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Specialist Materials for Industry and Science

MIDEL 7131 Retrofilling Guidance

MIDEL 7131 Retrofilling Guidance

One of the main applications of MIDEL 7131 after it was introduced in 1979 was to replace PCB based fluids in existing distribution transformers. This process is generally referred to as retrofilling.

MIDEL 7131 has been used extensively to retrofill PCB and mineral oil filled transformers worldwide. Retrofilling could help to improve the service life or reliability of the transformer, reduce environmental hazards or increase fire safety.

MIDEL 7131 is fully miscible with mineral oil in all proportions however care should be taken to reduce the residual contamination so that the superior performance of Midel is not compromised.

This page describes the practical procedures to be taken into consideration when retrofilling with MIDEL 7131. It is advisable to check that this process does not invalidate any manufacturer’s warranty prior to commencing any retrofilling project.

Retrofill of PCB-Filled Transformers

The retrofilling of PCB-filled transformers should only be carried out by a specialist contractor who is fully familiar with the regulations and who is licensed to handle PCB's. Details of such a contractor are available from M&I Materials on request.

Retrofill of Mineral Oil-Filled Transformers

MIDEL 7131 thermal expansion characteristics, cooling performance and electrical properties are similar to those of mineral oil. This ensures that there is no need to make any alterations to the design of a distribution transformer or to downrate it when retrofilling with MIDEL 7131.

MIDEL 7131 is also a good lubricant. Thus ensuring that components such as tapchangers or circulatory pumps, which tend to seize when in contact with silicone liquids, will not require changing when retrofilled with MIDEL 7131.

MIDEL 7131 is compatible with the materials of construction in general use with mineral oils. Its swelling effect on some rubbers and jointing materials however differs from that found with mineral oils and it may be advisable to change such materials if their dimensional stability is important for satisfactory transformer operation.

MIDEL 7131 is chemically and electrically compatible with mineral oil remaining in retrofilled transformers. However, an excess of residual oil can depress the fire properties of Midel and particular attention should be given to minimising oil residues. The following procedures should help in this task.


Drain the oil from the transformer through a base valve or, for greater speed use a small (0.5 H.P.) gear pump. Allow the unit to drain for at least two hours to permit the fluid to be released from the windings, cooling tubes, etc.. With very old oil, a draining time of longer than 2 hours would be desirable.

When discharging via a drain valve ensure that the valve is as close as possible to the base of the unit and, if feasible, tilt to enable as much oil as possible to escape. If tilting is impractical, no base valve exists, or greater speed is required, introduce a semi-flexible plastic tube (3/4" [20mm] ID polyethylene water piping) via a vent hole or filler cap at the top of the unit. Pump the fluid into a suitable container ensuring that the tube reaches the bottom of the transformer to enable the maximum amount of fluid to be evacuated from the vessel.

Take care not to damage the sensitive windings during the introduction or use of the piping. Such a probe should be inserted as far away from the winding, and as close to the tank wall as possible.


An additional 10% of the fill volume of Midel should be used to rinse the inside of the transformer. If using a pump, hose the windings etc. to enhance the removal of the mineral oil. As previously detailed, allow at least two hours for the washings to drain to the bottom before finally emptying, tilting where possible.

Solvents should not be used to remove the oil as such chemicals may affect the insulation material, certain paints and adhesives used in the manufacture of transformers.


When refilling the transformer it is important not to entrap air within the MIDEL 7131. Air entrapment can be minimised by refilling from the base of the unit and, if using a gear pump, ensuring that the flow rate does not exceed 3 gallons or 13.6 litres per minute. Another option is to fill the unit under vacuum.

Do not energise the unit for at least 12 hours so that trapped air can escape and so that the windings, etc. can be impregnated with the new fluid.


Throughout all stages of the retrofill operation it is essential that the introduction of moisture or particulate matter be avoided.

For more information about the benefits of retrofilling with MIDEL 7131 please click here.

If a pump is employed, the outlet side of the pump should be protected by a fine mesh or paper element filter. A Puralator Micronic Type MF146A or its equivalent will prevent any debris from the pump or particles from the transformer being introduced or reintroduced to the unit.

If it is suspected that the transformer contains wet oil, then a suitable molecular sieve drying unit may be beneficially employed.

  • Apiezon
  • Metrosil
  • Midel
  • Wolfmet

Products of M&I Materials Ltd.