thermal sand for underground cables

8/10/2019 Thermal Sand for Underground Cables

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Thermal Sand for UndergroundCables


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A significant source of problems withunderground cables is poor selection andinstallation of thermal backfill materials. Toprevent premature failures, you must ensureyou place cable systems in a hospitableenvironment.


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Importance of Thermally Stable BackfillAll the heat generated by an undergroundpower cable must be dissipated through thesoil. This is quantified by the soil thermalresistivity (or thermal rho, C-cm/W), whichcan vary from 30 to 500 C-cm/W.


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The ability of the surrounding soil to transfer theheat determines whether an operating cableremains cool or overheats. Improving theexternal thermal environment and accuratelydefining the soil and backfill thermal rhocommonly results in a 10% to 15% increase in

cable capacity and sometimes up to 30%.


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The use of a soil thermal rho of 90 C-cm/Whas become cable engineering practices. Soilstudies performed in the 1950s found this was asafe value for most moist soils. Howver fortransmission cables, it is assumed that thethermal backfill placed around the cables will

have a thermal rho of less than 90

C-cm/W.


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Thermal Backfills

Most moist soils (with the exception of organicclays and silts, volcanic soils, peat and fills withash and slag) have a rho of less than 90 C-cm/W. Sands when moist may even have a rhoof less than 50 C-cm/W.


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However many soils, especially uniform sands,can dry substantially when subjected to heatfrom the cables. The thermal rho of a dry soilwould exceed 150 C-cm/W, and possiblyapproach 300 C-cm/W for a dry uniform sand.Most contractors would use readily availablefine sand or concrete sand as the backfill as this

sand makes an inexpensive backfill material, butthermally, it is very poor because it dries outeasily under high cable loads.


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Poorly compacted trench backfill is anothermajor problem. Not only is the thermal rho ofuncompacted soil significantly higher, but theloose soil will dry more easily.


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Corrective Thermal Backfills

Native soils usually do not make good thermalbackfills because their thermal rho values arepoor. The operational reliability gained byplacing a properly constituted thermal backfillaround the cable has advantages over the

variability of re-compacted native soil.


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Yuleba Minerals ( www.yulebaminerals.com.au )has a graded and tested thermal backfill thathas been used in Roma, Miles and Surat basin

projects. There is a need for quality assuranceduring installation. If the gradation of thebackfill is not the correct size moisture or notenough compaction effort is applied then themaximum density will not be achieved and thethermal capability degraded.
http://www.yulebaminerals.com.au/http://www.yulebaminerals.com.au/


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Cement stabilized sand frequently has beenused as a cable trench backfill. A typical mix

design consists of 15 parts sand to 1 partcement, mixed with about 10 parts water.However, this backfill is quite strong and thus

would be difficult to excavate.


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Achieving soil density is needed in the restrictedtrench areas near cables or around cable pipegroups where proper compaction is difficult.Yet, it is precisely in these zones adjacent to thecables, where the heat flux is highest.


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Fluidized Thermal Backfills

Fluidized thermal backfills (FTB) is a slurrybackfill consisting of medium aggregate, sand, asmall amount of cement, water and a fluidizingagent. FTBs can be made with locally availablesand and aggregates. The componentproportions are chosen by laboratory testing of

trial mixes to minimize thermal resistivity andmaximize flow without segregating thecomponents.


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Fluidized thermal backfills do not have to becompacted; they flow in a fashion similar toconcrete. In fact, FTB is typically supplied fromconcrete trucks, and may be poured or pumped.

It solidifies to a uniform density byconsolidation, with excess water seeping to thetop. It hardens quickly so that the ground

surface may be reinstated the next day, but thelow strength (100 to 250 psi [0.7 to 1.8 MPa])allows it to be broken up with a backhoe ifrequired.


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If a higher strength is required, the cementcontent can be increased and the wateradjusted without degrading the thermalperformance.Backfills The Right Way The use of a well-designed thermal backfill canenhance the heat dissipation and increase the

allowable increased capacity of an undergroundpower cable, as well as alleviating thermalinstability concerns.


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The corrective backfill will reduce the heat fluxexperienced by the native soil so that it will notdry out; therefore, the stability of the native soilis no longer a concern. A good backfill should bebetter able to resist total drying and also have alow dry thermal rho if it is completely dried. It

should be available at a reasonable cost, and beeasy to install and easy to remove if required.


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The thermal backfill must be laboratory

evaluated and include specifications for mineralquality, gradation (sieve analysis), thermal dryout curve and optimum density. Typically, theentire trench width is filled with thermal backfillto a minimum height of 300 mm (12 inches)above the cables. For poor native soil conditionsor heavily loaded cables, the thickness of the

backfill can be increased to maintain a lowcomposite thermal rho. A fluidised thermalbackfill is the ideal way of providing a high-

quality cable backfill.


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For further information please visit

http://www.yulebaminerals.com.au/
http://www.yulebaminerals.com.au/http://www.yulebaminerals.com.au/

thermal sand for underground cables

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