All hydraulic systems have a reservoir. A reservoir is similar to an accumulator, except that the fluid pressure is constant over all fluid levels. A reservoir performs several functions. First and foremost, the reservoir holds fluid not required by the system under any given operating condition and accounts for fluid capacity needs over time in the system. Fluid volume needs will vary during different operational scenarios, such as gear extension. Secondly, the reservoir provides for thermal expansion of the fluid over the operational temperature range of the system. Thirdly, the reservoir provides fluid to the inlet side of the hydraulic pump. Reservoir pressurization levels are a critical aspect of reservoir installations.
Reservoirs consist of a container or volume, fluid inlet port, fluid outlet port, fill/drain port, and a means to pressurize the fluid in the volume. Reservoirs used in aerospace are pressurized using pressurized gas, through a piston which has high pressure hydraulic pressure on one side (commonly referred to a as a bootstrap reservoir), or through a mechanical piston and spring. The terms separated and non-separated are used with reservoirs. Separated means that the reservoir fluid is separated from the pressure source (spring, high pressure fluid or gas) though a separation device (usually a piston). Non-separated refers to a gas pressurized reservoir where there is no separation between the fluid and the gas.
Figure 1 shows a pressurized gas reservoir. For a gas reservoir a low pressure gas source is required. Engine bleed air, fed through a pressure regulator, is often used for pressurized gas reservoirs. A fixed pressure source, such as a charged nitrogen bottle, is another possible configuration. A bleed air configuration is shown in Figure 1. The pressure regulator setting will be in the 50-200 psi range.
Figure 1 Pressurized Gas Reservoir Schematic
In the pressurized gas reservoir, PAir = PFluid. The air does not mix with the fluid due to the density differences. However, air can escape from the fluid into the air providing some system removal of air. This characteristic does not replace system bleeding.
For a gas reservoir, the amount of gas should be sufficient to provide uninterrupted flow of fluid to the pump under all operating scenarios and vehicle attitudes. For example, a high nose up (pitch) attitude should not cause fluid to block the air inlet port. In addition, there should be enough gas to absorb high surge pressure that might occur with high return flow to the reservoir. Surge pressure should be less than the pressure relief valve setting and, of course, be much less than the proof pressure rating of the reservoir. The minimum recommended gas volume is 10% of the total reservoir volume. The inlet line should be located at a place so that inlet flow does create a vortex within the reservoir or cause foaming of the fluid. The fluid spin caused through vortex action will push fluid to the outside and can starve the reservoir outlet by opening the outlet up to air. Baffles or diverters can be used in the reservoir (as part of the reservoir design) to prevent vortexes or foaming. Lastly, if the air pressurization source is lost there should be a means to allow atmospheric air to enter the gas chamber of the reservoir.
A bootstrap reservoir is shown in Figure 2. A bootstrap uses a differential area piston where high pressure hydraulic pressure from the pump outlet is applied to the small area of the piston. This produces a low pressure on the reservoir side of the piston. A major advantage of bootstrap reservoirs is that reservoir pressurization is maintained during aggressive flight maneuvers, including negative g flight. Additional hydraulic plumbing and some components are required for bootstrap reservoir implementation (see examples in System Design, Hydraulic Power Generation). Also, note the check valve in the high pressure line. The purpose of this check valve is to maintain reservoir pressure after the pump has shut down so that the pump inlet is maintained when the engine driven pump is not rotating. Accumulators may also be used in this circuit to assist in maintaining pump inlet pressure. The accumulator will be between the check valve and the reservoir.
Figure 2 Bootstrap Reservoir
When the reservoir is at equilibrium P1A1 = P2A2. Since A1 >> A2, P1 << P2. The differential piston areas are set by the pump nominal pump outlet pressure and the required level of reservoir fluid pressure. Reservoir fluid pressure levels are discussed in Reservoir, Hydraulic – Sizing.
Figure 3 shows a mechanical piston reservoir. In this reservoir a spring with an appropriate preload and a low spring rate pushes on a piston and provides a fairly constant reservoir pressure.
