PIPE CONNECTION

Pipe connections are either threaded or unthreaded, threaded connections include straight threads and tapered threads, and unthreaded connections include socket welds, butt-weld, flanged, and grooved end fittings.

OUTLINE: 

1. Threaded Connections
     a. American National Standard
          i. Tapered
          ii. Straight
     b. British Standard Pipe
          i. Tapered
          ii. Straight
     c. Thread Pitch
2. Unthreaded Connections

Related links:
     Thread Types
     How to Identify Threads
     Thread Sealing Tips

 

THREADED CONNECTIONS:

American National Pipe and British Standard Pipe

American National Pipe threads and British Standard Pipe threads are in many ways comparable with one another.  American National Pipe is most commonly used in the United States and includes both National Pipe Taper (NPT) threads which as the names infers is a slightly tapered thread, and National Pipe Straight (NPS) which is a straight or parallel thread.  In comparison, British Standard Pipe threads and are used most often in countries other than the United States.    The two types of British Standard Pipes are British Standard Pipe Taper (BSPT) and British Standard Pipe Parallel (BSPP).  Although American National Pipe and British Standard Pipes are very similar it is not encouraged to mate the American National Pipes and British Standard Pipes together if it the pressure holding capacity is important.  The reason NPT/NPS threads and BSPT/BSPP is due to the difference in threads.  NPT/NPS threads have a 60o angle and have flattened peaks and flattened valleys, this is also known as a Sellers thread form.  BSPT/BSPP threads have 55o angle with rounded peaks and valleys, which is also referred to as Whitworth thread form. 


National Pipe Tapered: The National Pipe Tapered (NPT) is most highly use in the United States.  The use of a tapered thread provides an excellent seal when tightened together. There are both Male Pipe Tapered thread (MPT) and Female Pipe Tapered thread (FPT) types.  Male Pipe Tapered threads are external threads, and Female Pipe Tapered threads are internal threads.  There are specially modified threads, NPTF also known as Dryseal are designed to be less likely to leak without the use of a sealant. However, to assure a more leak free seal the use of Teflon tape is recommended.  Threads that are compatible with NPT threads are NPTF (Dryseal), NPSL (National Pipe Straight Locknut), and NPSC (National Pipe Straight Coupling).

National Pipe Straight:  The straight threads are particularly used use for mechanical joining so it is not excellent a seal on its own.  However, as a result a O-ring, hard metal seal, or soft metal seal.   Other straight thread pipes include NPSH (National Pope Straight Hose) thread, NPSM (National Pipe Straight Mechanical) thread, and NSPL (National Pipe Straight Locknut) thread.  Also a less common version of NPS threads is garden hose threads (GHT).  It is important to note that the pitch and size differ from National Pipe Tapered threads.

British Standard Pipe Tapered: The British Standard Pipe Tapered (BSPT) is a tapered thread that provides a water tight seal. 

British Standard Pipe Parallel: The British Standard Pipe Parallel (BSPP) thread is a standard for many countries it is primarily used for mechanical joining and is not capable of providing a water tight seal on its own.  To provide a leak free seal the use of an O-ring, hard metal seal, or soft metal seal in needed.


Difference in Thread Pitch

The purpose of determining the pitch of a fitting is to know the number of threads on that fitting.  The way to determine number of threads is to finds its pitch which is the distance from one peak of the thread to the next.   To do this you will need a thread pitch gauge which will specify which the width of the thread, make sure that the teeth of the gauge fit perfectly to assure that the gauge selected is accurate.  Pitch gauges are available for both metric and imperial measurements of fittings. Although the word “pitch” is used universally the measurement most often used is threads per inch (TPI).  To measure threads per inch you do not need a gauge but a ruler or a caliper to determine how many threads are in one inch.  If you are measuring a metric fitting it will be threads per centimeter.  A thread gauge, if used correctly tends to be more accurate than measuring Threads per Inch.  Finally, please note that although TPI and pitch are often used synonymously they are actually an inverse of one another.  So in the chart below you can see that a 1/16” pipe has a 27 pitch or thread per inch. What this means is that the actual pitch is 1/27” or 27 threads per inch.  Thread pitches (or threads per inch) for NPT/NPS and BSP are listed below. 


                                                                                     

Pipe Size

Pitch (Threads/ Inch)

NPT/NPS

BSPT/BSPP

1/16"

27

--

1/8"

27    

28

1/4"

18

19

3/8"

18    

19

1/2"

14

14

5/8"

--    

14

3/4"

14

14

1"

11 1/2

11

1 1/4"

11 1/2

11

1 1/2"

11 1/2

11

2"

11 1/2

11

2 1/2"

8

11

3"

8

11

3 1/2"

8

11

4"

8

11

5"

8

11

6"

8

11

8"

8

--


If you would like to learn more please see these related links:

Thread Types
How to Identify Threads
Thread Sealing Tips

 


UNTHREADED CONNECTIONS
:

Unthreaded Connections are not threaded, must be either welded or bolted together.
 
Socket-weld:  Socket welds require a pipe and socket connection.  The pipe is the male fitting and the socket is equivalent to a female fitting.  To provide a tight seal it is best to line the outside of the pipe and the inside of the socket with a sealant.

Flanged: A flange is a plate or ring that is attached to a pipe.  Two flanged pipe then create a tight seal by being bolted together.

Butt-weld: A butt weld is two pipes lined up with one another and welded along the contact edge to create a seal.

Grooved end: A grooved end fitting has a groove or shoulder along the edge.  This fittings groove allows for a seal without the need for welding.