I have had personal experience with two companies which have done on line filtering of oil filled transformers. These companies are:
-General Electric through one of their Field Service Shops and they are located around the US and the other is S.D. Meyers, Inc of Akron, Ohio.

In both instances the problem was water contamination, not sludge  which is associated with oil decay products or a problem associated with dissolved combustible gases.
This is a task that involved  a lot of investigation to qualify the companies. I would have preferred to have taken the unit off line for this corrective action but did not have that luxury.
I interviewed the individuals that would be responsible for the field work and had a detail review on the time involved and what criteria constituted a successful filtering operation.
Both instances were successful.

Thank you jack6238.
I think water contamination is not a problem. We have been able to extract around 12 to 15 liters of water from around 10000 liters of oil from transformers. But we have done it off line. Since we have not yet done online, I have not dared to do online, unless I know how online is done. I will try to contact both of your references. But in the mean time since you have done such work, can you explain the process? I mean compared to the size and rating how is the inlet discharge and outlet discharges are fixed? what are the other criteria? What precautions are to be taken? If I get these information, I will surely do one and inform the forum for advantages to all concerned.

Before you start this develope a contingency plan.
Find out where there is another transformer, wwhat it would take to get it to you and how you could hook it up if you had to.  Good contractors can do what you want, but if you have let the thing go for so long they may not be able to avoid something going wrong,
So when you let the blue smoke out of that one - have a plan in your pocket.

SD Myers specializes in this type of work.  I have used them and had very good results and they did it while the transformer was online.  They have self contained units that do a combination filtration/distallation of the oil to remove sludge and any condensation

Soorya
Now that I know where you are located, I'm concerned about doing this type of work on line. I mean I doubt that you have a lot of resources in that area. Actually I don't have the specific details on how to perform this work. All I have is a general writeup from SD Meyers. What you need are some specifics for your particular transformer. Is there anyone in your area that has demonstrated success in doing this job on line? And how did this transformer get in this shape? Are you sure you have a problem? What is the neutralization number in in KOH/GM? Have you retested to confirm the data. Remember that most bad tests are the result of bad samples.
There is an excellent book on the market that cost ~$100 US and it is A Guide to Transformer Maintenance and it is written by SD Myers and is available from Transformer Maintenance Institute, Division, SD Meyers, Inc., Akron, Ohio. If  you want I can get the ordering information for you. I have also written a guide for the Maintenance of Liquid Filled  Transformers. You can email me if you want.

dear soorya,

i think now the technology of online filtration is quiet well established and the activity can be undertaken. the inlet to  the filter machine is taken from the bottom of the transformer and the recycled oil is fed to the top of the transformer or to the conservator.

sludges do get dissolved in the transformer oil in some cases if the oil is maintained above 90 degrees centrigate but filtration of the oil particularly in hi-vacuum filtration machines is not recomended, as in parrafinnic base oils the aromatic contents tend to boil of in vacuum, as the oil is dependent for its characterestics on its various constituents it is not very reliably established regarding the logetivity of the oil reclaimed by oil online filtration process. I think this process is about 5-7 years old, any member having updated information please contact me at gmazumder@yahoo.com.

i have tried to study the process very closely but the details of ther process are closely gaurded and SD Myers division of Fluidex engineering based in south africa make this machines. other manufacturer of this equipment is enercon and you may check them up.

if you can send me a copy of the oil test results at gmazumder@yahoo.com i can try to be of some help.

hope this is of help.

Dear soorya,

one of the other eqiuipment manufacturers is enervac and you can find some details at www.enervac.com and not enercon as mentioned by me earlier.

if possible send me the oil test reports ans i can be of help. my email id is gmazumder@yahoo.com

regards

Suggestion: Try
http://www.thomasregister.com
and enter Oil filtering under Product / Service, which will return several companies you may like to contact.