When a treatment machine is running, the oil tank is the transfer station of the whole system: the oil arrives in it, is heated in it, circulates through it, is drawn off by the vacuum and returns to it. The pressure inside the tank is not steady during this process: it is pulled down while the vacuum is drawn, it rebounds when the pump stops, and there is a shock when the pump starts. If the tank were a completely closed vessel, those pressure changes would have nowhere to go; if it were completely open, the moisture and the dust in the air would reach the oil surface directly.
The breather is the component that resolves this contradiction. It is fitted to the vent of the oil tank and lets the pressure inside the tank balance with the outside automatically within a certain range: it vents when the pressure inside is too high, it draws in when the pressure inside is too low, and in between it stays closed and keeps the oil separated from the outside.
What this barrier actually holds back
The first thing held back is the backflow of moisture from the air. While the breather is closed the exchange between the tank and the outside is blocked and the damp air outside cannot get in. This matters particularly for insulating oil, because moisture is the factor that has the most direct effect on the breakdown voltage: if the machine stands for a week or two, the water content near the oil surface can be noticeably higher than at the bottom of the tank. Once the breather fails and sticks in the open position, this layer of protection is gone.
The second is the volatilisation of the light fractions. Transformer oil and lubricating oil both contain a certain amount of light hydrocarbons, and these components evaporate slowly when the oil surface is exposed to the air. What volatilisation carries away is the part of the oil most easily lost, and it shows as a slow fall in the oil level, a lower flash point and a slight change in viscosity. During installation this is an invisible bill: the oil is topped up slowly and consumed quickly, and the new oil that goes in mixes with old oil that has already lost its light fractions, so the quality is not uniform.
The third is dust. In the closed state the breather also acts as a seal and holds back most of the suspended particles that would otherwise settle directly onto the oil surface.
What the oil tells you when the breather has failed
Problems with the breather seldom show up as "the machine is broken"; more often they appear as a slow deterioration in the condition of the oil, so they have to be judged from the symptoms:
- The oil level falls with no obvious sign of leakage — this may be the volatilisation of the light fractions, or the breather may have lost its seal and the oil is being drawn off; the difference is established by checking whether the breather is stuck in the open position.
- The water content of a sample from the bottom of the tank differs clearly from a sample taken near the oil surface — this means moisture from the air is getting into the tank, so check whether the breather is open or stuck.
- The smell of the oil becomes faint and the flash point is low — part of the light fractions has been lost.
- There are oil traces around the breather — this means liquid is flowing back along the vent path, and the valve element may have been held by contaminants in the oil and lost its ability to reseat under its own weight.
| Observed symptom | More likely cause | First check |
|---|---|---|
| The oil level falls slowly, with no external leak | The breather is not sealing properly or is stuck in the open position | Check the valve element by hand for reseating and for being held by contaminants |
| The water content near the oil surface is higher than at the bottom | Backflow of outside moisture | Check whether the vent is open and whether the sealing ring is intact |
| The smell of the oil is faint and the flash point is low | Volatilisation of the light fractions | Establish whether the breather has been venting continuously |
| There are oil traces around the valve body | The valve element is stuck and liquid flows back | Remove and clean the valve element and the valve seat |
| An abnormal noise comes from the tank when the pump starts | Poor venting causes negative pressure inside the tank | Establish whether the breather passage is blocked |
Points to note in the selection and the installation
The venting capacity of the breather has to be matched to the volume of the oil tank and the rate at which oil enters and leaves. If the venting capacity is too small, the pressure inside the tank cannot fall when the pump draws a vacuum at start-up, and there will be difficulty in drawing oil, shaking in the pipework and even damage to the mechanical seal of the pump from the negative pressure; if it is too large the throttling effect is lost, the pressure inside the tank follows the outside, and the barrier is in effect not there.
The mounting direction depends on how the valve element resets under gravity. Some types use a counterweight so that the element falls back into the closed position when the pressures are balanced, and these normally have to be installed vertically with the valve body facing upwards; fitted the other way round they stay venting for long periods. Others are pressed closed by a spring, and the direction is less restrictive, but the vent still must not be pointed downwards, or oil sludge and condensate will run down the passage into the valve chamber.
The valve opening also has to be kept away from places where it is rained on directly. The sealing of most breathers is labyrinth type or diaphragm type: it keeps out mist but it does not keep out a heavy volume of water washing over it, and prolonged exposure to rain speeds up the ageing of the diaphragm noticeably.
The breather is one of the smallest parts on the machine, but the change in the condition of the oil during a period out of service usually starts with it.
The checks to carry out on a machine that is standing idle
If the machine is to be out of service for a long time, it is worth completing three things before it is shut down: first, bring the oil level in the tank up to the normal working level and do not drain it — with less oil the volume of air near the oil surface is larger and the breathing volume caused by temperature differences is greater as well; second, check that the breather is in the closed position and move it by hand to confirm that it returns; third, wrap the vent or fit a rain cover over it, without blocking the opening, since blocking it is as bad as leaving it open.
When the machine is brought back into service, add one check to the routine inspection of the oil itself: make sure there is no water in the bottom of the tank. Condensate that entered while the breather was ineffective settles at the bottom of the tank, and on a restart it is drawn straight into the system and vaporises in the vacuum chamber, which both lowers the vacuum level and makes the judgement on the condensate unreliable.