1. Size of steel tube
Divide 1 inch into 8 equal parts:
- 1/8 inch
- 1/4 inch
- 3/8 inch
- 1/2 inch
- 5/8 inch
- 3/4 inch
- 7/8 inch
Smaller dimensions are expressed in fractions of 1/16, 1/32 and 1/64, with the unit of measurement being inches.
When the numerator and denominator can be divided equally (for example, the numerator is 2, 4, 8, 16, 32), they should be divided equally.
Inches are indicated by two double quotation marks in the upper right corner, such as 1/2″.
For example, DN25 water pipe (25 mm) is equivalent to 1 inch water pipe in the English system.
The DN15 water pipe is equivalent to the English system 1/2 inch water pipe.
The DN20 water pipe is equivalent to the English system 3/4 inch water pipe.
2. Steel tube pattern:
There are two types of standards: English standards and international standards.
Steel Pipe Standard Table
| Tube type (drawing note code) | Nominal diameter (mm) | Outer diameter (mm) | Wall thickness (mm) | Internal diameter | Total area of the inner hole | Cross-sectional area in% of internal hole (mm²) | ||
|---|---|---|---|---|---|---|---|---|
| 33% | 27.5% | 22% | ||||||
| Electrical conduit (TC) |
16 | 15.87 | 1.6 | 12.67 | 126 | 42 | 35 | 28 |
| 20 | 19.05 | 1.6 | 15.85 | 197 | 65 | 54 | 43 | |
| 25 | 25:40 | 1.6 | 10:20 p.m. | 387 | 128 | 106 | 85 | |
| 32 | 31.75 | 1.6 | 28:55 | 640 | 211 | 176 | 141 | |
| 40 | 38.10 | 1.6 | 34.90 | 957 | 316 | 263 | 211 | |
| 50 | 50.80 | 1.6 | 47.60 | 1780 | 587 | 490 | 392 | |
| 15 | 8:75 p.m. | 2.5 | 15.75 | 194 | 64 | 53 | 43 | |
| 20 | 26:25 | 2.5 | 9:25 p.m. | 355 | 117 | 97 | 78 | |
| 25 | 32:00 | 2.5 | 27:00 | 573 | 189 | 157 | 126 | |
| Welded Steel Pipe (SC) | 32 | 40.75 | 2.5 | 35.75 | 1003 | 331 | 276 | 221 |
| 40 | 46.00 | 2.5 | 41:00 | 1320 | 436 | 363 | 290 | |
| 50 | 58.00 | 2.5 | 53.00 | 2206 | 728 | 607 | 485 | |
| 70 | 74.00 | 3.0 | 68.00 | 3631 | 1198 | 998 | 798 | |
| 80 | 86.50 | 3.0 | 80.50 | 5089 | 1679 | 1399 | 1119 | |
| 100 | 112.00 | 3.0 | 106.00 | 8824 | 2911 | 2426 | 1941 | |
| 15 | 9:25 p.m. | 2.75 | 15.75 | 195 | 64 | 54 | 43 | |
| 20 | 26.75 | 2.75 | 9:25 p.m. | 355 | 117 | 97 | 78 | |
| 25 | 33.50 | 3.25 | 27:00 | 573 | 189 | 158 | 126 | |
| Steel Pipe for Water Gas (RC) |
32 | 42.25 | 3.25 | 35.75 | 1003 | 331 | 276 | 221 |
| 40 | 48.00 | 3.50 | 41:00 | 1320 | 436 | 363 | 290 | |
| 50 | 60.00 | 3.50 | 53.00 | 2206 | 728 | 607 | 485 | |
| 70 | 75.50 | 3.75 | 68.00 | 3631 | 1198 | 998 | 798 | |
| 80 | 88.50 | 4:00 | 80.50 | 5089 | 1679 | 1399 | 1119 | |
| 100 | 114.00 | 4:00 | 106.00 | 8824 | 2911 | 2426 | 1941 | |
| 125 | 140.00 | 4.50 | 131.00 | 13478 | 4447 | 3706 | 2965 | |
| 150 | 165.00 | 4.50 | 156.00 | 19113 | 6307 | 5256 | 4204 | |
Observation:
- Pipes laid in natural soil and simple concrete must be fixed with water and gas pipes.
When using the steel pipe wall as a protective line, a welded steel pipe with a wall thickness of not less than 2.5 mm must be used in accordance with the national standard “Code for Construction and Installation Engineering Acceptance Electrical”.
- Steel pipes for water and gas must comply with (VB234-63).
3. Terminology of steel pipe dimensions
1. Size
A. Nominal Size: It is the size specified in the standards, the ideal size desired by both users and manufacturers and the size indicated in contracts.
B. Actual Size: Refers to the size obtained in the production process, which may be different and be larger or smaller than the nominal size.
C. Deviation: Refers to the difference between the actual size and the nominal size, which may result in the actual size being larger or smaller.
D. Weight per meter: It can be calculated as follows: weight per meter = 0.02466 * wall thickness * (outer diameter – wall thickness).
