Quick question: Do you know the differences between NH (NST), NPT and NPSH (IPT) when it comes to fire hose threading? If you aren’t familiar with fire hose couplings and threading, these questions can be pretty confusing.
And not understanding these differences can lead to mismatched parts, which leads to loss of money and time, not to mention plenty of frustration.
The truth is, many factors go into matching the correct fire hose devices together. And we’re here to clear up some of the mystery and give you some great resources to keep handy for when you do need to purchase and match the right products.
Defining Couplings and Threading
First, a hose coupling is simply a connector located on the end of a hose. It couples, or connects, with a hose, tap or water source and is typically made of brass, stainless steel or aluminum.
The key characteristics to understand with regard to proper thread attachment are size (3/4” to 6”), thread standard and gender — male coupling has threads on the outside, female coupling has threads on the inside. Some couplings are “sexless”: Storz, for example, is a quarter-turn quick connection.
The easiest way to identify what type of thread you need is by looking at the couplings on the hose you want to replace. There should be a stamp with the initials of the thread type. A water source fitting/adapter may also have the stamp. If you’re checking what to use on a fire hydrant, call the local fire department.
If there’s no stamp available, you can carefully count the threads per inch and measure the outside diameter of the male coupling; however, this can be a very tricky procedure.
Threading Alphabet Soup
What are the initials you should be using to identify the proper threading? There are many threads out there, but three are primarily used: NH or NST, NPT, and NPSH or IPT.
NH stands for National Hose and NST stands for National Standard Thread. This is often referred to as “fire hose thread” because fire departments use it the most. It’s popular on many fire hydrants and accessories.
Compatibility: Male NH/NST to Female NH/NST; Female NH/NST to Male NH/NST. Because these are not compatible with other thread types, adapters are used to connect the different types of couplings.
NPT stands for National Pipe Tapered and is mostly used in plumbing and industrial. It’s recommended to use plumber’s tape or thread-sealing paste.
Compatibility: Male NPT to Female NPT, NPTF, NPSM, NPSH; or Female NPT to Male NPT or NPTF.
NPSH or IPT refers to National Pipe Straight Hose or Iron Pipe Thread. NPSH is a straight thread and is popular on low-pressure water suction and discharge hose couplings.
Compatibility: Male NPSH to Female NPSH, NPSM or Female NPSH or Male NPSH, NPT, NPTF and NPSM.
Knowing what goes into fire hose threading and coupling might seem confusing at first, but knowing the basics and having some solid resources at your disposal takes the guesswork out of the process. For more information, check out One Stop Fire Products’ Thread Standards chart.