Understanding Filtration – Temperature and Viscosity
Understanding Filtration in Hydraulic Systems
When adding filtration to an hydraulic system, it’s important to understand the role of viscosity, pressure drop, fluid cleanliness and filter efficiency (beta ratios) in the overall performance of the system.
Temperature is also a key factor in determining the effectiveness of hydraulic filters.
What is Viscosity?
Viscosity is the measure of the resistance of a fluid to deform under sheer stress.
It is commonly perceived as thickness, or as a resistance to pouring. It may also be described as a fluid’s resistance to flow, or a measure of fluid friction.
Water has a low viscosity, and honey has a high viscosity.
However, viscosity can be affected by changes in the fluid temperature. For example, when heated, honey will flow more easily.
Understanding the behaviour of a fluid’s viscosity is an important part of understanding filtration technology.
Pressure drop is a measure of how much pressure is required to push a fluid through a restriction, or ‘orifice’.
A filter element acts as a restriction when fluid is flowing through it. The level of restriction is a product of the fluid flow, fluid viscosity and the element rating.
A restriction to flow will cause a higher pressure before the element, and a lower pressure after the element, i.e. a pressure drop.
Fluid viscosity changes with fluid temperature. Oil becomes less viscous, i.e. thinner at higher temperatures, which affects the pressure drop.
It is entitled “Effect of temperature and viscosity on flow rate and pressure drop”.
It is used to calculate the expected viscosity for mineral type hydraulic oil as temperatures vary.
ISO Ratings for Hydraulic Oil
The ISO rating given to a hydraulic oil is based on the kinematic viscosity of the fluid at 40 degrees celsius.
This is ISO 3104: 1994 – “Determination of kinematic viscosity and calculation of dynamic viscosity”. Definitions
The kinematic viscosity is the relationship between viscous and inertial forces in a fluid. Dynamic viscosity is the relationship between the shear stress and the shear rate in a fluid.
A centistoke (cSt) is the derived centimeter-gram-second (CGS) unit of kinematic viscosity.
This rating is based on the viscosity of the fluid at 40 degrees celsius.
For example, ISO 60 fluid will be 60 centistokes (cSt) at 40 degrees celsius.
ISO 32 fluid will be 32 centistokes (cSt) at 40 degrees celsius.
ISO 32 fluid will be 178 centistokes (cSt) at 10 degrees celsius.
For this reason, it is vital to consider the “cold start” condition of a hydraulic machine. i.e. What will the viscosity be at start up on a cold day?
ISO 32 fluid will be just over 11 centistokes (cSt) at 70 degrees celsius.
It is important to note that hydraulic fluid will lose lubricity if allowed to become too thin. This is dangerous.
The chart demonstrates the massive effect that temperature will have on the viscosity of an oil.
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