Malleable iron threaded fittings are the usual choice for small diameters in sprinkler systems, especially for connecting sprinklers and the final branch lines. They come in two main standards: American, with NPT threads, and British/European, with BSPT threads. Mixing them is a common cause of leaks. This guide explains the difference and how to choose.
Key points
- NPT and BSPT threads are not compatible, even if a fitting seems to tighten.
- Choose the standard to suit the pipe, sprinklers and equipment on the project, and keep to it throughout the system.
- In fire systems order listed and approved fittings, such as MECH’s UL / FM approved British-standard range.
- Use a suitable thread sealant without letting it into the bore.
The two standards in MECH products
| Item | American standard | British standard |
|---|---|---|
| Class / standard | Class 150 | BS EN 10242 / ISO 49 |
| Material | Malleable iron ASTM A197 | Malleable cast iron BS EN 1562 |
| Dimensions | ASME B16.3; plugs B16.14; unions B16.39 | To BS EN 10242 |
| Threads | ASME B1.20.1 (NPT) | ISO 7-1 (BSPT) |
| Approval | Per each fitting’s page | UL (EX6624) / FM, UL rated 500 psig for a number of fittings |
| Sizes | Up to 6″ depending on fitting | Depending on fitting, e.g. 1/2″ – 2″ |
Why NPT and BSPT do not mix
NPT threads have a 60° angle and BSPT 55°, and threads per inch differ in several sizes. A fitting may run in a few turns and seem tight, but the threads do not match along their full length, leaving a leak path that shows up at the pressure test or after commissioning.
Caution
Check the thread type of sprinklers, small valves and gauges before ordering threaded fittings. Many American sprinklers and devices have NPT threads.
Sealing the threads
- Clean oil and cutting swarf from the threads before assembly.
- Use PTFE tape or a pipe joint compound suitable for water service, or the method the designer specifies.
- Apply sealant to the male thread only, leaving the first one or two threads bare so none enters the bore.
- Tighten to an appropriate torque without overdoing it; overtightening can split the fitting.
- Pressure-test to the project requirements before commissioning.
Common mistakes
- Mixing American and British fittings in the same system.
- Too much sealant, which enters the pipe and reaches sprinkler orifices.
- Using unlisted fittings in a fire system that requires approval.
- Overtightening, causing cracks that may only show under pressure.
Frequently asked questions
Can an adaptor be used between NPT and BSPT?
Adaptors exist, but it is better to standardise the thread type across the system. Any adaptor must suit the pressure and be approved for the use.
What does BMI mean?
A common abbreviation for Black Malleable Iron — ungalvanised malleable iron.
Is hemp or jute used with compound?
It is a traditional method on some work, but the sealing material for the project is set by the designer; the key is that nothing enters the bore.
References
- ASME B16.3 — Malleable Iron Threaded Fittings: Classes 150 and 300.
- ASME B1.20.1 — Pipe Threads, General Purpose (Inch).
- BS EN 10242 — Threaded pipe fittings in malleable cast iron.
- ISO 7-1 — Pipe threads where pressure-tight joints are made on the threads.
- NFPA 13 — Standard for the Installation of Sprinkler Systems.
This guide is for technical awareness and does not replace the manufacturer’s data sheet or the requirements of the project’s approved designer.
