TM 5-848-1/AFM 88-12, Chap. 1
b. Engineering analysis. The engineering analy-
properties include:
sis will be in diagram form showing all connected
loads, flow quantities, pressures, and the location
(c) Weatherability.
of regulators. A complete set of supporting calcula-
(d) Strength-temperature relationship.
tions will be prepared. Notes will be included on or
(e) Toughness.
will accompany the diagram to indicate the specific
(f) Permeability.
gravity of the gas, the heating value of the gas in
(g) Flexibility.
Btu's per cubic foot, the base pressure and base
(h) Susceptibility to rodent damage.
temperature at which a cubic foot of gas is defined,
the flowing temperature, the normal atmospheric
While all these properties are important and must
pressure, and such other information as may be
be considered, only the long-term strength can be
pertinent.
defined well enough at the present time to be used
c. Plastic pipe engineering considerations.
as a primary basis for design. However, consider-
ASTM D 2513 presents engineering data for
ation of the effect of the other properties must be
thermoplastic pipe, tubing, and fittings intended for
given proper attention in determining suitability for
fuel gas service. ASTM D 2517 presents
an intended application and an overall factor of
engineering data for fiberglass pipe and fittings
safety.
including adhesives intended for fuel gas service.
(3) When selecting a plastic piping system, the
ASTM D 3350 covers the identification of
design engineer should relate installation and
polyethylene plastic pipe and fittings materials
service requirements with the characteristics of the
using a cell classification system. The specific
several plastic systems available. Some of the
plastic material chosen should be thoroughly
factors to consider are:
(a) Design pressure levels as a function of
investigated for the gas service and operating
conditions anticipated. It should be adequately
required wall thickness.
(b) Chemicals which may be naturally or
resistant to liquids and chemical atmospheres which
could be encountered in the gas distribution system
unavoidably present or added into the gas system.
(c) Effects of maximum and minimum ambi-
and tough enough to withstand the stresses which
may be imposed by external as well as internal
ent temperatures on installation practices, particu-
forces acting on complex piping system
larly at tie-in points.
(d) Installation requirements for a flexible
configurations.
(1) Engineering design concepts for plastics,
type vs. a semi-rigid type material.
(e) Suitability of material for making plastic-
including plastic pipe, are different from those used
when designing with metals. For metals at or near
to-plastic and plastic-to-metal connections.
(f) Special design or installation require-
ambient temperature, some considerable fraction of
the yield strength is widely used for design
ments.
(g) Availability of necessary fittings and tools
purposes. At ambient temperatures, plastics, unlike
structural metals, are influenced by the duration of
for fabricating pipe systems in the field.
(h) Capability and compatibility of material
stress as well as by relatively small changes in
temperatures. Therefore, design stress levels for
for development of satisfactory operating and
plastics are based on long-term strength as
maintenance procedures.
measured by plotting stress failure as a function of
9. Gas distribution system design.
time at a constant temperature. On a short-term
strength basis, design stress levels for plastics
a. Introduction. The design of a gas distribution
would be relatively low. Also, for plastics there is
system will be based on a demand (maximum
an appreciable increase in strength as temperatures
probable) equivalent to not less than 80 percent of
are reduced below design temperatures and an
the full (maximum possible) connected appliance
appreciable decrease in strength as temperatures
load. The actual consumption may never be equal
are raised above design temperatures. In fact, at
to the maximum probable, but such diversity
temperatures over 38 degrees & (100 degrees F)
factor as may exist at a military installation will
there could be significant differences among
not be as large as in a civilian community and
polyethylenes of the same type and grade. Plastic
would serve as a margin of safety if not included in
pipe users should obtain specific stress-time-
the design. The size of pipe in sections of a
rupture data for materials being considered.
distribution system that serves no more than 15
(2) In addition to the long-term strength, there
buildings in an isolated area, and in service lines to
are other properties which must be considered in
individual buildings, will be based on the full
selecting a material for fuel gas distribution. These
connected appliance load. In the various gas flow
Change 1
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