Figure 3 Basic Reservoir Schematic (Piston Spring Type)
Metal bellows is another (relatively new) type of reservoir. A metal bellows reservoir would be similar to a metal bellows accumulator (see Accumulator, Hydraulic – Description). Operation of a metal bellows accumulator would be similar to a piston/spring reservoir type.
The three basic types of reservoirs used in aerospace are shown in Figures 1 through 3. Additional features may be contained in reservoirs as shown in Figures 4 and 5.
Figure 4 shows a gas reservoir configuration with an emergency reserve capability. The tallest pipe would be used for normal operation and would therefore feed the normal system pump (usually the engine driven pump). The shorter standpipe would feed some type of auxiliary pump, such as a backup electric motor driven pump. The volume difference between the two standpipe inlets is the emergency reserve capacity for this reservoir. The auxiliary pump would be used for emergency services such as emergency braking and landing gear. For bootstrap reservoir systems, any emergency reserve would be contained in separate reservoir. System plumbing must provide proper isolation between normal operation system functions and auxiliary functions through the use of check valves, shuttle valves or shut off valves.
Figure 4 Single Reservoir With Emergency Reserve Capacity
When computing emergency reserve capacity, fluid returning to the reservoir should not be assumed. Therefore, the capacity should be what is required for emergency operation (e.g., to lower the landing gear and/or run emergency brakes). Military specifications recommend the minimum volume for emergency reserve be equal to 125% of the calculated emergency reserve needs. Emergency reserves can also be contained in a separate reservoir. For bootstrap reservoir systems any emergency reserve will need to be a separate reservoir since the reservoir would provide fluid to each system and piston movement would not pressurize beyond the to the normal system outlet standpipe.
Some hydraulic systems contain a reservoir with 2 compartments which feed 2 different pumps. This arrangement is only possible with a gas pressurized reservoir. An example is shown in Figure 5. The two compartments are connected during normal operation at normal fluid levels. However, should a leak develop in the main system, fluid will only drain from the main portion of the reservoir. Note that the return flow is prioritized to the backup compartment and only flows to the main system volume when the level is sufficiently high. Fluid would be retained in the backup system compartment should a leak develop in the main system. This arrangement is similar to the emergency reserve reservoir shown in Figure 4 but this design assumes fluid flow back to the reservoir during backup system operation. Therefore, full functionality is maintained for the backup functions in the hydraulic system. For this type of arrangement the hydraulic system plumbing must incorporate check valves and/or shuttle valves to isolate non-critical functions from the backup pump. For a 2 compartment reservoir, protection is provided against certain failures in the system – primarily loss of fluid in non-critical hydraulic lines and components. It also protects against a main system pump failure. Normal (or slightly less than normal if backup pump capability is below that of the normal pump) operation of the critical functions is maintained in this arrangement. In contrast a reservoir with emergency reserve only supplies enough for a one time application of critical functions.
Figure 5 Single Reservoir Feeding Normal and Backup Hydraulic Systems
A convenient means for filling and draining hydraulic reservoirs should be part of any reservoir design and installation. Typically, ground service panels are installed. Ground service panels provide a fluid connection to the reservoir so that the reservoir can be filled. A reservoir drain can also be installed into the ground service panel. Ground service panels also provide a means to connect hydraulic power cart pressure and return lines into the system. This function allows functional and operational checks of the system while the vehicle is on the ground. The pressure line from the cart will flow into the normal system pressure line through a tee fitting. The return line from the cart will connect into the normal system return line through a tee fitting. A shut off valve or check valve can be installed in the ground service line to prevent normal flow from flowing into the ground service lines. Installation of a check valve will prevent system loss should a leak develop in the ground service line. However, when using a valve it is important to not let fluid remain trapped in a ground service line as thermal expansion will lead to a pipe failure. Ground service panels are for ease of maintenance and are typically found on most commercial aircraft. A ground service panel (or equivalent) is required for each independent hydraulic system on the vehicle. In lieu of a panel, tubes with appropriate fittings (such as quick disconnect) can be located wherever convenient on the vehicle, such as maintenance bays or wheel wells. If a ground service panel is not used, the functions described above (filling, draining, external pressure line, external return line) are required for maintaining a system.
