- Couplings & Threads
Couplings & Threads
|National Standard||National Pipe Straight Hose|
|NH or NST||NPSH or IPT|
ODM = Outside Diameter of Male
TPI = Threads Per Inch
Abbreviations for tapered iron pipe thread is NPT (National Pipe Tapered) or TIPT (Tapered Iron Pipe Thread) - mechanical seal.
Abbrev. for straight iron pipe thread is NPSH (National Pipe Straight Hose), IPT or SIPT (Straight Iron Pipe Thread) - gasket seal.
Abbrev. for straight fire hose thread is NH (National Hose) or NST (National Standard Thread) - gasket seal.
For additional information on measuring and identifying threads from Dixon Powhatan and Red Head Brass see link below.
Couplings are the aluminum or brass ends connected to hose and threads are the grooves in or on the couplings that make connecting a hose to a water source or to another hose possible. The male coupling has threads on the outside and the female coupling has threads on the inside. Anodized hardcoated aluminum couplings are common for most firefighting, industrial and other uses today. When compared to brass, aluminum is less likely to crossthread as it is a harder metal, is lighter weight and more economical. Brass is still used where corrosion is an issue, especially in a saltwater environment.
How do you determine what thread type you need? The easiest way to determine thread type is to check the couplings on the hose you are seeking to replace for the initials of the thread type. Sometimes the water source fitting or adapter is stamped as well. If you are connecting to a hydrant or other fire equipment a call to the local fire department can help determine thread type. If you are careful and precise you may also count TPI or Threads Per Inch and/or measure the ODM or Outside Diameter of the Male coupling (this is tricky for NPT) to help determine thread type. Please see chart and links above for details.
There are three primary threads in use nationally today as well as a number of regional threads. Pennsylvania and Ohio for example are known to have quite a variety of "local" threads. The city of New York has at least two threads. Though there are some cases where different threads may be compatible, for the most part they are not and it is critical that the correct thread is specified. If a mistake is made however, or there is a desire to make an existing hose or outlet compatible with another thread, FireHoseDirect does offer a variety of adapters to make that possibe. Please see our "Adapters" category on the left side of any FireHoseDirect web page.
NH or NST: National Hose aka National Standard Thread is often referred to as “fire hose thread” as this is by far the most common thread in use today by fire departments. This is a “straight” thread which means fitting size is consistent from tip to collar and the seal is made via the gasket inside the female coupling mating against the square face of the male coupling, much as it does in common garden hose connections. The threads don't make the seal, they merely hold the two fittings tightly together.
NPT: National Pipe Tapered is common in plumbing and some industrial uses. Sometimes referred to as “pipe thread” because PVC pipe and schedule 40 pipe are typically threaded in NPT. As its name implies this is a tapered thread so that as the two couplings are threaded together the fitting size changes to force friction to be created and thus sealing by thread instead of gasket. This is why pipe compound or thread tape is often used on NPT threads to insure a good seal.
NPSH or IPT: National Pipe Straight Hose or Iron Pipe Thread has the same TPI as NPT but the thread does not taper. NPSH is a straight thread and has a gasket in the female coupling just like NH/NST. The same threads per inch means that an NPSH female coupling can thread on to a male NPT fitting. The seal should be good as long as the male fitting is square to make a good seal with the gasket. In the case of PVC pipe you may still want to consider using an aluminum adapter if the fitting will be threaded on and off frequently as the metal to metal connection should wear better than the metal to PVC. Since the female NPT fitting does not have a gasket to allow for sealing and a male NPSH fitting is not tapered this combination is not compatible.
Finally there is GHT or Garden Hose Thread (actually there are quite a few more we won't get into here). This thread seals via washer in the female coupling and is not compatible with the other threads mentioned above. GHT is for 3/4 inch hose and we sell adapters to convert from GHT to NH or NPSH. A typical conversion would be GHT to 1 inch NH for example.
Adapter connectors are available to convert from one thread spec to another as well and increase or reduce diameter. When looking for threaded adapters you may see the letter M or F or sometimes FM in front of the thread type. This stands for Male or Female. For example 2.5" FM NPT x 1.5" M NH threaded adapter will allow you to connect and 1.5 inch NH coupled fire hose to what was a 2.5" male NPT supply outlet.
When shopping for a fire hose nozzle make sure it is compatable with your hose coupling. We offer both NH/NST and NPSH/IPT nozzles at FireHoseDirect. Please see our nozzle section for both economy nozzles and full fledged fire department grade everyday firefighting nozzles.
Higbee: This is the short flat triangular fade at the tip of a male NST/NH thread, as opposed to a thin thread beginning on NPT, that allows attaching a fire hose coupling quickly to another coupling or to the water supply outlet without jamming via cross-threading. There is typically a notch on one of the male and female coupling rocker lugs to locate the "Higbee", called a "Higbee Cut" (see pic at bottom). Line these notches up to make joining two lengths of hose quick and easy. NFPA 1963 requires a Higbee on NH thread. Please see photo at the bottom of this page for an example.
"Higbee Cut" and Fire Hose thread fade