Pipe Materials & Specifications
Pipe schedules, material grades, NPS sizing, specifications, and selection criteria for various service conditions.
- Explain pipe schedule numbers and their relationship to wall thickness
- Describe NPS (Nominal Pipe Size) and how it differs from actual dimensions
- Identify common pipe material grades for different service conditions
- Select appropriate insulation materials for personnel protection
- State the standard thread pitch and taper angle for NPT pipe threads
Leçon 1
Pipe Schedules, NPS & Wall Thickness
Pipe Schedule System
The exam asks: What schedule of pipe has the thickest wall? The answer is the highest schedule number - Schedule 160 (or XXS/double extra strong). Pipe schedule is a numbering system that indicates the wall thickness of a pipe for a given nominal size.
Common schedules include: 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160. Schedule 40 is considered standard wall and is the most commonly used in general service applications. Schedule 80 is extra-heavy wall used for higher-pressure applications.
NPS - Nominal Pipe Size
NPS (Nominal Pipe Size) is the designation system for pipe sizes. For pipes 14 inches and larger, NPS corresponds to the actual outside diameter (OD). For pipes smaller than 14 inches, NPS does not match the actual OD - it is a nominal designation from historical conventions.
The key fact is that for any given NPS, the outside diameter remains constant regardless of schedule. As schedule increases, the wall thickness increases inward, reducing the inside diameter (ID).
NPT Thread Specifications
The exam tests pipe thread knowledge. The typical angle for a standard pipe thread taper is 1 degree 47 minutes (written as 1 degrees 47'). This creates a 3/4-inch taper per foot. The standard thread pitch for 1-inch NPT pipe is 11.5 threads per inch.
NPT Thread Engagement
Hand-tight engagement plus 2-3 turns with a wrench provides a proper NPT joint. Thread sealant (tape or pipe dope) is required because the taper creates the seal.
Outside diameter - actual dimensions:
For piping 14 inches and larger, the NPS equals the actual outside diameter. For smaller pipes, the OD is larger than the NPS designation (a legacy of when NPS referred to inside diameter of early cast iron pipe). Key dimensions to know:
| NPS | Actual OD (inches) |
|---|---|
| 1/2 | 0.840 |
| 3/4 | 1.050 |
| 1 | 1.315 |
| 1-1/2 | 1.900 |
| 2 | 2.375 |
| 3 | 3.500 |
| 4 | 4.500 |
| 6 | 6.625 |
| 8 | 8.625 |
For Schedule 40 pipe, the inside diameter can be found by subtracting twice the wall thickness from the OD. For NPS 2 Schedule 40: ID = 2.375 - (2 x 0.154) = 2.067 inches.
Higher schedule numbers mean thicker walls. Schedule 40 is standard, Schedule 80 is extra-heavy. NPS designations keep the OD constant while the ID changes with wall thickness. NPT threads have a 1 degree 47 minute taper angle. For pipes smaller than 14 inches NPS, the actual OD is larger than the NPS designation.