Pressure Relief Devices

These devices are the transformers’ last line of defense against excessive internal pressure. In the event of a fault or short circuit, the resultant arc instantly vaporizes surrounding oil, causing a rapid buildup of gaseous pressure. If the pressure relief device does not operate properly and pressure is not sufficiently relieved within a few milliseconds, a catastrophic tank rupture can result, spreading flaming oil over a wide area. Two types of pressure relief devices are discussed below. Consult your transformer’s instruction manual for specifics.
If the pressure relief devices have been painted, they should be replaced. It is virtually impossible to remove all paint from the mechanism and be certain the device will work when needed.

CAUTION: Never paint pressure relief devices because paint can cause the plunger or rotating shaft to stick. Then, the device might not relieve pressure, which could lead to catastrophic tank failure during a fault. Look at the top of the device. On newer units, a yellow or blue button should be visible. If these have been painted, the button will be the same color as the tank. On older units, a red flag should be visible. If it has been painted, it will be the same color as the tank.

Newer Pressure Relief Devices

Newer pressure relief devices are spring-loaded valves that automatically re-close following a pressure release. The springs are held in compression by the cover and press on a disc which seals an opening in the tank top. If pressure in the tank exceeds operating pressure, the disc moves upward and relieves pressure. As pressure decreases, the springs re-close the valve. After operating, this device leaves a brightly colored rod (bright yellow for oil, blue for silicone) exposed approximately 2 inches above the top. This rod is easily seen upon inspection, although it is not always visible from floor level. The rod may be reset by pressing on the top until it is again recessed into the device. The switch must also be manually reset. A relief device is shown in the open position in Picture 1. Picture 2 also shows a pressure relief device with the yellow indicating arm.

Picture 1: Pressure Relief Device

CAUTION: Do not re-energize a transformer after the pressure relief device has operated and relays have de-energized the transformer, until extensive testing to determine and correct the cause has been completed. Explosive, catastrophic failure could be the result of energization after this device has operated.

Picture 2: Photograph of a Pressure Relief Device


CAUTION: Bolts that hold the device to the tank may be loosened safely, but never loosen screws that hold the cover to the flange without referring to the instruction manual and using great care. Springs that oppose tank pressure are held in compression by these screws, and their stored energy could be hazardous.

Once each year, and as soon as possible after a known through-fault or internal fault, inspect pressure devices to see if they have operated. This must be done from a high-lift bucket if the transformer is energized. Look at each pressure relief device to see if the yellow (or blue) button is visible. If the device has operated, about 2 inches of the colored rod will be visible. Each year, test the alarm circuits by operating the switch by hand and making sure the correct annunciator point is activated. If the relief device operates during operation, do not re-energize the transformer; Doble and other testing may be required before re-energizing, and an oil sample should be sent for analysis. Every 3 to 5 years, when doing other maintenance or testing, if the transformer has a conservator, examine the top of the transformer tank around the pressure relief device. If oil is visible, the device is leaking, either around the tank gasket or relief diaphragm. If the device is 30 years old, replace the whole unit. A nitrogen blanketed transformer will use a lot more nitrogen if the relief device is leaking. They should be tested as described below.
A test stand with a pressure gauge may be fabricated to test the pressure relief function. Current cost of a pressure relief device is about $600, so testing instead of replacement may be prudent. Have a spare pressure relief device on hand so that the tank will not have to be left open. If the tank top or pressure relief device has gasket limiting grooves, always use a nitrile replacement gasket. If there are no grooves, use a cork-nitrile gasket. Although relief devices themselves do not leak often, the gasket may leak.

Older Pressure Relief Devices

Older pressure relief devices have a diaphragm and a relief pin that is destroyed each time the device operates and must be replaced. The relief pin determines operating pressure; a number, which is the operating pressure, normally appears on top of the pin. Check your specific transformer instruction manual for proper catalog numbers. Do not assume you have the right parts or that correct parts have been previously installed—look it up. If the operating pressure is too high, a catastrophic tank failure could result.

CAUTION: Replacement parts of an older pressure relief device must be replaced with exact duplicate parts; otherwise, the operating relief pressure of the device will be wrong.

On older units, a shaft rotates, operates alarm/trip switches, and raises a small red flag when the unit releases pressure. If units have been painted or are more than 30 years old, they should be replaced with the new model as soon as it is possible to have a transformer outage. Once each year, and as soon as possible after a through-fault or internal fault, examine the indicator flag to see if the device has operated. The flags must be examined from a high-lift bucket if the transformer is energized. A clearance must be obtained to test, repair, or reset the device. See the instruction manual for your specific transformer. Test alarm/trip circuits by operating the switch by hand. Check to make sure the correct annunciator point activates.
Every 3 to 5 years, when doing other maintenance or testing, examine the top of the transformer tank around the pressure relief device. If the transformer has a conservator and oil is visible, the device is leaking, either around the tank gasket or relief diaphragm. The gasket and/or device must be replaced. Before ordering, make sure that the new device will fit the same tank opening. Contact the manufacturer to obtain a correct replacement.