There is often
confusion amongst new users of industrial plastic pipe as to the actual
measurements of the internal bore and external diameter. Over the decades, and
indeed centuries, there have been many adapted and modified standards, some of
which have been long abandoned, but others which have remnants in today’s
specifications.
To understand
how plastic pipe is measured, the user must first understand the methods of
production and consider the way that industrial plastic pipes are generally
assembled:
The machines
that manufacture lengths of plastic pipe heat plastic pellets and then squeeze
them through a die. This allows the wall thickness and
diameter to be controlled. The problem comes in the cooling process, where,
like all materials, the hot plastic contracts. If this were left unchecked, the
finished product would have a varying external diameter and bore, which would
result in a poor fitting product that is next to useless. The pipe manufacturers
concentrate on maintaining a fixed external diameter, and let all the shrinkage
happen to the internal diameter / bore of the pipe. It is here that we need to
appreciate how industrial plastic pipe is assembled. In general, and this is
true for both solvent weld pipework such as pvc pipe and electrofusion welded
materials such as polypropylene, assembly is by inserting a pipe into a socket.
The external diameter of the pipe is therefore critical, as any variation will
make a poor joint with the socket. The internal diameter is less critical as it
does not have to perfectly match anything else.
The plastic pipe
manufacturers therefore control the outside diameter of the pipe. The external
diameter of the pipe therefore remains constant, and the internal diameter
varies depending on the pressure rating (due to a thicker wall). It is
important to note that even with the same manufacturer, wall thicknesses for
the same specification of pipe can vary slightly, resulting in a slight bore
variation (usually less than 0.5mm). This is certainly the case between
different manufacturers and the internal bore should not be relied on for
machining purposes.
With metric
pipe, and this knowledge, determining the pipe dimensions is simple; te outside
diameter of the pipe measures the same as the stated size; so a 50mm pipe has a
50mm outside diameter, a 90mm pipe a 90mm outside diameter etc. Unfortunately
the same cannot be said for imperial (inch) pipe. This is an area of great
confusion as the size of the pipe refers to the “nominal bore” (or to put it
another way “approximate bore”) of the pipe. So, a 2 inch pipe has an
approximate bore of 2”. As the outside diameter of the pipe needs to remain
fixed, to allow for fitting into sockets etc. the wall thickness of the plastic pipe changes with pressure rating and so, as a result, does the bore…which is
why it is referred to as “nominal”. So a 2” pipe does not actually measure 2”
anywhere! Many inexpereicnced users will order a 2 inch pipe and expect it to
have an outside diameter of 50.8mm but in fact the OD is 60.3mm.
The general
rules of thumb are:
Metric pipe –
will measure the stated diameter as an external diameter
Imperial pipe –
Will not measure the stated diameter anywhere so the user must check first in
the table below:
Pipe size (inches)
|
Outside diameter (mm)
|
3/8
|
17.1
|
½
|
21.4
|
¾
|
26.7
|
1
|
33.6
|
1 1/4
|
42.2
|
1 1/2
|
48.3
|
2
|
60.3
|
2 ½
|
75.2 (Note that for North
American Pipe 2 ½” = 73mm)
|
3
|
88.9
|
4
|
114.3
|
5
|
140.2
|
6
|
168.3
|
8
|
219.1
|
10
|
273.0
|
12
|
323.9
|
