The Transtek Guide to NPT and BSP Fittings - Details, Similarities and Differences
Throughout the worlds of pneumatics, hydraulics, resources, engineering, manufacturing and plumbing, fittings and adapters are the unsung heroes connecting everything together. Two of the most common types are NPT (National Pipe Taper) and BSP (British Standard Pipe). Although not readily interchangeable, both are designed to connect hoses, pipes and equipment. This comprehensive guide explores and explains NPT and BSP fittings, and hopefully provides you with the knowledge to make informed decisions on projects where leak-free, reliable or safe connections are required.
First Things First: What are Fittings? What Do They Do?
Fittings are components used to connect equipment, or to terminate or change the direction of pipes, tubes, and hoses in various systems. They come in a vast array of shapes, sizes and materials, with each one designed for a specific application. The correct fitting will provide a connection that's appropriate and safe for the project or task in hand.
NPT Fittings: An Introduction
NPT (National Pipe Taper) fittings are defined by the American National Standard for Tapered Pipe Threads (ANSI/ASME B1.20.1), which ensures consistency across all aspects of NPT threads. NPT fittings are designed to provide a leak-free connection, primarily through their tapered design. This taper creates a metal-to-metal, pressure-tight seal.
NPT Thread Sizes: NPS, Diameter and TPI
NPT fittings are sized based on a nominal pipe size (NPS). However, the NPS doesn't directly correspond to the diameter of the NPT thread. Instead, it relates to the inside diameter of a Schedule 40 pipe. The outer diameter and threads per inch (TPI) can be found in the basic chart below:
| Nominal Pipe Size (Inches) |
Actual Outer Diameter (Inches) |
Threads Per Inch (TPI) |
|---|---|---|
| 1/8 | 0.405 | 27 |
| 1/4 | 0.540 | 18 |
| 3/8 | 0.675 | 18 |
| 1/2 | 0.840 | 14 |
| 3/4 | 1.050 | 14 |
| 1 | 1.315 | 11.5 |
| 1 1/4 | 1.660 | 11.5 |
| 1 1/2 | 1.900 | 11.5 |
| 2 | 2.375 | 11.5 |
| 2 1/2 | 2.875 | 8 |
| 3 | 3.500 | 8 |
| 4 | 4.500 | 8 |
NPT Threads: Tapered or Straight?
While the defining characteristic of NPT threads is their taper, NPS fittings with straight threads are also available:
- NPT (National Pipe Taper): The “standard” NPT fitting. Threads are tapered and create a seal when tightened. This is the most common type of NPT thread and used for secure, pressure-tight connections.
- NPS (National Pipe Straight): Unlike NPT, NPS threads are straight / parallel and defined by ANSI/ASME B1.20.3. They do not provide a leak-tight seal on their own and require a gasket or other sealing method. Because of this, NPS threads are often used for mechanical connections and are less common in pressure applications.
NPT Fitting Materials: Which Works Best for You?
A range of materials provides a choice of properties and benefits:
- Brass: Used for plumbing or low pressure applications. Corrosion-resistant and inexpensive.
- Steel: Strong and durable. Often used in high pressure applications.
- Stainless Steel: Excellent corrosion resistance. Suitable for harsh environments, aggressive media and hygienic applications.
- Plastic (PVC, CPVC): Lightweight and corrosion-resistant. Used in plumbing and irrigation.
NPT Fittings: Common Types
- Elbows: Change the direction of a hose or pipe, or position equipment for easy access
- Tees: Create a branch in a hose or pipe, or connect multiple pieces of equipment
- Couplings: Connect two hoses or pipes together
- Adapters: Connect pipes or devices with different thread types or sizes
- Nipples: Short fitting with male threads to connect two female threaded ends
- Plugs: Close off the end of a hose or pipe
- Bushings: Reduce the size of a fitting
We also supply Universal Hose and Fittings Kits that simplify pressure calibration work and adapt to a wide range of connections in the field.
Ralston QTHA-KIT4-SS Universal QT Hose & Adapter Kit
Need help navigating the world of NPT fittings? Contact a Transtek product expert here to find out more.
BSP Fittings: An Introduction
BSP (British Standard Pipe) is another common type of threaded fitting or adapter, regularly used in Australia and other Commonwealth countries. Like NPT, BSP threads can be straight (BSPP, as defined by ISO 228/1) or tapered (BSPT, as defined by ISO 7/1).
