Fire Service, NPT, NPSH, and Delayed Shipments

by Kochek | May 9, 2013 | News

One of the most common causes of delayed shipments or extended lead times in the fire industry is a lack of understanding that leads to questions which must be answered before a product can be built. It can take days to track down the answer to,”Is this really what you want?” I give you a single, yet poignant, example: NPT threads.

National Pipe Thread is, of course, found on most pipe fittings and the like, used in the construction of shopping centers, ships, trucks, and so on. It is engineered to be a permanent connection. In other words, used to build something engineered to be around for a long time. Like shopping centers… ships… trucks… and so on… With NPT threads, a seal is made by the contact between female and male threads. Because the male thread is tapered, it gradually gets too large as it is screwed together with the female and the threads bind/seal. Without a gasket to deteriorate, this will be a sound connection for a very long time, and because it’s stationary the seal won’t likely be corrupted. Not only that, it can facilitate the max system pressure of many a project due to the fine character of the thread itself, (the purpose for which it was designed).

This image shows the male taper of NPT
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Now the NPSH or National Pipe Straight Hose thread incorporates the same fine thread, mimicking the same pitch and thread count per inch of it’s high pressure sister, except the female thread incorporates a gasket at the base of the threads. The non-tapered male of the NPSH thread screws all the way in to meet the gasket and seals. Bound to need gasket replacement eventually, yes; but hose couplings are put together and taken apart regularly and a female NPT cannot swivel. So the gasket is a small concession in exchange for adding the the convenience of a “high pressure temporary portable threaded connection”.

OK, so how does this relate to my adapters not getting to me in a timely fashion??? Give me what I came here for!

The US Forestry Service enlists a fire fighting theory that oftentimes uses high pressure to accomplish it’s objectives. Naturally, because the “nationally established” high pressure thread is NPT, the US Forestry Service went about as far as an organization can go to make NPT the “standard” for all of their operations. Well there always seems to be some confusion as to which thread was appropriate for a given application in the Forestry Service because, “I’ve got two national pipe threads and they don’t work so good”. It is because you have a thread designed to be screwed together once and left alone, (NPT), and a hose coupling thread, (NPSH), meant to be connected and re-connected time and time again.
So let’s fix at least one problem today, shall we?
If you screw a tapered male pipe thread, (NPT), into a NPT female, (no gasket), the male thread wedges against the female thread and makes the seal, permanently.
If you screw the same tapered male pipe thread, (NPT), into a NPSH female, (w/gasket, no taper ), the male thread wedges against the gasket and makes the seal, temporarily.
If you screw a straight pipe thread, (NPSH), into a tapered female pipe thread, (NPT), it will not work.

So here’s the moral of the story:
NPSH Females work with NPT and NPSH Males!

So….. Unless you are in fact building shopping centers, ships, trucks, or things of this nature…
Always order NPSH (P) females and NPT (N) males!

Don’t be the next to suffer delays in shipment because of a preventable situation.

One last tip…
_eBook_12-12 When figuring out what your part number should be for a Kochek fitting with pipe or any threads not NH, remember sizes are stated in this order: first- Storz size, next- Female thread size, and lastly- Male thread size. You can also use this legend to help:

The style number represents the configuration. In this case, it is a female to male rigid rocker lug reducer, (37R). This is going to be a female 4.5″ special thread x 2.5″ NPT male. Special threads denoted in part numbers by (X) must have the thread specification in the description of the purchase order and stated as diameter x threads per inch. Example “x = 4.063 x 8”

See ya on the big one…..