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3 Causes of Pressure Drop in Hydraulic Systems

Hydraulic Piston System
Hydraulic power uses the powerful nature of fluids as a means of transferring force from one system to another. The highly effective nature of hydraulics gives it a surprisingly wide range of uses, from the world of manufacturing to the world of automotive design. Yet a hydraulic system may not live up to expectations unless it is carefully designed.
One of the most common problems associated with hydraulic systems comes in the form of pressure drop. Pressure drop reduces the amount of power at the output end of a hydraulic system, and it can also lead to excess heat and internal damage. If you would like to learn more about hydraulic system design to reduce pressure drop, keep reading. This article covers three frequent causes of pressure drop.

1. Fluid Viscosity

One of the most immediate — and the most changeable — factors affecting pressure drop involves the viscosity of the hydraulic fluid in your system. Many people mistakenly assume that all hydraulic fluid has the same chemical make-up. Yet hydraulic fluids, much like motor oils, come in a wide range of viscosities.
Viscosity denotes the level of internal resistance of a fluid. The higher the viscosity, the more resistance it experiences on a molecular level. Overcoming higher levels of resistance takes a greater amount of effort. As a result of this additional friction, thicker fluids promote greater pressure drops than do thinner fluids.
Using an excessively viscous fluid not only increases pressure drop, but it can also lead to cavitation and other serious forms of damage. Always be sure to stock your hydraulic system with an appropriately rated fluid. If you don't know the ideal viscosity, contact a professional to help you determine which fluid to use.

2. Operating Temperature

The topic of hydraulic fluid becomes more complicated when you consider that viscosity changes in response to temperature. Generally speaking, the higher the temperature, the lower the viscosity of the fluid becomes. This rule holds true for all hydraulic fluids, regardless of what their base viscosity rating may be.
Therefore, a designer must factor average operating temperatures into the overall equation governing pressure drop. If a particular hydraulic system experiences relatively high operating temperatures, the fluid's viscosity will have less of an effect on pressure drop. That said, the system will struggle to generate the desired level of pressure when starting cold.

3. Pipe Roughness

Friction plays a key role in pressure drop. While some friction happens within the fluid itself, as discussed above, friction between the fluid and the pipe walls also plays a factor. All pipes exert some level of friction on the fluid passing through them. Generally speaking, the longer any give pipe, the more it contributes to pressure drop.
Yet an even more important consideration has to do with the roughness of the pipe walls. The rougher the pipe, the more it slows down the fluid, resulting in pressure loss. Engineers express pipe roughness using two terms: absolute and relative roughness. Absolute roughness relates directly to the material used to construct the pipe.
For instance, concrete pipes have a much higher degree of absolute roughness than iron pipes, which in turn are much rougher than PVC pipes. While useful, absolute roughness does not provide a complete picture of the effect of roughness on pressure drop. Instead, designers must calculate the relative roughness.
To determine relative roughness, simply divide the absolute roughness of the material by the internal diameter of the particular pipe in question. An engineer can then plug the resulting value into the Darcy-Weisbach equation to determine the exact friction loss that will occur in the pipe.
Pressure drop significantly alters the dynamics of a fluid system. For more information about how to prevent pressure drop from negatively affecting your hydraulic set-up, please contact our experts at Quad Fluid Dynamics Inc.
Quad Fluid Dynamics, Inc.
2826 Westway Drive
Brunswick, OH 44212
Phone: 330-220-3005