Hydraulic reservoirs for mobile equipment are expected to perform the same functions as their industrial counterparts - but usually under more adverse and less predictable operating conditions. Machine motion (which makes complex baffling systems necessary to prevent fluid sloshing) and extreme ambient temperatures are just two examples of the special problems faced by designers of hydraulic systems for mobile equipment.

Cold temperatures can cause water to condense on the interior surfaces of the reservoir. Unless a means is provided to remove condensate from the hydraulic fluid, that water will act as a contaminant to degrade system performance and reliability and shorten the life of the fluid and hydraulic components. In addition, reservoirs constructed of steel would be prone to rust and the high potential for fluid contamination it poses. Rust can be prevented simply by constructing the reservoir of engineered plastics, composites, non-ferrous metals, or steel with a corrosion-resistant coating.

Size and weight limitations may require mobile equipment to operate with reservoirs as small as the volume a pump discharges in a minute. This is roughly a third the size of a reservoir traditionally used in industrial applications. The space and shape limitations mobile equipment place on reservoirs usually requires that they be custom designed. Cost, size, and weight must be minimized, while still maintaining adequate performance and efficiency.

Internal or external filters?

Return filters are often placed inside the tank to save space and to provide integral diffusion. One advantage of in-tank return filtration is that filling the tank through the filter helps ensure system cleanliness. However, be sure contaminants cannot fall into the reservoir when a return filter element is changed. Placing filters within the tank provides a neat design but may promote contaminating an area that is difficult to keep clean. While more difficult to plumb, external return filters keep contamination outside the tank, and they are more easily accessible for servicing.

Magnets should be placed in the reservoir to trap ferrous particles. Dams and suction strainers also can be added to increase the effectiveness of the reservoir as a solid contaminant controller. Particle dams, placed between the return and suction areas of the tank, help contain heavier particles that may have bypassed the return filters. Dams commonly consist of an angle plate that extends across the floor of the tank. The dam should be high enough to contain particles until the reservoir is routinely cleaned, but low enough to prevent fluid from having to cascade over it. Dams also provide ideal mounting surfaces for magnets.

Locating a pump at or above fluid level and far away from the tank (more the rule than the exception with mobile equipment) usually prohibits the use of pump inlet filters. Suction strainers or filters should be considered as a form of last-chance pump protection when positive pressure can be provided at the pump inlet &emdash; as with a charge pump or pressurized reservoir. When sizing suction filters, pay attention to fluid temperature (especially during startup) if equipment will operate in cold climates and pumps cannot be disengaged during startup.