In gas transmission projects, what truly causes anxiety for purchasers, contractors, and even end-users is never “price,” but a more practical question:
Who is responsible in the event of a leak? What will the cost be?
This article avoids abstract theory and helps you solve three core problems:
- Why do welds leak?
- How to achieve near-100% weld integrity?
- How to instantly identify high-risk products during procurement?
I. Why are gas pipelines most vulnerable to weld problems?
The biggest risk in welded steel pipes lies not in the base material, but in the weld.
- Weld = Naturally weak area
The welding process introduces:
- Inhomogeneous microstructure (grain changes)
- Residual stress
- Microcracks or lack of fusion
These defects may be initially invisible, but they deteriorate rapidly under the following conditions:
| Influencing Factor | Risk Outcome |
|---|---|
| Internal pressure fluctuations | Weld fatigue cracking |
| Gas corrosion (e.g., H₂S) | Stress Corrosion Cracking (SSC) |
| Temperature variations | Crack propagation due to thermal expansion and contraction |
| External soil corrosion | Pitting and perforation |
II. The Real Pain Point for Clients: Not “Buying Steel Pipes,” but “Buying Safety”
From the perspective of actual projects, clients typically face these four hidden risks:
- Invisible Quality Issues (Most Critical)
- Appears to be acceptable on the surface
- But actually contain internal defects (porosity, slag inclusions, incomplete penetration)
- These are completely undetectable during normal inspections.
- Price competition leads to shoddy workmanship
Common problems:
| Cost Reduction Method | Consequence |
|---|---|
| Reducing welding heat input | Lack of fusion in the weld seam |
| Lowering inspection ratio | Missed defect detection |
| Using lower-grade steel strip | Insufficient strength |
| Skipping heat treatment | High residual stress |
These problems won’t surface immediately, but will manifest in accidents within 1–3 years.
- Testing “done, but not enough”
Many suppliers will say:
- UT (ultrasound testing) was done.
- RT (radiographic testing) was done.
But the key question is:
What is the testing rate?
| Inspection Ratio | Practical Implication |
|---|---|
| 10% Sampling | Extremely high risk |
| 20% Sampling | Still not fully controllable |
| 100% Inspection | Only way to truly ensure reliability |
- Incorrect Selection (More Common Than Quality Issues)
Examples:
- Using ordinary structural pipes instead of gas pipes
- Ignoring PSL rating requirements
- Failure to consider corrosive environments
- These types of problems lead to “systemic risks,” not single points of failure.
III. How to Achieve “Weld Integrity”?
- Raw Material Control (Source Control)
Select materials that meet gas standards, such as:
- API 5L (PSL2 preferred)
- Low sulfur, low phosphorus
- Control carbon equivalent (CE)
Purpose: To reduce weld crack susceptibility
- Welding Process Control (Core Element)
| Process Parameter | Control Objective |
|---|---|
| Welding Temperature | Prevent lack of fusion |
| Welding Speed | Avoid porosity |
| Squeezing Force (ERW) | Ensure weld seam compactness |
| Online Heat Treatment | Eliminate residual stress |
- 100% Non-destructive Testing
Key Requirements:
| Inspection Method | Required or Not |
|---|---|
| Ultrasonic Testing (UT) | Mandatory |
| Radiographic Testing (RT) | Required for high-spec projects |
| Magnetic Flux Leakage (MFL) | Recommended |
| Hydrostatic Testing | Mandatory |
- Post-treatment and Corrosion Protection
No matter how good the weld is, leaks will still occur if corrosion protection is inadequate:
- 3PE corrosion protection (mainstream)
- FBE coating
- Cathode protection system
Especially suitable for:
- Welded Steel Pipes for Gas
- High humidity/high salinity areas
IV. ERW vs LSAW: How to Choose a Safer Weld for Gas Projects?
Which type of weld is more reliable?
| Comparison Item | ERW Steel Pipe | LSAW Steel Pipe |
|---|---|---|
| Weld Type | High-frequency electric resistance welding | Submerged arc welding |
| Weld Quality | High (automated) | Very high |
| Defect Probability | Low | Lower |
| Cost | Low | High |
| Applicable Scenarios | Urban gas, medium to low pressure | Long-distance pipelines, high pressure |
V. Procurement Pitfalls Avoidance Guide
5 Questions You Must Confirm with Suppliers:
- Are welds 100% UT inspected?
- Is a complete (traceable) inspection report provided?
- Does the PSL2 standard apply?
- Is online heat treatment performed?
- Do you have actual gas project case studies?
