NPT vs NPS fittings
- Thread starter NYSail
- Start date
Primary concern is not mixing fittings with different threads. I recently replaced some of my fittings with Groco bronze fittings with BSPP (British straight pipe parallel) threads. Thru hull, valve, & tail pieces with bspp threads can be ordered directly from Groco ensuring that all of the threads are matched.
NY,
Having a mechanical piping design background, here's some info for thought in decision making:
Understanding NPT, NPTF & NPS Thread Connections
NPS, NPT & NPTF
NPS = National Pipe Straight
NPT = National Pipe Thread
NPTF = Nation Pipe Thread Fuel
Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight (parallel). Both threads have a 60° included angle and have flat peaks and valleys. Tapered Pipe Thread is commonly found on ends of pipe, nipples and fittings (ex: couplings, elbows, tees, etc.).
Sharply angled threads are very critical to joint being tight with no leaks. Thread sealant or tape is typically required to complete the seal. Straight Pipe Threads need a gasket or O-ring to create a seal. While NPT and NPS threads will engage, they do not seal properly with each other.
NPT and NPTF
The requirements for NPT (National Pipe Thread) are spelled out in ANSI B1.20.1
The requirements for NPTF (National Pipe Thread Fuel) are spelled out in ANSI B1.20.3
Both NPT and NPTF have the same threads-per-inch, pitch diameters, and taper-per-inch. The differences come in the major and minor diameters, the root and crest of the threads.
The NPTF thread crests fall with in the parameters of the NPT requirements, but have a smaller range than the NPT.
The NPTF thread roots are different than the NPT thread roots. NPTF thread roots are designed to interfere with the crest of the mating thread with the intention of creating a mechanical seal through thread form deformation at assembly. NPTF threads have two classes identified: Class 1 and Class 2. NPT thread roots are designed to allow clearance with the mating thread crests on assembly.
Having a mechanical piping design background, here's some info for thought in decision making:
Understanding NPT, NPTF & NPS Thread Connections
NPS, NPT & NPTF
NPS = National Pipe Straight
NPT = National Pipe Thread
NPTF = Nation Pipe Thread Fuel
Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight (parallel). Both threads have a 60° included angle and have flat peaks and valleys. Tapered Pipe Thread is commonly found on ends of pipe, nipples and fittings (ex: couplings, elbows, tees, etc.).
Sharply angled threads are very critical to joint being tight with no leaks. Thread sealant or tape is typically required to complete the seal. Straight Pipe Threads need a gasket or O-ring to create a seal. While NPT and NPS threads will engage, they do not seal properly with each other.
NPT and NPTF
The requirements for NPT (National Pipe Thread) are spelled out in ANSI B1.20.1
The requirements for NPTF (National Pipe Thread Fuel) are spelled out in ANSI B1.20.3
Both NPT and NPTF have the same threads-per-inch, pitch diameters, and taper-per-inch. The differences come in the major and minor diameters, the root and crest of the threads.
The NPTF thread crests fall with in the parameters of the NPT requirements, but have a smaller range than the NPT.
The NPTF thread roots are different than the NPT thread roots. NPTF thread roots are designed to interfere with the crest of the mating thread with the intention of creating a mechanical seal through thread form deformation at assembly. NPTF threads have two classes identified: Class 1 and Class 2. NPT thread roots are designed to allow clearance with the mating thread crests on assembly.
Start with the thru-hull and seacock primer article: https://marinehowto.com/category/plumbing/
Problem is in some situations it is not possible to use sea cocks due to design inadequacies (lack of space). in my case these areas are not subject to any impacts. But that was not my question...... just looking for opinions. They make both: NPTnamd NPS. Is on better / preferred over the other and why.
Gre
Gre
The threads on thru hulls are NPS and must be used with a NPS ball valve which are harder to find. Home Depot/lowes carry NPT ball valves. I installed flanged adapters that have a female NPS that the thru hull threads into ,however the male end is NPT which lets you use a standard NPT ball valve. The adapters are made by Groco.
I also install G10 pads that the adapter bolted to , but not necessary This also adds safety as you cannot snap the thru hull due to a seized ball valve and meets ABYC standard for insurance
I also install G10 pads that the adapter bolted to , but not necessary This also adds safety as you cannot snap the thru hull due to a seized ball valve and meets ABYC standard for insurance
The key is one side or the end is fixed and you get the seal by torque on the free side, which forces a thread to thread seal. With the proper torque, no thread dope or tape is needed, except on Stainless Steel.
Using NPS, both sides are free and if you are piping, like natural gas, your coupling must remain in tension set by proper torque.
If that torque tension is ever released, you loose the thread to thread seal.
____
Through hulls, IMHO, should be NPT.
Jim...
NPT threads interfere with each other at the pitch diameter. There is still clearance at the root diameter and thus a leak path. You need a sealant with NPT threads to fill that tiny space. A thru Hull needs a straight thread so when the sea cock or flange adapter is installed the flange is in full contact with the backing plate and the flange of the thru Hull is in full contact with the Hull on the outside. A tapered thread thru Hull will either prevent full flange contact or the flange will prevent full thread engagement. Maine Sail’s link he provided gives all the info you need to do it right.
Having installed the new GROCO BSPP strait thread bronze ball valves on the existing euro straight thread thru hulls I can assure you that you will get a leak-free fitting with LOTS of thread engagememt. I did not use any type of pipe thread tape or dope. Once the fitting hoses and valves are hooked up nothing is moving enough to unthread a fitting with a 10 thread fit.
One of the aspects of failure demonstrated by Maine Sail was that small and inadequate thread engagement will cause a fitting to fail. I also don’t like the concept of basically cross-threading valves that have a potential for future replacement.
One of the aspects of failure demonstrated by Maine Sail was that small and inadequate thread engagement will cause a fitting to fail. I also don’t like the concept of basically cross-threading valves that have a potential for future replacement.
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seepage of salt water into the root diameter clearance likely then results in salt crystals forming in the threads preventing further leakage. Might make removal difficult at a later date. A 1 inch diameter thread ten threads engaged is about 2.5 feet of length before any seepage will appear. Not much pressure on the fluid either at only a few feet underwater.
Yes, GROCO does recommend a pipe dope or tape to seal the BSPP threads against rotation. I have to do some hull bottom fittings in an open bilge area and will use a pipe dope to tighten up those fittings and totally seal the thread against seawater intrusion.
I like to think of it as follows: Take a walk out in the world some day, maybe through a field or the woods, a desert, anywhere. How much metal do you see just naturally occurring in your travels? None? It takes energy to make metals into the forms we see them in the products we use. So metals are at a higher energy state than how they naturally occur in nature. Energy flows from high potential to low potential (heat transfers from hot to cold for instance) so metals are always trying to go back to a lower energy state. Thus we see corrosion, rusting, etc. as the metals combine with other elements to create more stable compounds. Given enough time (eons) our steel will be iron ore again. Without a picture I don't know if your Chia growth is just a patina from the copper in your bronze or if someone sprinkled chia seeds from a chia pet on your thru hull. https://www.quora.com/Why-does-bronze-and-brass-turn-green
