Hydraulic fluid selection criteria include the expected range of operating temperatures, available means of temperature control and fluid physical properties at expected temperature levels. Where ambient and structural temperatures are above hydraulic fluid flash and/or fire points in a compartment, the potential fire hazard must be considered.

Fluid stability is affected by thermal stress, which can result in changes in viscosity and formation of volatile components, insoluble materials and corrosive deposits. Hydraulic system efficiencies are reduced by high fluid viscosity at lower temperatures, which results in inlet problems with pumps, sluggish response of critical actuators, power loss in transmission and weight penalties due to line size. At high temperatures, low fluid viscosity can cause internal leakage and slippage in pumps, actuators and valves.

Compressibility of a fluid increases with pressure and temperature and results in loss of volume output of pumps. In control systems, compression of fluid provides a mass-spring condition that limits system response. For analytical purposes, the assumption of uniform temperature throughout a hydraulic circuit usually is quite accurate. When a pressure drop occurs without external work resulting, i.e. losses through orifices and tubing, the hydraulic fluid temperature rises by 7° F per 1,000 psi drop for each circuit. However, this rise normally is dissipated as the fluid passes through the system.

Low power level systems (low pressure of 1,000 psi or less and low flow (of 5 gpm or less) generally do not require any special cooling equipment. Large high power level systems operating in relatively warm ambient temperature regions may require heat exchangers in order to maintain fluid temperatures below upper limits of 135° C. For hydraulic systems working under operating temperature range –40 to 100° C with a maximum temperature range –54 to 135° C, mineral based hydraulic fluids are preferable. For higher operating temperature applications, fire resistant synthetic hydraulic fluids are more suitable. However, these fluids become highly viscous below –20° C.

Contributed by Harshavardhan Joshi, Aircraft Hydraulic Systems Engineer, CSIR – National Aerospace Laboratories, India