Compression Set Analysis of Sealing Materials After Immersion in Transformer Oil

Compression set analysis after immersion in transformer oil is a critical durability test that evaluates the ability of elastomeric sealing materials to recover after prolonged compression while exposed to mineral insulating oil. This article explains the test principles, applicable standards such as ASTM D395, ASTM D471, and ISO 815-1, specimen requirements, test procedures, and evaluation criteria. It serves as a technical reference for engineers involved in transformer sealing design, material qualification, and long-term reliability assessment.

Technical Overview

Compression set, also known as compression permanent deformation, refers to the residual deformation that remains in an elastomeric material after it has been held under compression for a defined period and then released. For sealing components installed in power transformers, this property is of decisive importance because a seal that cannot recover its original thickness will lose contact pressure, allowing insulating oil to leak and moisture to ingress.

Transformer seals, including flange gaskets, O-rings, and molded sealing plates, operate in continuous contact with hot mineral insulating oil for service lives of twenty to thirty years. The oil environment introduces additional degradation mechanisms that are not present in dry-air compression set testing: physical swelling caused by oil absorption, extraction of plasticizers and processing oils, post-vulcanization reactions, and chemical attack at elevated temperatures. For this reason, compression set testing performed with the specimens immersed in transformer oil provides a far more realistic representation of field service conditions than conventional oven testing in air.

The test is typically performed in accordance with ASTM D395 (Method B, constant deflection), with the immersion environment established by ASTM D471 and the transformer oil itself specified according to IEC 60296, ASTM D3487, or GB 2536. Equivalent international procedures include ISO 815-1 and GB/T 7759.1. The compression set value is calculated as CS = (t0 − t1) / (t0 − ts) × 100%, where t0 is the original specimen thickness, ts is the spacer (compressed) thickness, and t1 is the recovered thickness measured after release. A lower percentage indicates better elastic recovery and sealing capability.

Testing Samples

Cylindrical Compression Buttons

Standard molded cylindrical specimens per ASTM D395 are used for the baseline compression set determination. The preferred Type 1 geometry is 29.0 ± 0.5 mm in diameter and 12.5 ± 0.5 mm in thickness, while the smaller Type 2 geometry is 13.0 ± 0.2 mm in diameter and 6.3 ± 0.3 mm in thickness. Specimens must be free of porosity, surface defects, and molding flash.

O-Rings and Radial Seals

O-rings used in transformer valves, butterfly valves, and gas relay connections are tested in dedicated compression fixtures per ASTM D1414, which allows the compression set of actual seal cross-sections to be evaluated rather than that of a laboratory plaque.

Flange Gaskets and Sealing Plates

Full-size or scaled gaskets made of nitrile rubber (NBR), fluororubber (FKM), silicone rubber (VMQ), EPDM, cork-rubber composites, or nitrile-bonded cork are tested either as die-cut discs of defined diameter or in dedicated flange simulation fixtures. These samples represent the actual sealing products used on transformer tank flanges, radiator connections, bushing turrets, and conservator joints.

Material Compound Slabs

For material qualification and compound development, vulcanized sheets are supplied to accredited third-party laboratories, from which compression buttons are cut or molded under controlled cure conditions.

Testing Items

The core test item is the compression set value measured after a defined exposure period at a defined temperature while the compressed specimen is fully immersed in transformer oil. In most technical specifications, the following supplementary measurements are performed on companion specimens to build a complete picture of oil resistance:

• Compression set percentage after immersion in transformer oil (constant deflection, ASTM D395 Method B)
• Volume change and mass change resulting from oil swelling (ASTM D471)
• Hardness change in IRHD or Shore A before and after immersion (ISO 48 / ASTM D2240)
• Change in tensile strength and elongation at break after immersion (ISO 37 / ASTM D412)
• Visual assessment of surface cracking, blistering, tackiness, or delamination
• Optional condition monitoring of the oil itself, including breakdown voltage, acid number, and dissipation factor, to detect interaction between seal and fluid

Testing Methods

The standard test sequence is as follows. First, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours in accordance with ISO 23529 or ASTM D1349. The initial thickness t0 is measured at three points using a dial gauge or laser gauge with an accuracy of 0.01 mm.

Each specimen is then placed between clean, polished plates of a compression fixture and compressed to the specified deflection, most commonly 25%, using fixed spacers that define the compressed thickness ts. The assembled fixtures are fully immersed in unused mineral transformer oil conforming to IEC 6026-class specifications or the equivalent national standard, inside a sealed glass or stainless-steel vessel. Natural ester fluids per IEC 62770 may be used where the seal is intended for ester-filled transformers.

Commonly specified exposure conditions include 70 °C, 100 °C, and 125 °C, with durations of 24 h, 72 h, 168 h, or 336 h. For long-term aging assessment and lifetime estimation, extended exposures of 1000 h or longer are applied. Upon completion of the exposure period, the fixture is removed from the oil bath and allowed to cool for 30 minutes at room temperature while still compressed, in accordance with the standard recovery procedure. The specimen is then released and rested for a further 30 minutes before the final thickness t1 is measured and the compression set percentage is calculated.

Key procedural precautions include the following:

• Maintaining oil volume-to-specimen-surface-area ratio within the limits of ASTM D471 to ensure immersion severity is comparable between laboratories
• Replacing the oil between test series to prevent accumulated extract from altering the immersion medium
• Testing a minimum of three specimens per condition and reporting the median value
• Verifying fixture parallelism so that deflection remains uniform across the specimen face
• Excluding specimens exhibiting bubbling, bond failure, or non-uniform swelling from the reported data set, with such observations documented separately

Applications

Oil-immersed compression set analysis is applied throughout the transformer industry for several purposes:

• Qualification of gasket and O-ring materials for new transformer designs, on-load tap changers, bushings, and radiator packages
• Incoming inspection and lot acceptance testing of sealing components delivered by gasket suppliers
• Comparative evaluation of elastomer compounds, such as NBR versus FKM versus silicone, for high-temperature oil service
• Accelerated aging studies supporting seal life prediction and transformer maintenance interval planning
• Failure analysis of leaking transformer flanges, conservator joints, and valve stems, where excessive compression set is frequently identified as the root cause
• Research and development of oil-resistant rubber formulations, including peroxide-cured and low-compression-set compounds

FAQ

What compression deflection is typically used? The most common deflection is 25% of the original thickness, applied under constant deflection per ASTM D395 Method B. Some specifications use 15% or 20% to simulate low-stress flange conditions.

Why test in transformer oil instead of air? Transformer oil penetrates the elastomer network, causing swelling and plasticizer extraction, while the hot oil environment accelerates chemical degradation. Air-oven compression set values are consistently lower than oil-immersed values and can substantially overestimate seal performance in service.

What acceptance criteria are commonly applied? Requirements vary by manufacturer and service temperature, but typical specifications for high-grade FKM sealing materials require compression set not exceeding 25% to 35% after 168 h at 125 °C in oil. NBR materials are more commonly evaluated at 100 °C, with limits of 35% to 50%.

Can actual gaskets be tested instead of molded buttons? Yes. Finished parts can be tested in purpose-built fixtures that reproduce the flange geometry and bolt torque, which is particularly valuable for large rectangular gaskets used on transformer tank covers.

How do ester fluids affect the test? Natural and synthetic esters have different solvency characteristics than mineral oil, and some elastomers that perform well in mineral oil swell significantly in esters. Seals intended for ester-filled transformers should always be validated by immersion testing in the actual service fluid.