Another important feature for a reservoir is fluid level indication. Indications should be provided when the reservoir fluid level is low, when the reservoir is overfilled and when fluid level is ok. Ideally, fluid level indication would be continuous throughout the range of fluid in a reservoir. This could be accomplished through a sight gauge located directly on the reservoir. The drawback of gauges located directly on the reservoir is that the reservoir may not be easily accessible in the vehicle. Also, on pre-flight checks flight crews don’t normally like to access maintenance bays. Electronic means through a continuous sensor and indicator are preferred. The indication could then be provided in the cockpit and the ground service panel. Another indication method is to install switches on the reservoir at the low level point, fill point and overfill point. The switches can then be used to power indication lights on a ground service panel. Switches are a less expensive alternative and are appropriate for hydraulic systems whose criticality is less severe (i.e., not required for continued safe flight and landing). The drawback of switches is the potential for a latent failure in a switch or indicator light. Since fluid volume varies with temperature, electronic sensors can provide fluid level indications that are compensated for fluid temperature. For flight safety critical hydraulic systems, the indication system should be fail safe such that no single failure in the indication system will lead to an erroneous indication.
In addition the above, reservoirs should provide over pressure protection and also provide a means to relieve pressure on the reservoir for servicing. Over pressure protection is normally done using a pressure relief valve – for fluid or air depending on type of reservoir. A manual relief valve is used to release reservoir pressure for servicing. In some applications, a bleed port for bleeding air from the system may be contained in the reservoir. A bleed valve would only be required in the reservoir could collect trapped air and/or is located in the system such that entrained air would migrate to the reservoir.
Reservoirs are normally made from aluminum. High strength materials are not required since operating pressures are low. Non boot strap reservoirs are typically cylindrical in shape, but may also resemble a box with rounded corners. Bootstrap reservoirs will consist of two separate cylinders connected together along a flat surface (see Figure 3). Materials will be corrosion resistance. Externally, there will be an inlet fitting and outlet fitting plus connections for fill and drain ports. The inlet fitting should be below the minimum fluid level. Fittings are normally internally threaded bosses. Permanent welded or swage connections are not used on reservoir connections. No weight or preloading should be carried by connecting tubes. In some cases, a standpipe would protrude a small amount above the bottom of the reservoir so that debris would be trapped in the bottom and not pulled into the system. The outlet should be located below the minimum required fluid level for obvious reasons.
Baffles and diverters are also common to prevent fluid from sloshing around and also to stop any swirling action from flow coming into the reservoir. Baffles can be installed in the up/down direction or side to side.
The indication system will also be part of the reservoir. Indication systems can be sight glass, switches located at the fill level and full level, or a capacitance based sensor which provides an analog signal proportional to fluid volume.
Reservoirs are sized to have sufficient capacity to provide flow under all operating conditions, account for expansion and contraction of fluid at different temperature and system leakage over time (between maintenance intervals). See Reservoir, Hydraulic – Sizing for details on computing capacity requirements.
Reservoir Fluid Pressure
The pressure setting for a reservoir is determined by pump inlet pressure requirement and the pressure loss characteristics between the reservoir and the pump. Longer tube runs, tube bends, tube diameter, difference in elevation between pump and reservoir, fitting/connector losses and shut off valve losses all contribute to pressure loss between the reservoir and the pump. The best designs minimize these losses as much as possible. On large vehicles where there is large separation between reservoir and pump and/or large change in elevation, boost pumps are installed in the pump inlet line. The boost pump could be driven from high pressure hydraulic fluid or through an electric motor. Alternatively, inlet impellers powered by the pump shaft can be part of the pump to boost inlet pressure and accelerate fluid into the pump. More information on determining the reservoir pressurization level can be found in Reservoir, Hydraulic – Sizing.
Another aspect of reservoir pressurization is the ability of the reservoir to maintain pump inlet pressure during negative g flight. Negative g flight is usually only a transient phenomenon so the system need only be designed for short durations of negative g. For highly maneuverable vehicles where longer durations of negative g flight are required, a bootstrap reservoir should be used.
