① Nominal size and actual size A, nominal size of **steel pipe**: is the standard size specified in the standard, the user and the manufacturer want to get the desired size, but also the contract specified in the size of the order.

B, the actual size: the actual size of the production process, the size is often greater than or less than the nominal size. This phenomenon is greater or less than the nominal size of the phenomenon known as the deviation.

② deviation and tolerance

A, Deviation: In the production process, because the actual size is difficult to achieve nominal size requirements, that is often greater than or less than the nominal size, so the standard specifies the actual size and nominal size allows a difference between. The difference is positive for the positive deviation, the difference is negative for the negative deviation.

B, tolerance: the provisions of the standard positive and negative values of the sum of the absolute value is called tolerance, also known as “tolerance zone.”

The deviation is directional, that is, “positive” or “negative”; tolerance is no direction, therefore, the deviation of the value of “positive tolerance” or “negative tolerance” is wrong.

③ delivery length

Delivery length, also known as the length of the user requirements or contract length. There are several rules for the length of the delivery:

A, usually length (also known as non-length length): Where the length of the standard length of the provisions of the length and no fixed length requirements, are called the usual length. For example, the structure of the pipe standard: hot (extrusion, expansion) steel pipe 3000mm ~ 12000mm; cold drawn (rolled) steel pipe 2000mmm ~ 10500mm.

B, length of length: length should be in the normal length of the length of the contract is required in a fixed length size. But the actual operation are cut out the absolute length of the length is not possible, so the standard length of the length of the provisions of the allowable positive deviation.

The structural pipe standard is:

Production length of the length of the tube than the average length of the pipe into the rate of decline is greater, the production enterprises to raise the price requirements are reasonable. Increase the rate of the enterprise is not consistent, generally based on the base price increase of about 10%.

C, the length of the length: the length of the length should be within the normal length range, the contract should be marked with a single length and the length of the total length of the composition (for example, 3000mm × 3, that is, 3000mm 3 times the total length of 9000mm). In practice, should be based on the total length of the allowable positive deviation of 20mm, plus the length of each monogon should stay incision margin. To structural tube, for example, the provisions of the incision margin: diameter ≤ 159mm for the 5 ~ 10mm; diameter> 159mm for 10 ~ 15mm.

If there is no deviation in the length of the standard deviation and cutting allowance, the supply and demand sides should be negotiated and specified in the contract. Times the length of the same length with the length of the production will bring a significant reduction in production rate, so the production enterprises to raise the price is reasonable, the increase in the rate of increase with the length of the same length of the same.

D, the length of the range: the length of the range in the normal length range, when the user requires a fixed range of length, the need to specify in the contract.

For example: usually length of 3000 ~ 12000mm, and the length of the length of 6000 ~ 8000mm or 8000 ~ 10000mm.

It can be seen that the length of the length is longer than the length and length of the length, but it is much more severe than the usual length, and the production rate is reduced. Therefore, the production enterprises to raise the price is justified, the increase in the price increase in the base price of about 4%.

④ uneven wall thickness

Steel pipe wall thickness can not be the same everywhere, in its cross-section and longitudinal pipe body on the objective of the existence of wall thickness, that is, uneven wall thickness. In order to control this non-uniformity, in some steel pipe standards in the provisions of the wall thickness of the allowable indicators, the general provisions of not more than 80% of wall thickness tolerance (after consultation between the supply and demand sides).

⑤ ellipticity

In the cross section of the circular steel pipe there is a phenomenon in which the outer diameter is not equal, that is, there is not necessarily perpendicular to each other, the maximum outer diameter and the minimum outer diameter, the difference between the maximum outer diameter and the minimum outer diameter is ellipticity Roundness). In order to control the ellipticity, some steel pipe standard provides the allowable index of ellipticity, the general provisions of not more than 80% of the outer diameter tolerance (after consultation between the supply and demand sides).

⑥ curvature

The steel pipe is curved in the length direction, and the curve is expressed by the number of curvature. The curvature specified in the standard is generally divided into the following two types:

A, local bending degree: with a meter long ruler by the maximum bending in the steel pipe, measured the chord height (mm), that is, local bending value, the unit is mm / m, that method such as 2.5mm / m The This method also applies to the bending of the pipe end.

B, the total length of the total curvature: with a string, from the tube at both ends of tension, measuring the maximum bending height of steel pipe (mm), and then converted into length (in meters) percentage, The length of the curvature.

For example: steel pipe length of 8m, measured the maximum string height 30mm, then the tube full length of curvature should be:

0.03 ÷ 8 m x 100% = 0.375%

⑦ size tolerance

Size tolerance or size of the standard deviation beyond the standard. Here the “size” mainly refers to the outer diameter of the steel pipe and wall thickness. Often it is called the size of the tolerance is called “tolerance outrageous”, this deviation and tolerance is equivalent to the name is not tight, should be called “deviation”. The deviation here may be “positive”, it may be “negative”, rarely in the same batch of steel pipe in the “positive, negative” deviation are out of the phenomenon.

**Source: China ****Steel Pipe**** Manufacturer** – Yaang Pipe Industry Co., Limited (www.yaang.com)