Insulating Oils, Fluids, and Gases:Insulating Liquid Sampling Procedures

Insulating Liquid Sampling Procedures

The validity of the test results is dependent upon the sampler being certain that the oil sample is truly representative of the oil in the equipment. Glass bottles are recommended as containers for samples because they can be easily inspected for cleanliness. The glass bottles may be either cork or glass stoppered or fitted with screwcaps having cork or aluminum liners (inserts). Corks should be of good quality. Do not use rubber stoppers. Clean, new, rectangular-shaped, 1 quart (qt) cans with screwcaps have been found to be satisfactory containers for shipping samples. Samples should be taken from the equipment in accordance with ASTM D 923, Standard Test Method for Sampling Electrical Insulating Liquids.

Containers should be rinsed in lead-free gasoline (which is flammable and should be used out-of-doors only) or chlorothene (a nonflammable solvent), dried, and washed in strong soapsuds. Then they should be thoroughly rinsed with water, dried in an oven at about 105°C for several hours, and removed from the oven. As the bottles cool, they should be sealed by dipping the necks in wax, and then stored for future use. These bottles should be opened only when the bottle temperature and the ambient temperature are the same or nearly so.

Sampling Oil from Transformers

General sampling instructions are as follows:

At least 2 qt of oil should be taken as a sample for dielectric, acidity, and IFT tests. Allow space at the top of the container for expansion. If two 1 qt bottles are used for a sample, label the bottles as 1 of 2 and 2 of 2.

Samples from outdoor apparatus should be taken on clear days when the humidity is near normal and the oil is at least as warm, or warmer than the surrounding air. Cold oil may condense enough moisture from a humid atmosphere to seriously affect its insulating properties. Therefore, this precaution must be observed in sampling spare transformers.

Samples should never be drawn in rain or when the relative humidity of the atmosphere exceeds 70%.

Guard against wind and dust.

When taking samples from an opening, such as a valve, clean the valve thoroughly and allow enough liquid to run out (about 1 qt) to remove any moisture or foreign material.

In a sealed transformer, which has a vacuum, be sure to vent the trans- former before drawing the sample.

Place the sample in the freezing compartment of a refrigerator overnight.

If the sample is cloudy when viewed the next day, it contains free water. Since free water is undesirable, take another sample to determine whether water is in the oil or was in the sample container.

Sampling Oil from Drums or Shipping Containers

The oil drum should remain undisturbed for several hours before drawing the sample.

A glass or Pyrex thief is recommended for sampling because it can be easily inspected for cleanliness. A glass tube approximately 36 in. long, 1 in. in diameter, and tapered at both ends is recommended for the sampling thief.

The thief should be cleaned before and after sampling in the same manner as for cleaning sample containers. When not being used, the thief should be corked at both ends.

Discard the first full thief of oil.

Draw the sample in the following manner:

With the top end covered with the thumb, lower the tube to within approximately 1/8 in. from the bottom of the drum.

Remove the thumb from the top opening until the thief is filled with oil.

Replace thumb over top of thief and remove thief full of oil to the sample container. Release thumb to permit oil to run into the container.

4.4.3 Taking Oil Samples for Gas-in-Oil Analysis

This procedure has been developed to maintain uniformity of all oil samples taken in the field for a laboratory gas-in-oil analysis. Special stainless-steel containers are used for collecting samples of oil for gas-in-oil analysis using gas chromatograph. These stainless-steel containers are not to be used for any other purpose and should be kept clean to eliminate all contaminants and purged with dry air for shipment to the field.

Use a can to catch overflow oil from the stainless-steel container.

Obtain two lengths of Tygon clear plastic tubing and attach one to each end of the stainless-steel container. Make certain that the tubing between transformer and container is as short as possible.

Attach the tubing from one end of the stainless-steel container to the sample valve cock on the transformer.

Hold the stainless-steel container in a vertical position with the length of tubing on the outlet end in the can to catch the overflow oil.

Open the sampling valve on the transformer. Open valve on the inlet side of container.

Open valve on the outlet side of container and allow the stainless-steel container to fill and overflow into can. At least 1 pint should over- flow to assure removal of all bubbles in the sampling system.

Close top valve (outlet side) first to ensure a contamination-free sample.

Close bottom valve (inlet side) and then close sampling valve on the transformer.

Do not wrap any kind of tape around valves or nozzles of the stainless- steel container.

Forward the sample to the laboratory.

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