When describing or ordering specific steel pipe dimensions in a factory, doubts or complications can arise. There may also be misunderstandings, since, although there are established measures and a code for them, they may not be known or may be expressed in a different way.
If we buy the wrong size tubes, many problems can occur later. The best thing to do first is to find out about the dimensional characteristics of a pipe and the types of pipes available. This is the subject we will be discussing today. Finally, we will also look at how to describe the dimensions of these tubes correctly.
How is a steel pipe made?
Pipes are basically divided into 2 types: welded and seamless. Although in our blog we have already explained how seamless pipes were manufactured, that is to say, without welding, let’s refresh a little the process of creation of these 2 types of pipes.
Seamless tubes are manufactured by melting metal and introducing it into a mould with the shape and size of the part we want to create (in this case a tube). After this step, a die is introduced to create the internal hole of the part (the extrusion). The striker inserted into the hole presses the walls of the tube against the mould and creates the internal cavity. This type of seamless pipes are highly valued and are widely used in sectors that need very resistant parts to the internal pressure that they can carry or contain (such as gas or oil).
Welded steel pipes involve a much more traditional process. The sheet material has to be rolled, folded and cut into the shape of a tube with cylinders that mould the sheet. When the desired shape is achieved, the joint is closed with a weld bead.
How to understand the dimensions of steel pipes
Before going completely into the subject, it is necessary to know certain technical specifications in order to understand the measurement system of a steel pipe. The 3 basic things to be clear about are the outside diameter (OD or Outer Diameter , wall thickness (WT) and the length we want the tube to be according to the use we are going to make of it (the most normal measurements are 6 or 12 metres). Each of these factors will be explained below.
With this data, the weight of the pipe, the pressure it can withstand and the cost of manufacturing it (due to its length) can be calculated. When selecting a steel pipe, these are the key points to consider. Therefore, these are the characteristics to be aware of when selecting the correct size tube.
Steel pipe dimensions: standard and norms
There are several standards to describe the size of a steel pipe, the OD and the WT. These are set out in a number of standards, 2 in particular: ASME B 36.10 and ASME B 36.19. Both are suitable for the dimensions of steel pipes and fittings.
ASME Standard B36.10M. This standard refers to and standardises steel tube dimensions and sizes, either with or without seams or used at low or high pressures and/or temperatures. This standard indicates that the outside diameter of the pipe has to be less than 32.385 cm (NPS 12, DN 300). Therefore, the actual pipe diameter would be larger than the nominal pipe size (NPS) or nominal diameter (DN).
Pipe sizes smaller than NPS 12 (DN 300) have a different OD due, firstly, to the standardisation and norms used in the USA. There, all pipe sizes are expressed in NPS. Secondly, this condition is also due to the fact that these sizes are determined by the OD standard. Although it was established to determine the size of the pipe, it should be borne in mind that pipes will always have a wall thickness and internal diameter that will vary in size. In a small tube, for example, there will be a difference in size between OD and ID (wall thickness). However, even if the pipe dimension is larger, the OD and ID will hardly vary in size because there is no direct relationship between the old thickness standard (OD) and the nominal size. That is why ASME B 36.10 M accepts both points.
In ASME B36.10M, pipe sizes are identified by a standard (extra strong (XS), double extra strong (XSS)) or by pipe numbers (5, 10, 20, 30, 40, 80, 120, 160).
ASME Standard B36.19M. This second standard is specifically tailored to the dimensions of stainless steel tubes, whether welded or seamless. The standard size is almost the same as ASTM B36.10M. However, it has some differences. As the wall thickness (thickness) of the tubes is different from B36.10M, the suffix “S” is added:
– For NPS 14 to NPS 22 (DN 350-550), schedule 10S.
- NPS 12 of Schedule 40S.
- NPS 10 and 12 of Schedule 80S.
What is the Schedule?
The Schedule is a method for classifying the dimensions of a steel tube. It is represented as ‘Sch’ (short for Schedule) and is set out in ASME B 36.10. is used as numbering in the tube series. For example, if we find Sch 80, i.e. schedule 80, the number refers to a table or chart within standard B 36.10.
Depending on the use of the pipe and the material it is made of (steel, iron…), its internal diameter will be of one size or another to withstand the pressure. This is calculated on the basis of the nominal diameter. If the pipe is made to carry liquids under pressure, it must have sufficient strength, toughness and wall thickness to contain it. This level of thickness is the code and identifies the pipe. There is a formula for knowing this measure:
Sch.=P/[ó]t×1000
P is the designated pressure, expressed in MPa. The term ‘[ó]x’ is the fatigue of the material under the designated temperature (thermal fatigue), expressed in MPa.
To describe the parameter of a steel tube, the steel pipe is usually described by the steel pipe dimension. It is a method that expresses the wall thickness of the pipe as a figure. However, the tube cross-section (SCH) is not the same as the wall thickness: it represents the tube within a series of thicknesses. Each schedule means a different wall thickness in pipes of the same diameter. The higher the number in the table, the higher the pressure resistance.
The most common sizes are 40 and 80, as these are the most commonly used by the main industries. As they are capable of withstanding quite high pressure, they are often purchased in large quantities due to their practicality and price. We have already talked about the fluctuations in steel prices, so always be aware of market fluctuations and buy wisely.
The tubes with this profile can be manufactured in many types of materials: stainless steel, carbon steel, stainless steel… Depending on what you need, a piece can be cut to the size and material you require, always with the help of professionals in the sector.
What is Nominal Pipe Size (NPS)?
Nominal Pipe Size (NPS) is a North American standard measurement for pipes used at high or low pressures and temperatures. Two non-dimensional numbers are used: a nominal size (NPS) expressed in inches, and a schedule (Schedule or Sch).
What is the Nominal Diameter (DN)?
The nominal diameter is, in other words, the outside diameter. As the tube wall itself is quite thin, the outer and inner diameter are almost the same. The average value of the two is used to determine the tube diameter. The DN is an interchangeable value, as the nominal diameter of the pipe and its fittings can be interconnected. The fact that it has a value almost equal to the inside diameter of the pipe does not mean that it is the actual diameter of the pipe.
The nominal diameter value is represented by the abbreviation ‘DN’ together with the unit in millimetres. A DN50 pipe would therefore have a nominal diameter of 50 mm. If you ever come across a measurement in inches, we leave you with a conversion table that converts them into DN (bearing in mind that 1 inch = 25.4 mm).
How to describe the size and dimensions of steel pipes
The best we can do is to use an example again to learn how to describe these characteristics of a tube.
Tube outside diameter x wall thickness: Φ 88.9mm x 5.49mm (3 1/2″ x 0.216″). 114.3mm x 6.02mm (4 1/2″ x 0.237″), 6m or 12m length, single or double random length.
We hope this guide to understanding the parts of a pipe and choosing pipe dimensions will help you the next time you need to choose a pipe size for your project. If you need assistance or are looking for a metal cutting company (iron, steel, copper, brass, titanium, aluminium) at Initube we are specialists in cutting and machining tubes and profiles, with more than 40 years of experience in the metallurgical sector.
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