Dissolved Gas Analysis-Gas Chromatography - Part 2

Now that we have extracted the sample of gas from the oil it must be injected into the chromatograph for analysis and identification of the gases. Gas chromatography machines use tubes packed with special material to separate the gases. These tubes are called columns. Columns come in varying lengths packed with varying materials depending on exactly what you’re looking for. You can’t just inject your gases into any old chromatograph and expect it to tell you what’s in the sample.  The column is in the “oven “section of the machine and it’s heated to a specified temperature. This allows the gases to expand and stratify within the tube.

After a period of time long enough for the gases to separate and stratify they are “pushed” through the tube with an inert gas (usually argon) and then burned at the end of the line.

The chromatograph prints out a line graph corresponding to temperature and time. It looks sort of like a pencil line drawing of a series of mountains.  Looking at time on the bottom horizontal axis tells you what gas you’re looking at (remember they stratified or layered and came out in a certain order). When you calculate the area under the peak you can evaluate the percentage of that particular gas.  Now all that remains is to translate all of that data into parts per million of dissolved gas in the sample using the total amount of oil used along with the amount of gas extracted. That part is simple math.

All of the above is chemistry and can be performed by any qualified technician. The real art is interpreting the results. This requires usage of several different methods of evaluation along with a good field knowledge of the equipment and other reasoning skills.

I like to use Rogers Ratio, Sacramento State, Dornenberg Ratios, Key Gas and any other method available to come up with then verify my conclusions. No one method works best or all of the time.  Conclusions need to be determined by gathering as much onsite physical data and equipment history as possible. In many cases when a problem is discovered it is only a snapshot.  Another snapshot (test) should be performed at a later date to verify the problem and the extent of it.  That could be as soon as a week or as long as six months depending on the problem.  

That’s all for today.

Please write or comment on any subjects that may interest you for further postings.

Thank you,

Bob

Today’s quote is from  Billy Connoly

"My definition of an intellectual is someone who can listen to the William Tell Overture without thinking of the Lone Ranger."

Dissolved Gas Analysis-Gas Chromatography

Dissolved Gas Analysis-Gas Chromatography

Gas Chromatography is always an interesting topic to write on. There is so much technology involved from sampling, to extracting and testing and analyzing it can fill volumes. 

I have gone over some sampling procedures in earlier posts so here I will only mention that the sample has to be obtained carefully and it’s a good idea to never let it be exposed directly to the atmosphere.

Extraction is a job in itself. Once a sample has reached the lab the testing technician has to extract the gases from the oil in order to be able to put them into the chromatograph for testing.  An accurate measure of the oil used for extraction is needed first. The tech will usually use 35 to 40 milligrams of oil. This is normally in a syringe which is easy to measure or sometimes in a steel bulb which is a little harder. Oil poured out from a bottle can be tested but exposure to the atmosphere can skew the results.

In the lab a measured amount of oil is injected into a glass vessel under vacuum. The oil is then stirred by means of a magnetic stirring device under the vessel. Observation shows the oil will foam like a bottle of ginger ale when first injected. It will continue to bubble while being stirred. The stirring action exposes all of the oil surface to the vacuum and helps force the gases out of the liquid faster.  It is very much akin to opening a bottle of ginger ale. The contents,  under pressure holds gas, carbon dioxide, in the liquid. When the pressure is removed by opening the bottle the gas bubbles form inside the liquid and float to the top. Since the gas is compressed into the liquid you don’t see a volume change.  Once equilibrium is reached the gases no longer bubble out and the ginger ale turns “flat”. Put it into a vacuum and it will again begin to bubble.  

Once the oil is “flat” and won’t give up any more gas the collection portion of the apparatus is sealed off with a valve. The extracted gas is then compressed using liquid mercury for a piston.  At a given pressure the volume is measured. This enables the technician to calculate how much gas was in the given amount of oil. This forms the base for the parts per million of total gas found in the sample.  Most of the gas is then extracted from the chamber using a syringe then immediately injected into the gas chromatograph.

Sounds simple, but when working with vacuum it is very difficult to keep leaks from forming. The glassware used has to be specially designed for the purpose, and all hazardous materials need to be contained and handled carefully.

The glass vessel is then removed from the apparatus to dispose of the oil. The magnet is retrieved and put back into the vessel. The sealing edges of the vessel are then coated with vacuum grease. The apparatus is then tested to make sure it’s ready for the next sample.

Any comments or suggestions are always welcome. If you have a question my email address is: transformerbob@gmail.com

Quote of the month:

"My mom said she learned how to swim. Someone took her out in the lake and threw her off the boat. That's how she learned how to swim. I said, 'Mom, they weren't trying to teach you how to swim.' " --Paula Poundstone

Transformer Oil Reclaiming and Heat

Product News! Product News! Product News! Product News! Product News! Product News!

Before you read my blog….

Have you ever been to the doctor for a routine checkup and been given bad news?  If you’re at all like me the first thoughts that form in your mind will be “I’m going to get a second opinion!”  

