The Role of Hydraulic Fluid Viscosity in Pump Performance
When buying a new or remanufactured hydraulic pump, always check the specifications for the recommended fluid viscosity. Selecting a unit that’s incompatible with the fluids you’re already using could be an expensive mistake.
Viscosity is a measure of resistance to flow. It’s easy to assume that lower viscosity means less resistance and, therefore, higher efficiency, but the reality is more complex.
Details depend on pump type, but most hydraulic power transmission systems perform better overall with higher viscosity fluids. Further complicating matters, viscosity varies with temperature. With some fluids, the variation is small, but for others, it’s potentially a much bigger issue. This is why pump manufacturers recommend either a viscosity range or an ISO Viscosity Index number for their equipment.
Understanding the Viscosity Index
Viscosity is measured in terms of absolute and kinematic viscosity. The hydraulics industry generally uses kinematic viscosity since fluid density is better for indicating how a fluid will flow.
Kinematic viscosity is measured in units of centiStokes (cSt), which are equivalent to mm2/s. You may sometimes also see Saybolt Seconds Universal (SSU), which is an older way of expressing flow resistance.
Whichever units are used, they are always given for fluid at a defined temperature. As temperature rises, fluids flow more easily/become less viscous, and at lower temperatures, they are thicker.
This complicates fluid selection because most hydraulic equipment has to operate over a range of temperatures (especially if used outdoors). The industry addresses this by using the ISO “Viscosity Index” (VI).
The Viscosity Index shows, in a single number, how the viscosity of a particular fluid varies with temperature. The larger the VI, the smaller the change of viscosity in proportion to temperature change. You could think of this as the slope of a line plotted on a temperature viscosity chart.
For the mineral oils used in most hydraulic systems, the preferred VI is around ISO 100.
What Viscosity Impacts
Flow Rate & Pressure
A thicker, more viscous fluid needs more force to move it through pipes and openings of a particular diameter, which is why systems with higher viscosity fluids usually use larger diameter pipes.
Comparatively, installing a pump designed for less viscous fluids in a high-fluid viscosity system will result in lower flow rates. The same applies to pressure. If you expect a particular pressure from a pump but use a high-viscosity fluid, the pressure achieved will be lower.
So, from an energy efficiency perspective, lower viscosity is beneficial. The impact on volumetric pump efficiency is different, though.
Pump Efficiency
Assuming you have a positive displacement pump, such as a piston, gear, or rotary screw pump, a low-viscosity fluid is more prone to leaking through the various clearances. Conversely, high-viscosity fluids leak less. In hydraulics, we call this “slip,” and higher-viscosity fluids slip less and so have better volumetric efficiency.
Lubrication Issues
Another problem with low viscosity is that it does a poorer job of lubricating moving surfaces, leading to more friction, generating heat, lowering viscosity, and accelerating wear.
Cavitation
Using a fluid of the wrong viscosity can also cause cavitation, which is the inclusion of air in the fluid. This is a serious problem because it makes the fluid somewhat compressible. It can also affect lubrication and increase wear.
Cavitation can occur when fluid of too high a viscosity is used, making it harder for the pump to draw fluid in, and once incorporated into the fluid, bubbles take longer to come out. However, low-viscosity fluids can suffer from turbulence that also causes cavitation.
So, the bottom line is that many pump problems are caused by using the wrong viscosity of fluid. Always check what’s recommended for the pumps you use.
Determining Viscosity Recommendations
Pump manufacturers understand that users of hydraulic equipment don’t want to be locked into a particular fluid. Most would rather standardize on one or just a few fluids for all their pumps and systems, as this saves on storage space and simplifies purchasing and maintenance. Plus, the manufacturers can’t anticipate every service environment their units will go into.
Manufacturers address this by specifying pump performance in terms of how they handle fluid of a reference viscosity. This is usually water.
Obviously, water doesn’t make a very good hydraulic fluid because its viscosity is too low, it doesn’t lubricate very well, and it can cause corrosion. So, a suitable hydraulic fluid must be selected. This is done by determining the pressure and flow needed and the expected temperature range, then applying correction factors for flow rate, pressure drop, and efficiency to the baseline data.
These new numbers must then be compared against the baseline performance specs to check that the pump and/or fluid will still perform as needed. If not, it’s time to look for a different pump or alternative fluid.
Maintaining Viscosity
Sudden shifts in the viscosity of a hydraulic fluid can be extremely damaging and even dangerous. Pump performance will change, altering flow rate and pressure and possibly causing motors and actuators to behave unexpectedly.
Preventative actions include:
- Temperature Monitoring: Measure fluid temperature in the reservoir, shutting down the process or system when it goes out of range.
- Install Temperature Control: Heat and/or chill the hydraulic fluid reservoir to keep it within defined limits.
- Avoid/Prevent Contamination: Condensation can put water into the fluid, so insulate tanks and pipes as necessary. Use desiccants, separators, and filters to remove contamination.
- Store Fluid Properly: Understand manufacturers’ recommendations and shield fluid from direct sunlight and temperature extremes.
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Founded over 25 years ago, Panagon Systems specializes in manufacturing aftermarket hydraulic pumps, motors, and replacement components and providing comprehensive repairing and rebuilding solutions. Please visit our blog for more advice and recommendations.
