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Until you come across the choice of a pipe, it seems that there is nothing complicated about it. After all, there is a size, according to which it is necessary to select the material. So it is, but you can measure something both inside and outside. So what is the correct measurement? Oddly enough, both. There are standards according to which pipes are marked according to the outer size, there are ones in which the inner size is indicated. And the diameters of steel pipes are only in a certain set. So with the same outer diameter, we can have a pipe with different throughput and operating at different pressures.
Understanding the terminology
In our country, the metric number system. And we measure everything in meters, centimeters and millimeters. Traditionally, the diameter of steel pipes is measured in millimeters. But the pipe has two diameters – inner and outer, and is also characterized by wall thickness. So what diameter are we talking about when talking about the size of steel pipes? Depends on the standard to which they are made. In some cases, this refers to the outer diameter, in others, the inner. This is how difficult it is.
Nominal and conditional diameter
Pipes are used at different pressures. For a higher one, greater strength is required and it gets due to the wall thickness. In this case, the outer diameter of the pipe is left fixed. Otherwise, it will not be possible to connect the segments, there will be difficulties with threads, fittings, etc. So the outer diameter is only an external parameter. Therefore, such a concept as a conditional pass was introduced. In general, this is an outdated name and, according to modern standards, they say “nominal diameter”.
Nominal pipe passage (nominal diameter) is a calculated value that is calculated during design. This value roughly corresponds to the inner diameter of the pipe in millimeters. Approximately, because the walls, with an equal outer size, are of different thicknesses. And this means that the clearance is changing. In order to somehow coordinate all this, they introduced the nominal diameter. This is a specific list of quantities defined by GOST 28338-89. They are shown in the table. Actual dimensions are rounded to the nearest nominal.
The designation of this value is DN, sometimes they put the Russian version – Dn. After these letters there is some number without metric signs: DN30 or DN150. This is read as a nominal pipe diameter of 30 or 150, or a nominal diameter of 30 and 150. There are no units of measurement, since this is a conditional value.
Once again: all existing elements of water and gas pipeline systems are marked in accordance with the list of standard values - nominal diameters Dn. The actual size of the internal section of the pipe or fittings may be larger or smaller. It is rounded up to the nearest normative value.
In fact, the conditional passage of the pipe or the nominal size is a value that displays the throughput. It is approximately equal to the inner diameter. After all, when you assemble a system, you use elements from different materials, with different wall thicknesses. Therefore, it is more reasonable to focus not on the diameters of the inner part, but on the conditional passage. This will make it possible to ensure the same throughput of all elements of the system.
Diameters of steel pipes according to the standard 3262-75
This document describes water and gas pipes (the abbreviation VGP is common). They can be made of galvanized steel and ordinary. They can have a thread – short or long, while the edge should be even, with a small amount of burrs (5%). The edge of a pipe without a thread can be smooth or designed for welding – with a chamfer of 35-45 °.
Steel pipes manufactured according to the GOST 3262-75 standard, with an equal outer diameter, can have three wall thicknesses. Depending on the wall thickness, the pipe is called:
- lightweight (the thinnest wall);
- ordinary (medium in thickness);
- reinforced (the thickest).
With the same outer diameter and different wall thicknesses, the inner diameter changes. But in order not to be confused, they use the concept of nominal diameter or nominal diameter. It used to be designated as Du, now it is correct to write DN. Sometimes it is considered that the conditional passage is equal to the inner diameter. Sometimes this is so, but not always. Rather, it rarely happens. More often, the actual internal section of the pipe is different – the walls of different thicknesses. How to control the quality of the product? According to the wall thickness and outer diameter.
How to check the quality
Since the internal clearance can be different, it is necessary to control the external dimensions and wall thickness. More important is the thickness. It must be measured at different points, from both ends of the pipe. Approximately this parameter can be tracked by the mass of a running meter.
By the way, deviations in wall thickness can be considerable. The standard allows deviations both up and down. If the diameter of the steel pipes is not more than 40 mm, the permitted deviations are up to 0,4 mm upwards and up to 0,5 mm downwards. There are two categories of pipe production: conventional and high precision. For products of increased accuracy, deviations in the direction of reducing the wall thickness are slightly less.
Examples of designations for pipes that are manufactured in accordance with this standard: pipe 20 * 2,8 GOST 3262-75. It should be read as a pipe with a standard diameter of 20 and a wall thickness of 2,8 mm. According to the table, it can be determined that this is a pipe of ordinary strength with an outer diameter of 26,8 mm. There you can also find the approximate mass of a meter, depending on the thickness of the walls.
