Threaded Pipe Connections

     Pipe threads are either tapered or straight (parallel). The two styles are not compatible.

     Tapered threads (as the name implies) have a slight taper. When mated together and tightened, the threads compress to form a seal. The most widely used pipe threads in North America are National Pipe Taper threads (NPT). Some confusion may result from the use of NPT, FPT, and MPT in describing threads. Both FPT and MPT are NPT threads, with FPT meaning female threads (internal) and MPT meaning male threads (external). NPTF (Dryseal) threads are modified NPT threads which are less likely to leak without a sealant. To assure a leak-free seal, we recommend using a sealant compound or Teflon tape. You can use NPTF threads with NPT threads, but you'll lose some of the leak-free characteristics.

     There are a number of threads that are compatible with NPT threads. They are NPTF (Dryseal), NPSC (National Pipe Straight Coupling), and NPSL (National Pipe Straight Locknut).

Tapered Thread

     

     Straight (parallel) threads are used for mechanical joining. They serve one purpose—to hold a fitting in place. As a result, an O-ring (elastomer), hard metal seal, or soft metal seal is required. Straight pipe threads include NPSM (National Pipe Straight Mechanical), NPSL (National Pipe Straight Locknut), and NPSH (National Pipe Straight Hose) threads. Unified screw threads (UN/UNF) conform to SAE (Society of Automotive Engineers) specifications. Sizing and pitches differ from the NPT threads. Less common straight threads are garden hose (GHT) and fire hose coupling (NST).

Straight Thread

NPT and BSPT/BSPP Threads
     The European equivalent to NPT tapered threads is BSPT (British Standard Pipe Taper) threads. BSPP (British Standard Pipe Parallel) threads are straight (parallel) threads and have the same angle, shape, and threads per inch (pitch) as BSPT threads. BSPT and BSPP threads are not compatible with and should not be substituted for NPT threads.
     NPT and BSPT/BSPP threads have different angles, shape, and (in most cases) threads per inch (pitch). The thread angle is 60° for NPT threads; 55° for BSPT/BSPP threads. NPT threads are flattened at the peaks and valleys, while BSPT/BSPP threads are rounded.
Pipe Size

Pipe OD

Threads per Inch

NPT

BSPT/

BSPP

 1/8"

0.405"

27

28

 1/4"

0.540"

18

19

 3/8"

0.675"

18

19

 1/2"

0.840"

14

14

 3/4"

1.050"

14

14

1"

1.315"

11 1/2

11

1 1/4"

1.660"

11 1/2

11

1 1/2"

1.900"

11 1/2

11

2"

2.375"

11 1/2

11

2 1/2"

2.875"

8

11

3"

3.500"

8

11

3 1/2"

4.000"

8

11

4"

4.500"

8

11

5"

5.563"

8

11

6"

6.625"

8

11

8"

8.625"

8

11

NPT

Flattened Peaks and Valleys

BSPT/BSPP

Rounded Peaks and Valleys

Unthreaded Connections
Socket-weld fittings can help you to save the time and effort needed to thread your pipe.Simply weld these fittings to your system for a tight seal by applying primer and PVC cement to the outside of the pipe end and inside of the socket. Butt-weld fittings are useful when you want even flow in sealed systems. Unlike threaded fittings, their smooth interior improves flow as well as minimizes the need for cleaning and servicing. To install, butt fittings up to your pipe and weld along the point of contact.
Flanged fittings have rotating flange connections (pipe has fixed flange connections) for easy installation. Flange gaskets sold separately. Grooved end fittings make it quick and easy to assemble and disassemble piping systems. Standard couplings with EPDM gaskets provide a seal to help maintain pipe pressure ratings when joining grooved-end pipe and fittings. Each coupling is coated with a rust-inhibiting, water-based orange paint and includes a two-piece housing; EPDM gaskets; and two zinc-plated nuts and bolts for securing coupling assembly to pipe. The gasket allows the coupling assembly to expand and contract as temperatures change, so there's no need for expansion joints.