A steel pipe is a cylindrical tube made from steel and is the most utilized product in the steel industry. The primary use of steel pipes is in the transport of products—including oil, gas, and water over long distances. Common household appliances such as fridges use steel pipes, as well as heating and plumbing systems. Steel pipes come in a variety of sizes and can also be used for structural requirements like hand rails and pipe bollards.
William Murdoch is thought to be the pioneer of steel pipes, when he joined barrels of muskets together to invent a coal lamp burning system in 1815. Murdoch used his innovative piping system to transport coal gas to individual lamps on the streets of London.
Since the 1800’s great strides have been made in the technology of steel pipes including improving manufacturing methods, developing applications for their use, and the establishment of regulations and standards that govern their certification.
How are steel pipes made?
Whether the pipe is produced by welded or seamless methods, the first step for either process is to convert raw steel to a more usable form.
Raw steel is produced by melting raw materials in a furnace and controlling the composition through the addition of alloys and removal of impurities. Molten steel is poured into molds to make ingots, or transferred to a continuous casting machine to make plate, billets, and blooms. This raw steel must be converted to a form that can be rolled into pipes of specific dimensions or stretched into a hollow tube.
There are two main intermediate steel products that are used to make steel pipes:
- Steel skelp is made from slabs that are heated to 2,200˚F. The heat causes a scale to form on the surface, which must be removed through a scale breaker and high pressure cleaning. Once cleaned, the steel slab is hot rolled into thin, narrow strips of steel called skelp. Skelp is pickled (surface cleaned) with sulfuric acid, washed with water and rolled onto large spools as the raw material for pipe making. The width of the skelp determines the diameter of the pipe that can be made.
- Steel billets can be produced directly from a continuous casting machine, or as a secondary product from cast ingots. The ingot is rolled and stretched through grooved and stacked rollers called “two high mills” to form blooms. To convert a bloom to a billet requires another sequence of rolling and stretching to form a solid billet with the required dimensions for the pipe specifications.
Skelp is unwound from the spool, heated, and rolled through grooved rollers, which bend the edges of the skelp upwards. This process produces a cylindrical tube where the two edges have been bent right around to meet each other forming a long cylinder. A welding process joins the edges together and seals the pipe.
- Continuous welding
In a continuous welding operation, welding rollers press the edges of the pipe into each other—forming a forged weld due to the heat that has already been applied to the skelp. No metal is added during welding, and the final rollers reduce the diameter and wall thickness of the pipe to the specifications.
- Electric resistance welding
Electric resistance welding follows a similar process to continuous welding, except that the skelp is cold rolled into the pipe shape. Current is supplied to the pipe edges by revolving copper disks, which heat the edges up to the weld temperature. Welding rollers join the pipe edges to create the forged weld.
- Spiral welding
Spiral welding and double submerged arc welding use more conventional welding techniques and the addition of weld material to form the bond.
The equipment and infrastructure required to produce vast quantities of welded steel pipe is significant, as depicted in this video look inside an oil pipe manufacturing plant.
Seamless pipes are produced without the need for welding. They are created by heating and stretching a solid steel billet, before piercing it to form the hollow center. Billets are heated to extreme temperatures making them white hot and then rolled under high pressure causing the billet to stretch out. A bullet shaped piercer is used to make the hollow center regular according to its dimensions. A series of milling operations follow to conform the pipe to the required specifications.
A fantastic overview of the entire seamless pipe manufacturing process—from steelmaking to the finishing processes—can be seen in this seamless steel tubes production process video.
Pipes may be put through a straightening machine as a final process step before being fitted with joints at the end. Small bore piping is usually fitted with threaded joints, but larger bore piping is normally fitted with flanges which are welded onto the end of the pipe. Measuring machines check the dimensions of the finished pipe, and stamp the details on the side of the pipe for quality control purposes.
How are steel pipes used?
There are two main categories of applications for steel pipes: structural use and transport use. All pipes are sized according to their outer diameter, while the inner diameter will vary depending on the wall thickness.
Structural uses of steel pipes can be found in building and construction. These pipes are also commonly referred to as tubes.
Steel pipes are used in construction to provide additional strength to foundations in a process called piling. Steel tubes are driven deep into the earth before the foundation is laid to provide stability for a high building, or construction on ground that is not secure.
