Sulphur hexafluoride in electrical insulating oil

[ScottyUK]

Chemical engineering isn't my discipline, so if I'm abusing some of the terminology you guys are intimately familiar with, I apologise.

Background:
I have a large power transformer which has suffered a mechanical failure on one of its bushings. The bushing is the insulator assembly which brings the electrical connections out of the transformer. The transformer is filled with mineral-based insulating oil. The external side of the bushing connects to a bus duct which is filled with SF6 gas at 6.2 BarG. The bus duct is basically a tube of approx 12" diameter with a tubular busbar suspended within it. The gas is an excellent electrical insulator, allowing the separation of the electrical conductors to be reduced and thus making the apparatus very compact when compared to an air-insulated equivalent.

Problem:
A seal on the bushing has failed, allowing SF6 to enter the transformer tank where it has contaminated the oil. I am hearing two schools of thought about removing the SF6 from the oil: one is that it is more-or-less insoluble and will not affect the oil, and the other saying that it is very tricky to remove and it is probably economical to scrap 38,000 litres of oil rather than reprocess it. As standard, insulating oil is processed through an oil processor which we know as an 'Ilovac' prior to being used. The Ilovac uses heat and high vacuum to remove light hydrocarbons and water from the oil, but I am being told that this will not remove the SF6.

I would appreciate some opinions.



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If we learn from our mistakes,
I'm getting a great education!

 

[pkpbip]

I'm not sure of the quality needed for your insulating oil, but SF6 has a boiling point of -63C so it is extremely volatile. I don't have any idea of the solubility of SF6 in typical hydrocarbons, but I would imagine that very little would stay with the liquid hydrocarbon if the solution was vented in some way.

If there is very little SF6 in the oil then maybe that doesn't matter (depends on the purity needed).

 

[ScottyUK]

Thankyou for the response.

If I get a definitve answer I will post it back here.



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If we learn from our mistakes,
I'm getting a great education!

 

[bchoate]

bchoate
An issue not mentioned in any of the comments is that SF6 is a fully fluorinated compound and as such falls under the Kyoto protocols for greenhouse gases. Although SF6 has a low boiling point, venting it to the atmosphere is probably not an option. Scrapping mineral oil contaminated with SF6 is not likely to be as easy as one would like either.

The mineral oil would have to be removed in a closed system into a process where vacuum could be applied along with low heat to drive off the SF6. The SF6 would have to be condensed for collection and disposal or resuse. There is the possibility, if any arcing has occurred in the system, that a variety of SF6 byproducts may have formed.

I have seen one listing for SF6 reclamation but the company may not handle mineral oils.

http://www.sf6.net/

 

[ScottyUK]

bchoate,

You are absolutely correct, SF6 is very bad news environmentally, breaking down in the high atmosphere under the intense solar radiation. It has a far greater ODP than, say, CO2. I wonder if the difficulty in prcessing the oil is not removal of the SF6 per se, but how to recover it once it is removed under vaccuum.

Arcing has not occurred in this system; arcing is (hopefully!) confined to the circuit breaker chamber itself, where products of SF6 breakdown such as metallic fluorine are deposited. These form their own hazards, the most serious I'm aware of being the formation of HF when in contact with water.



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If we learn from our mistakes,
I'm getting a great education!