3.6.3 Type A-2, Steel Casing Conduits. S t e e l casing sections are units
c o m p l e t e l y insulated and coated, prefabricated at the factory in unit sections
o f standard length (usually 21 feet). The sections are shipped from the
f a c t o r y with an extension of bare pipe at each end for field welding.
P i t c h i n g of the pipe is provided by the slope of the trench bottom. Sections
are composed of the following items.
(a) Pipe(s) for either a single or a multiple pipe installation.
( b ) Layer of premolded or preformed insulation of the required thickness
( r e f e r to chapter 7).
( c ) Open mesh cloth insulation jacket.
( d ) Intermediate pipe supports. These supports are designed to permit
f r e e expansion or contraction of the pipe without damage to the insulation
Also, they allow free air circulation and drainage from one end of the system
t o the other.
p i p e insulation and the inner surface of the exterior metallic casing for the
e n t i r e length of the system.
( e ) Annular airspace (l-inch minimum) between the o u t e r surface of the
( f ) Zinc or other corrosion-resistant coated steel
( g ) Hiqh temperature melting point asphalt or coal t a r c o a t i n g . T h i s i s
a waterproof-coating with low permeability which neither softens nor flows at
temperatures below 220F.
( h ) Asphalt saturated asbestos felt wrapping.
( i ) Reinforcing end collars welded at the factory to each end of the
c o n d u i t section.
3 . 6 . 4 Configurations of Steel Casing Conduits. S t e e l casing conduits are of
two types: c o r r u g a t e d steel conduits and spiral welded steel conduits.
3 . 6 . 5 Corrugated Steel Conduits. T h e s e sections have helically corrugated,
z i n c - c o a t e d , steel conduits with spiral seams welded to ensure a waterproof
p r e s s u r e - t i g h t casing. T h e conduit is designed for strength to prevent
d e f o r m a t i o n in service. T h e insulation on the pipe(s) at each end of the
c o n d u i t sections is usually protected with a heavy asbestos felt cap wired
on. The insulation caps are permanent and remain in place when the pipe and
c o n d u i t joints are made. Joint closures are made by welding the bare ends of
t h e pipes. A f t e r welding and testing for leaks, insulation is applied to the
pipe. F i n a l l y , a connector band or coupling, which fits snugly over the
r e i n f o r c i n g e n d c o l l a r s , i s w e l d e d a i r t i g h t . A f t e r welding, waterproofing
s i m i l a r to that of the section is applied.