Application of Bentonite on UG cablese

[CKent]

Good day to all!!!

One turnkey project agreement by our company and one worldwide recognize power/engineering company involved the use of bentonite in 115 kV UG lines. These lines are inside the substation and would connect the 115 kV transmission lines outside to the primary of the power transformers. Bentonite will have to be pumped inside the conduits after the cables have been installed. We have a lot of similar substations but this will be the first one that will involve the use of bentonite as some sort of filler in the conduit of the cable. The company involved would not provide technical info except that they say bentonite could somehow dissipate the cable's heat partially, thus providing more ampacity for the cable.

Has anyone have any experience similar to this application? We wanted to know what could be the pros and cons of using such.

 

[itsmoked]

I have zero experience with this but the bentonite will allow heat removal via conduction instead of the usual:
cable-convection-conduit-conduction-earth. You would get:
cable-conduction-conduit-conduction-earth.

Conduction is generally much better than convection in removing heat.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jghrist]

I agree that the bentonite would increase heat removal and ampacity. I'm not sure if this makes more sense than direct burial with thermal backfill and a barrier above the cable for preventing dig-ins. An advantage of installation in conduit is to allow cable replacement in the event of cable failure. The bentonite would prevent this. Conduit installation would be more expensive than direct burial - what is the advantage?

 

[BJC]

I think someone is confused. Bentonite is ( or was ) commonly used as a pulling lubricant. Bentonite/water slurry is very slick snd works well as a lubricant.
Jghrist is correct. The heat of the cable will bake the bentonite it will get hard and dry. The only way to remove cable that were pulled using bentonite is to fill the conduit with water and let them soak for a few days. If the raceway is totally filled there is no way to get water in to soak the bentonite. The only way to replace the cable is with a backhoe.

 

[CKent]

Thanks for the replies!

That is the reason why wanted to know more especially know more about actual experience. It is our standard practice to lay the UG cables inside conduits, with the primary advantage of course of making it easier to replace/repair damaged portion/section of the cable. However, as mentioned, the contractor insists on using their practice of injecting bentonite to sort of fill in the cable ducts.

From what I have researched of bentonite, it is primarily used as a digging mud. It is a good material in retaining moisture and is often used also in waterproofing. It particularly swells when wet and contracts when dried. Although it is great in holding moistures, it could be hard to get it wet or soak again once dried up. So there might be a possibility that the heat of the cable will eventually dryup the bentonite and hardened it which could pose bigger problems when maintaining or repairing the involved cables...

 

[BJC]

The experience I have had with bentonite was with the residual material left from using it as a pulling lubricant. In these case the clay would collect in low points and would be on the bottom of the cable. To pull the cable out it was necessary to fill the duct with water and let it set for 2 or 3 days. The force ot breake the cable loose was high enough ot damage the cable.
AS I said earlier if the duct was packed with bentonite there is no chance of getting the cable out.
Most cable companies list acceptable pulling lubricants. I am sure they will still list bentonite as acceptable. Write them and ask their opinion on filling the duct with bentonite.
If you want a heat transfer medium whats wrong with water?

 

[jghrist]

I'm not sure how good a thermal fill bentonite would be. I'm familiar with its use as an enhancement for lowering electrical resistivity for grounding. It depends on maintaining some minimum level of moisture.

If bentonite is inside a conduit, ground moisture will not keep it moist. I don't think filling the conduit with bentonite would be very effective long-term because it would dry out and have a high thermal resistivity and also shrink away from the cable leaving air gaps.