Sizes

Pipe thread sizes are based on an inside diameter (ID) or flow size. For example, “1/2–14 NPT” identifies a pipe thread with a nominal inside diameter of 1/2 inch and 14 threads to the inch, made according to the NPT standard. If “LH” is added, the pipe has a left hand thread. The most common global pipe thread forms are:

Plastic injection molded thread forms are manufactured to ANSI B2.1 and SAE J476 standards.

The word “tapered” in several of the above names points to the big difference between many pipe threads and those on bolts and screws. Many pipe threads must make not only a mechanical joint but also a leakproof hydraulic seal. This is accomplished by the tapered thread form of the male matching the thread form of the female tapered thread and the use of pipe sealant to fill any voids between the two threads which could cause a spiral leak. The bottoms of the threads aren't on a cylinder, but a cone; they taper. The taper is 1/16 inch in an inch, which is the same as 3/4 inch in a foot.

Because of the taper, a pipe thread can only screw into a fitting a certain distance before it jams. The standard specifies this distance as the length of hand tight engagement, the distance the pipe thread can be screwed in by hand. It also specifies another distance – the effective thread, this is the length of the thread which makes the seal on a conventional machined pipe thread. For workers, instead of these distances, it is more convenient to know how many turns to make by hand and how many with a wrench. A simple rule of thumb for installing tapered pipe threads, both metal and plastic, is finger tight plus one to two turns with a wrench. Torque installation values can be determined per application, but due to the variations involved in pipe joints such as disimiliar materials of male and female threads, type of sealants used, and internal variations in product wall thickness, a standard torque specification cannot be generically applied .

This table shows the distances and number of turns called for in the standard. A tolerance of plus or minus one turn is allowed, and in practice threads are often routinely cut shorter than the standard specifies. All dimensions are in inches.

American Standard Taper Pipe External Thread

Taper/Parallel Threaded Joints

Despite the standards created to maintain uniform fittings, tapered pipe threads are inexact and during the course of use and repair the threads can become damaged and susceptible to leakage. The area where the crest and the root of the thread meet can form a spiral leak path no amount of tightening will eliminate.

A pressure tight joint is achieved by the compression in the threads resulting from tightening. This compression and sealing occurs in the first few turns of the internal thread. As wrenching takes place, material from both the male and female threads deform into each other. This ensures full thread contact which minimizes spiral leakages. Variations between injection-molded plastic and machined metal thread forms can occur due to different manufacturing processes.

Pipe threads were originally designed as machined thread forms. With the use of thermoplastics and plastic injection molding in the manufacture of plastic pipe thread forms, mold shrinkage and plastic sink make it difficult to insure leak free joints. For this reason, the use of a Teflon based sealant is recommended on all plastic pipe threads. The most common form of sealant is Teflon tape wrapped 2 to 3 turns around the male thread before assembly. Liquid Teflon based sealants are also used successfully to ensure a pressure tight seal. It is always important to use care when applying sealants to avoid introducing the sealant material into the system flow path.

The following sections show examples of how different threads are used and issues that can arise in attempting to create a leak free connection.

When a BSPT tapered male thread is tightened into a straight female thread (BSPP) the seal can only be made at the base of the female port with 1 or 2 threads. Sealing is compromised by the lack of thread form control in BSP specifications. Variation in crests and roots may cause a mismatch in the thread and create a spiral leak. Thread sealant is required to seal this combination.

Using both tapered male and female BSPT threads would offer a better chance of sealing since you are now matching the taper of the male and female thread. This offers more threads a chance of sealing against spiral leakage. Crest and root control is still missing, but with thread sealant, a pressure tight joint would be easier to accomplish.

A number of variations of the NPT thread have been introduced to overcome the problem of spiral leakage and are known as Dryseal threads (See SAE standard J476). The best known is the NPTF (F for Fuel). With this thread design, there are controls on the crests and roots of both the male and the female threads to ensure the crest crushes or displaces material into the root of the mating thread. The interference fit between the crest of one thread and the root of the other, along with the thread flanks matching, seals against spiral leakage.

A variation of the Dryseal thread is the NPSF (National Pipe Straight Fuel). It is used for internal threads and a NPTF external thread can be screwed

GLOBAL THREAD

Works with Inch or Metric Threads

"Global Thread" fittings can be assembled with female threads produced to the following standards:

Tapered NPTF

Parallel (BSPP, BSP, ISO 7)

Tapered (BSPT, PT, ISO7, ISO 228)