CONCLUSIONS - CR98

fication simulations were 24 to 48 hours, depend-

here the water flow was "stopped" at three times

ing upon the time step, length of time modeled,

in the simulation: minimum water temperature

and complexity of the mesh. This was on a '486 66-

(MWT), minimum air temperature (MTT) and

MHz system with 16 Mb of RAM. There are much

maximum frost penetration (MFP). At MWT, the

pipe never reaches 0C. The top of the pipe drops

faster computers available now and soon to be

to about 0.6C, and the sides and bottom drop to

available that could significantly reduce these

about 1.2C, then all slowly rise over the next 200

times. For example, during the preparation of this

report, some of the above scenarios were run on a

hr. For both MTT and MFP, the pipe never reaches

0C, even 200 hr after stopping the water.

233-MHz system with 64 Mb of RAM. On this sys-

tem a 4-day run on the '486 66-MHz system was

To determine what type of protection the shield

reduced to about 6 hr. The smaller scenarios were

offered over an unshielded pipe, several numeri-

reduced to 1 hr or less, depending upon their size

cal simulations were performed using the Went-

and complexity.

worth Street physical configuration, without the

The boundary conditions required for design-

shield present, and again stopping the water flow

ing a shield should be conservatively selected to

at MWT, MTT, and MFP times. At MWT, the pipe

top reached 0C approximately 37 hr after stop-

allow for some uncertainty in the exact physical

situation present that is being designed for. The

ping the flow. With a shield, the water temperature

never dropped to 0C. At MTT, the pipe dropped

designer will need to choose the appropriate

gradually to about 0.4C compared to staying at

boundary conditions (in this case coldest air tem-

about 2 with the shield. At MFP without the

peratures on record and recorded water tempera-

shield, the pipe essentially reached 0C after about

tures) and set the failure criteria he is designing

around. The failure criterion of the pipe reaching

160 hr where it remained for the rest of the 200-hr

0C is probably too conservative in that there is a

evaluation time. With the shield, the bottom of the

pipe dipped to about 0.25C approximately 25 hr

large amount of latent heat to be used before the

water in the pipe will turn to ice, but in these veri-

after the water stopped and then it started to warm

fication runs we had decided to be conservative.

up. The above points out the added protection the

Frost-shielding configuration possibilities can

shield provides under the described conditions.

vary depending upon geographical location,

extent of pipeline shielded, type of pipe shielded

CONCLUSIONS

(sewer or water), material characteristics at the

site, available room for a shield, and the economics

The CPAR project demonstrates the viability of

of excavating for different configurations. A shield

burying water lines in the frost penetration zone

around a pipe with relatively warm water will al-

by designing and installing an insulation shield to

low shallower possibilities and a thinner insula-

protect them from freezing. Using the finite ele-

tion thickness than one around a pipe with a tem-

ment method to model a buried frost shield ap-

perature barely above freezing. A strong point for

pears to give reliable and useful information about

finite element modeling is the ability to perform

the configuration and effectiveness of the shield.

several parametric design simulations to assess the

The studies show that with the proper boundary

effect of any parameter the designer chooses to

conditions and material properties the program

look at. In addition, the graphical contour output

predicted temperatures very close to what was

from the modeling can give an extremely useful

measured in-situ. As with all numerical modeling,

picture to the designer to assess exactly what im-

its accuracy is highly dependent upon the accuracy

pact design changes have on the shield perfor-

of the material properties and boundary conditions.

mance. The studies have demonstrated that a

It would be expensive to develop standards to

shield will provide thermal protection to forestall

cover every possible combination of climate, soil

the freezing of a buried water line under the condi-

condition, pipe burial depth, and pipe tempera-

tions described for Berlin, N.H.

ture that could be encountered throughout the

Something not investigated completely during

frost-susceptible areas of the United States. The fi-

this study was the use of alternative shield config-

nite element procedure described above allows the

urations. The inverted-U design worked well in

local designer, who is most familiar with the area

this study because of the limited lateral space

of concern, to determine the pertinent variables

available, but other configurations may be more

and assess the effect of each variable upon the pro-

appropriate for different situations.

posed design.

It was mentioned earlier in the report that con-

The simulation run times used during the veri-