What Is NPT?
NPT (National Pipe Taper) threads are standardized tapered threads used for pipes and fittings in North America. The taper allows for a tight, leak-proof seal when combined with thread sealants like PTFE tape or pipe dope. NPT threads have a 60-degree thread angle and a taper of 1° 47′ 24″ (approximately 0.75 inches per foot) .
NPT Thread Size Chart
Understanding NPT sizes is crucial, as the nominal pipe size (NPS) does not directly correspond to the actual diameter. Below is a chart detailing common NPT sizes:
|
Nominal Pipe Size (NPS) |
Threads per Inch (TPI) |
Outside Diameter (in) |
Pitch (in) |
Hand-Tight Engagement (in) |
|
1/16" |
27 |
0.3125 |
0.03704 |
0.0644 |
|
1/8" |
27 |
0.405 |
0.03704 |
0.0806 |
|
1/4" |
18 |
0.540 |
0.05556 |
0.1142 |
|
3/8" |
18 |
0.675 |
0.05556 |
0.1177 |
|
1/2" |
14 |
0.840 |
0.07143 |
0.1575 |
|
3/4" |
14 |
1.050 |
0.07143 |
0.1626 |
|
1" |
11.5 |
1.315 |
0.08696 |
0.2006 |
|
1 1/4" |
11.5 |
1.660 |
0.08696 |
0.2074 |
|
1 1/2" |
11.5 |
1.900 |
0.08696 |
0.2124 |
|
2" |
11.5 |
2.375 |
0.08696 |
0.2219 |
Measuring NPT Threads
To accurately identify NPT sizes:
-
Measure the Outside Diameter (OD): Use calipers to measure the OD of the male thread or the Inside Diameter (ID) of the female thread.
-
Determine Threads per Inch (TPI): Use a thread pitch gauge to count the number of threads per inch.
Refer to a Thread Chart: Match your measurements to a standard NPT thread chart to identify the nominal size.
Important: NPT sizes are based on nominal pipe size, not actual measurements. For example, a 1/2" NPT fitting has an OD of approximately 0.84 inches
NPT vs. Other Thread Types
While NPT is prevalent in North America, other thread types are used globally:
-
BSPT (British Standard Pipe Taper): Similar to NPT but with a 55-degree thread angle. Common in the UK and Asia.
-
BSPP (British Standard Pipe Parallel): Parallel threads requiring an O-ring or washer for sealing.
-
Metric Threads: Used in European equipment, specified by diameter and pitch (e.g., M5x0.8).
-
UNF (Unified National Fine): Straight threads used in precision applications.
Tips for Selecting Pneumatic Fittings
-
Application Requirements: Consider pressure, temperature, and environmental factors.
-
Material Compatibility: Ensure the fitting material is compatible with the media (e.g., air, water, chemicals).
-
Sealant Use: Apply appropriate thread sealant to tapered threads to prevent leaks.
-
Regular Inspection: Periodically check fittings for wear or damage to maintain system integrity.
Here are three frequently asked questions about pneumatic fittings:
-
What types of pneumatic fittings are available?
Common types include push-to-connect, barbed, compression, and threaded fittings—each suited for different tubing and application requirements. -
How do I choose the right size pneumatic fitting?
Selection depends on the tubing’s outer diameter (OD), the thread type (e.g., NPT, BSPT), and the pressure rating of the system. Using a fitting size chart helps ensure proper fit and performance. -
Can pneumatic fittings be reused?
Some fittings, like threaded metal ones, can be reused if undamaged, but push-to-connect fittings may wear or lose grip strength over time and should be replaced to maintain a secure seal.
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