Selection of Welded Steel Pipes for Gas Pipeline Applications

Literature Overview

The 2005 paper by Huo Qun, published in Tractors and Farm Transporters (Vol. 32, No. 3, p. 59), addresses the selection criteria for welded steel pipes used in gas pipeline systems, particularly in the context of high-pressure gas transmission. The paper compares spiral-welded steel pipes (SSAW) and longitudinal-welded steel pipes (LSAW and HFW/ERW) for gas pipeline applications, considering factors such as pressure rating, corrosion resistance, cost, and availability.

Core Technical Content

Pipe Type Comparison for Gas Pipeline Service

Parameter	Spiral-Welded (SSAW)	Longitudinal Submerged Arc (LSAW)	High-Frequency Welded (HFW/ERW)
Typical diameter range	Φ219–Φ3600 mm	Φ406–Φ3000 mm	Φ21–Φ273 mm
Wall thickness range	4–40 mm	6–40 mm	1.5–12 mm
Weld orientation	Helical (20°–30°)	Longitudinal	Longitudinal
Typical material grades	API 5L Gr. B, X42, X52, X60, X70	API 5L Gr. B, X42, X52, X60, X65, X70, X80	API 5L Gr. B, X42, X52, X60
Hydrostatic test pressure	1.5 × SMYS × D/t	1.5 × SMYS × D/t	1.5 × SMYS × D/t
Cost index (relative)	1.0	1.1–1.3	1.2–1.5
Maximum practical pressure	Moderate	High	Limited by diameter
Weld inspection methods	UT, MT, radiography (limited)	UT, MT, RT	UT, MT

Selection Criteria Framework

The selection of welded steel pipe for gas pipeline applications should follow a systematic evaluation based on the following criteria:

1. Pressure and Diameter Requirements

For high-pressure gas transmission (≥6 MPa) with large diameters (≥Φ406 mm), LSAW pipes are preferred due to their superior weld quality and proven performance in high-pressure service
For medium-pressure distribution (1–4 MPa) with moderate diameters (Φ219–Φ610 mm), SSAW pipes offer an economical solution with adequate performance
For low-pressure distribution (<1 MPa) with small diameters (<Φ273 mm), HFW/ERW pipes provide the most cost-effective option

2. Weld Quality Considerations

The weld orientation in spiral-welded pipes introduces additional complexity:

The helical weld is subjected to combined stress states under internal pressure
Weld defects in SSAW pipes are oriented at an angle to the pipe axis, which can be advantageous for crack arrest but complicates inspection
LSAW pipes have welds aligned with the pipe axis, making inspection more straightforward but potentially allowing axial crack propagation

3. Material Grade Selection

Application Category	Recommended Grade	Design Pressure	Minimum Yield Strength
Long-distance transmission	API 5L X65/X70	6–10 MPa	450–485 MPa
Regional distribution	API 5L X52/X60	2–6 MPa	360–415 MPa
Local distribution	API 5L Gr. B / X42	0.4–2 MPa	245–290 MPa
Industrial gas supply	API 5L Gr. B	≤1.6 MPa	245 MPa

Quality Assurance Requirements

For gas pipeline applications, the following quality assurance measures are mandatory:

Quality Requirement	Specification	Standard Reference
Chemical composition	C ≤ 0.25%, Mn ≤ 1.4%, P ≤ 0.030%, S ≤ 0.030%	API 5L
Mechanical properties	Tensile strength, yield strength, elongation per grade	API 5L
Hydrostatic test	1.5 × SMYS × D/t, hold for 5 minutes minimum	API 5L
UT inspection of weld	Full-length, phased array preferred	API 5L
MT of weld surface	100% coverage on both sides	API 5L
Flattening test	Per API 5L requirements	API 5L
Charpy V-notch impact	3-point average ≥ 40 J at minimum service temperature	API 5L

Engineering Considerations

Corrosion Protection: Gas pipelines are typically buried and require cathodic protection combined with external coating systems (3LPE, 3LPP, or FBE). The coating system selection should consider:

Soil resistivity
Groundwater table depth
Presence of stray currents
Design life requirements (typically 20–30 years)

Joint Design: Field welding of gas pipeline sections requires:

Pre-qualified welding procedures per ASME B31.8 or GB 50351
100% radiographic or UT inspection of field welds
Hydrostatic testing of the completed pipeline section
Cathodic protection potential verification after coating and burial

Study Insights and Reflections

This paper, while concise, addresses a fundamental engineering decision that carries significant consequences for pipeline safety and economic performance. The selection between spiral-welded and longitudinal-welded pipes is not merely a matter of cost but involves a careful balance of technical requirements, inspection feasibility, and long-term reliability. Engineers should recognize that the weld in a gas pipeline is the weakest link in the system, and the choice of pipe type directly determines the nature and inspectability of this critical feature. The paper's emphasis on matching pipe type to pressure and diameter requirements reflects sound engineering practice, but modern engineers should also consider additional factors such as availability of advanced inspection technologies (PAUT, MFL), lifecycle cost analysis, and evolving regulatory requirements that may favor certain pipe configurations over others.