This article examines NPT and NPS fittings, which are common acronyms used in the plumbing industry. NPS is the acronym for National Pipe Straight, and NPT is the acronym used to refer to National Pipe Tapered; both acronyms refer to a standard of thread used in the plumbing industry. Subsequent paragraphs of this article will examine the differences and similarities between NPT and NPS fittings and some of the features that separate one from the other.
National Pipe Thread is a classification for approved fittings that have been adopted and standardized by the American National Pipe Standard. National Pipe Tapered (NPT) and National Pipe Straight (NPS), as well as National Pipe Taper for Fuel (NPTF) and National Pipe Straight Mechanical (NPSM), are types of threaded fittings under the National Pipe Thread. There is a common misconception that NPT stands for National Pipe Thread, but National Pipe Tapered and National Pipe Thread are different.
The fundamental difference between NPT and NPS fittings lies in the application, and their different applications rely on their physical appearance/design. As its name suggests, an NPT fitting has a tapered thread when viewed from the side, and the threads are more prominent at the end of the threaded system than at the open end of the thread. On the other hand, the threads of an NPS fitting are straight and parallel. Unlike NPT fitting, the threads of an NPS fitting are equal, and the diameters of each thread do not vary.
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Differences Between NPT vs. NPS Fitting
National Pipe Tapered (NPT) and National Pipe Straight (NPS) fittings are fundamentally different threaded fittings that serve different purposes.
Applications and Usage
NPT is commonly used on the ends of pipes, nipples, and other fittings such as elbows, tees, couplings, etc. NPT threaded fittings are usually used to secure threaded fittings together. For example, for a ball valve to attach to the end of a through-hull fitting, an NPT is best used to secure a watertight connection between the ball valve and the through-hull fitting.
On the other hand, NPS threaded connections are often used to mate into surfaces with varying degrees of thickness. For example, in the instance of a through-hull fitting, the nut that holds it on the inside of the hull will have to be flexible enough to accommodate different thicknesses of holes. In such a situation, an NPS is a good choice because the nut can move up and down the length of the NPS without restriction (as a result of the equal diameter of the threads of an NPS) and clamp where necessary. NPS is also used for ballast system solutions.
Thread sealant or PTFE (polytetrafluoroethylene) tape makes the seal stronger and ensures a leak-free, watertight seal. NPT threaded fittings make for tighter seals and connections because of their physical build (outfitted with threads that progress from smaller to larger, from the end of the thread to the top of the NPT). This physical build ensures a secure connection when the NPT is mated with another threaded fitting.
On the other hand, NPS threads cannot seal without the help of an O-ring or a gasket to create the much-needed seal. This inability of NPS threaded fittings to seal independently results from its straight threads.
Similarities Between NPT vs. NPS Fitting
There are few similarities between National Pipe Straight (NPS) and National Pipe Tapered (NPT). This lack of numerous similarities is because they are used for different purposes and in various areas of plumbing.
Sub-Categories of the National Pipe Thread
National Pipe Straight (NPS) and National Pipe Tapered (NPT) are two sub-categories of the National Pipe Thread. The National Pipe Thread is a means of classifying threaded fittings recognized by the American National Pipe Standard. These threads interconnect fittings and pipes by joining an internal thread (also called ‘female’) with an external thread (also known as ‘male’).
Angle, Pitch, and Other Features
NPT and NPS fittings have the same thread angle at 60° included angle, and they have the same pitch (threads per inch). NPS and NPT are similar in terms of shape; they both have valleys and flat peaks.
NPT and NPS are not interchangeable, and this boils down to their physical differences. NPT threads are very secure when properly sealed, and the use of sealant or plumbing tape makes them virtually impenetrable, so they are often used to move liquids and gas. NPS threads are not as secure as NPT threads and are not used to transfer liquids and gas unless they are sealed with an O-ring. One must keep in mind that attempts to mate NPT and NPS together will not be successful. NPT and NPS threads will engage each other for a few turns before the tapered threads of the NPT prevent them from sealing together. Thus, it is essential to note that you are dealing with two different threading fittings if you have a male (external) and female (internal) thread that screws together but remains loose and do not seal properly.