Reservoirs are considered items of mass and must be mounted to meet g-loading and crashworthiness requirements. No loads should be imparted to connecting tubes.
Reservoirs should be located as close as possible to the pump(s). The primary installation design goals are to minimize the length of the pump inlet line and fittings/components in the line to minimize pressure losses. Elevation differences between the reservoir outlet and the pump inlet should be kept as small as possible. However, reservoirs must not be located in any rotor noncontainment zone. Shut off valves should be installed in pump inlet line to shut off flow when required and provide for engine fire protection. Shut off valves should also not be located in a rotor noncontainment zone.
Check valves are normally in the pressure line to the pump so that fluid would be held in the pump inlet line during reservoir pressurization transients. These transients could occur during engine start or at other conditions when reservoir pressure is momentarily lost. Check valves also maintain fluid in the line to minimize fluid flow delays that could result from an empty tube. Negative g flight is another reason to install a check valve in the pump inlet line.
Reservoirs also remove heat from the system. The amount of heat removed depends on the reservoir size and the location in the aircraft. If the reservoir does not transfer sufficient heat to the environment, then a heat exchanger is required (see Thermodynamics, Hydraulic Systems).
As a summary of the above, when selecting a reservoir, the following factors should be evaluated:
Capacity – Reservoir capacity should meet design requirements. Capacity determination is discussed in Reservoir, Hydraulic – Sizing.
Type of Reservoir – Separated or non-separated? Will reservoir be pressurized through mechanical means (spring), pressurized gas or hydraulically pressurized (bootstrap)?
Pressurization System Aspects – The choice of reservoir type has a big affect on power generation system design and possible interface to other systems. For example, a pressurized gas reservoir will require interfacing to the engine bleed air system (see Figure 1).
Emergency Reserve Capacity – Does the reservoir require an emergency reserve capacity? If so, what is this capacity over and above the normal capacity?
Fill and Drain Features – How is the reservoir filled with fluid? How is the reservoir drained for maintenance? How is the reservoir pressure bled off?
Fluid Level Indication – Fluid level indication method should be compatible with the vehicle indication system. This system may need to interface with crew compartment indication systems.
Pressure Rating – Ensure the reservoir is rated appropriately for the system pressure
Pressure Relief Valve – Is an overpressure relief valve incorporated into the reservoir or is this installed in the pressure line?
Temperature Rating – Reservoir should be rated for maximum rated fluid temperatures in combination with applicable environmental temperatures
Materials – Reservoir materials must be non-susceptible to corrosion from hydraulic fluid, any fluid contamination and any possible moisture that could be in the system. In addition, reservoir strength should be sufficient to pass proof and burst testing and vibration.
Seals – Seals should be appropriate for the hydraulic fluid used. Different fluids may require different seal materials.
Mounting – Method of mounting the reservoir to vehicle structure should be robust. Reservoirs are considered items of mass and the attachments are subject to substantial g loading design requirements.
Connections – What interfaces are provided (type of fitting) for connections with the hydraulic tubing?
Operational Characteristics – The reservoir needs to provide sufficient fluid to the pump without cavitation. In addition there should be no froth or foam characteristics in the fluid exiting the reservoir. To accomplish this requirement, reservoir design should include baffles, diverters, sufficient inlet and outlet flow tube size (diameter), sufficient level of pressurization. These aspects in the design should be review carefully and operational tests completed to prove out operating characteristics under all foreseeable operating conditions.
See Qualification - Hydraulic Components for discussion on reservoir qualification and required certification testing. For a reservoir, many of the categories listed will not apply due to low pressure and simplicity of the component. At a minimum, temperature, proof and burst pressure, vibration, and endurance should be run. Some specifications may require an immersion test to verify materials are not subject to corrosion or breakdown in service.
Operational tests should also be run on the pressurization system and reservoir operation at minimum and maximum fill levels. Reservoir capacity should be verified by test. Relief valve operation and indication systems should also be verified by test.