|by Carol Fey
When things fit together, why do they sometimes leak?How much
have you thought about the threads on the controls you work
with? I was in a test lab recently, and the engineer, Joe Beagen,
was pointing out that there are NPT threads on an air eliminator.
“Wait a minute,” I said to Joe. “Say that again.”
“About the threads.”
“I said the threads are NPT.”
“Doesn’t NPT just mean it’s threaded?” I asked.
“No,” Joe said. “NPT stands for National Pipe Tapered. It’s
a tapered thread so that when we tighten the joint, it makes
a metal-to-metal seal. You still might use Teflon tape to make
sure it doesn’t leak, but tapered thread really helps.”
I knew all threads weren’t the same. I’d learned that from an
aged workbench I’d bought at a farm auction long ago. I thought
I’d found a treasure of spare nuts and bolts. With the many
little drawers full of nuts and bolts and screws and washers,
I should have been set for life. But was I ever wrong. None
of the nuts fit on any of the bolts. Even when they looked the
same size, they wouldn’t go together.
That’s the same auction where I bought 10 buckets, and when
I got them home, found that every last one of them had a hole.
But the point is something that we all know very well — not
all threads are created equal. How does it work, that so many
things do fit together? There’s a story there. But when things
fit together, why do they sometimes leak? There’s a story there,
too. Let’s take a look.
Enlarge this picture
Figure courtesy of http://www.engineeringtoolbox.com/npt-national-pipe-taper-threads-d_750.html.
Screw threads were invented a long, long time ago. The idea
seems to have been first recorded by Archimedes, a third-century
B.C. mathematician. That’s a really long time ago. By the
first century B.C., screws were common enough, roughly cut
from wood or filed by hand into metal. The ancient Greeks
and Arabs documented what they knew about screws. Leonardo
da Vinci left sketches on how to cut screw threads by machine.
There’s evidence that screws were used to fasten suits of
armor. (Whoa! I guess there was no getting out of your armor
in an emergency!)
In the 16th century, screws appeared in German watches. But
here’s the crazy part — screws were cut by hand until the
18th century. There was no standardization. Even with the
Industrial Revolution when screws were machine-made, parts
didn’t fit together unless they were specifically made to
Standardization came painfully slowly. In the mid-1800s, England
standardized on the Whitworth thread. This thread had rounded
rather than pointed crests (top of the thread) and roots (bottom
of the thread). The angle between threads was 55 degrees.
Twenty years later, the United States also standardized —
on something else entirely. The American Sellers standard
had a 60-degree thread angle, enough to make a difference.
This became the American National Standard.
Thread incompatibility was enough of a problem during World
War II — especially in building and repairing airplanes —
that the two countries plus Canada were motivated to standardize.
The result in 1948 was the United Screw Thread. This incorporated
the compromise of the American 60-degree thread angle, and
the English rounded thread crests and roots. (Visit the following
Web link to view a graphic portrayal of the angle and crest
form of American and English threads: www.sizes.com/tools/thread_american.htm.)
Pipe threads evolved similarly to screw threads. The American
Na-tional Standard for Pipe Threads was developed in 1882
by Robert Briggs, and was originally known as Briggs Standard.
Two general thread types have been approved. It includes two
general pipe threads: tapered and straight. The tapered threads
became known as National Pipe Taper (NPT). The straight is
known as NPS.
For any of you non-plumbers like me, let me explain what tapered
means. At first I thought “tapered” had something to do with
the slant of the threads. Not so. Tapered means that the diameter
of the pipe or fitting where the threads are starts out smaller
at the beginning of the threads and gets larger by the end
of the threads. Both the male pipe threads (MPT) and female
pipe threads (FPT) are tapered.
On the other hand, “straight” is just as it sounds. The diameter
remains the same from the beginning to the end of the threads.
The advantage of tapered threads, or NPT, is that they can
be used for sealing as well as joining the two components.
Straight is good only for joining.
As you know, ordinary NPT doesn’t provide a perfect seal.
A sealant compound — pipe dope — or Teflon tape must be added
for a no-leak seal.
However, there is a modified thread called Dryseal American
National Standard Taper (NPTF), which needs no sealant or
tape for sealing.
Here are some NPT characteristics:
The taper from the beginning to the end of the thread is
1/16 inch for every inch. That can also be expressed as 3/4
inch for each foot, or as 1 degree and 47 minutes.
Thread crests (tops) and roots (bottoms) are flat.
The angle between threads is 60 degrees.
The number of threads per inch depends upon the diameter.
That’s called “pitch” (more on that in a moment).
NPT Thread Sizes
Now here’s another amazing piece of information for a controls-person-who-is-not-a-plumber.
Thread sizes are based on the inside diameter (ID). That’s
where it counts, because that’s where the flow is. For example,
“3/4-14 NPT” means 3/4 inch inside diameter, 14 threads per
inch, with the NPT characteristics above.
As you can see in the chart, you can expect this 3/4-14 NPT
to have an outside diameter (OD) of 1.050 inches.
The number of threads per inch is called “pitch.” For NPT,
there are a standard number of threads per inch. In the chart
above, notice that 1 inch NPT, 1 1/4 inches NPT, 1 1/2 inches
NPT, and 2 inches NPT all have 11-1/2 threads per inch.
Why some things look like they fit but don’t: Some threads
seem like they’re interchangeable, but they aren’t. For example,
NPT tapered threads don’t fit well with NPS straight threads.
Here’s another mismatch — NPT and ISO (International Standards
Organization) threads look similar, but NPT threads have a
60-degree taper angle and the pitch is measured per inch.
ISO threads have a 55-degree taper angle, and are measured
in millimeters. So the pitch is different.
Why some things fit but leak: There are many standards besides
the NPT. For example, the British Standard Pipe taper (BSP)
has descended from the English Whitworth standard. It has
an angle of 55 degrees and a 1 in 16 inch taper. The NPT has
a 60-degree thread angle. Both have 14 threads per inch in
the 1/2- and 3/4-inch sizes. They’ll connect. But because
the thread angles and the crest and root tolerances are different,
there can be leakage, called “spiral leakage.”
So, this “spiral leakage” is why threads leak even if they’re
tapered. The solution is to use a sealant. Like Joe the engineer
says, NPT helps prevent leakage. And you may still need the