ASTM A178 Electric Resistance Welded (ERW) Pipe (Tube)
Basic Overview
A standard specification for electric-resistance-welded carbon steel boiler tubes. A178 covers welded (ERW) tubes for boiler and superheater applications, making it a welded alternative to seamless boiler tube standards like A192 and A210.
Important: This is a tube standard (for heat transfer applications), not a pipe standard (for fluid transport).
Key Features of A178
| Feature | Description |
|---|---|
| Material Type | Carbon Steel |
| Manufacturing | Electric Resistance Welded (ERW) |
| Main Use | Boiler and superheater tubes |
| Grades | A, C, D (Grade A is most common, similar to A192) |
| Key Advantage | Cost savings over seamless tubes for suitable applications |
| Temperature Limit | ~425°C (800°F) max (standard carbon steel limit) |
Chemical Composition (Grade A - Most Common)
| Element | Content (%) |
|---|---|
| Carbon (C) | 0.06 - 0.18 |
| Manganese (Mn) | 0.27 - 0.63 |
| Phosphorus (P) | 0.035 max |
| Sulfur (S) | 0.035 max |
| Silicon (Si) | 0.25 max |
Mechanical Properties
| Grade | Tensile Strength Min | Yield Strength Min | Comparison to Seamless |
|---|---|---|---|
| A | 325 MPa (47,000 psi) | 180 MPa (26,000 psi) | Matches A192 |
| C | 380 MPa (55,000 psi) | 205 MPa (30,000 psi) | Intermediate strength |
| D | 415 MPa (60,000 psi) | 255 MPa (37,000 psi) | Matches A210 A-1 |
Comparison Table: A178 (Welded) vs. Seamless Boiler Tubes
| Aspect | ASTM A178 (ERW Tube) | ASTM A192 (Seamless Tube) | ASTM A210 A-1 (Seamless Tube) |
|---|---|---|---|
| Manufacturing | Electric Resistance Welded | Seamless | Seamless |
| Longitudinal Weld | Yes (critical inspection point) | No | No |
| Cost | Lower (more economical) | Higher | Higher |
| Grade A Strength | Matches A192 exactly | Same as A178 Grade A | Higher |
| Primary Concern | Weld integrity & quality | Homogeneous structure | Balanced strength |
| Typical Use | Non-critical boiler sections, low-pressure boilers | High-pressure boiler tubes | General boiler tubes |
Common Applications
Industrial boiler generating tubes (lower pressure sections)
Heat recovery steam generator (HRSG) tubes
Low-pressure hot water boiler tubes
Heat exchanger tubes for non-severe service
Economizer sections of boilers
Critical Technical & Selection Notes
1. The Weld is Everything
The ERW longitudinal weld is the critical factor determining tube integrity:
Modern HF-ERW (High Frequency) produces high-quality welds.
The weld must be heat treated (normalized) to eliminate the cast structure and achieve properties matching the base metal.
Non-destructive testing (NDT) of the weld seam is mandatory (usually eddy current).
2. When to Use A178 (The Economic Choice)
Choose A178 ERW tubes when:
The boiler operates at lower pressures (e.g., industrial, commercial).
The application is non-critical (e.g., hot water heating).
Cost is a significant driver.
The design allows for a weld efficiency factor (typically 0.85 or 0.9 for ERW vs. 1.0 for seamless in code calculations).
Avoid A178 for:
High-pressure utility boilers (power plants).
Critical superheater sections with high metal temperatures.
Cyclic or fatigue service.
Where the project specification explicitly requires seamless tubes.
3. Fabrication & Installation
Can be bent and expanded, but with more care than seamless due to the weld seam.
The weld seam should ideally be positioned on the neutral axis during bending.
Weldable with standard methods.
Tube expanding into tube sheets must be done carefully to avoid cracking at the weld.
4. Code Acceptance & Restrictions
ASME Boiler and Pressure Vessel Code, Section I: Permitted, but with restrictions and often requiring a weld joint strength factor.
Design Responsibility: The designer must account for the welded construction, which may require thicker walls compared to seamless for the same pressure.
Inspection: More stringent inspection of the weld seam is typically required.
Industry Context: The Economics of Boiler Tubes
A178 represents the cost-effective segment of the boiler tube market:
Typical Tube Selection Logic:
High-Pressure Power Boiler: A192/A210 seamless (no risk tolerance).
Industrial Process Boiler: A178 Grade A/C ERW (acceptable risk, lower cost).
Hot Water Heating Boiler: A178 ERW (standard choice).
Rule of Thumb: Many package boiler manufacturers use A178 ERW tubes for the generating bank and lower-temperature sections to reduce cost, while using seamless tubes (A213) for the superheater.
Important Distinction from Other ERW Pipe
| A178 (ERW) | A53 / A135 (ERW) | |
|---|---|---|
| Product | Boiler/Heat Exchanger TUBE | General Service PIPE |
| Standard Focus | Heat transfer, high-quality weld | Fluid conveyance, structural |
| Testing & Quality | More stringent (NDT on weld, flattening tests) | Standard hydrostatic or NDT |
| Application | Inside boilers/fired heaters | Plumbing, fencing, low-pressure lines |
Final Takeaway: ASTM A178 is the electric-resistance-welded carbon steel boiler tube standard. It offers a cost-saving alternative to seamless tubes for lower-pressure and non-critical boiler and heat exchanger applications. Its acceptance hinges on modern, high-quality ERW manufacturing with proper heat treatment and inspection. For critical high-pressure/high-temperature service, seamless tubes (A192, A210, A213) remain the standard.
