The problem
Water is one of the most damaging contaminants that can migrate into a hydraulic or lubrication system. It can cause catastrophic failures, accelerated wear, part corrosion, and degradation of the oil's viscosity and lubricity.
Water can exist in oil in three distinct states (
see Figure 1):
- Dissolved water (1): This is the least harmful state; the individual water molecules are dispersed within the oil, and there is no visual difference in the appearance of the oil.
- Emulsified water (2): As the water content increases, the oil will approach its saturation point. As it does, its appearance will become milky or cloudy; this is known as an emulsion.
- Free water (3): This is the most harmful state. The oil is now past its saturation point, and a phase separation has occurred, leading to water forming freely in globules within the oil.
Tank corrosion
As well as water existing within the fuel or oil, it can also be present as a vapour in the air space at the top of the tank. The source of this vapour can be the water within the oil or fuel itself or humid air drawn into the tank as the liquid level drops. This water vapour can be particularly damaging as it can condense on the tank walls into free water droplets when the system cools down, and it can lead to corrosion in steel tanks.
Any corrosion will increase particulate contamination in the oil or fuel, significantly reduce the tank's life, and increase the risk of unplanned downtime. Referring to Figure 2, note the corrosion (4) above the fluid line (5). The area within the fluid area has no corrosion (6).
The solution
Introducing dry nitrogen effectively removes water from the oil in hydraulic circuits, gearboxes and other oil-based lubrication systems with a reservoir equipped with an atmospheric breather. The process can remove water present in all three phases of saturation and the vapour trapped in the tank air space.
The principle is straightforward: dry nitrogen is fed into the air space at the top of the reservoir in a continuous, replenishing stream exiting through the breather. The nitrogen fills the air space, purging the oxygen and creating a protective and dehydrating atmosphere above the oil or fuel. Because nitrogen has a much lower dew point than air, around -43°C, it will naturally draw the moisture (and harmful dissolved gasses) from the oil by a process known as mass transfer and then transport it out of the tank into the atmosphere via the breather. By replacing the warm, moisture-rich air at the top of the tank with a constantly replenishing cloud of dry nitrogen, the continuous dewatering process can reduce and maintain water contamination levels at values below 100 parts per million.
A secondary benefit of dry nitrogen, which is particularly pertinent when using the system on older, legacy tank installations, is that by purging the headspace of oxygen, and then replacing it with inert nitrogen, it prevents or stops any corrosion of the internal tank walls. Rust is an iron oxide formed by the reaction of iron and oxygen in the catalytic presence of water or moisture in the air. The N2-Dry system removes two of these components, virtually eliminating the ability for rust to form. This will both increase the life of the tank and reduce the level of particulate contamination in the oil or fuel.
The Filtertechnik N2-Dry
The Filtertechnik N2-Dry system utilises a feed from a standard site compressed air supply (or a supply from a small compressor if site air isn't available) and passes it through a specially designed nitrogen separation membrane.
Nitrogen is produced by channelling the air supply (78% nitrogen, 21% oxygen, 1% other gasses) through a specially developed, continuous-use membrane that splits nitrogen from oxygen and other gasses. Using the principle of Selective Permeability, the oxygen passes through this specially developed membrane whilst the nitrogen is retained within the core. This process effectively splits the nitrogen from the oxygen. The nitrogen then exits through a port at the end of the membrane housing, and the enriched oxygen through another located on the side. A needle valve on the nitrogen outlet controls the flow and the purity of the nitrogen being generated.
The benefit of using a nitrogen-generating membrane over a nitrogen source such as a compressed gas cylinder is that no additional nitrogen is being introduced into the local atmosphere. Therefore, the overall composition of the resident air in the room is not changed. However, it is recommended that the N2-Dry system only be installed in rooms with adequate ventilation.
