Reservoir Considerations
Closed Loop Hydraulic Systems & Mobile Equipment
Dwell Time -- Traditionally, dwell time – the time the hydraulic fluid “dwells” in the reservoir between inlet and outlet during operation – has been suggested as a minimum of one minute for mobile equipment. The purpose of this dwell time is to allow, in a well designed reservoir, time to purge entrained and entrapped air and to allow contaminant to settle out of the working fluid.
Reservoir Capacity – In order to insure one minute dwell time in the reservoir, the fluid volume contained in the reservoir should be a minimum of the volume of the fluid flow rate through the reservoir per minute of time. That is, if you have a 10 gpm flow rate through the reservoir, you need 10 gallons of fluid in the reservoir at all times during operation of the machine.
Reservoir Flow Rate – It should be noted that for traditionally designed closed loop hydraulic systems (hydrostatic transmissions) the flow rate through the reservoir is NOT the working flow rate of the system, but is the flow rate of the CHARGE system – usually in the range of 10% of the working flow rate.
Note: If the machine has supplemental open loop systems for operating cylinders or other open loop applications, which is often the case in mobile equipment, the maximum flow rate of the open circuit system must be summed with the closed loop system charge flow rate to establish the total volume of oil flowing through the reservoir.
Expansion Capacity – It is customary to add a minimum of 10% to the calculated minimum fluid volume necessary in the reservoir to allow for thermal expansion of the fluid, etc. Therefore, if a 10 gallon fluid volume is indicated by the flow rate through the reservoir, a calculated volume of 11 gallon for the cubic capacity of the reservoir should be used in reservoir design.
Note: In systems utilizing cylinders of significant volume the change in volume of fluid in the reservoir should be taken into account when sizing the reservoir such that in the condition of least volume in the reservoir – cylinders extended – the minimum volume for one minute dwell time will be accommodated. In Addition sufficient expansion capacity should be designed into the reservoir such that the additional volume can be accommodated when all cylinders are retracted.
Baffling, etc. – Good reservoir design practice should be followed. Internal baffling should be provided such that flow “streamlining” between the reservoir inlet and outlet is prevented and dwell time is assured.
Location and Orientation – In mobile equipment design reservoir location should be considered such that heat transfer to the reservoir from hot objects such as the engine exhaust system is avoided. Further, mobile equipment tends to be subject to severe orientation angles during operation. Care should be taken in location and design of the reservoir such that in times of adverse vehicle orientation the reservoir outlet (inlet to the pump(s)) is NEVER exposed to air. In mobile equipment subject to severely varying orientations reservoirs of deep design are usually preferred to reservoirs of shallow design.