Stainless steel nipples are essential components in plumbing, industrial piping, and fluid-handling systems. These short pipe sections with male threads on both ends are used for connecting two other fittings or pieces of equipment. While the material offers corrosion resistance and strength, the thread type on stainless steel nipples is equally critical for system compatibility and leak-proof operation.
In this comprehensive blog, we’ll explore various thread types such as NPT, BSPT, BSPP, and others commonly used in stainless steel nipples. By the end, you’ll be equipped with the knowledge to choose the correct thread type for your application.
Why Thread Types Matter in Stainless Steel Nipples
Thread types define how a stainless steel nipple connects to another pipe or fitting. If incompatible threads are used, it can lead to:
- Leaks
- Poor pressure handling
- Cross-threading or damage
- System failure
Hence, understanding the differences in thread types, standards, and regional preferences is crucial for selecting the right stainless steel nipple.
1. NPT – National Pipe Thread (Tapered)
Origin: United States
Standard: ANSI/ASME B1.20.1
What is NPT?
NPT stands for National Pipe Tapered thread, a thread type widely used in North America. It features a tapered profile which means the diameter of the thread decreases slightly along its length. This design allows for a mechanical seal when tightened.
Characteristics:
- 60° thread angle
- Tapered thread ensures leak-tight connections (with sealant)
- Commonly used in oil & gas, water, and industrial piping
Applications:
- Plumbing systems
- Hydraulic lines
- Industrial process piping
Example: A 1/2” NPT stainless steel nipple will taper to form a tight seal as it is screwed into an NPT threaded fitting.
2. BSPT – British Standard Pipe Tapered
Origin: UK/Commonwealth countries
Standard: ISO 7-1, EN 10226-1
What is BSPT?
BSPT is the British equivalent of NPT but has different thread angles and dimensions. Like NPT, it’s also tapered, designed to provide a pressure-tight seal. However, BSPT uses a 55° thread angle instead of 60°.
Characteristics:
- 55° thread angle
- Tapered for sealing
- Not compatible with NPT threads
- Used heavily in Asia, Europe, and Australia
Applications:
- Compressed air systems
- Water pipelines
- Gas transmission
Important Note: A BSPT stainless steel nipple should never be paired with an NPT fitting without an adapter.
3. BSPP – British Standard Pipe Parallel
Standard: ISO 228
What is BSPP?
Unlike BSPT, BSPP threads are parallel (straight). They do not create a seal by themselves and usually require an O-ring or bonded seal washer to achieve a leak-free connection.
Characteristics:
- Parallel threads
- Also 55° angle
- Requires sealing washer or face seal
- Common in hydraulic systems
Applications:
- Hydraulic and pneumatic systems
- Marine equipment
- Fire suppression systems
Example: BSPP stainless steel nipples are used where frequent disassembly or sealing via face fitting is required.
4. Metric Threads (ISO Metric)
Standard: ISO 261, ISO 965
What are Metric Threads?
Metric threads are often used in European machinery and automation industries. They’re defined by pitch (distance between threads) and thread diameter in millimetres.
Characteristics:
- Parallel threads
- Various pitches available
- Common in equipment and valves
Applications:
- European-manufactured pumps and machines
- Robotics and automation systems
Though not very common in general plumbing, metric threads can be found in specialized stainless steel nipples used in imported systems.
5. UN/UNF (Unified National/Fine Threads)
These are used primarily in the US for mechanical connections, not piping systems. While rare in stainless steel nipples used for fluid transmission, you might encounter them in certain industrial components.
Thread Identification – How to Tell Them Apart
Here are a few practical tips to help you identify the thread type on a stainless steel nipple:
| Parameter | NPT | BSPT | BSPP |
|---|---|---|---|
| Tapered/Parallel | Tapered | Tapered | Parallel |
| Thread Angle | 60° | 55° | 55° |
| Sealing Method | Thread sealant | Thread sealant | Washer or O-ring |
| Common Regions | USA, Canada | UK, Asia, Europe | Europe, Australia |
Use a thread gauge, calipers, or consult a sizing chart to match the pitch and diameter to the correct standard.
Tips for Selecting the Right Thread Type in Stainless Steel Nipples
- Check Regional Standards:
Use NPT for North America, BSPT/BSPP for UK/Europe/Asia unless otherwise specified. - Know the Application:
Tapered threads (NPT, BSPT) work best for pressure-tight seals. Parallel threads (BSPP) are ideal when dismantling or maintenance is frequent. - Verify with Drawings or OEM Specs:
Ensure compatibility with the rest of the system using design documents or equipment manuals. - Use Thread Sealants Wisely:
Apply PTFE tape or thread sealant paste for NPT and BSPT threads to avoid leaks. - Avoid Mixing Thread Types:
NPT and BSPT may appear similar but are not interchangeable—doing so could damage threads or lead to leaks.
Common Industries Using Various Thread Types
- Oil & Gas: NPT and BSPT
- Chemical Processing: BSPP for high-pressure equipment
- Water Treatment: BSPT/NPT depending on system design
- Marine: BSPP with face-seal washers
- HVAC Systems: Mixture of BSPT and BSPP
In these industries, stainless steel nipples are used due to their durability and corrosion resistance—especially in environments exposed to chemicals, moisture, or high pressure.
Conclusion
Understanding the thread types in stainless steel nipples is essential to ensuring a reliable, leak-proof, and efficient piping system. Whether it’s NPT for North American setups or BSPT/BSPP for European and Asian systems, the correct choice depends on the application, pressure requirements, and regional standards.
By familiarizing yourself with the differences between NPT, BSPT, and BSPP threads, you can avoid compatibility issues, increase system performance, and reduce downtime.
Always consult with manufacturers or certified piping engineers if you are unsure of the correct thread type. After all, a simple mismatch in threads can lead to significant operational issues.