IEC 60567 SAMPLING OF GASES AND OF OIL FOR ANALYSIS OF FREE AND DISSOLVED GASES
IEC 60567 OIL-FILLED ELECTRICAL EQUIPMENT - SAMPLING OF GASES AND OF OIL FOR ANALYSIS OF FREE AND DISSOLVED GASES - GUIDANCE
4 Sampling of oil from oil-filled equipment
4.1 General remarks
It is important to bear in mind that receiving a qualitative and a representative sample is crucial for obtaining a reliable diagnosis of the electrical equipment. Even the most sophisticated extraction or diagnosis methods cannot overcome faulty samples.

Warning: When sampling oil, precautions should be taken to deal with any sudden release of oil and avoid oil spillage.

Of the three methods described below, the method of sampling by syringe is recommended. The other two methods are alternatives to be used in case of difficulties.

Sampling into glass sampling tubes is also suitable provided they are fitted with sufficient lengths of rubber tubing acting as expansion devices.

Stainless steel sampling tubes fitted with valves are very robust and are not affected by large temperature changes and can be used without expansion devices.

Sampling in glass bottles is also adequate provided the bottles are fitted with a suitable cap which allows oil expansion. Sampling into bottles is simple, requires little skill, and is adequate for many purposes such as routine sampling on a large scale from equipment on site. The use of bottles (0,5 l to 2,5 l capacity) may be preferred where comparatively large samples of oil are required. When using the glass bottle sampling method, care should be taken to minimize air contact with the sample.

The methods described are suitable for large oil-volume equipment such as power transformers. With small oil-volume equipment, it is essential to ensure that the total volume of oil drawn off does not endanger the operation of the equipment.

The selection of points from which samples are drawn should be made with care. Normally, the sample should be taken from a point where it is representative of the bulk of the oil in the equipment. It will sometimes be necessary, however, to draw samples deliberately where they are not expected to be representative (for example, in trying to locate the site of a fault).

Samples should be taken with the equipment in its normal condition. This will be important in assessing the rate of gas production.

Some of the dissolved oxygen present in the oil sample may be consumed by oxidation. The reaction can be delayed by exclusion of light (for example, by wrapping a clear glass sampling vessel in an opaque material) but, in any case, the analysis should be carried out as soon as possible after sampling.

NOTE 1 When sampling from bushings, the manufacturer’s instructions should be followed carefully. Failure to do so may lead to serious damage and bushing failure. The oil sampling should be carried out on de-energized bushings. When sampling, precautions should be taken to deal with any sudden release of oil. Samples should be taken with the off-load equipment in its normal position in order to assess correctly the bushing condition.

NOTE 2 For transformers with two sampling valves, the following procedure should be used: open the outer valve first, followed by the second one. This is particularly important to avoid entrance of air into the transformers.

4.2 Sampling of oil by syringe
4.2.1 Sampling equipment
a) Impermeable oil-proof plastic or rubber tubing to connect the equipment to the syringe. This should be as short as possible. A three-way cock should be inserted in the tubing.

The connection between the tubing and the equipment will depend upon the equipment. If a sampling valve suitable for fitting to a tubing has not been provided, it may be necessary to use a drilled flange or a bored oil-proof rubber bung on a drain or filling connection.

NOTE Sampling by syringe is the procedure recommended for bushings by IEC SC36A. In the case of bushings fitted with a sampling point at the mounting flange, the described procedure applies.

In the case of bushings not fitted with a sampling point at mounting flange, it may be possible to take a sample from the top of the bushing. The manufacturer's instructions should be consulted to determine a suitable position. Insert one end of the sampling tube into the bushing, from the top, and connect the other end to the three-way stopcock on the syringe, using plastic coupling, then follow the same procedure.

In the case of bushings pressurized at ambient temperature, the procedure is not applicable, and reference should be made to the instructions of the equipment manufacturer.

b) Graduated gas-tight syringes of a size suitable for containing an adequate oil sample volume (20 ml to 250 ml) fitted with a cock or an obturator so that it may be sealed. See item b) of 3.2.1 for checking the gas-tightness of the syringe. The size of sample required depends on the likely concentration of gas in the sample, the analytical techniques and the sensitivity required.

c) Transport containers which should be designed to hold the syringe firmly in place during transport but allow the syringe plunger freedom to move and prevent its tip from contacting the container whatever its position during transportation.

4.2.2 Sampling procedure
See Figure 4.
a) The blank flange or cover (11) of the sampling valve is removed and the outlet cleaned with a lint-free cloth to remove all visible dirt. The apparatus is then connected as shown in Figure 4a, and the equipment sampling valve (5) opened.
b) The three-way cock (4) is adjusted (position A) to allow 1 l to 2 l of oil to flow to waste (7) (or less, see note).
c) The three-way cock (4) is then turned (position B) to allow oil to enter the syringe slowly (Figure 4b). The plunger should not be withdrawn but allowed to move back under the pressure of the oil.
d) The three-way cock (4) is then turned (position C) to allow the oil in the syringe to flow to waste (7) and the plunger pushed to empty the syringe. To ensure that all air is expelled from the syringe, it should be approximately vertical, nozzle upwards, as shown in Figure 4c. Confirm that the inner surfaces of the syringe and plunger are completely oiled.
e) The procedure described in steps 3) and 4) of this subclause is then repeated until no gas bubble is present. Then the three-way stopcock (4) is turned to position B and the syringe filled with oil (Figure 4d).
f) The cock (2) on the syringe and the sampling valve (5) are then closed.
g) The three-way stop-cock (4) is turned to position C and the syringe disconnected (Figure 4).