How to deal with import symbols
There are not only products of domestic manufacturers on the market. There are pipes marked according to the American system. To begin with, they distinguish between two types of pipes: pipes and tubes. Both words are translated as a pipe, but they are intended for different systems and the requirements for them are different.
Pipes
A type Pipes — electrowelded and seamless. They are designed to transport liquids and gases. So this is just the type that can be used in our heating and plumbing systems. The main characteristic of pipes type pipes – inner diameter. There are two strength standards in this group, which determine the wall thickness and working pressure.
- Schedule 40 or standard. The designation may contain st (as in the figure below). These are products with a standard wall thickness.
- Schedule 80 or extra heavy. The designation is EX. It is a material for use in high pressure pipelines.
As you understand, with an equal outer size, the clearance will be different. Consider, for example, a two-inch pipe. It is designated as NPS = 2 ″ inner diameter in different versions is different:
- standard (standard) schedule 40 – 2.067 inches (which is approximately equal to 5,25 cm);
- extra heavy (heavy) schedule 80 – 1.939 inches (approximately 4,925 cm).
These categories standardize the wall thickness and the maximum working pressure. The outer diameter remains constant, while the actual inner diameter changes with the wall thickness. That is, NPS=2″ describes the inner diameter, which will be about two inches, but will vary with wall thickness. Here the situation is similar to our standards: there is a certain list of values to which the actual parameters are rounded when marking. Once again: if we are talking about a two-inch pipe of the Pipe type (NPS is in the marking), you need to understand that we are talking about the inner diameter, but exactly two inches will not be there. It will either be a little more or a little less. Similarly, with other sizes in inches.
Type Tubes
The word Tubes refers to pipes that are marked by outer diameter. The internal will depend on the wall thickness. Therefore, this standard still has the concept of gauge, which can be translated as caliber. It indicates just the thickness of the wall.
ASTM is affixed in the marking. The numbers following the abbreviation describe the outside diameter. In this group, copper pipes may be of interest to us.
Correspondence table for pipe diameters in inches and millimeters
Let’s say right away that pipes are measured not in ordinary inches, but in pipe ones. If an ordinary inch is 25,4 mm, then pipe according to GOST is 33,249 mm. And pipes with such an outer diameter (or close to it) are usually called inch. But the inner diameter can be very different: the wall thickness of an inch pipe can be 2,5 mm, or maybe 8 mm. Correspondence of nominal diameter in inches and millimeters is given in the table. Note! These are not outer/outer dimensions. This is the correspondence between the nominal diameters of our standard and the DIN standard.
If we consider the correspondence table, it can be seen that when calculating the conditional diameter, the usual inch is used. We use pipe when it is necessary to translate the symbol into the outer diameter. But even knowing the size of the pipe inch will not help to get the exact value of the outer diameter. It gives an approximate correspondence by which you can navigate. The exact data is shown in the table below.
So, let’s sum up. How to convert pipe dimensions in inches to centimeters and vice versa? If you translate Du into inches, you need to divide by the usual inch and round the resulting number to the nearest standard. Diameters of steel pipes of running dimensions are summarized in tables. If there is no table, the outside diameter can be approximated using “pipe inch”. We multiply the pipe size in inches by 33,249, we get a number close to the table value. Why not accurate? Because we are trying to translate the nominal (read – conditional) diameter into a real value – the external size. Therefore, the result is only close to the table.
How to measure pipe diameter
If you are not a professional plumber, you can hardly determine the size of the pipe by eye. And you need to know this parameter, since the diameters of steel pipes are an important parameter. This value will be required, for example, when replacing a pipeline, installing new fittings, etc. You will need a diameter. It is easier to determine the wall thickness – most often it is 4 mm. Its parameters are sufficient for any household pipeline: water supply (cold or hot), heating. You can measure it on the saw. But if you check old pipes, it may well be that the walls have become thinner.
So, to determine the diameter of a pipe, the easiest way is to measure its cross section. If you have access to the cut, use a ruler or centimeter tape. If there is no access to the cut, a drawing pen can help.
If there is no caliper, you will need a centimeter tape or tape measure with a flexible web. It is necessary to measure the circumference and write the resulting figure in millimeters. To calculate the diameter of the pipe, you need to divide this figure by 3,14 (number π).
For example, you have measured 4,4 cm, that is, 44 mm. We divide this figure by 3,14, we get: 44 / 3,14 u14d 13,5 mm – this is the outer diameter of the pipe. We look at the table, which shows the diameters of steel pipes. Most of all, it looks like our pipe is 8 mm in diameter, with a nominal diameter of XNUMX mm. A small discrepancy will be attributed to the inaccuracy of measurements.