Definition and Standard Background
Welded steel pipe for low pressure fluid transportation is a general purpose welded pipe used to convey water, gas, air, oil, heating steam and similar low pressure fluids. In the Chinese system the product is specified by GB/T 3091, which covers both plain welded pipe, commonly called black pipe, and hot dip galvanized welded pipe, commonly called white pipe. The older GB/T 3092 designation for general welded pipe has been withdrawn and its scope absorbed into GB/T 3091, so both black and galvanized products are now ordered against the same standard.
Materials and Manufacturing
The pipe is formed from carbon steel strip or coil, welded by electric resistance welding, then sized, straightened and tested. Common base grades include Q195, Q215, Q235 and Q355 as specified in GB/T 700 and GB/T 1591. Galvanized pipe is produced by hot dip galvanizing after welding, so the coating covers the outside surface and, when specified, the inside surface as well.
Specifications
| Parameter | Typical range or reference |
|---|---|
| Size designation | Nominal diameter DN, an approximate value of the inside diameter and often quoted in inches |
| Common sizes | DN 8 to DN 150 in regular production, larger sizes by agreement |
| Wall schedule | Ordinary wall and thickened wall |
| End form | Plain end, threaded end to GB/T 7306.1 or GB/T 7306.2, grooved end on request |
| Surface finish | Black as welded, or hot dip galvanized |
| Delivery length | Random lengths, commonly 4 m to 12 m |
| Testing | Hydrostatic test to the pressure given in the standard, plus coating mass verification for galvanized pipe |
Applications
Typical uses are municipal water supply and drainage, gas distribution networks, building heating and ventilation, compressed air systems, fire protection branches, agricultural irrigation, greenhouses and general structural or support pipework. Galvanized pipe is preferred outdoors and in damp service, because the zinc coating gives sacrificial protection at cut edges and damaged spots.
Ordering and Inspection Points
State the nominal diameter, the wall schedule, the end form, the surface finish and the delivery length, because these choices are not interchangeable: an ordinary wall pipe cannot be substituted for a thickened wall pipe at the same design pressure. For threaded pipe, confirm the thread type and the coupling or cap requirement, since parallel and tapered sealed threads are not interchangeable. For galvanized pipe, confirm whether the inside surface must be coated and which coating mass applies, and verify the marking, the hydrostatic test record and the coating test report before shipment. Keep end protectors in place during transport to avoid damage to threads and bevels.
FAQ
Q: What is the difference between black pipe and white pipe?
Black pipe is the plain welded pipe as it comes from the mill, while white pipe is the same pipe after hot dip galvanizing. The two names describe the surface finish, not a different pressure rating.
Q: Which standard governs this product?
GB/T 3091 covers welded steel pipe for low pressure fluid transportation, including both plain and galvanized forms. The withdrawn GB/T 3092 designation has been merged into this standard.
Q: What does nominal diameter mean?
Nominal diameter DN is a convenient size designation close to the inside diameter. It is not an exact dimension, so the outside diameter and wall thickness are the values to use for design calculations.
Q: What is the difference between ordinary wall and thickened wall pipe?
The thickened schedule has a heavier wall for the same nominal diameter, which raises the pressure capacity and the stiffness of the pipe. The two classes are not interchangeable in a pressurised line.
Q: Can the pipe be supplied with threads?
Yes. Plain end and threaded end pipes are both available, and the thread form should be stated in the order because differing thread standards cannot be mated safely.
Q: Where is galvanized pipe preferred?
Galvanized pipe is preferred for outdoor, humid or intermittent wet service such as water supply, irrigation and general plant utilities, where the zinc coating limits atmospheric corrosion.