Label carefully the sample (see Clause 5).

NOTE 1 In the case of small oil-volume equipment, the procedure described in step b) of this subclause is not applicable, and lower volumes should be removed. Reference should be made to the instructions of the equipment manufacturer.

NOTE 2 It is good practice to avoid contamination of the outer surface of the plunger and inner surfaces of the syringe by dust or sand. Those particles can affect the sealing properties of the syringe. This kind of contamination can be originated by dusty winds or from the handling of the syringe.

NOTE 3 In the case of sealed transformers, if a bubble appears in the syringe directly after sampling, it is recommended to resample.

4.3 Sampling of oil by sampling tube
4.3.1 Sampling equipment
a) Impermeable oil-proof plastic or rubber tubing to connect the equipment to the sampling tube. This should be as short as possible.

The connection between the tubing and the equipment will depend upon the equipment. If a sampling valve suitable for fitting to a tubing has not been provided, it may be necessary to improvise by using a drilled flange, or a bored oil-proof rubber bung on a drain or filling connections.

b) Glass or metal sampling tube, typically of volume 250 ml to 1 l. It may be closed either by cocks or pinch cocks on impermeable oil-resistant tubing or by valves.

A sampling tube and its seal design is acceptable if the loss of hydrogen of the sample contained is less than 2,5 % each week.

The size of sample required depends on the likely concentration of gas in the sample, the analytical technique and the sensitivity required.

c) Transport containers, which should be designed to hold the sampling tubes firmly in place during transport.

4.3.2 Sampling procedure
See Figure 5.
a) The blank flange or cover (11) of the sampling valve is removed and the outlet cleaned with a lint-free cloth to remove all visible dirt. The device is then connected as shown in Figure 5.
b) The cocks (2) on the sampling tube (28) are opened and the equipment sampling valve (5) is carefully opened so that oil flows through the sampling tube to waste (7).
c) After the sampling tube (28) has been completely filled with oil, about 1 l to 2 l are allowed to flow to waste (7) (see Note 2).
d) The oil flow is then closed by shutting off firstly the outer cock (2), then the inner one (2) and finally the sampling valve (5).
e) The sampling tube (28) is then disconnected and the sample carefully labelled (see Clause 5).

NOTE 1 If a glass sampling tube with integral glass cocks is used, it is preferable to drain 1 ml or 2 ml of oil from it prior to transporting it back to the laboratory in order to avoid breaking the tube in the event of it being exposed to a rise in ambient temperature. Record on the label that this has been done.

NOTE 2 In the case of small oil-volume equipment, the procedure described in step c) of this subclause is not applicable. Reference should be made to the instructions of the equipment manufacturer.

4.4 Sampling of oil by bottles
4.4.1 Sampling equipment
a) Impermeable oil-proof plastic or rubber tubing to connect the equipment to the bottle. This should be as short as possible.
The connection between the tubing and the equipment will depend upon the equipment. If a sampling valve suitable for fitting to a tubing has not been provided, it may be necessary to improvise by using a drilled flange, or a bored oil-proof rubber bung on a drain or filling connections.
b) Glass or metal bottles capable of being sealed gas-tight, typically of volume 0,5 l to 2,5 l. Suitable bottles have, for example, screwed plastic caps holding a conical polyethylene seal (see Figure 6b). A bottle and seal design is acceptable if it permits losses of hydrogen of less than 2,5 % each week.
c) Transport containers, designed to protect the bottle during transport.

4.4.2 Sampling procedure
See Figure 6a.
a) The blank flange or cover (11) of the equipment sampling valve is removed and the outlet cleaned with a lint-free cloth to remove all visible dirt.
b) Connect the oil-proof plastic or rubber tubing (3) to the equipment.
c) The sampling valve (5) is carefully opened and about 1 l to 2 l of oil allowed to flow to waste (7) through the tubing (3) ensuring that all gas bubbles are eliminated before the oil sample is collected (see the note at the end of this subclause).
d) Place the end of the tubing (3), with the oil still flowing, at the bottom of the sampling bottle and allow the bottle to fill slowly.
e) Allow about one bottle volume to overflow to waste (7), then withdraw the tubing (3) slowly with the oil still flowing.
f) Close the sampling valve (5) and disconnect the tubing.
g) Tilt the bottle to allow the oil level to fall a few millimetres from the rim so as to leave a small expansion volume. Place the bottle cap securely in position and label the sample (see Clause 5).

NOTE In the case of small oil-volume equipment, the procedure described in steps c) and e) of this subclause is not applicable. Reference should be made to the instructions of the equipment manufacturer.

4.5 Disposal of waste oil
Waste oil shall be disposed of according to local regulations.