Pipes & Tubes
Complete Guide to Pipes and Tubes
Seamless pipe and welded pipe are most common type of metal pipes. Nowadays, the most common material for pipes tends to be metal and, in particular, steel (either stainless or carbon steel). As a hollow tube, a pipe will have a round cross section so that it can be used later for various products such as powders, gas, pellets, and more. Although many use the terms interchangeably, the words ‘pipe’ and ‘tube’ do differ as tubing can be used for many different processes whilst pipes are generally used within a piping system. When we use the term ‘pipe’ here, we generally mean the ASME B36.10 Welded and Seamless Wrought Steel Pipe as well as the ASME B36.19 Steel Pipe.
Generally there are six types of tubular goods.
- Standardard Pipe: Ultimately, there are three different types of standard metal pipes - welded (ERW Pipe), seamless pipe, and galvanized pipe. Although schedule 40 steel pipes are the most popular for thickness, there are many different ranges for both the ERW and seamless pipes. When you are considering anything bigger than 12 inches, you will need to differentiate between the standard pipe and the true schedule 40 steel pipe as they will offer something different. They represent 10% of all tubular products.
- Line Pipe: Used primarily in Oil and Gas Applications. Line pipe includes ERW, FW, SAW and DSAW Pipe. They are manufactured to API 5L Specification and are available in X42, X50, X60 etc. grades. They represent 21% of all tubular products.
- Oil Country Tubular Goods (OCTG): This includes drill pipes, tubing and casing. It is used in drilling and completion of Oil and Gas wells. OCTG are produced by ERW and Seamless manufacturing. OCTG represent 35% of all tubular goods.
- Pressure Tubing: used for industrial and pressure application. Pressure tubing are produced using seamless manufacturing. They represent 2% of all tubular goods.
- Mechanical tubing: used for mechanical and structural application and is produdced by ERW and seamless manufacturing. They conform to ASTM specification. Mechanical Tubing represent 17% of all tubular goods.
- Structural Tubing: used for support or retention purpose. This tubing can be round or square and are produced by ERW manufacturing. They are used for fences, construction and other misc. support needs. They represent 15% of all tubing goods.
Which One? - When dealing in this area of the industry, many will ask whether they should use a pipe or a tube so let’s take a look at the key differences.
- With a pipe, nominal pipe size (NPS) is the key characteristic whilst schedule number (SCH) will be used to identify wall thickness. If the wall thickness is 2.77mm, an NPS of 1/2 and SCH of 40 will be around the same as a 21.3mm outside diameter.
- With a tube, its outer diameter will define it as opposed to NPS and there are two methods of determining the wall thickness - thousandths of an inch or Birmingham wire gage (BWG). If the wall thickness is 1.5mm, the outside diameter of 12.7mm can be found with 1/2” x 1.5.
Materials - Within a piping system, most engineering companies will employ engineers that work to determine the materials that should be used. Although it does depend on the role it will play, most pipe will be made from carbon steel which is manufactured to a certain set of standards - ASTM.
Over the years, the whole industry has realized that carbon-steel is not only strong but also machinable, weldable, durable, ductile, and generally cheaper than many other materials. As long as the manufacturer can ensure that it meets the required levels of temperature, corrosion, pressure, hygiene, and resistance, it will be the main choice.
Aside from this, iron is also a popular option and will be made from ductile- and cast-iron. For the most part, they will be used for sewage lines in addition to water and gas. If particularly corrosive or hazardous gases will be seen, plastic provides another option. Finally, you may also find a plethora of metals and alloys such as nickel, brass, copper, lead, aluminum, and more. However, these will be a more expensive option despite offering their own unique benefits. For example, they might add resistance to chemicals, they may be able to protect against heat, or they can excel at high temperatures. Recently, stainless steel has seen increased usage for food processing, instrument lines and heat transfer jobs but they still haven't reached the popular levels of copper alloys and copper.
ASTM Materials - Here, there are many different types of materials so let’s take a look;
A335 - In this specification, high-temperatures will be covered for seamless ferritic alloy-steel pipe.
A106 - In this specification, high temperature service will be the goal once again but this time with carbon steel pipe. For seamless pipe, Grade B within this specification is most common.
A312 - This time, general corrosive service is important alongside high temperature service for straight-seam welded pipe, seamless pipe, and any cold worked welded austenitic stainless steel pipe.
A333 - Finally, low temperatures will be the target for weldon carbon, wall seamless, and alloy steel pipe.
Line Pipe - When it comes to line pipe systems, often you will see many materials combined together to achieve the best results. In order to withstand chemical attacks, an applicable material will line a carbon steel pipe so it can handle corrosive fluids. Most commonly, this will be Teflon and it will be applied after the piping has been fabricated.
In addition to Teflon, many manufacturers choose to go for glass, concrete, plastics, and some even test coatings such as Zink, Epoxy, and Bituminous Asphalt. As we have seen already, the material has to be suitable for the job and this is normally the most important factor. Sometimes, the strongest material isn't the best option if it cannot handle what is required.
Nominal Pipe Size - When high or low pressures and temperatures are used, Nominal Pipe Size (NPS) will be the standard sizes used across North America. Essentially, this name comes from the Iron Pipe Size (IPS) which is its predecessor.
Initially designed to establish the pipe size, IPS used inches to approximately measure the inside diameter of a pipe. Therefore, if a pipe had an IPS of four, this means that the inside diameter of the pipe was around four inches long. Over time, the name was shortened and would come to be known as 4inch or 6inch pipes. At first, every single pipe was designed with exactly the same thickness which could be described as standard or standard weight (STD or STD.WT. respectively). After this, the outside diameter was the same for all sizes so a six inch schedule 40 steel pipe, for example, would see a 6.065 ID and a 6.625 OD in inches.
