Dehydrating breathers, sometimes referred to as silica gel breathers, prevent moisture from ambient air coming into contact with an electrical transformer’s insulating liquid. The breather contains hydrophilic (attracted to water) crystal or bead shaped silica gel. Within the transformer industry, silica gel is the preferred drying agent although other options are available.
Breathers are fitted to all conservator type liquid immersed transformers. A hermetic liquid insulated transformer does not require a dehydrating breather because the transformer tank is completely sealed and the insulating liquid has no contact with ambient air.
Function of Transformer Dehydrating Breather
Dehydrating breathers are simple yet critical pieces of equipment required for maintaining a healthy electrical transformer. As the transformer load varies, so too does the temperature. As the temperature increases, the insulating liquid volume increases and air is forced out of the conservator tank. As the temperature decreases, the oil volume decreases and air is drawn back into the conservator tank via the silica gel breather (see animation below).
Transformer ‘Breathing’ Due to Changing Temperature
If the transformer tank was not vented (open to atmosphere), the change in temperature would create a positive pressure within the transformer tank as the temperature increases, and a negative pressure when the temperature decreases. However, venting to atmosphere creates additional complications (see below).
Ambient air contains many undesirable foreign bodies and these must be prevented from coming into contact with the transformer insulating liquid. Moisture contained within the air is particularly hazardous to transformer insulating liquid as it has a detrimental effect upon the insulating liquid’s dielectric strength and can cause a significant reduction in the transformer’s useful working life.
How Dehydrating Breathers Work
The below video is an extract from our Introduction to Electrical Transformers Online Video Course.
As the transformer cools, the insulating liquid volume decreases and air is drawn to the conservator tank via the breather. Moisture within the air is absorbed by the silica gel as it passes through the breather. Silica gel can absorb approximately 20% of its weight in moisture before it becomes saturated e.g. 5 kg of silica gel can absorb approximately 1 kg of water.
Transformer Inhaling Air
In addition to its dehydrating function, the breather has a second function. An oil trap located beneath the breather isolates the silica gel from ambient air when there is no/little pressure difference between the conservator tank and the ambient air. The oil trap forms a barrier through which air cannot freely flow, thus the silica gel is not constantly absorbing moisture and the intervals between regenerating (drying-out) the silica gel can be extended. The oil trap also attracts dust particles from the air when air does enter the breather; this reduces the likelihood of insulting liquid contamination.
When to Change the Silica Gel
The silica gel will absorb moisture from the air until it becomes saturated, at which point it will no longer be effective. Fortunately, it is very easy to know when silica gel must be replaced or regenerated (a process known as regeneration). As the silica gel become saturated, it will change colour, starting at the bottom and spreading upwards. Typical colours employed today are orange (dry) changing to clear (saturated).
Silica Gel Breather Becoming Saturated
Or, purple (dry) changing to pink (saturated).
Silica Gel Breather Becoming Saturated
It is generally advised to keep a 5-10 cm (2-4 inches) ‘buffer’ of dry silica gel at all times, but regenerating or replacing the silica gel when it is 1/3 is saturated is best practice. Having less than 1/3 dry silica gel within the breather increases the likelihood that some moisture will reach the transformer insulating liquid and contamination will occur.
Silica Gel Regeneration
Many people replace the silica gel when it becomes saturated although this is usually not necessary. The absorbed moisture will liberate itself from the silica gel if exposed to elevated temperatures, thus it is possible to restore the silica gel to its former non-saturated state. Typical means for regenerating silica gel include placing the silica gel beads on a hot surface and allowing for them to dry.
How to Check the Breather is Working
There are two easy ways to ensure the breather is working correctly.
- Look for bubbles passing through the oil trap.
- Check the silica gel colour is changing over time.
If the silica gel does not change colour over time, there is a strong possibility that the transformer has a leaking gasket or seal, and air is entering and exiting through the leak. Leaking gaskets and seals should be found and replaced as soon as possible.
Breathers are purchased based upon their drying capacity per volume of insulating liquid. For example, a transformer utilising 5,000 litres of insulating liquid will require a smaller breather than a transformer utilising 30,000 litres. The breather size is usually determined by the transformer original equipment manufacturer (OEM).