Sizes of tubes and pipes
Dimensions of tubes belong to the essential characteristics of tubes. In this regard tube dimensions must be given in such a way that can completely determine the tube.
For tubes with circular cross section the three main dimensions except for the length are: outside diameter (OD), inside diameter (ID) and wall thickness (WT). For circular tubes are given two of these values. According to the type of tubes also belong the appropriate dimensions and the dimensional tolerances.
Dimensional series of steel tubes
Dimensions of the individual tubes are not randomly produced, but are arranged according to dimensional series based on certain systems. Tube dimensions are in mm. In the USA and some other countries dimensions are given in inches (German “Zoll”). In this case, the tubes are divided into two groups so called TUBE. These tubes are used for mechanical usage and energy facilities and in inches are given dimensions of the outside diameter. PIPE are tubes for pipelines for different types of media.
The PIPE sizes are given as the nominal tube size. For sizes up to 12 inches the approximate size (clearance) of the inside pipe diameter is given. From the PIPE dimensions after their conversion to millimeters, is created the first preferred line of the outer diameter of steel tube (1st line is in EN 10220, DIN 2448, etc.). This does not mean that the tube dimensions in the 2nd and 3rd series are not used. Dimensions in series 2 and 3 (for use in Europe are modified by the rounded off dimensions in mm) are standards for TUBE used in the construction of energy facilities and for tubes for mechanical application.
Series of wall thicknesses of steel tubes
Series of wall thicknesses have its origin in the Imperial Unit system (inch), which for expression of sizes uses fractions. Wall thicknesses in PIPE are created by “Schedule” (40, 60, 80, 120 ….), connected in some dimensions to the weight class (STD, XS, XXS). These values, converted to millimeters form a part of series of wall thicknesses.
(Note: size – the value of Schedule for example 40 is not constant, but depends on the outside diameter of tube).
For the Tube the wall thickness values are derived from the “scales” of BWG, SWG eventually others. After conversion into mm these values are another part of series of wall thicknesses for steel tubes. For the precision steel tubes used in Europe and in countries using the SI system the dimensional line is created with rounded off dimensions of the outside diameters and wall thicknesses.
Tolerances for outside diameter, inside diameter and wall thickness are valid for two ordered values only. Tolerance of third dimension is possible to agree at precision tubes only.
Ovality of steel tubes
Ovality (out of roundness), is the difference between max. and min. diameters readings taken in the same given section. Ovality shall be within OD tolerance.
O = Dmax–Dmin (absolute value in mm)
O = 100.(Dmax–Dmin)/D (in %)
Eccentricity of steel tubes
Eccentricity (mean deviation), is a measure of the distance between centres of the outside and inside diameters. Eccentricity shall be within WT tolerances and is calculated from the WT in one cross section using formula:
E = (tmax–tmin)/2 (absolute value in mm)
E = (tmax–tmin)/(tmax+tmin).100 (in %)
Tube straightness deviation (e)
Tube straightness deviation indicates the maximum deviation of the tube from a line joining its two ends, where L is the length of the tube. It is given in percentages per unit length. Usually required straightness is 3 mm for 1 meter.
e/L x 100 %