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loop.  This maintains maximum pressure at branches and outlets and provides
two-way distribution to consumers.  The use of a closed loop equipped with
conveniently located segregating valves permits a partial shutdown of the
s y s t e m for inspection or repairs.  Generally, provisions are made for bleeding
each part of a loop between segregating valves.  The loop pipe should be of
sufficient size to prevent an excessive pressure drop at any outlet regardless
o f the direction of airflow around the loop.  Branches shall be at the top of
the main to prevent carryover of condensate and foreign matter.
6 . 2 Piping Materials.
6.2.1 Low-Pressure and Medium-Pressure Systems. Black steel pipe in
appropriate thicknesses conforming to pipe Schedule 40, ASTM A 53 or A 120 is
used in these systems and is joined preferably by welding.  For special
c o n d i t i o n s , stainless steel pipe and copper tubing with appropriate fittings
are also used.  Connections to removable equipment are always flanged, except
when using small threaded pipe.  I n all cases, pipes, f i t t i n g s , and valves
shall be in accordance with NAVFAC Specification 21Y (Latest Revision), Steam
Power Plant, Heating, and Ventilating Equipment and Piping; and
ANSI/API 510-1983, Pressure Vessel Inspection Code - Maintenance, Inspection,
Rating, R e p a i r , and Alteration.
6.2.2 High-Pressure Systems.  Seamless steel pipe conforming to ASTM A 53 or
A 120, with Schedule determined in accordance with ANSI/API 510-1983, is
commonly used for high-pressure air systems.  E x c e p t in very small sizes, butt
welded fittings are employed. Screw fittings, when used, have their ends
sealed by fillet welds and exposed pipe threads covered with weld. Unions are
o f the forged steel ground joint type.  When economic considerations dictate
i t s use, small systems may employ stainless steel pipe with special fittings
t o assure a continuous supply of high quality air. Shore activities,
p a r t i c u l a r l y torpedo workshops, f r e q u e n t l y require copper tubing,
S i l v e r - b r a z e d , bronze fittings and valves, and bronze unions with steel ring
nut, plastic ring gasket, and silver-brazed ends are used with copper tubing.
Pipes and fittings shall comply with NAVFAC Specification 21Y (Latest
Revision), Steam Power Plant, Heating, and Ventilating Equipment and Piping;
and ANSI/API 510-1983, Pressure Vessel Inspection Code - Maintenance.
I n s p e c t i o n , Rating, R e p a i r , and Alteration.
6 . 3 Valves.  R e f e r to chapter 6 for description and application of the
d i f f e r e n t types of valves used in air distribution systems. Globe valves used
i n high-pressure horizontal air lines should be installed so that the stem
p r o j e c t s horizontally.  T h i s prevents restrictions to line drainage which
could be caused by the elevated valve seat.  I f the valve must be installed
w i t h the stem in the vertical position, o r when the valve interferes with line
drainage, a drain connection for the pocketed space should be provided.
6.4 Expansion and Contraction.  Expansion and contraction of pipes carrying
compressed air is normally not a serious problem.  However, consideration must
b e given to failure of an aftercooler which would allow air at elevated
temperatures to enter the system and cause thermal expansion. Piping in the
immediate vicinity of the air compressors can also be hot enough to warrant
c a r e f u l checking of flexibility. Wherever possible, expansion is controlled
by a change in direction of pipe runs or by the use of expansion bends and


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