An important design consideration is whether to specify a vented or pressurized reservoir. The major deciding factors are the location and inlet requirements of the pumps. The fluid level of the reservoir in many mobile applications is below the pump inlet. At best, if there is vacuum at the pump inlet, the pump may have to be derated. If inlet line losses are great enough, cavitation will occur. In these cases, pressurizing the reservoir will help maintain pump performance.

There are three ways to pressurize a reservoir on most mobile equipment:
if available, use regulated compressed air from a machine's pneumatic system. This is the most effective method
trap the air within the reservoir clearance volume (above the fluid) and depend on thermal expansion of the fluid to compress this air, and thus pressurize the reservoir. A reservoir pressure cap holds pressure within the tank and relieves any excess pressure, or
tap pressurized air from the scavenge pump of a two-cycle diesel engine.

With pressurized reservoirs, be sure to calculate stresses on reservoir walls, because even low pressures can exert substantial loads against large areas. For example, an internal pressure of only 3 psi applies a force of 1800 lb on a 20- by 30-in. wall. This force, combined with weight of hydraulic fluid, plus G forces involved in mobile equipment, can produce stresses high enough to actually work harden a metal reservoir. Work hardening makes the metal more brittle, which eventually will cause leakage when the metal is exposed to continued stress.

Wall stresses should also be calculated for vented reservoirs. High stresses develop quickly in large areas of flat plate. And again, weight of the fluid can cause large deflections. Furthermore, mounting peripheral equipment — such as ladders — to a reservoir increases the need to specify stiffening members and thicker plate.

For vented reservoirs, strong consideration should be given to using filtered breathers to reduce ingression of airborne contaminants. If equipment will be operated in cold weather, use of a filtered breather with desiccant should be considered to reduce the humidity of air drawn into the reservoir, thereby reducing the amount of moisture that can condense on interior surfaces of the reservoir. If a filtered breather is used, be sure to also specify a vacuum breaker. This type of relief valve allows air to enter the reservoir if the filter becomes clogged. A clogged filter could cause the pump to pull a vacuum in the reservoir, which could lead to pump cavitation or other problems.

In cases of extremely heavy airborne contamination, you may want to consider a breather that prevents ambient air from coming in contact with the fluid. These breathers use a bladder or diaphragm to keep the ambient air separated from the hydraulic fluid and the reservoir's interior surfaces.

Cleaning and maintenance

Access for reservoir servicing must also be taken into account. There should be provisions to drain both return and suction areas of the tank, especially if a dam is installed to separate them. Pipe couplings often are used for such drains, but SAE O-ring ports provide better sealing. Valving should also be installed to close off inlet lines when replacing pumps or other components that are mounted below fluid level.

This is often wishful thinking, but access should be provided for cleaning and maintaining the interior of the tank. Ideally, hatches should be large enough to provide enough room for service personnel to maneuver cleaning tools.

Submitted by Brian Burgess, product manager, fixed-displacement products, Parker Hannifin Corp., Hydraulic Pump & Motor Div..