2. Deviation and tolerance
A. Deviation: In the production process, it is often difficult to achieve the nominal size specified in the standards. As a result, the actual size may be larger or smaller than the nominal size. This difference between the actual size and the nominal size is called deviation. A deviation that results in a larger actual size is called a positive deviation, while a deviation that results in a smaller actual size is called a negative deviation.
B. Tolerance: Tolerance refers to the allowable deviation range specified in the standards. It is expressed as the sum of the absolute values of the positive and negative deviation values. Tolerance is direction-free, meaning it has no positive or negative directionality. It is incorrect to refer to deviation values as “positive tolerance” or “negative tolerance”.
3. Duration of delivery
Delivery length is also referred to as the length required by the user or the length specified in the contract. The standard specifies the delivery length as follows:
A. Normal Length (also known as Non-Fixed Length): Refers to all lengths within the range specified in the standard and without fixed length requirements. For example, the standard for structural pipes specifies the following:
- Hot rolled steel tube (extruded, expanded): 3000mm to 12000mm
- Cold drawn steel tube (rolled): 2,000 mm to 10,500 mm
B. Fixed Length: Fixed length must be within the normal length range and is a fixed dimension specified in the contract. However, it is not possible to achieve an absolute length in practice. As a result, the standard specifies an allowable positive deviation value for the length.
The production rate of fixed-length pipes has decreased significantly compared to that of normal-length pipes, which makes it reasonable for manufacturers to increase prices. The price increase varies from company to company, with an average increase of approximately 10% compared to the base price.
C. Multiple Length: Multiple lengths must be within the normal length range. The contract must specify the single length and the multiple of the total length (e.g. 3,000 mm x 3, i.e. three times 3,000 mm, for a total length of 9,000 mm). In practice, a positive deviation of 20 mm should be added to the total length and an incision margin should be reserved for each length.
For example, for structural tubes, an incision margin of 5 to 10 mm is required for outer diameters less than or equal to 159 mm, while outer diameters greater than 159 mm require a margin of 10 to 15 mm. If the standard does not specify the length deviation and cutting margin, they must be negotiated by both parties and noted in the contract.
Double length is similar to fixed length and significantly reduces the finished product rate of the producing company. It is therefore reasonable for the producing company to increase its prices, with an average increase similar to that of fixed length.
D. Gap Length: The gap length is within the normal range. If the user requires a fixed track length, this must be indicated in the contract. For example, the normal length is generally 3000-12000mm, while the range length may be 6000-8000mm or 8000-10000mm. Gap length is less strict than fixed length and double length, but more strict than normal length, which also reduces the producer's income. As a result, it is reasonable for the manufacturer to increase their prices, with an average increase of around 4% above the base price.
4. Uneven wall thickness
The wall thickness of the steel pipe cannot be completely consistent. There is an objective occurrence of uneven wall thickness in its cross-section and longitudinal body, which results in uneven wall thickness.
To regulate this irregularity, certain standards for steel pipes have established an allowable index for wall thickness non-uniformity, which is normally no more than 80% of the wall thickness tolerance (agreed through negotiations between the supplier and the purchaser). .
5. Ovality
In the cross section of a circular steel tube, there is a difference in the outer diameter, resulting in a maximum and minimum outer diameter that are not necessarily perpendicular to each other. This difference between the maximum and minimum external diameter is called ovality or irregularity.
To regulate ovality, certain standards for steel pipes have established an acceptable ovality index, which is typically no more than 80% of the outer diameter tolerance (determined through negotiations between the supplier and purchaser).
6. Curvature
The steel tube is bent along its length and the degree of curvature is quantified using numbers, known as bending.
Folding is generally divided into two types according to the pattern:
A. Local bending: The bend at the maximum point along the length of the steel pipe is measured using a meter long ruler. The chord height (mm) at maximum bending is referred to as the local bending value, expressed in mm/m. For example, it can be expressed as 2.5 mm/m.
This method can also be used to measure the curvature at the end of the pipe.
B. Total bending of the entire length: The maximum rope height (mm) at the bend of the steel tube is measured by squeezing a thin rope at both ends of the tube. This value is then converted into a percentage of the length (in meters), which represents the total curvature of the steel tube along its length.
For example, if the length of the steel tube is 8m and the maximum chord height is 30mm, the total curvature of the tube would be:
0.03 ÷ 8m × 100% = 0.375%.
7. Size out of tolerance
The allowable deviation from the tolerance or standard for dimensions.
The dimensions referred to here mainly refer to the outer diameter and wall thickness of the steel pipe.
It is common for people to refer to size deviations as “out of tolerance,” but this is not always a strict definition of deviation and tolerance. The correct term is “out of specification”.
The deviation can be positive or negative. It is unusual for positive and negative deviations to be outside the standard in the same batch of steel pipes.
Note: Formula for calculating the theoretical weight of common profiles:
m=F×L×ρ
- m – mass kg;
- F – sectional area m 2 /m;
- l – length m;
- ρ— Density * Kg/m 3
Where: Method of calculating the area of section F:
1. Square steel F=a 2
2. Steel tube F=3.1416 ×$ (D – $)
- D – diameter;
- $- thickness;
3. Steel sheet, flat steel F=a ×$ a – width
