sign. A phenomenon noticed in the Berlin studies
nearby boundary condition temperatures than
was a phase shift of the water temperatures in the
would the nodes farther away. By performing sev-
pipe compared with the surface temperatures. Fig-
eral years of simulation, the FE program internally
corrects this start-up error.
ure 3 shows this shift from the Labossiere Street
Once the start-up error is removed, the second
data. It is evident that in the summer the water in
step is to check the length of time it takes for the
the pipe reaches its peak temperature about 600 to
pipe to freeze. The temperature boundary condition
700 hours after the surface temperature peak. A sim-
at the pipe is removed (simulating no water flow),
ilar or longer lag is evident during the cooling sea-
and the simulation is run until either the frost pene-
son. This shift is probably caused by a combination
tration reaches its
of factors. The water
maximum or the zero-
temperature at the
degree temperatures
source does not re-
touch the pipe. If the
act as quickly to air
0C isotherm reaches
temperature chang-
the pipe, the amount
es as the surface
of time that it took to
does, due to its mass
do so can be calculat-
and specific heat. In
ed and compared to
addition, if the wa-
the desired time of
ter flow rate is low
protection. If the out-
and it stays in the
come is not satisfacto-
pipe for an appre-
ry, a different design
ciable amount of
can be tried, using the
time, it will tend to
first previous results
equilibrate with the
to guide the design.
temperature of the
The desired time of
soil at the pipe
protection is deter-
burial depth. This
mined by the design Figure 3. Temperatures recorded at surface of Labossiere Street temperature will be
engineer for the loca- and water temperature in pipe below. Note lag between time of out of phase with
tion and level of ac- peak surface temperature and peak pipe temperature.
the surface temper-
ceptable risk.
ature because of the
insulating effect of the soil above. If the water tem-
peratures within the pipe are very cold, the timing
BERLIN FIELD STUDIES
of this shift can have an effect upon the time to
freeze.
Shield design and thermocouple layout
General design considerations
Labossiere Street
The surface material above the pipeline can
Two independent insulated pipe test sections
have a noticeable impact upon surface tempera-
were installed in the field tests in Berlin, N.H. The
tures. This material should be accounted for (e.g.,
first design was built in the summer of 1994 on La-
by using an n factor) when designing from air tem-
bossiere Street in an area where ledge was present
peratures. The coldest surface temperatures ex-
to the surface along most of the pipeline. This was
pected should probably be used in the design, but
the first and purposely conservative shield design
that is a decision the designer will have to make.
until some experience and data were acquired to
All of the CPAR test pipelines were installed in the
verify the model simulation. The background of
street right-of-way and have asphalt as a top sur-
this design is explained fully in Coutermarsh
face material, which is plowed in the winter. They
(1997), with a synopsis below.
therefore did not get any benefit from snow or
The basic physical configuration of the pipeline
grass cover, which can provide a warmer surface
and shield along with the thermocouple (TC) mon-
than a plowed road does.
itoring locations is shown in Figure 4. The pipe is a
The water temperature in the pipe can have a
0.2033-m- (8-in.-) diameter ductile iron pipe about
large impact upon the success of a design in that a
123 m (405 ft) long on a dead-end water line with
relatively warm water supply can provide energy
low flow, and it is buried about 1.5 m (5 ft) deep. A
to the soil within the shield. Accurate water tem-
0.15-m- (6-in.-) thick, inverted-U shield was placed
peratures are therefore important to the shield de-
over the pipe with the bottom of the legs at or
4
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