point after reassembling the valve. I f the valve does not close tightly after
i t has popped and has relieved considerable fluid, it is hanging up because
the blowdown adjustment is probably too low. Blowdown should be set between 2
and 4 percent of the set popping pressure. I f the valve chatters, the
probable cause is low blowdown adjustment, or excessive back pressure caused
by defective installation.
8 BRANCH VALVING AND CONNECTIONS. Takeoffs from supply and return mains to
b u i l d i n g s are usually taken at the t o p o f t h e l i n e s . These branch connections
a r e located at fixed points of the m a i n s , at or near anchor points, to prevent
movement. V a l v e s are not required a t each takeoff where a branch is short.
Where the branches are of considerable length, however, or where several
b u i l d i n g s are served, takeoffs should be provided with valves. When the
l e n g t h of the branch requires provision for expansion, the expansion loop or
j o i n t is commonly located inside the building.
9 FITTINGS. F i t t i n g s are used to provide bends in pipelines, branches, and
c o n n e c t i o n s ; to seal pipes; and to attach fixtures.
( a ) S t e a m D i s t r i b u t i o n . W e l d e d construction is preferred for steam
distribution for the following reasons:
q There are no joints that may leak and cause trouble.
( b ) H T W D i s t r i b u t i o n . S c r e w e d joints are not permitted; welded joints
are used throughout the system. Flanged joints are kept to a minimum.
R e f e r to chapter 5 of this manual.
( c ) Compressed Air Distribution.
10 REFERENCE DESIGN SPECIFICATIONS FOR FITTINGS.
q For steam supply and condensate return consult NAVFAC Specification
21Y (Latest Revision), Steam Power Plant. Heating, a n d Ventilating
Equipment and Piping; and ANSI/API 510-1983, Pressure Vessel
Inspection Code - Maintenance. Inspection, Rating, Repair, and
A l t e r a t i o n , for minimum design requirements of distribution systems.
q High Temperature Water.
S e e references listed in this paragraph.
q Compressed Air.
NAVFAC Specification 21Y applies. Also, ANSI/API
510-1983, Section 2, " I n d u s t r i a l Gas and Air Piping"; and Section 8,
" G a s Transmission and Distribution Piping Systems."
MAINTENANCE OF FITTINGS.
The following items should be checked.
( a ) Leakage. C h e c k all flanged fittings, elbows, and tees twice a year
f o r leakage and any loosening of bolts. I f leaks are permitted for an
e x t e n d e d period of time in heat distribution systems, wire-drawing of flanges