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Arabic (عربي) What Happens When Oil Turns to Gas
An internal arc inside a transformer vaporises the surrounding oil almost instantly. A litre of mineral oil converts to roughly a thousand litres of gas. In a sealed tank with no route of escape, the pressure rise is measured in milliseconds, not seconds — and the tank is not designed to contain it. Without a controlled release path, the weakest point of the tank fails: a weld splits, a gasket blows out, or in severe cases the tank ruptures and burning oil is expelled.
The pressure relief device exists to decide where that release happens. It is a spring-loaded valve mounted on the tank cover, calibrated to open at a pressure below the mechanical limit of the tank and to discharge the excess through a controlled outlet. Once the pressure falls, it closes again and the tank remains sealed.
This is a protection of last resort. Every other device — Buchholz relay, winding temperature indicator, differential protection — is intended to detect a fault and disconnect the transformer before the pressure ever reaches this point. The relief device operates when they have not been fast enough, or when the fault energy was too great for any protection to prevent the pressure rise.
How It Operates
1 — Sealed
Under normal service the valve disc is held against its seat by calibrated springs. The seal is oil-tight and gas-tight; normal thermal expansion and contraction of the oil produce nowhere near enough pressure to lift it.
2 — Opening
When internal pressure exceeds the set point, the disc lifts. As soon as it moves, the exposed area increases sharply, so the opening force grows and the valve snaps fully open — typically in under two milliseconds. Oil and gas discharge through the outlet.
3 — Resealing
Once the pressure falls below the closing value, the springs return the disc to its seat and the tank is sealed again. Only a small quantity of oil is lost, and no moisture or air enters afterwards — the transformer is protected while awaiting investigation.
The resealing behaviour is the whole point of a modern relief device. It limits the volume of oil expelled, keeps the fire risk contained, and — crucially — leaves the tank closed so that the insulation system is not exposed to atmospheric moisture in the hours or days before an engineer arrives.
Why Sealed Transformers Depend on It
In a conservator-type transformer, a pressure rise has somewhere to go before it becomes dangerous: the connecting pipe leads to the conservator, and the Buchholz relay in that pipe detects both the gas and the surge. There is a path, and there is a device watching it.
Hermetically sealed transformers have neither. The tank is completely filled and welded closed, with corrugated walls absorbing thermal expansion. There is no pipe, no conservator, no gas space. A fault that vaporises oil raises the pressure against a rigid, closed boundary.
This is why the pressure relief device is not an accessory on a sealed unit — it is the only mechanical route by which fault energy can leave the tank in a controlled way. On DATSAN sealed transformers it is fitted as standard above a defined rating, and supplied on request below it.
Relief Device or Explosion Vent
Older transformers used an explosion vent: a length of pipe rising from the tank cover, closed at the top by a thin glass or bakelite diaphragm. On overpressure the diaphragm shattered and the pressure escaped. It worked, but the shortcomings are significant.
| Aspect | Explosion vent | Pressure relief device |
|---|---|---|
| Operating pressure | Depends on the diaphragm; scatters widely with age and temperature | Calibrated and repeatable, set by spring load |
| After operation | Tank left open to atmosphere until the diaphragm is replaced | Reseals automatically; tank stays closed |
| Oil loss | Substantial — discharge continues until pressure equalises | Limited to the volume released before resealing |
| Moisture ingress | Certain — the insulation system is exposed | None |
| Signalling | None — operation is discovered by inspection | Mechanical indicator plus contacts for alarm or trip |
| Reuse | Diaphragm must be replaced | Reusable after resetting the indicator |
Indication and Signalling
A relief device that operates unnoticed is of limited value: the transformer has suffered a fault severe enough to raise tank pressure past the set point, and that needs to be known. Three levels of indication are available.
Mechanical indicator
A brightly coloured pin rises when the valve operates and stays raised until reset by hand. It records the event even if no electrical connection exists, and is visible from ground level during a routine inspection.
Electrical contacts
One or two changeover contacts operate with the valve, wired to alarm or to trip. On unattended substations this is the only way the event becomes known in time to act on it.
Discharge pipe
An optional outlet pipe directs expelled oil away from bushings, cable boxes and walkways towards a safe collection point. Recommended for indoor substations and wherever personnel access is close to the tank.
Whether the contacts are wired to alarm or to trip is a decision for the protection scheme, not for the device. On a distribution transformer feeding non-critical load, trip is usual: the pressure rise indicates a fault serious enough that continued operation is unjustifiable. Where an immediate outage carries its own risk, the contact is wired to alarm and the decision left to the operator.
Typical Specifications
| Parameter | Typical value |
|---|---|
| Opening pressure | 0.35 bar standard; 0.15–0.70 bar available to order |
| Closing pressure | Approximately 80 % of the opening pressure |
| Opening time | Under 2 ms from set point to full opening |
| Discharge capacity | Depends on orifice size, commonly 100–400 m³/h |
| Contacts | One or two changeover contacts, 5 A |
| Indicator | Mechanical pin, manually reset |
| Ambient temperature range | −40 °C to +120 °C |
| Enclosure protection | IP65 for the contact housing |
| Mounting | Bolted flange on the tank cover, in the gas space above oil level |
| Seal material | Nitrile or fluorocarbon elastomer, oil resistant |
Selection and Installation
Set pressure follows the tank, not the transformer rating. The opening pressure must lie below the pressure the tank can withstand without permanent deformation, and comfortably above the highest pressure reached in normal service — including the peak that occurs when a cold transformer is loaded rapidly. A device set too low causes nuisance operations; set too high, it opens after the tank has already yielded.
Mount in the gas space, not below oil level. The device belongs on the tank cover, above the oil. Mounted low, it discharges a far greater volume of oil for the same pressure event, and the valve may not reseal cleanly.
One device is not always enough. Larger tanks, and tanks with a high ratio of oil volume to cover area, may require two devices to achieve the necessary discharge capacity. The calculation is part of the tank design, not an accessory selection.
Test the contacts, not the valve. The electrical contacts can and should be verified during commissioning using the manual test facility. The valve itself is factory calibrated and sealed; field adjustment of the spring setting invalidates the calibration.
Note: Values in this article are typical and given for guidance. DATSAN fits pressure relief devices as standard to hermetically sealed transformers above a defined rating, and on request below it. Set pressure, discharge capacity, contact configuration and the need for a discharge pipe are determined at the design stage according to the tank design and the customer's protection scheme, and confirmed in the test report.