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Old 2nd Jun 2018, 11:17 pm   #1
simpsons
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Default Bush A640 LOPTX tropicalisation

I have a pitch version of the above LOPTX which after almost 6 months perched on top of central heating boiler itself enclosed in a cupboard, is working very well.

It required this treatment having been inadvertantly left insitu in my Celotex insulated shed, known to me as my shack.

Just in case the tar is now permeable, can I seal it with high voltage varnish?

I have seen an example using polythene but cannot fathom out how the open ends are sealed.

Any suggestions?

Chris
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Old 3rd Jun 2018, 3:00 am   #2
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Default Re: Bush A640 LOPTX tropicalisation

Quote:
Originally Posted by simpsons View Post
Just in case the tar is now permeable, can I seal it with high voltage varnish?
If this was my transformer I would probably dissolve off all the tar/pitch with turpentine (or white spirits in the UK in a bath over a week) and replace it with multiple layers of varnish (by repeated dipping into a container of varnish) to a thick layers of 1 to 2mm or more, because pitch is untrustworthy in the long run. I know for sure this works well on LOPTYs & overwinds to about 15kV at least. But I have not tested this method at higher voltages like 20kV though. Marconi used this method to insulate the LOPTY's/overwinds in some of their 1960's vintage studio monitors and its much better than pitch for longevity and they practically never fail. I don't know the details of your transformer, voltages etc.

However, if you want to keep the pitch, you could attempt to seal it with varnish, such as marine grade spar varnish. The solvent in that dissolves into the pitch surface, and it would probably seal it, the only thing is that the mixture at the surface probably would not harden normally, remain a bit tacky and attract dust, but it would probably work.

Another option would be a very soft artists brush and lightly brush the pitch surface with white spirit. The surface will microscopically melt, helping to seal any small crevices or microscopic holes.

Likely though, the pitch there is very dry, shrinking and cracking putting the entire transformer at risk. Do not attempt to remove the pitch by either heating it, or picking at it in any way, or likely the transformer will be damaged. The best way to remove it is by dissolving it off in a bath very slowly over a few days to a week.

Your other option, since its working without trouble currently, is to simply leave it as it is, which might be a less risky move if you are not confident with any of the other suggested treatments. If you haven't done something before, like dissolving and replacing the pitch on a lopty with another substance, then you might want to avoid it, until there is a less important or non working lopty for you to experiment with.

I was quite surprised when I joined this forum there were so many people struggling with pitch covered loptys and doing things like heating them to get the pitch off. With old pitch it won't flow properly with heating as it has lost its lower molecular weight solvents and it just bakes itself onto the surfaces. I first saw pitch being dissolved off transformers with mineral turpentine for re-coating/varnishing and repairs in NZ in the 1970's, so to me it was just old hat that this was the way to do it.
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Old 3rd Jun 2018, 10:27 am   #3
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Default Re: Bush A640 LOPTX tropicalisation

Thank you Argus25 for your informative response.

It must be the chemical breakdown of the original tar which contrary to expectation, causes it to become permeable.

After all, given the "Outstanding quality of refined tar-based coatings is their extremely low permeability to moisture and there high dielectric resistance, both of which contribute to the corrosion resistance. (Munger, 1984)" something about the particular coating used may be the reason why this particular BUSH transformer's failure rate is greater than its predecessors.

Dissolving the tar coating and repainting the windings with varnish is not my first choice and the reforming of the top layer by disolving it I will look into.

One way I wondered was to overcoat the overwind with liquid roof tar which is available as a DIY repair kit. Unfortunately, my only contact who understands anything to do with oil is currently in South America but, he only helps find the stuff using satellites and drones!!

If push comes to shove, I'll diary to bring the TX in from the cold during the winter and park it in a warm space. No recommendations necessary!!

Chris
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Old 3rd Jun 2018, 12:23 pm   #4
Argus25
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Default Re: Bush A640 LOPTX tropicalisation

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Originally Posted by simpsons View Post
One way I wondered was to overcoat the overwind with liquid roof tar which is available as a DIY repair kit.
That sounds quite reasonable and a good source for it.

I found it very difficult to get tar. When I was a teenager I needed some to repair the tops of some LeClanche cells and some tar top car batteries. In the end some friendly road workmen dispensed a pint of it from their truck into a large old dog food tin for me. Over the years I used it up and now do not have any left.
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Old 3rd Jun 2018, 1:13 pm   #5
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Default Re: Bush A640 LOPTX tropicalisation

They still sell solid bitumen in slabs for repairing roofs.
https://www.google.com/search?client...iw=993&bih=572
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Old 4th Jun 2018, 12:58 am   #6
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Default Re: Bush A640 LOPTX tropicalisation

Yes its a very common roofing product in AU these days and easy to get. But back in NZ, at least in my neighborhood in the 1970's it was seldom seen on a roof and none of the local hardware stores had it in stock where I lived.
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Old 4th Jun 2018, 1:02 am   #7
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Default Re: Bush A640 LOPTX tropicalisation

Perhaps it does not work as well in your climate.
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Old 4th Jun 2018, 7:32 am   #8
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Default Re: Bush A640 LOPTX tropicalisation

Why not have a day out at Ironbridge Gorge Industrial Revolution museum?
One of the many interesting exhibits there is the Tar Tunnel, an old bitumen mine, through which the public are allowed to walk. The large pools of tar are not accessible, but there's plenty of the stuff running off the walls.
I don't know if it's officially allowed, but I'm sure no-one would notice if you collected a sample.
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Old 4th Jun 2018, 10:09 am   #9
simpsons
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Default Re: Bush A640 LOPTX tropicalisation

deseymo1, thank you for your suggestion. Before I comment, it just so happens I worked for ITT Europe and I had the fortune to understand the processes used in undersea telephone cable manufacture. Part of this process included covering part of the cable in a bitumen product.

During the winter, it was believed that at night some emplyees would huddle around the open vats to keep warm and doze off. No knowledge of carcinogens then; but I digress.

The difficulty in using solid tar in which a transformer is to be dipped is more than getting the block to heat to make it suitable to use.

Notwithstanding the physical process of dipping a sphere with insulated wires exiting the object at different places into molten tar, just how hot and for how long? The buffs who thought of the process must have thought this through, manufactured a jig and given a time for the proceedure.

So, not quite back to the drawing board but here is food for thought. In using lead alternative flashing, the liquid goo used to adhere the flashing to the roof is waterproof and as the adveert runs, sticks to sh1t. I just wonder if this is the holy grail I am looking for? I will have a word with the manufacturers.

In the meantime, please, no more jokes about tar and Aussies. Looking at Google pictures of Maroockydore and Worksop suggests that they have both a lot to offer but for different reasons.

Chris

Last edited by simpsons; 4th Jun 2018 at 10:10 am. Reason: missing text
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