Darn Right you are!

Because there’s a lot riding on it! And as we all know, doctors are only human, they are not infallible. Well the same holds true for your electrical equipment.  When your equipment gets an annual physical checkup from your testing company do the results sometimes come in less than good?  Along with those results do you get a list of options proposing “Major $urgery” on it?  Maybe it’s time to reach out for a second opinion.  Luckily you may not have to reach too far.  If you have my dissolved gas analysis program in your toolbox you can verify those results immediately…or not! Think about the upbeat calls you’ll get from your clients when you tell them that you’re going to “anesthetize” their transformer for a half day or so to perform major repairs . OR……Think about how cheery they’ll be if the analysis said it was okay and the unit blows up two weeks later.  Believe me it can and does happen.  All of the good engineers I know have an inborn instinct to minimize risk, and I’m sure that you are one.  One of the golden rules in preventive maintenance is maximizing safety and reliability along with minimizing costs.

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Attention:

Transformer Oil Reclamation Continued

Hot Topic

In this post we need to talk a bit about heat. Heat is an extremely critical element in transformer oil reclamation. The filtering media will strip the sludges and acids from the oil but unless the oil gets hot enough the sludge that is deposited on the interior surfaces of the equipment will stay there. If it is not removed it will harden and begin to crack along with the insulation that it has formed onto. Essentially hot oil melts the sludge deposits in the transformer.  The transformer oil aniline point is right around 160 degrees Fahrenheit. Aniline point meaning the point at which is begins to act as a solvent. Transformers are cleaned much the same as if you run clean water into a muddy bucket. The more water that you flush it with the cleaner it gets. Cold oil will dissolve sludges over time. Evidence of this is available from test results taken after an oil change in a dirty transformer. Over a period of time the new oil will pick up some of the sludge in solution and it will show up in the tests by a rapidly dropping IFT reading. I use the term rapidly relatively. It should take about two years for the IFT in a lightly loaded unit (25C) to stabilize. At that point the oil is usually saturated and you start deteriorating where you left off at the oil change. You can change the oil again but this is an expensive and wasteful way to clean one out.

Heating filtering and re-circulating really works best in this case. Keep re-circulsating until the test results stabilize on the oil and you will have rid the unit of most of the sludge contamination. The temperature of the oil coming out of the reclamation unit needs to be between 180 and 200 degrees. That because it’s going into colder oil and the entire apparatus needs to reach at least the 160 degree mark. Remember that the cooling fins are normally functioning while the transformer is being treated and they will cool the oil somewhat. Ideally your oil processer should block off the fins to delete any “cold” spots that would allow sludge to accumulate or not be dissolved.

Heat also makes the filtering elements more active. In the case of fullers earth it becomes much more active and efficient at removing acids at elevated temperatures. The same goes for the vacuum system that is removing the moisture from the system. Oil will hold more moisture in solution at elevated temperatures and therefore is likely to change the equilibrium reached inside the unit between the oil and the cellulosic insulation. That can cause the insulation to unlock and give up some of it’s moisture to the oil. That oil then goes back to the processing equipment to be vacuumed.

There are still a few engineers out there who are not proponents of energized processing. Since this has been done successfully on a regular basis since the 1960’s I don’t agree with them. Energized processing gives you the extra boost of the internal heat generated by the transformer, and that helps drive moisture out of the insulation into the oil to be removed in the processer. As soon as you shut the unit down the moisture in solution will start to migrate over to the insulation degrading the integrity of it. It may not be much but with today’s design criteria it doesn’t take much.

As for those who think it’s bad business to circulate oil in a running transformer, I suggest that you look inside the next time your sampling technician takes the top off of a small energized unit with cooling fins (especially a hot one).  You can actually see the oil moving through the unit. As the oil is heated by the core and coils it rises to the top and then as it passes into the cooling fins becoming more dense as it cools it drops to the bottom only to repeat the process. If it weren’t ok to move the oil around in a transformer the big ones wouldn’t have circulation pumps in the cooling fins. The trick is to keep the level constant with a relatively gentle flow. As long as you don’t get gas bubbles you will be ok. I should also note here that the oil is drawn into the processor from the bottom of the transformer and reintroduced through the top. Drawing from the bottom alleviates the problem of stirring up sediment. In most cases it goes into the processer to be removed and never gets stirred up around the current carrying parts of the equipment. Energized hot oil processing is safe and effective when performed on qualified equipment by experienced technicians.

That’s about all I have for today.

Any comments or suggestions are always welcome. If you have a question my email address is: transformerbob@gmail.com

Today’s quote comes from the boss-

“Employees are like mules, Some you stand in front of and coax along with a carrot. Some you stand behind And kick them in the ass. The key to management is knowing which mules are which!”

Don’t forget to order a copy of my transformer oil gas analysis program. It’s a great tool to have. You won’t regret it. When you need it,  it will be there.

Thanks,

Bob