9.
ECONOMIC FACTORS. The designer of the piping system should obtain
estimates of fabrication costs. Items made from less used materials are
often not as available as is first indicated. A delivery quotation should be
obtained with the cost quotation for every component that is not readily
available.
Section 4.
CONSTRUCTION CONSIDERATIONS
1.
MECHANICAL STRENGTH. When necessary for mechanical strength to prevent
damage, collapse, excessive sag or buckling of pipe due to superimposed loads
from supports or other causes, the wall thickness of the pipe should be
increased; or, if this is impractical or would cause excessive local
stresses, the superimposed loads or other causes shall be reduced or
eliminated by other design methods. It shall be the responsibility of the
designer to determine if the completed piping system must be subjected to a
detailed stress analysis. Should a stress analysis be required, it shall be
performed in accordance with the requirements of the B 31.1 Code (Reference
2).
2.
PIPE SUPPORTS. Careful consideration must be given to the mechanical
support of all pipe, tubing, and system components. The strength of all
mechanical supports must be adequate for all reasonably anticipated
externally applied loads as well as the planned functional loads. This will
include the applied force when valves are being operated or regulators are
being adjusted as well as loading from actuating pressure relief devices and
normal human operator abuse. If a section of pipe is located where it is
likely to be used as a step, a handhold, or a ladder, it should be protected
or braced accordingly.
The spacing of pipe supports is frequently left to the art and skill of the
field installation crew; however, the system designer should provide some
guidance for the support spacing. A good rule of thumb for this is,
"supports should be located at each end of a pipe or tubing run with
intermediate supports spaced at approximately 50 diameters of the supported
pipe."
The supports must be arranged so that they do not impose excessive loads on
the system when the pipe changes size in response to pressure or temperature
changes.
3.
BENDING. Detailed instructions and minimum radii for the hot or cold
bending of pipe and tubing are given in MIL-STD-1627, Bending of Pipe or Tube
for Ship Piping Systems (Reference 9).
General guidance for bending requires that the minimum wall thickness at any
point in a completed bend shall not be less than the computed wall thickness
for the design pressure and temperature.
A simplified table for the required minimum wall thickness before bending is
presented below. It is based on the theoretical minimum calculated wall
thickness for a straight pipe section, (tm).
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