Now that you understand API and ASTM pipe standards, let’s focus on pipe dimensions, schedules, and tolerances, which are essential for selecting the right pipe size for different applications.
1. Understanding Pipe Dimensions
Steel pipes are classified by outer diameter (OD), wall thickness (WT), and length (L).
🔹 Outer Diameter (OD) – The external measurement across the pipe
🔹 Wall Thickness (WT) – The thickness of the pipe wall
🔹 Nominal Pipe Size (NPS) – The standardized pipe size system
Example: NPS 6 (6-inch pipe)
- The actual outer diameter (OD) is 6.625 inches (168.3 mm)
- The wall thickness (WT) depends on the schedule
✅ Pipes are measured by OD and WT, not just the nominal size.
2. Pipe Schedule (SCH) – What Does It Mean?
Pipe schedule (SCH) indicates the wall thickness of the pipe.
| Schedule | Wall Thickness Increases as Schedule Increases |
|---|---|
| SCH 5 | Thin-walled pipe |
| SCH 10 | Used in low-pressure systems |
| SCH 20 | Slightly thicker but still for light-duty |
| SCH 40 | Most common for general applications |
| SCH 80 | Thicker wall, used for high-pressure systems |
| SCH 160 | Extra thick, used in extreme pressure conditions |
| XXS (Double Extra Strong) | Maximum thickness, for highest pressure |
Example: 6-inch pipe (NPS 6)
- SCH 40: 0.280” (7.11 mm) wall thickness
- SCH 80: 0.432” (10.97 mm) wall thickness
- SCH 160: 0.562” (14.27 mm) wall thickness
✅ The same pipe OD can have different schedules, which affect pressure rating.
3. How to Select the Right Pipe Schedule?
The pipe schedule depends on pressure, temperature, and application.
| Application | Common Pipe Schedules |
|---|---|
| Low-pressure water & gas | SCH 10, SCH 20 |
| General construction piping | SCH 40 |
| Oil & gas pipelines | SCH 40, SCH 80 |
| High-pressure steam | SCH 80, SCH 160 |
| Extreme pressure systems | SCH 160, XXS |
✅ For high-pressure applications, thicker schedules (SCH 80, SCH 160) are required.
4. Pipe Tolerances (Manufacturing Accuracy)
Steel pipes have allowable tolerances for diameter, wall thickness, and length.
(1) Outer Diameter (OD) Tolerance
- Seamless Pipes: ±1% of the OD
- Welded Pipes: ±0.5% of the OD
(2) Wall Thickness (WT) Tolerance
- API 5L, ASTM A106: -12.5% (minimum WT must be at least 87.5% of the nominal value)
- ASTM A53: -10% (used in general piping applications)
(3) Length Tolerance
- Standard pipe length: 5m to 12m (random length RL) or fixed length (FL)
- Allowable tolerance: ±0.5% on specified length
✅ Tighter tolerances are required for precision applications (oil & gas, high-pressure systems).
5. Pipe End Types
Steel pipes have different end finishes depending on their connection method.
| End Type | Description | Application |
|---|---|---|
| Plain End (PE) | Square-cut ends, no threading | Welding, slip-on fittings |
| Beveled End (BE) | Angled cut (30° or 37.5°) for welding | Butt-weld joints |
| Threaded End (TE) | External threads (NPT or BSP) for screwing | Low-pressure pipelines |
| Grooved End (GE) | Grooves for mechanical couplings | Fire protection, HVAC systems |
✅ Oil & gas pipelines typically use beveled ends for strong welded connections.
6. Pipe Weight Calculation Formula
The weight of a steel pipe depends on OD, wall thickness, and length.
Weight Formula for Carbon Steel Pipe
Weight (kg/m)=0.02466×(OD−WT)×WT\text{Weight (kg/m)} = 0.02466 \times (\text{OD} – \text{WT}) \times \text{WT}Weight (kg/m)=0.02466×(OD−WT)×WT Weight (lbs/ft)=10.69×(OD−WT)×WT\text{Weight (lbs/ft)} = 10.69 \times (\text{OD} – \text{WT}) \times \text{WT}Weight (lbs/ft)=10.69×(OD−WT)×WT
Example: 8-inch SCH 40 pipe (OD: 8.625”, WT: 0.322”)Weight=10.69×(8.625−0.322)×0.322=34.72 lbs/ft\text{Weight} = 10.69 \times (8.625 – 0.322) \times 0.322 = 34.72 \text{ lbs/ft}Weight=10.69×(8.625−0.322)×0.322=34.72 lbs/ft
✅ Knowing pipe weight helps in transportation, installation, and load calculations.
🔹 Summary of Lesson 6
- Pipes are classified by OD, WT, and NPS, with wall thickness determined by the schedule.
- Higher schedules (SCH 80, SCH 160) mean thicker walls and higher pressure ratings.
- Pipes have manufacturing tolerances for OD, WT, and length to ensure quality.
- End types (PE, BE, TE) depend on the connection method required.
- Weight calculation helps in handling and installation planning.