BSP Thread Sizes: A Different System
BSP fittings also use a nominal size designation, but this differs from NPT. It's crucial to consult a BSP thread chart to determine actual dimensions:
| Nominal Size | Threads per Inch (TPI) | Major Diameter (mm) | Minor Diameter (mm) |
|---|---|---|---|
| 1/16" | 28 | 4.191 | 3.556 |
| 1/8" | 28 | 6.35 | 5.493 |
| 1/4" | 19 | 9.728 | 8.255 |
| 3/8" | 19 | 13.157 | 11.174 |
| 1/2" | 14 | 16.662 | 14.289 |
| 3/4" | 14 | 21.337 | 18.464 |
| 1" | 11 | 26.99 | 23.217 |
| 1 1/4" | 11 | 33.72 | 29.043 |
| 1 1/2" | 11 | 42.38 | 36.107 |
| 2" | 11 | 54 | 46.726 |
BSP Threads: Tapered or Parallel?
BSP fittings are available with a choice of thread types, which suit different applications:
- BSPT (British Standard Pipe Taper): Similar to NPT, both male and female threads are tapered. However, the taper angle and thread form are different from NPT.
- BSPP (British Standard Pipe Parallel): BSPP fittings use straight / parallel threads. They require a gasket or other sealing method to create a leak-tight seal.
BSP Fitting Materials: Which Works Best for You?
Like NPT, BSP fittings are also available in brass, steel, stainless steel and plastic. As with NPT, material suitability depends on the application requirements.
BSP Fittings: Common Types
BSP fittings share most common types with NPT. Elbows, tees, couplings, adapters, nipples, plugs and bushings are all available.
We also supply Universal Hose and Fittings Kits that simplify pressure calibration work and adapt to a wide range of connections in the field.
Crystal CPF x BSPP Hose and Fittings Kit
Need help finding your way through the world of BSP fittings? Talk to a Transtek specialist here for some expert advice.
NPT and BSP Fittings FAQs
Should I use NPT or BSP fittings? Which is better?
Which countries and industries use NPT fittings?
Which countries and industries use BSP fittings?
What material should I use for fittings?
Can NPT and BSP fittings be used together?
NPT vs. BSP: Crucial Differences
Beyond the issue of inter-compatibility, the key differences between them are:
- Thread Angle: NPT threads have a 60-degree angle. BSP threads have a 55-degree angle.
- Thread Form: NPT threads have sharp, flattened peaks and valleys. BSP threads have rounded peaks and valleys.
- Sealing: NPT relies on its taper and thread sealant for sealing. BSPP (parallel) relies on an O-ring, seal, washer or gasket for sealing. Like NPT, BSPT (tapered) relies on its taper and thread sealant for sealing.
Choosing the Right Fitting: A Step-by-Step Guide
As mentioned, selecting the correct fitting is crucial. Our step-by-step guide is designed to help:
- Identify the Thread Type: Determine whether you need NPT or BSP fittings. This is the first and most critical step as they are not interchangeable.
- Determine the Size: Measure your connection point(s) and determine the appropriate nominal size(s) by using a chart for NPT or BSP.
- Choose the Material: Material compatibility will be dictated by geographical location, application environment, media being used and pressure requirements.
- Choose the Fitting Type: Your connection requirements will determine the fitting type(s) required; elbow, tee, coupling, etc.
- Consider Pressure and Temperature Ratings: Ensure all fittings are appropriately rated for the pressure and temperature of the system.
Working with NPT and BSP Fittings: Best Practices
- Thread Sealant: Always use a suitable thread sealant on tapered NPT and BSP threads to ensure a leak-free seal. Apply the recommended level of sealant to the male thread only. Washers, seals and gaskets are typically used with parallel threads.
- Tightening: Avoid over-tightening NPT and BSP threads as this can cause galling and thread damage, which can lead to leaks. Hand-tightening plus a few turns with a wrench is usually sufficient for tapered threads. Parallel threads often require specific torque settings.
- Measurement: Use appropriate tools (e.g. thread gauge or calipers) to accurately measure NPT or BSP threads to ensure correct identification and compatibility.
- Cleanliness: Always ensure all threads are undamaged, clean and free from debris before assembly.
- Compatibility: Double-check thread types and sizes before connection. Do not attempt to force mismatched threads together.
Transtek Pty Ltd has proudly been providing specialist equipment and customised solutions to industries throughout Australasia since 1988.
If you have any questions about NPT or BSP fittings, or need help to determine which adapters are right for you, please get in touch. Our experienced team will help you find the right connection.
Scott Hall
Operations Manager
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