Terms related to steel pipes

Dimension

A. Nominal size: It is the nominal size stipulated in the standard, which is the ideal size expected by users and manufacturers, and also the order size indicated in the contract.

B. Actual size: the actual size obtained in the production process, which is often greater than or less than the nominal size. This phenomenon of greater or less than the nominal size is called deviation.

C. meter weight: weight per meter =0.02466* wall thickness * (OD - wall thickness)

Deviation and tolerance

A. Deviation: In the production process, because the actual size is difficult to meet the requirements of the nominal size, that is, it is often greater than or less than the nominal size, so the standard stipulates that there is A difference between the actual size and the nominal size. A positive difference is called a positive deviation, and a negative difference is called a negative deviation.

B. Tolerance: The sum of absolute values of positive and negative deviations specified in the standard is called tolerance, also called "tolerance zone".

Deviation is directional, that is, "positive" or "negative"; Tolerances are not directional, so it is wrong to call the value of the deviation "positive tolerance" or "negative tolerance".

Delivery length

Delivery length is also known as user required length or contract length. The delivery length in the standard has the following provisions:

A. Normal length (also known as non-fixed length) : where the length is within the length range stipulated by the standard and there is no fixed length requirement, it is called the normal length. For example, the structural pipe standard stipulates: hot rolled (extruded, expanded) steel pipe 3000mm ~ 12000mm; Cold-drawn (rolled) steel pipe 2000mm ~ 10500mm.

B. Fixed length: The fixed length shall be within the usual length and shall be a fixed length as required in the contract. However, it is impossible to cut absolute length in practice, so the standard specifies the allowable positive deviation value for length of length.

The standard of structural pipe is:

It is reasonable for the manufacturer to raise the price because the yield of regular length pipe decreases greatly. The range of price increases varies from enterprise to enterprise, generally on the basis of the base price increases of about 10%.

C. Length of the double ruler: the length of the double ruler shall be within the usual length range. The length of the single ruler and the multiple of the total length shall be indicated in the contract (for example, 3000mm×3, namely three multiples of 3000mm, with a total length of 9000mm). In practice, the allowable positive deviation of 20mm should be added on the basis of the total length, plus the notch allowance should be left for each single size length. Taking structural pipe as an example, the allowance for incision is specified as 5 ~ 10mm with outer diameter ≤159mm; Outside diameter > 159mm is 10 to 15mm.

If there is no deviation of the length of the double ruler and the allowance of cutting in the standard, the two parties shall negotiate and indicate in the contract. If the length of double scale is the same as the length of fixed scale, it will greatly reduce the rate of finished material for the manufacturer. Therefore, it is reasonable for the manufacturer to raise the price, and the price increase range is basically the same as that of the length of fixed scale.

D. Range length: The range length is within the usual range. When the user requires a fixed range length, it shall be indicated in the contract.

For example, the length of a range ruler ranges from 3000 mm to 1200 mm. The range ruler ranges from 6000 mm to 8000mm or 8000mm to 10000mm.

It can be seen that the range length is looser than the length of fixed scale and double scale, but it is much stricter than the usual length, which will also reduce the yield of production enterprises. Therefore, it is reasonable for manufacturers to raise prices, and the range of price increases is generally about 4% on the base price.

Uneven wall thickness

The wall thickness of steel pipe cannot be the same everywhere, and there is an objective phenomenon of unequal wall thickness in its cross section and longitudinal pipe body, that is, uneven wall thickness. In order to control this non-uniformity, the allowable index of non-uniformity wall thickness is specified in some steel pipe standards, which generally does not exceed 80% of the wall thickness tolerance (implemented after negotiation between the supply and demand parties).

Ovality

The phenomenon of unequal outside diameters exists on the cross section of the circular steel pipe, that is, there are maximum outside diameters and minimum outside diameters that are not necessarily perpendicular to each other, so the difference between the maximum outside diameters and the minimum outside diameters is ovality (or unroundness). In order to control the ovality, the allowable index of ovality is specified in some steel pipe standards, which is generally not more than 80% of the outer diameter tolerance (implemented after negotiation between the supplier and the supplier).

Curvature

Steel pipe in the direction of length curve, with a number to indicate the curve is called bending degree. The bending specified in the standard is generally divided into the following two types:

A. Local bending degree: Measure the chord height (mm) at the maximum bending point of the steel pipe with A meter long ruler, that is, the value of local bending degree. The unit is mm/m, and the expression method is as follows: 2.5mm/m. This method also applies to the bending of pipe end.

B. Total bending degree of the full length: use a thin rope to tighten from both ends of the pipe, measure the maximum chord height (mm) at the bending point of the steel pipe, and then convert the percentage of growth (in meters), that is, the full length bending degree of the steel pipe in the direction of length.

For example, if the length of the steel pipe is 8m and the maximum chord height measured is 30mm, the bending degree of the full length of the pipe should be: 0.03÷8m×100%=0.375%