Many functions in hydraulic systems are provided by reservoirs. First, a reservoir holds the fluid in a convenient spot for the pump inlet. It supplies extra fluid to the circulating system in the event of leakage or cylinder extension. In addition, most reservoirs are called upon to perform a fluid-conditioning role, in which the turbulent fluid returning from the hydraulic system is allowed to settle and deaerate. And to complete the complex list, many users expect a reservoir to exchange heat with the outside air, thus cooling the heated fluid.

From these complex and occasionally conflicting requirements have grown a body of folklore about reservoirs. Elegant rules of thumb proclaim that the reservoir should be at least three to five times the size of the per-minute flow; accordingly, a 50-gpm hydraulic system is thought to need at least a 150-gallon reservoir. Actually, most of these intricate rules are based on the thought that the reservoir must dissipate much of the system heat. If a heat exchanger is not available, the rules are generally accurate. If a heat exchanger is used, the reservoir can be as small as one time the per-minute flow -- or, a 50-gallon reservoir for the example system. With astute design of internal baffles and settling areas, the reservoir can be much smaller, perhaps as little as one-half to one-third the flow. And, of course, certain types of closed hydraulic systems require almost no reservoir at all, using instead an external makeup pump to put more fluid into the system as required.

Rules of thumb aside, most applications require a standard industrial reservoir that performs the standard gamut of tasks. In general, the reservoirs can be categorized as integral, dual purpose, or separate.

Integral reservoirs are those built into the structural members of the hydraulic system or machine. For example, space within a machine base can often be made fluid tight at little extra cost, or tubular structural members in a machine can be used as reservoirs, thus eliminating the need for extra additional space.

Integral reservoirs offer maximum performance in minimum space and usually provide an excellent cosmetic appearance. They must be designed carefully to surmount possible operating problems such as localized heating and poor accessibility.

For example, localized heating of a machine base by high-temperature hydraulic fluid can cause thermal distortions that produce machine inaccuracies. Accessibility for cleaning of the reservoir and servicing intake filters may be difficult or even impossible.

Dual-purpose reservoirs are those in which a common reservoir is used for hydraulic fluid and lubricating fluid. For example, a tractor-transmission case is also used as the reservoir for the tractor hydraulic system. Primary benefit is the space saved.

Offsetting this are several limitations. The fluid must meet the requirements of both the hydraulic system and the transmission gears. In some high-performance systems, these requirements may be almost mutually exclusive. In addition, temperature control of the fluid may be difficult because there are two sources of heat for the reduced total amount of fluid. If a separate heat exchanger must be added, space required tends to offset the saving.

Separate reservoirs are most commonly used for industrial jobs. They may be rectangular (with pump and motor mounted on the top), vertical, or L-shaped. With a rectangular system, the pump inlet line is short and can be opened without difficulty for servicing. The inlet filter is readily accessible. A vertically mounted motor and pump can be combined to reduce space requirements. The drive motor extends above the tank top, and the pump is supported on the underside of the top. Although this arrangement protects the pump and keeps lift requirements to a minimum, maximum size of the system is limited. The motor pump cover and any attached controls must be removed as a unit for servicing the reservoir.

L-shaped packages have a tall, narrow rectangular tank with pump and motor mounted beside the tank on a common base. The pump suction line enters the tank through the side, either below or above the fluid level. If the line enters below the fluid level, the reservoir fluid provides a positive feed to the pump; however, a shutoff valve must be included to permit servicing the pump without draining the tank.

A rectangular tank placed above the pump and motor, in an overhead configuration, again provides positive inlet pressure for the pump and permits easy tank access for servicing. However, access for pump service is limited, and a shutoff valve must be included to permit servicing the pump without draining the reservoir.

As mentioned earlier, the traditional rules of thumb for determining reservoir size are inadequate. The only accurate method is to determine the heat balance of the hydraulic system, calculate the amount of heat that will be generated in it through lost work, then determine the amount of space available and the amount of heat that can be dissipated in that amount of reservoir space. Such a calculation will quickly reveal the necessity for a heat exchanger, if it exists. After a decision is made on a heat exchanger, the reservoir size can be determined. Whatever the size and type of reservoir used, experts recommend that extra features be included.