The final determination allowing the designer to set the final pipe
thickness requires that a design pressure and temperature be chosen.
This is not as simple a task as it might seem. The buried pipe group
is relatively less specific as to what design pressure is in the codes,
whereas the aboveground piping group has specific definitions of these
terms.
The form of the equations in either group of codes may be manipulated
algebraically to calculate any one of the three variables––pressure,
stress, or thickness––when the other two are known. This most often
comes into play in the buried pipelines. It should be recalled that
pipelines are intended to cover miles of terrain, with potential elevation
changes, which would lead to variations from the base pressure.
B31.11 uses the term maximum steady-state operating pressure. It
stipulates that pressure to be the sum of static head pressure, pressure
to overcome friction losses, and any backpressure. The steady-state
pressure is also used in B31.4. Code B31.8 uses a somewhat backward
definition by invoking in Paragraph 803.212 the design pressure as the
maximum pressure permitted by this code, as determined by the procedures
applicable to the materials and locations involved.
Remember that in most cases there is not a temperature problem to
be dealt with. Only Code B31.8 recognizes temperatures above 250°F
and then to only 450°F where the reduction is less than 14 percent. In
those portions of a line that are buried, the temperature would be very
steady and would not fluctuate drastically.
Essentially the above discussion boils down to this: The design pressure
must, of necessity, be outside the scope of the code to predetermine,
as the line must be laid out to find elevation differences and must be
sized to calculate friction losses, and the end conditions must be established
to determine any backpressure.
The maximum allowable working pressure would be a calculation
based on the materials, the thickness of the pipe, or the pressure rating
of the component to determine that pressure. Then one must be sure to
not operate above it.
There are surge-type rules and conditions for which the code defines
limitations. In some situations because of some outside factor, the
operating conditions have changed in such a way that the line has to
have a lowered operating pressure. This lowering of pressure is some-times
permanent but often temporary until a particular condition can
be corrected.