Transformer Oil Effect on Seal Ring Deformation: Testing Methods and Evaluation

Transformer oil can cause rubber sealing rings to swell, soften, or develop permanent deformation, directly threatening the leak-tightness of transformers and other oil-filled electrical equipment. Laboratory immersion and compression-set tests based on standards such as ISO 1817, ASTM D471, and GB/T 1690 provide a quantitative way to evaluate how transformer oil affects seal deformation. This article covers sample preparation, test conditions, measured parameters, acceptance criteria, and typical applications of the test.

Technical Overview

Sealing rings made of elastomeric materials such as nitrile rubber (NBR) and fluoroelastomer (FKM) are used extensively in power transformers to seal flanged joints, bushings, radiator valves, gas relays, conservators, and manhole covers. These seals remain in continuous contact with hot mineral transformer oil throughout the service life of the equipment, which may exceed twenty or thirty years.

Prolonged exposure to transformer oil can physically and chemically alter the elastomer. Aromatic hydrocarbons in the oil may be absorbed into the polymer network, causing swelling and softening; plasticizers and additives may be extracted, causing shrinkage and hardening; and sustained compressive stress at elevated temperature produces permanent set. The combined result is a loss of contact pressure at the sealing interface and, ultimately, oil leakage.

The test for the effect of transformer oil on seal ring deformation evaluates this compatibility by immersing seals or standard elastomer specimens in transformer oil under controlled temperature and time conditions, then measuring volume change, mass change, hardness change, mechanical property change, and compression set. The principal reference standards include ISO 1817 and GB/T 1690 for liquid resistance testing, ASTM D471 for the effect of liquids on rubber, ISO 815-1, GB/T 7759.1, and ASTM D395 for compression set, and IEC 60296, ASTM D3487, and GB 2536 for the specification of the transformer oil itself.

Testing Samples

O-Ring Seals

O-rings are the most common sealing elements in transformer construction. Samples are typically selected in standard cross-sections per ISO 3601-1 or GB/T 3452.1, in materials such as NBR (medium or high acrylonitrile content) and FKM. Whole O-rings are tested in their as-molded condition whenever possible, because the deformation behavior of a finished seal cannot always be reproduced by flat specimens.

Flat Gaskets and Molded Seals

Rectangular gaskets, strip seals, molded flange gaskets, and custom-shaped seals used at bushing turrets, valve flanges, and manhole covers are tested either as complete parts or as representative sections cut from production parts. The sealing surface finish and molding skin of the actual part are retained so that the test reflects real service conditions.

Standard Laboratory Specimens

When the aim is material characterization rather than part qualification, standard specimens are used: dumbbell specimens per ISO 37, GB/T 528, or ASTM D412 for tensile testing, and cylindrical buttons or ring specimens per ISO 815-1 or GB/T 7759.1 for compression-set testing. Specimens are molded from the same compound as the production seals and cured under production conditions.

Testing Items

The test program is designed to capture every dimension of oil-induced deformation and degradation:

• Volume change (swelling or shrinkage) after oil immersion
• Mass change after oil immersion
• Hardness change (Shore A) per ISO 48, GB/T 531, or ASTM D2240
• Tensile strength change per ISO 37, GB/T 528, or ASTM D412
• Elongation at break change
• Compression set (permanent deformation) under compressive stress in hot oil
• Dimensional change of the seal cross-section or thickness
• Visual condition of the surface after exposure: cracking, blistering, tackiness, or deposits

Testing Methods

Conditioning and Initial Measurement

All specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours in accordance with ISO 23529 before testing. Initial dimensions, mass, hardness, and tensile properties are measured and recorded as baseline values. For O-rings, the cross-section diameter is measured at several positions using a calibrated optical or contact method.

Oil Immersion Exposure

Samples are fully immersed in unused transformer oil conforming to IEC 60296, ASTM D3487, or GB 2536 in sealed glass or stainless-steel vessels. The oil volume is maintained at a minimum ratio of about 15 mL of oil per gram of rubber so that the liquid composition is not significantly altered by extractables. Common test conditions include 100 ± 2 °C for 24 h, 72 h, and 168 h for routine qualification, and extended exposures such as 125 °C for 336 h or 1000 h for high-temperature sealing classes. When in-service compatibility is being studied, tests may be repeated in aged oil taken from operating transformers, because oxidation products in aged oil can accelerate elastomer degradation.

