Discussion:
Pipe thread question, NPT vs NPSF, MIP, FIP and IPS
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Jeff Wisnia
2003-08-07 17:23:06 UTC
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I'm putting together a custom compressor and I have a question relating
to the nomenclature of threaded fittings. The pressure switch is spec'd
as having four "NPSF" ports. All the rest of my plumbing is NPT. My
question is whether or not a 1/4" NPT pipe nipple can mate with the NPSF
ports on the switch. I thought abbreviations starting with NPS meant
straight non-tapered threads, but I can't figure out why this one
component would be threaded differently than all my other pneumatic
parts which are NPT.
NPSF is the female "fuel service" or "dryseal" thread ( "F" for fuel) which
can be mated with a plain NPT pipe nipple to produce a metal to metal seal
without the "spiral leak path" which occurs when two regular NPT threads are
mated. But, to be closer to perfection the male thread should be a NPTF, not
just a plain old NPT, IIRC the "fuel" designation comes from the fact that
early pipe dopes weren't hydrocarbon proof, and often disolved if used in
fuel service.

A common misunderstanding is that NPT threads seal because "the tapered
threads jam together". That's not true, and a properly made up NPT threaded
joint will predictably leak unless some sort of pipe joint compound or tape
is used, at least until the spiral leak path gets plugged by corrosion or
crap in whatever is being carried in the plumbing. It's sometimes possible
to overtighten NPT threads so that metal deformation forces a seal,
particularly with brass fittings, but that's not the way they're supposed to
work.

Here's a pretty good primer on this stuff:

http://www.webster-inst.com/techinfo/pipeconn/pipefr/pipeconn.htm
Second question: I was at the Home Depot getting a few extra fittings,
when I realized that the cast brass parts (my preference) were labeled
in MIP and FIP rather than NPT. Are they the same as NPT?
Yes, a holdover from earlier days. "Male Iron Pipe and "Female Iron Pipe".
"Iron Pipe" threads became NPT threads.
Lastly, what is IPS threading? The MSC catalog has a lot of brass
fittings and nipples marked as IPS sizing. Is this compatible with NPT
or just with itself?
IPS ("Iron Pipe Straight") eqivalent to NPS. Not really compatible with NPT.
-Adam
Jeff (Who wonders how long it's going to remain PC to refer to threads and
electrical connectors by gender specific terms....What a world.)

P.S. If you're going to use a return addy which asks us to see your sig
line, why not follow through and give us something to "see".

JW



--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to
place the blame on."
adam smith
2003-08-09 03:20:25 UTC
Permalink
Post by Jeff Wisnia
A common misunderstanding is that NPT threads seal because "the tapered
threads jam together". That's not true, and a properly made up NPT threaded
joint will predictably leak unless some sort of pipe joint compound or tape
is used
Yes, I've found this out the hard way trying to plumb a shower head
without the tape. Leak city!
Post by Jeff Wisnia
Lastly, what is IPS threading? The MSC catalog has a lot of brass
fittings and nipples marked as IPS sizing. Is this compatible with NPT
or just with itself?
IPS ("Iron Pipe Straight") eqivalent to NPS. Not really compatible with NPT.
There may be a case of multiple meanings here. In the last couple of
days, I've called MSC and a plumbing supply house, and they both told me
that IPS is "iron pipe size". The MSC customer support guy assured me
that all their fittings sized as "IPS" in the catalog do in fact have
NPT threads (ANSI B1.20.1). That said, however, I did find a couple of
references online that talked of NPS straight pipe as IPS. Perhaps this
is a case of the IPS designation refering to pipe size independent of
threading type? Or maybe a few things are confusingly called IPS for
different reasons?

-Adam
adam at airraidsirens dot com

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