When replacing hydraulic fluid, it is tempting to believe that the new oil will be clean and free of contaminants, and that it can be put straight into the reservoir without any problems. Unfortunately, this is not always the case. If you use hydraulic oil from a large drum, there is a high chance that it already contains some water and dirt particles; new hydraulic oil typically has a cleanliness level of ISO 4406 23/21/18, which is more than most hydraulic systems will tolerate. If the system has a rating of say, 20/18/15, then the new hydraulic oil is already too contaminated, as a single digit increase in any of those numbers is effectively a doubling of the contamination level for each micron size.
We can see, therefore, that it is a good idea to filter new hydraulic oil before it enters the system taking with it contamination that will potentially lead to problems with the system. Most hydraulic system failures can be traced back to contaminants in the oil causing friction, high temperatures and a loss or build-up of pressure that can cause serious damage to the components within. Avoidable problems should not be encouraged by cutting corners when replacing hydraulic fluid as it is a false economy.
If you usually replace the hydraulic oil straight into the reservoir, you can add a filtration cart or a kidney loop system to clean the fluid before it gets into the system itself. Even if you have a filter downstream it is a good idea to still keep a filtration system in the reservoir too, to ensure that the downstream filter does not have to work too hard and retains the lifespan it is expected to have, cutting down on element changes. A kidney loop system is ideal for filtering hydraulic fluid in the reservoir and runs independently of the equipment itself, meaning it can still be cleaning the oil even when the equipment is not being used. This means that the fluid can be filtered thoroughly before the machinery is switched back on and also offers a higher level of filtration throughout the life of both the hydraulic fluid and the equipment itself.
Dual filter elements are usually used in kidney loop systems to filter out particles of different sizes and ensure that the filter does not become clogged too early. This also allows for better element change schedules as they can both be done at the same time, rather than replacing the first, then the second, then the first again and so forth more frequently. The dual filter elements in a kidney loop system also perform better than in-system filters, as they are not exposed to any pressure and can retain contaminants more effectively.
Alternatively, new hydraulic fluid can be filtered into the system via the return filter. If the application is very sensitive, it may be best to stick with a kidney loop filter, but if this is not possible due to the nature of the hydraulic equipment, the return filter route is a good option. A tee needs to be installed in the return line above the filter, and one branch connected to a drum pump discharge hose via a quick connector. When it is time to filter the new oil in the drum pump, it is attached to the return line and the oil gets pumped through the return filter and into the reservoir.
Not filtering new hydraulic oil into a system basically opens the door wide to dirt and water getting in, and undermines maintenance activities and careful user behaviour designed to keep the equipment in full running order. If you need to change how you replace the hydraulic fluid or add a filtration component into the system, the cost of doing so should be weighed against the savings in unnecessary maintenance and repairs due to contamination related damage.