Hydraulic Reservoirs

A reservoir stores a liquid that is not being used in a hydraulic system. It also allows gases to expel and foreign matter to settle out from a liquid.

Construction. A properly constructed reservoir should be able to dissipate heat from the oil, separate air from the oil, and settle out contaminates that are in it. Reservoirs range in construction from small steel stampings to large cast or fabricated units. The large tanks should be sandblasted after all the welding is completed and then flushed and steam cleaned. Doing so removes welding scale and scale left from hot-rolling the steel. The inner surface then should be sealed with a paint compatible with the hydraulic fluid. Nonbleeding red engine enamel is suitable for petroleum oil and seals in any residual dirt not removed by flushing and steam-cleaning.

Shape. Figure 2-11 shows some of the design features of a reservoir. It should be high and narrow rather than shallow and broad. The oil level should be as high as possible above the opening to a pump’s suction line. This prevents the vacuum at the line opening from causing a vortex or whirlpool effect, which would mean that a system is probably taking in air. Aerated oil will not properly transmit power because air is compressible. Aerated oil has a tendency to break down and lose its lubricating ability.

Size. Reservoir sizes will vary. However, a reservoir must be large enough so that it has a reserve of oil with all the cylinders in a system fully extended. An oil reserve must be high enough to prevent a vortex at the suction line’s opening. A reservoir must have sufficient space to hold all the oil when the cylinders are retracted, as well as allow space for expansion when the oil is hot.

A common-size reservoir on a mobile machine is a 20- or 30-gallon tank used with a 100-GPM system. Many 10-GPM systems operate with 2- or 3-gallon tanks because these mobile systems operate intermittently, not constantly. For stationary machinery, a rule of thumb is that a reservoir’s size should be two to three times a pump’s output per minute.

A large size tank is highly desirable for cooling. The large surface areas exposed to the outside air transfer heat from the oil. Also, a large tank helps settle out the contaminates and separates the air by reducing recirculation.

Location. Most mobile equipment reservoirs are located above the pumps. This creates a flooded-pump-inlet condition. This condition reduces the possibility of pump cavitation—a condition where all the available space is not filled and often metal parts will erode. Flooding the inlet also reduces the vortex tendency at a suction pipe’s opening.

The location of a reservoir affects heat dissipation. Ideally, all tank walls should be exposed to the outside air. Heat moves from a hot substance to a cold substance; heat transfer is greatest when there is a large temperature difference. Reservoirs that are built into front-end loader arms are very effective in transferring heat.

Ventilation and Pressurization. Most reservoirs are vented to the atmosphere. A vent opening allows air to leave or enter the space above the oil as the level of the oil goes up or down. This maintains a constant atmospheric pressure above the oil. A reservoir filter cap, with a filter element, is often used as a vent.

Some reservoirs are pressurized, using a simple pressure-control valve rather than a vented one. A pressure-control valve automatically lets filtered air into a tank but prevents air release unless the pressure reaches a preset level. A pressurized reservoir takes place when the oil and air in a tank expand from heat.

Line Connections. A pump suction and a tank’s return lines should be attached by flanges or by welded heavy-duty couplings. Standard couplings usually are not suitable because they spread when welded. If a suction line is connected at the bottom, a coupling should extend well above the bottom, inside the tank; residual dirt will not get in a suction line when a tank or strainer is cleaned. A return line should discharge near a tank’s bottom always below the oil level. A pipe is usually cut at a 45-degree angle and the flow aimed away from a suction line to improve circulation and cooling.

A baffle plate is used to separate a suction line from a return line. This causes the return oil to circulate around an outer wall for cooling before it gets to the pump again. A baffle plate should be about two-thirds the height of a tank. The lower corners are cut diagonally to allow circulation. They must be larger in area than a suction line’s cross section. Otherwise the oil level between a return and a suction side might be uneven. Baffling also prevents oil from sloshing around when a machine is moving. Many large reservoirs are cross-baffled to provide cooling and prevent sloshing.

Maintenance. Maintenance procedures include draining and cleaning a reservoir. A tank should have a dished bottom that is fitted with a drain plug at its lowest point; a plug fitting should be flushed with the inside of a tank to allow for full drainage. On large tanks, access plates may be bolted on the ends for easy removal and servicing. A reservoir should have a sight gauge or dipstick for checking the oil level to prevent damage from lubrication loss.

The strainers on a pump’s suction line may not require as much maintenance. However, an element in a filter in a return line will require regular changing. Therefore, that filter should not be inside a reservoir. When a reservoir is pressurized by compressed air, moisture can become a maintenance problem. A tank should have a water trap for moisture removal; it should be placed where it can be inspected daily.

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