Underground Steam-Line Construction
DONALD H. BRANDT*
For many years the basic design of underground
lines by Consumers Power Company consisted of
laying the supply and return mains in a concrete
or cellular tile duct. Underdrains were used or
omitted depending on soil conditions. Minor variations in design were employed until about 195I
when the Company made its first main extension
using a granular pour-in-place hydrocarbon insulation.
Improvements and refinements have been added
since the 1951 installation until at the present time
the Company is installing' a large majority of its
main extensions and steam service runs in basically
the following manner:
Step I. The trench is cut as nearly as possible to
the minimum required width for the pipe
and proper cover of insulating material
which of course varies with pipe size, steam
conditions and other factors.
Step 2. The bottom of the trench is leveled to
proper ~Tade and precast concrete-pipe
support pads. containing- a piece of reinforcing rod bent u-shape and imbedded
in the pad with the prongs downward. are
laid in the trench at approximately 10-foot
intervals.
Step 3. The pipe and expansion fittings are laid,
alig;ned and welded in place.
Step 4. The pipe anchors and ~uides are installed.
These are constructed of 50-pound railroad rail and 4-inch angle iron respectively, imbedded in concrete at predetermined intervals.
Step 5. The insulation is poured to prescribed
thickness of cover and tamped in place
followed by back filling of the trench.
Although the above is overly simplified in this
writinlt, it covers the basic procedure presently
used by all contractors the Company employes for
steam-main construction. In most installations the
Company furnishes the major permanent materials
required.
Inasmuch as distribution steam pressures range
from 125 psi downward, it has not been deemed
necessary to pressure test each section of line before
covering. However, a clause provided in the
standard construction contract protects the Company in the event a leak is detected when the
acceptance test is made or any subsequent failures
of the line proven to be due to faulty construction.
Step 3 has been simplified recently by the development of a special enclosure for a standard
corrugated expansion joint. The enclosure, welded
to the pipe at one end and fitted with a packing
gland at the other end, can be installed in much
the same manner as a section of pipe of equivalent
length. This permits the contractor to proceed to
Step 4 much more quickly than in the past when
FIG. I-Two Recently Purchased Expansion Joints.
The near joint shows the stationary end of the
expansion fitting. The unit has been fitted by the
contractor with an elbow and 0 nipple in preparation
for installation.
FIG. 2-Expansion Fittings Installed in line.
This picture was taken while the contractor was engaged in Step No. 4 of the description of construction
procedure. In this picture the anchor has been
positioned and the concrete poured; however, the
guides were not installed at the time the photograph
was taken.
FIG. 3-Expansion Fitting Prepared for Covering.
In the foreground the insulating material has already
been poured and tamped as described in Step No. 5.
The form boards have been advanced and are being
posit:oned preparatory to pouring the insulating
material and back-filling the next section of line.
concrete enclosures of various designs were required
for each expansion joint.
In the opinion of the writer, the Company is
presently using a method of underground steam-line construction that has several major benefits:
1. Low initial cost.
2. Locally available contractors can properly
install it.
3. The external surface of lines unearthed and
inspected have shown no signs of external
corrosion.
