BEYONDVALVES
BY GREG JOHNSON
Putting the “P” in PVF
Of course, the “P” in PVF stands for
pipe, those long round things with
the hole down the middle! Learning a
little bit more about the pipes that connect to your valves will help you as a
valve professional.
First, we need to understand how
pipe is sized. Pipe is measured by utilizing a dimensionless number known as
nominal pipe size or “NPS”. These sizes
run from ¼ inch to above 72 inches. If
you take a caliper to a 1 inch NPS pipe
you will be surprised to see that it is not
1 inch in diameter, rather, it is actually
1.315 inches in diameter. This oversize
dimensioning applies to pipe through
NPS 12 inches. For diameters 14 inches and above, the NPS is the actual outside diameter. When it comes to dimensions, pipe should not be confused with
tubing. Tubing size is equal to its outside diameter.
ing systems on record, in ancient Rome,
2000 years ago, were made of lead. The
industrial revolution and the Iron Age
ushered in the era of iron pipe, which
peaked in the 1920s. Although steel is
the primary choice for industrial applications, non-metallic piping materials
have made huge headway in the water
and wastewater industry where iron
once was king.
Steel pipe material specifications are
detailed in ASTM documents, and some
of the more common ones are listed in
the table below:
ASTM A106
Designation
ASTM A53
ASTM A335
Pipe Schedules
Since the strength of pipe is proportional to its wall thickness, pipe is also categorized in this way. Initially pipe dimensions were fairly standardized. In fact
the wall thickness was called “
standard” (STD), or “standard wall.” As
pressures increased, the designations
were amended to include thicker walled
pipe called “extra-strong” (XS) or
“extra-heavy” (XH). Eventually the
term “double extra-heavy” (XXH) or
“double extra-strong” (XXS) was
added to the range of pipe wall thicknesses as they grew thicker to accommodate even higher pressures. The three
pipe thicknesses STD, XS and XXS
were inadequate to handle an ever-increasing range of fluid pressures, so a
new system based upon numbers called
“pipe schedules” was devised.
These schedules are the approximate
expression of 1000 P/S, where P is the
service pressure and S is the allowable
stress, both measured in PSI. Pipe
schedules begin with 5 and currently
run all the way up to 160. The most
common schedules in use today are 40,
80 and 160. The old terms—standard,
extra-heavy and double extra-heavy—
are still used and they correspond to
various pipe schedules, depending upon
the NPS of the pipe. Schedules with an
“S” suffix, such as SCH 20S, refer to
stainless-steel pipe, dimensioned in
accordance with ASME B36.19M.
Steel pipe is dimensioned in accordance
with ASME B36.10M.
ASTM A312
API 5L
Material Notes
Seamless & welded
steel pipe
Seamless carbon
steel pipe
Alloy steel pipe
(Cr/Mo, etc.)
Stainless-steel pipe
Line pipe for petroleum pipelines
Materials and Specifications
Today, the predominant material for
industrial piping is steel. However, that
was not always the case. The oldest pip-
How It’s Made
Another important identifier in steel
pipe is the process by which it is made,
either welded or seamless. Welded pipe
is made from plate, which is rolled into
a circle and then welded down the
length of its longitudinal seam. Seamless pipe is made by squeezing and
rolling round bar around a pointed mandrel that pierces the inside of the bar,
creating the pipe bore. Due to the equipment costs involved in seamless pipe
manufacturing, it is primarily used in
the smaller sizes. For critical applications, however, seamless pipe is almost
always specified, and it is commonly
manufactured in sizes through NPS 24.
