6.3 Oil tanks
On modern units, the main oil tank has a capacity of around 75 m3 for the lubricating oil system oil inventory, with a normal working level volume of 50 m3: a typical arrangement is shown on Fig 2.59. The tank is designed with the oil return and the pump suction separated by baffle plates, to assist in de-aeration and settlement and also to prevent the formation of stagnant pockets of oil.
The quantity of oil in the tank and the internal design of the tank provide around seven minutes oil transit time from return to suction, with the normal time limits being between five to ten minutes.
The trend on modern systems is to provide a self-contained section of the overall oil tank for the hydrogen seal-oil system, the objective being to eliminate the risk of hydrogen gas passing into the main lubricating oil system. Where a self-contained system is used, a back-up supply of oil to the hydrogen seals is provided. Where the lubricating oil system provides the hydrogen gas-side seal-oil, special precautions are taken to ensure that any entrained hydrogen is released and vented to atmosphere. A separate seal-oil loop detraining chamber is used to ensure that all hydrogen is removed before the oil is mixed with the main oil system.
Coarse-mesh strainers are provided on all oil returns to the tank to assist de-aeration and to catch any large foreign material. Problems have arisen in the past with corrosion at the air/oil interface in the main oil tank due to the presence of water/water vapour in the oil. The main oil tank is now fabricated either in stainless steel or mild steel with a special phenolic paint protection against corrosion.
On the top of the oil tank, in addition to the suspended oil pumps and the bearing oil pressure relief valves, two vapour extraction pumps are also mounted. One to remove water and oil vapour from the tank and the other to extract hydrogen and oil vapour from the detraining tank.