They are defined as tubes suitable for conveying fluids without dispersion and constitute, as mentioned, the great majority of the tubes produced. Almost all of these have a circular section. Generally, pipes for hydraulic applications follow precise dimensional standards; the most commonly used standard is ANSI B36, which establishes some concepts of universal use.
The pipes are classified according to a nominal diameter which is, to some extent, the internal diameter of the pipe. The nominal diameters constitute a series of unchangeable values. Pipes with a given nominal diameter have a constant outside diameter, regardless of thickness. In the US use (and generally in the petroleum industry) the nominal diameters (abbreviated with the abbreviation NB, i.e. Nominal Bore, nominal bore) are expressed in inches, while in European use they are expressed in millimeters and designated by the abbreviation DN , i.e. nominal diameter; the external diameter values are in any case the same, and there is a correspondence (originating from the ANSI B36.10 standard).
Classification by material
The tubes are cylindrical profiles that can take on different dimensions and have different characteristics according to the functions they are called upon to perform. The choice of a pipe to be used in a pipe takes into account the
- material of which the tube is formed
- construction type of the tube
Once these two parameters have been selected, the choice of the tube requires the identification of the diameter (in relation to the flow rate of fluid to be transported) and the choice of the thickness. As regards the latter, reports are adopted that allow it to be calculated both in the operational phase and in the case of hydraulic testing. Below is a list of the types of pipes, distinguished according to the material, which are frequently used in industrial and civil practice.
Steel pipes
There are different models of steel pipes that can be distinguished according to the characteristics of the fluid to be transported, its temperature and pressure and the characteristics of the environment in which the pipe will be placed.
Threadable tubes
They are pipes in unalloyed steel (Fe 330) that can be easily machined. They can have smooth or threaded ends with conical or cylindrical threads. Sizes range from DN 10 to 150 and pressures up to PN 25.
Galvanized pipes
Particular pipes that are used in those cases in which it is desired to avoid oxidation of the material which would lead to pollution of the transported fluid. The pipes in question are placed in special zinc baths and the excess material is eliminated with a subsequent processing.
Pipes for mechanical applications
They are quality unalloyed steel tubes typically used in heat exchangers or ovens.
Pipes for pipelines
This category includes carbon steel pipes which are generally used in the transport of water, petroleum products or natural gas.
Commercial pipes
Non-alloy steel pipes (Fe 320) suitable for transporting fluids for which watertight integrity is required and sizes up to DN 600.
Special pipes
They are stainless steel alloy pipes suitable for working in critical conditions and therefore with fluids that have a high temperature or a high chemical aggressiveness.
Copper pipes
They are pipes made of copper and have high thermal conductivity characteristics and a certain workability (annealed physical state). They are small-medium in size and due to their characteristics are frequently used in domestic systems for the transport of water (drinking water, traditional and radiant heating), fuel gas, diesel oil and fluids for solar systems. Copper pipes are also used in the field of air conditioning and refrigeration, as well as for the transport of medical gases. Pipes in copper alloys, brass and cupronickel are also used, especially in the marine sector.
Spheroidal cast iron pipes
Tubes obtained from the centrifugation of magnesium in gray cast iron. They have good corrosion resistance but on the other hand have a high weight per unit of length. This parameter must be taken into account, for example, when dimensioning the supports that will have to support the pipes. They can withstand pressures up to PN 10 and due to their characteristics are used in the transport of water, petroleum products or natural gas, in particular in underground pipes.
Plastic pipes
They are pipes that are increasingly used thanks to the characteristics of lightness, flexibility, corrosion resistance, dielectric properties. The limit of these pipes lies in the low values of pressure and temperature that they can withstand. The most used materials in the construction of these particular pipes are:
- PVC (polyvinyl chloride)
- PE (polyethylene)
- PP (polypropylene)
- PVDF (polyvinylenfluoride)
It should be borne in mind that these pipes undergo aging when exposed to sunlight. Due to their properties they are used in the transport of drinking or waste water, food liquids, chemical products.
Glass tubes
Glass tubes are used in special applications, for example for handling strongly acidic substances.
Normative requirements
- UNI EN 1333: 2007
- DM LL.PP. 12.12.1985
- Circular Ministry LL.PP. 20.03.1986 n. 27291
Recommendations of the Italian Institute of Plastics: Plastics in irrigation (PDF)
Nominal pressure
The nominal pressure value (symbol PN) is a conventional indication that represents the permissible internal pressure (expressed in bar) of a pipe for a temperature of 20 ° C.
The nominal pressure of a pipe is defined as the sum of the following two values:
- Maximum allowable working pressure (PMA – pression maximale admissible): maximum pressure value p that can occur in axis with the pipes for the heaviest hydraulic operation of the system, including any overpressures Δp determined by foreseeable operating conditions, even if consequent to transient phenomena (overpressure of various motion);
- Equivalent pressure (Po): Especially in large diameter pipes, tensile stresses arise which are not connected with the hydraulic operation of the system, such as the ground load, support conditions, external overloads (dynamic and static), thermal variations and any other actions including seismic ones. Equivalent pressure is defined as the axial pressure which gives the pipe maximum traction stresses equal to those determined on the basis of the specific conditions indicated above. If the equivalent pressure value is considerably reduced compared to that of the operating pressure, only the PMA value can be considered instead of PN.
The nominal pressure is the basic element for choosing the series or classes of pipes, joints (e.g. flanges), special pieces making up the pipes and related accessories (e.g. hydraulic equipment) intended for the operation of a pipeline in pressure.
The nominal pressures are indicated with the abbreviation PN followed by a numerical value (in bar).
The selectable values of PN, according to the UNI EN 1333 standard are:
2 5 6 10 16 25 40 63 100
Nominal diameter
In hydraulics, the nominal diameter (symbol DN) is a conventional value that serves as a reference to determine the size of the various elements that can be coupled in a pipe (pipes, flanges, valves, special pieces, etc.).
Generally, the DN expresses approximately the dimension in millimeters of the effective internal diameter of the element, for metallic and cementitious materials, and of the effective external diameter, for plastic materials.
The nominal diameters are indicated with the abbreviation DN followed by the numerical value (in mm) which, for pipelines of a certain importance, is:
DN Diameter in mm 10 (Øe 17.1mm)
15 (Øe 21.3mm)
20 (Øe 26.7mm)
25 (Øe 33.4mm)
32 (Øe 42.2mm)
40 (Øe 48.3mm)
50 (Øe 60,3mm)
65 (Øe 73,0mm)
80 (Øe 88.9mm)
100 (Øe 114.3mm)
125 (Øe 141,3mm)
150 (Øe 168.3mm)
200 (Øe 219.1mm)
250 (Øe 273,0mm)
300 (Øe 323,8mm)
350 (Øe 355,6mm)
400 (Øe 406.4mm)
450 (Øe 457,2mm)
500 (Øe 508,0mm)
550 (Øe 558.8mm)
600 (Øe 609.6mm)
700 (Øe 711.2mm)
800 (Øe 812.8mm)
900 (Øe 914.4mm)
In US use (and generally in the petroleum industry) the nominal diameters (abbreviated with the abbreviation NB, that is Nominal Bore, nominal bore) are expressed in inches.