There are two fundamental types of pile foundations:
- End bearing piles have the bottom end resting on a layer of especially strong soil or rock. The load of the building is transferred through the pile onto the strong layer.
- Friction piles transfer the load of the building to the soil across the full height of the pile, by friction. The entire surface of the pile helps to transfer the forces to the soil.
Guard rails are also made from steel tubes, creating an aesthetically pleasing safety feature to stairs and balconies.
Scaffolding poles are made from steel tubes with a design that enables them to be connected in a pattern. This allows construction workers to access areas of the building that are out of reach and high above ground level.
The most common use of steel pipes is for the transport of products. The material is well suited for long-term installations. It can be buried underground due to its hardiness and resistance to breakdown.
Low pressure applications do not require steel pipes to have high strength as they are not exposed to significant stresses—these product pipes will have a low wall thickness and will be cheaper to manufacture. Some applications even operate at atmospheric conditions (sewers and gravity fed water systems). More specialized applications—such as pipes used in the oil and gas industry—require more stringent specifications. The hazardous nature of the product being transported demands a high strength characteristic, and therefore higher wall thickness, with its associated higher costs. Specifications for these applications are strictly enforced and quality control and documentation forms a critical part of the manufacturing process.
How are steel pipes characterized?
There can be confusion about the way steel pipes are specified and the exact characteristics of the pipe based on these specifications. The American Society for Testing and Materials (ASTM) along with The American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API) are some of the most referenced organizations for piping specifications.
Specifications can be broken down into three main categories:
Steel pipe sizes
Pipe size is quoted as a “Nominal Pipe Size” or NPS. The origin of the NPS numbers for smaller pipes (< NPS 12) is different to the origin for larger diameter pipes. However, all pipes of a specific NPS number have the same external or outer diameter (OD). The internal diameter will vary depending on the wall thickness of the metal. The reason for this is so that the same structural supports can be used for all piping of a specific NPS number regardless of the wall thickness.
Steel pipe schedules are a way to describe the wall thickness of the pipe. This is a critical parameter as it is directly related to the strength of the pipe and the suitability for specific applications. A pipe schedule is a dimensionless number and is calculated based on the design formula for wall thickness, given the design pressure and allowable stress.
Examples of schedule numbers are as follows: 5, 5S, 10, 20, 30, 40, 50, 60, 80, 100, 120, 140, 160, STD, XS, and XXS—with the most common being schedules 40 and 80. As the schedule number increases, the wall thickness of the pipe increases. The schedule number of a pipe therefore defines the internal diameter, as the OD is fixed by the NPS number.
The weight of a pipe can be calculated based on the NPS which defines the outer diameter, and the schedule which defines the wall thickness. The formula uses the theoretical weight of steel of 40.8 pounds per square foot per 1 inch of thickness to determine the constant.
W = 10.69 x t (OD – t)
W = weight (in pounds per foot)
OD = outer diameter
t = thickness
The following table from Engineering Toolbox shows the measurements of OD, wall thickness, and weight for pipes of different NPS. Both schedule 40 and schedule 80 measurements are shown.
Based on ASTM A53 – Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless.
Certification of steel pipes
Manufacturers issue a Material Test Report or Mill Test Report to validate that the product meets the chemical analysis and mechanical properties specification. The MTR will contain all relevant data to the product and will accompany the product through its lifecycle.
The following are typical parameters that may be recorded on an MTR:
- Chemical composition including carbon content, alloys, and sulfur
- Material size, weight, identification, and grade
- Material heat number, which ties back to the processing batch
- Mechanical properties like tensile strength, yield strength, and elongation
For steel bollards, the most common specifications cited are ASTM A53 and ASTM A500.
Steel pipe bollards
Reliance Foundry supplies pipe bollards that are made from steel pipes. Bollards are vertical pipe lengths installed in the ground to protect people, buildings, and surrounding infrastructure from vehicle collisions.
Steel pipe bollards must conform to safety specifications to ensure they are strong enough to resist the impact of vehicle collisions. Schedule 40 and schedule 80 steel can be used to make steel pipe bollards depending on the application.
Steel pipe bollards are often covered with stainless steel, plastic, or other metal decorative covers for aesthetic appeal and to protect the steel pipe from corrosion.