Over the years, the industrial requirements in this area have changed when dealing with high-pressure fluid and they now boast thicker walls which can be categorized as extra strong (XS) or extra heavy (XH). As the high-pressure requirements advanced even more, the double extra strong and heavy options were introduced but the outside diameters remained the same.
Pipe Schedule - At the very beginning of proceedings when IPS was used, there were only three thicknesses in use. At the end of the 1920s, however, a new system was created by the American Standards Association which saw smaller increments of sizes. Suddenly, the IPS system was gone and had been replaced by the NPS. In order to note the difference between the nominal wall thickness of pipe, ‘schedule’ was introduced as a term. Within this system, there are now a range of thicknesses including SCH 5, 10s, 40, 80, 140, STD, XXS, and many in between.
When using NPS, you have to realize that it is completely dimensionless and therefore shows a standard pipe size as long as there is a number that follows without the inch symbol. With this in mind, it is easy to see that NPS 6 shows a pipe that has an outside diameter of 168.3mm (6-7/16 inches). Although not completely accurate, there is a resemblance between NPS and the inside diameter in inches. When you get above 14 inches, the NPS will be roughly the same as the inch size.
Regardless of the NPS, the outside diameter will remain the same but the wall thickness will expand with a larger schedule number. For the inside diameter, this will ultimately depend on the wall thickness which is determined by the schedule number.
Conclusion - SCH and NPS, non-dimensional numbers, can be used to specify the size of a pipe. As these two combine, the inside diameter will be determined. Furthermore, ASME B36.19 can be used to determine the size of stainless steel pipes with the outside diameter and the schedule wall thickness. With these, an ’S’ suffix will be used and when this suffix is missing, the ASME B36.10 is followed for carbon steel pipes.
Different Types and Lengths of Pipes - When we say the term ‘pipe manufacturing’, it normally refers to the processes seen within the mill rather than how each piece connects to the end product for the pipeline. When a pipe is made in a mill, they will be a length or joint piece but there will also be pipes that ship as ‘double joints’ which is where two parts have been welded together already in an attempt to save time. When it comes to gas and oil pipelines, these will be longitudinally welded or seamless. If the diameter is larger than normal, a spirally welded pipe may also be an option. For steel pipes, there are four types - seamless pipe, spiral welded, electric resistance welded (ERW pipe), and longitudinally welded SAW.
Welded Pipe - As a tubular product, a welded pipe will be one that has been formed from flat plates (skelp) before then being bent and prepared. Nowadays, the longitudinal seam weld is the most common for pipes that have a large diameter. Alternatively, a spiral welded pipe can be used and this will allow for larger diameters to be made with thinner skelp. When defects occur in the spiral welded pipe, these mainly arise from the SAW weld.
ERW and HFI Pipe - To make ERW pipes, manufacturers would previously install a butt weld after using resistance heating. However, the majority of pipe mills have now switched to HFI since it offers more consistency and a better level of control. Even though the process has changed, ‘ERW’ is still a used term for it even when the weld is made using HFI.
Often, laminations and narrow weld line defects will be the most common within HFI and ERW pipes during the strip production. When the manufacturers attempt to fuse where there is insufficient pressure or heat, this causes defects and realignment of nonmetallic inclusions will also lead to hook cracks. After trimming, the weld line isn't visible which means that huge lengths of weld can be produced all with poor fusion as soon as the required parameters drop beneath the set limits.
In the early days, ERW pipe would also have another problem in that it wouldn't stand the pressure that it had done during the testing phase of production; this is known as pressure reversal. During the pressure testing, crack growth can occur from lack of fusion and poor weld line strength.
Seamless Pipe - Mandrel Mill - At around one to six inches in diameter, this process would be utilized for smaller seamless pipes. Before piercing, the steel is heated right up to 1,300 degrees centigrade and a mandrel will be enter the tube. After passing through the rolling mill, the thickness of wall will be reduced as rollers push down on either side. If the diameter needs to be reduced, it will then go through a second stage in a stretch-reducing multi-stand mill. After cutting the pipe to the particular length, it will be heated, straightened, inspected and then go through hydrostatic testing.
Seamless Pipe - Plug Mill - When the pipe is large than six inches and smaller than sixteen inches, this process is chosen instead and it will be heated at the same temperature as before. Using a rotary ‘elongator', the hole in the shell is widened and a bloom will occur - short thick-walled tube.
Forcing through the bloom, a plug the size of the diameter will enter and go through the rolls of the mill. As the thickness reduces, it will rotate 90 degrees before passing through again so it gains the round characteristic. Finally, it will go through a reeling and reducing mill so it evens out and reaches the desired dimensions. Then, it will be cut as before and then put through the finishing process.
Seamless Pipe - Extrusion - For tiny pipes, this will be the third and final process used and it is heated to a little under the previous method at 1,250 degrees centigrade. After this, it is sized and descaled before going through a steel die. By using a multi-stand reducing mill, the final dimensions can be achieved.
Length - Although piping lengths are not exact from the factory, there are two main sizes in single random length and double random length. For the single, this will be between 5-7 meters whilst the double makes up 11-13 meters. Although different lengths can be achieved, these ones are often cheaper and easier to calculate.
Ends - In terms of ends, there are three main options in threaded ends (TE), beveled ends (BE), and plain ends (PE). For smaller pipes, PE pipes are appropriate and are commonly used with socket weld fittings/flanges as well as slip-on flanges. As the name suggests, TE will be used for small pipes using threaded fittings and flanges.
Beveled and Root Face - Finally, BE can be used with a wide range of diameters of buttweld fitting and flanges. Using this, they can be welded directly to one another or to the pipe itself. Plus or minus five degrees, the beveled angle will be at around 30 with a 1.6mm root face.