Post-Immessment Measurement of Deformation

After the exposure period, specimens are removed, drained, and blotted lightly to remove surface oil without squeezing out absorbed liquid. Volume is determined by hydrostatic weighing (Archimedes principle) or by dimensional measurement, and mass is measured on a balance with 0.001 g resolution, within the time window specified by ISO 1817 and GB/T 1690 to minimize error from evaporation of absorbed oil. Hardness and tensile tests are then performed on the exposed specimens. Volume change and mass change are calculated as (value after immersion minus value before immersion) divided by the value before immersion, expressed as a percentage.

Compression Set Test in Transformer Oil

Compression set is the most direct indicator of deformation-related sealing failure. Using the constant-deflection method of ISO 815-1, GB/T 7759.1, or ASTM D395, the seal or cylindrical specimen is compressed between polished plates to a defined deflection, typically 25 %, using spacers to control the compressed height. The fixture with the compressed specimen is immersed in transformer oil at the test temperature (for example 100 °C or 125 °C) for 22 h, 24 h, 70 h, or 168 h. After release, the specimen recovers for 30 minutes at room temperature before the final thickness is measured. Compression set is calculated as (t0 − tr) / (t0 − ts) × 100 %, where t0 is the original thickness, ts is the spacer thickness, and tr is the recovered thickness. A lower value indicates better recovery of the sealing force.

Acceptance Criteria

Acceptance limits depend on the product standard, the elastomer class, and the temperature classification of the transformer. Typical industrial guidance for mineral transformer oil is a volume change not exceeding about +20 % for NBR and +10 % for FKM, a hardness change within roughly −10 to +10 Shore A degrees, and a compression set not exceeding approximately 25 % for NBR and 15 % to 20 % for FKM at the rated temperature. Any visible cracking, blistering, or sticky surface after exposure is grounds for rejection regardless of numerical results. The governing product or purchaser specification always takes precedence over these indicative values.

Applications

The test is applied throughout the life cycle of transformer sealing systems:

• Qualification of new elastomer compounds and finished seals by transformer manufacturers before design release
• Incoming batch inspection of O-rings and gaskets to verify consistency of supplied material
• Selection of retrofit seals during transformer maintenance, overhaul, and leakage repair
• Compatibility studies when converting equipment from mineral oil to natural or synthetic ester insulation fluids specified in IEC 61099 or ASTM D6871
• Failure analysis of leaking transformers, in which seals recovered from service are tested to identify swelling, excessive set, or thermal aging as the root cause
• Research and development of improved elastomer formulations, including blends and surface treatments with enhanced oil resistance

FAQ

Which elastomer performs best in transformer oil?

NBR with medium to high acrylonitrile content is the most widely used and cost-effective material for mineral transformer oil, offering a good balance of oil resistance and mechanical strength. FKM provides superior resistance at high temperatures and in chemically aggressive service but at higher cost. EPDM and silicone rubber are generally not recommended for continuous immersion in mineral oil because they swell significantly.

What temperature and duration should be used for the test?

For general-purpose transformer seals, immersion at 100 ± 2 °C for 24 h, 72 h, or 168 h is common. For seals in high-temperature locations such as bushing turrets or near windings, extended tests at 125 °C for 168 h to 1000 h are used. The chosen condition should correspond to the hottest continuous temperature the seal will experience in service, usually with a safety margin.

How much oil absorption and swelling are acceptable?

Moderate swelling is normal for NBR, and values up to about +20 % volume change are commonly accepted provided compression set and mechanical retention remain within limits. Excessive swelling indicates insufficient crosslink density or inadequate acrylonitrile level, while shrinkage indicates extraction of plasticizers and often predicts hardening and cracking.

Does aged in-service oil give different results from new oil?

Yes. Oxidized oil contains acids, peroxides, and sludge precursors that can accelerate elastomer degradation, and depleted oxidation inhibitors reduce protection. For maintenance decisions or failure analysis, testing in both new oil and representative in-service oil provides a more complete picture of compatibility.

Where can this test be performed?

The test can be carried out by accredited third-party laboratories as well as by in-house materials laboratories of transformer manufacturers, provided the facility maintains calibrated instruments, controlled-temperature baths, and procedures traceable to ISO 1817, ASTM D471, and GB/T 1690. Independent accredited testing is recommended for supplier qualification and dispute resolution.