NPT and NPTF are standard thread designs that enjoy wide levels of adoption in industry. For pipes, the NPT threading requirements should be as per ANSI B1.20.1, while NPTF requirements follow ANSI B1.20.3. In this article, we explore the differences of NPT vs NPTF, as well as compare NPT with other thread designs including NPS, JIC, and MIP.
Differences Between NPT vs NPTF
NPT stands for National Pipe Taper, while NPTF means National Pipe Taper for Fuels. Generally, both thread design applications range from basic electrical conduits to high-pressure piping that carries hydrocarbons and aggressive liquids. Because of the thread taper on both thread designs, their fasteners provide solid and vibration-resistant connections. Another similarity is that they share the same taper-per-inch, threads-per-inch, and pitch diameters. However, NPT and NPTF have several differences across parameters as the following table illustrates.
|Parameter||Difference Between NPT and NPTF|
|Function||NPT is general-purpose tapered pipe thread. On the other hand, NPTF is a dry seal tapered pipe thread.|
|Thread Crest||Generally, both thread crests have the same parameter requirements. However, NPT has a larger range.|
|Thread Root||The design of NPT thread roots allows clearance with mating thread crests on assembly. But NPTF thread roots interfere with mating crests.|
|Seal||To be leak-proof, the NPT requires a sealant due to the allowance between its root and crest. For NPTF, there is a mechanical seal as the crest and root interfere. Thus, there is no need for any sealant.|
|Gauge Requirement||The L1 gauge requirements are the same for NPT and NPTF for sizes 3/8 inch and below. For ½ inch and above, the requirements differ mainly in the Ring Gauge Minor Diameter and the Plug Gauge Major Diameter.|
|Inspection||Checking the size for NPT requires a single L1 plug or a single thin L1 ring for internal or external threads respectively. But NPTF requires multiple plugs and gauges for additional L2 and L3 threads in assembly.|
Because of the similar taper design, NPT and NPTF fit into each other. Although, due to the differences in the major and minor diameters, as well as the crest and root, a sealant is required to avoid leakage in interchanged fittings. However, interchanging them is not advisable, especially for thread sizes of ½ inch and above, due to the varying gauge requirements.
NPT vs NPS
Both the NPT and the NPS (National Pipe Straight), also known as the NPSM (National Pipe Straight Mechanical), have the same thread angle, which is 60°. In addition, both designs have the same threads per pitch with flat peaks and valleys. Nevertheless, they have a fundamental difference with the NPT being tapered and the NPS having a parallel cross-section as the figure below indicates.
As a result, the NPS needs a gasket or O-ring to accomplish a seal. However, it still does not provide an effective enough seal in comparison with tapered threads. Thus, engineers prefer tapered threads in applications involving fluid transport. When it comes to the possibility of interchanging both fittings, it is important to note that their threads will engage because they have identical thread angles. However, they do not seal properly as there is a variation in cross-section and other thread parameters. Thus, it is not advisable to interchange their fittings.
NPT vs JIC
Generally, NPT and JIC (Joint Industry Council) are fundamentally different in that they stem from different standards. Moreover, these standards are the SAE J514 and MIL-DTL-18866 for the JIC. ANSI B1.20.1 specifies the NPT fittings guidelines. The table below highlights other differences in these thread forms.
|The thread is tapered at an angle measured from the central axis of the fitting.||These fittings run parallel to each other.|
|Each successive thread has an angle of 60° between.||Threading comes in variable forms including coarse, fine, or extra-fine thread sizes.|
|The sealing is dependent only on the tapering of the threads.||Has a 37° flare angle at the end of its fittings. As a result, has better sealing capabilities.|
|Its sealing capacity is relatively less, so its ideal for low-pressure applications.||Possesses superior sealing, so the preferable option in high pressure applications.|
Although it may be possible to fit a male NPT fitting to a female JIC fitting of a similar size, the integrity of the seal would not be reliable. Additionally, putting a male JIC fitting to a female NPT fitting is not possible. Thus, these thread forms are not interchangeable.
NPT vs. MIP
Generally, MIP stands for Male Iron Pipe and its compliment FIP is Female Iron Pipe. Because of the thread dimensions of MIP and that of NPT are the same, they are interchangeable. However, there are some differences between them, which the table below highlights.
|Material||Plastic and PVC.||Iron, copper, steel, PVC, and plastic.|
|Application||Low pressure applications including residential water systems and hydraulic systems.||High-pressure applications such as industrial and commercial piping systems as well as some residential water systems.|
|Shape||Angle between taper and central axis.||Straight|
|Truncation of roots and crests||Always flat.||Could be flat or not.|
|Recommended sealant||Sealant compound or PTFE (Polytetrafluoroethylene) tape.||Pipe dope or Teflon tape. Also uses O-ring or gasket to prevent leakage.|