel, carbon steel, copper, or plastic, depending on the specific requirements of the application.
Pipes are available in various sizes (designated by Nominal Bore size- NPS), schedules (wall thicknesses), and types. The size and schedule of a pipe are determined based on factors such as the flow rate, pressure, temperature, and the properties of the fluid or gas being transported. The type of pipe chosen depends on factors such as the nature of the fluid or gas, the operating conditions, and any applicable codes and standards.
What is Nominal Pipe Size (NPS)?
Nominal Pipe Size (NPS) is a standard designation used to indicate the approximate inside diameter (ID) of a pipe used in piping systems. It is a numerical value that does not represent the actual dimensions of the pipe but provides a convenient reference for pipe sizing.
The NPS system is primarily used in North America and is based on the inch-pound units of measurement. It is widely accepted and standardized by various organizations, including the American National Standards Institute (ANSI) and the American Society of Mechanical Engineers (ASME).
The NPS value does not directly correspond to the actual physical dimensions of the pipe. Instead, it represents a standardized size based on historical conventions and compatibility with fittings and other components. The outside diameter (OD) and wall thickness of the pipe vary depending on the specific schedule (wall thickness) associated with the NPS value.
For example, an NPS 2 pipe does not have an actual inside diameter of 2 inches but rather an inside diameter that corresponds to the specified wall thickness for that particular NPS and schedule. The actual outside diameter and wall thickness are determined by the pipe schedule, such as Schedule 40, Schedule 80, and so on.
What is Pipe Nominal Diameter (DN)?
Pipe Nominal Diameter (DN) is a metric-based standard designation (same as NPS but in mm) used to indicate the approximate inside diameter (ID) of a pipe used in piping systems. It is widely used in many countries and is based on the International System of Units (SI) measurement system.
The DN system provides a standardized way to classify and specify the size of pipes based on their inside diameter. The DN value does not represent the actual physical dimensions of the pipe but provides a convenient reference for pipe sizing.
In the DN system, the size of a pipe is specified using a numerical value followed by the letters “DN.” For example, DN 50 represents a pipe with a nominal diameter of 50 millimeters.
Unlike the NPS system, which has a non-linear relationship between the nominal size and the actual dimensions of the pipe, the DN system is more closely related to the actual inside diameter. The DN value corresponds to the approximate inside diameter of the pipe, which may have slight variations depending on the specific pipe standard and manufacturing tolerances.
The DN system is commonly used in Europe, Asia, and other regions that follow metric measurements. It is standardized by organizations such as the International Organization for Standardization (ISO) and various national standards bodies.
Pipe Schedule (Wall thickness)
In piping engineering, the schedule is a system used to indicate the wall thickness of pipes. It provides a standardized way of specifying the thickness for different pipe sizes. The schedule number is typically represented by an integer value and is associated with a specific wall thickness.
For carbon steel pipes, the schedule numbers commonly used are: 5, 10, 20, 30, 40, 60, 80, 100, 120, and 160. These numbers represent the approximate wall thickness of the pipe in relation to the nominal diameter. For example, schedule 40 has a thicker wall compared to schedule 10 for the same nominal diameter.
For stainless steel pipes, a slightly different schedule system is used, typically represented by the letters “S” following a numerical value. The commonly used stainless steel schedules are: 5S, 10S, 20S, 30S, 40S, 60S, and 80S. These schedules also indicate different wall thicknesses corresponding to the nominal diameter.
The weight series for pipe wall thickness includes:
- STD (Standard): This represents the standard wall thickness for a given pipe size.
- XS (Extra Strong): XS pipes have a thicker wall compared to STD pipes, providing higher strength.
- XXS (Double Extra Strong): XXS pipes have an even thicker wall compared to XS pipes, offering the highest level of strength.
Pipe Manufacturing methods
There are several methods used for manufacturing pipes, including seamless processes and welded processes. Here are some common manufacturing methods:
- Seamless: In the seamless manufacturing method, pipes are produced without any welding or joining processes. The seamless pipes are typically created through hot rolling or cold drawing processes.
- Hot Rolled: This method involves heating a solid cylindrical billet and then rolling it to form a seamless pipe. The hot rolling process results in a rough outer surface.
- Cold Drawn: In this method, a solid cylindrical billet is pulled through a die to reduce its diameter and form a seamless pipe. Cold drawing provides a smoother surface finish compared to hot rolling.
- Electric Resistance Welding (ERW): This method involves the use of electric current and pressure to create a weld between two pipe edges. In ERW, no filler material is added during the welding process. The edges of the pipes are heated and fused together using an electric current, resulting in a welded seam along the length of the pipe.
- Electric Fusion Welding: In this method, also known as electric fusion or fusion welding, the edges of the pipe are heated and fused together using an electric arc. Unlike ERW, filler material is added during the welding process to facilitate the fusion of the pipe edges. Electric fusion welding is commonly used for larger diameter pipes.
Piping Codes & Standards
- ASME B31.1 – Power Piping: This code specifically addresses the requirements for power plant piping systems, including those in thermal and nuclear power plants.
- ASME B31.2 – Fuel Gas Piping: This standard covers the design, materials, fabrication, installation, inspection, testing, and safety aspects of fuel gas piping systems.
- ASME B31.3 – Process Piping: This widely used code covers the design, materials, fabrication, installation, inspection, examination, and testing of process piping systems typically found in chemical, petroleum, and similar industries.
- ASME B31.4 – Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids: This standard focuses on the design, construction, operation, maintenance, and inspection of liquid pipeline systems used for transportation within the petroleum industry.
- ASME B31.5 – Refrigeration Piping: This code applies to refrigeration systems, covering the design, materials, fabrication, installation, and testing of piping used for refrigerants in commercial and industrial applications.
- ASME B31.8 – Gas Transmission and Distribution Piping Systems: This standard provides guidelines for the design, construction, operation, and maintenance of natural gas and other fuel gas transmission and distribution systems.
- ASME B31.9 – Building Services Piping: This code covers the design, construction, operation, and maintenance of piping systems within buildings for various services such as heating, cooling, and plumbing.
- ASME B31.11 – Slurry Transportation Piping Systems: This standard focuses on the design, construction, operation, and maintenance of piping systems used for the transportation of solid-liquid mixtures, known as slurries.
These ASME codes and standards provide comprehensive guidelines and requirements to ensure the safe and reliable design and installation of various types of piping systems in different industries and applications.
Pipe vs Tube
| Feature | Pipes | Tubes |
|---|---|---|
| Shape | Generally round | Can be round, square, rectangular, or other shapes |
| Size | Designated by nominal pipe size (NPS) | Designated by outside diameter (OD) or dimensions |
| Wall Thickness | Usually thicker walls | Can have thin or thick walls, depending on application |
| Manufacturing | Seamless or welded | Seamless, welded, or extruded |
| Applications | Conveyance of fluids or gases | Structural applications, heat exchangers, mechanical uses |
| Strength | Generally stronger and more rigid | Varies depending on size, material, and wall thickness |
| Surface Finish | Rougher surface finish due to manufacturing processes | Smoother surface finish, especially in seamless tubes |
| Tolerances | Less stringent tolerances for dimensions and roundness | More stringent tolerances for dimensions and roundness |
| Cost | Typically less expensive | Often more expensive due to manufacturing processes |
