The Importance Of Fluid Selection

Fluid Selection

The selection of a hydraulic fluid on the basis of viscosity is always a compromise. (The chemical properties are usually caused by additives, and they have little affects on the fluid. This is discussed later in this lesson.)  When a system starts up, its temperature is at the temperature of the surrounding air. Consequently, its fluid viscosity may be very high if the system is subjected to low ambient temperatures, or be very low if it is subjected to high ambient temperatures. Low surrounding temperatures require a suitable means of heating the fluid, while high temperatures require a suitable means of cooling the fluid. As the system warms up, the fluid viscosity decreases. If the same fluid were used both in the summer and winter, the viscosities of the fluid would become extremely low as the temperature increased. That is why it is common practice to use low viscosity fluids during the winter, or when operating temperatures are comparatively low, and to use high viscosity fluids in the summer or when running temperatures are higher. Changing the fluid periodically in this manner is probably more economical than either providing complex heating and cooling methods or using more expensive fluids that have a higher viscosity index. If the hydraulic system is inside a plant, temperature change is seldom a problem.

Usually, complete adaptability of the fluid viscosity to the system is not required. Fluids normally heat up as they move through a hydraulic system. As long as fluids are selected for the operating temperatures at which they are efficient, they will be satisfactory for operation during start up. Component manufacturers design each piece of equipment to perform well over a reasonable rang of specific operating conditions. On this basis, pumps, which are probably more critical than other components, may be designed to perform efficiently at fluid temperatures and viscosities that usually exist in the hydraulic reservoir. Valves and actuators then can be adapted to function better at the somewhat higher temperature conditions and lower fluid viscosity conditions that exist within the system.  Even though the components can accept some fluid viscosity differences, it doesn’t mean that you can fill a system with a hydraulic fluid close to the recommended type and expect it to work.

As the viscosity of the fluid changes during system operation, it affects the hydraulic system in general. When the viscosity of a fluid is too low, it usually decreases the overall efficiency of a hydraulic system in one or more or the following ways:

1. Increased leakage within valves and actuators, resulting in the loss of precision control and some power
2. External leakage developing at gaskets, mechanical connections, and seals.
3. Increased pump slippage resulting in loss of pressure and lower volumetric efficiency.
4. Increased wear on moving parts, especially control valves and actuators.
5. Increased fluid temperatures and shortened fluid service life.

When the viscosity of a fluid is too high, it also affects the operating efficiency of the system.  Many of the effects of high fluid viscosity are similar to those of low viscosity. Some of them are as follows:

1. Increased overall pressure drop and increased pressure drop in each component because of increased flow-resistance.
2. Increased drag in the system and poorer system-response.
3. More sluggish system operation , especially at low operating-temperatures
4. Lower mechanical efficiency.
5. Greater vacuum at the pump intake, causing cavitations and reduce pump-efficiency.
6. Increased power consumption because of increased flow-resistance in the system and reduced pump-efficiency.
7. Higher fluid-temperatures and shortened service-life
8. Higher system noise-level.