Air typically enters the hydraulic system through the the pump inlet and, under certain conditions, past the rod seal of a double-acting cylinder.
But air can also invade the system through joints in pressurized plumbing.
When fluid travels through a pipe or hose at relatively high velocity – in a pressure line for example, and has to change direction through a tee or elbow, a venturi effect can be created.
Because the sealing arrangement of the hydraulic connector is designed to withstand positive pressure – but not negative pressure, air can be drawn into the system – even when the plumbing has no apparent leaks.
If you made a glass model of a pipe elbow and connected a measuring point in the middle of the angle, you would see a negative pressure when fluid passed through the elbow at high velocity.
And if you looked carefully, you’d likely see air bubbles entering the system through the seal of the measuring connection.
What it comes down to is use as few sharp angles – tee-pieces, elbows, etc in hydraulic plumbing as possible. a
There is another intermittent and problematic source of noise in hydraulic systems – decompression.
This problem arises because hydraulic oil is NOT incompressible. The ratio of a fluid’s decrease in volume as a result of increase in pressure is given by its bulk modulus of elasticity.
The bulk modulus for hydrocarbon-based hydraulic fluids is approximately 250,000 PSI, (17,240 bar) which results in a volume change of around 0.4% per 1,000 PSI (70 bar).
When the change in volume exceeds 10 cubic inches (160 cubic centimeters) decompression must be controlled.
The compression of hydraulic fluid results in storage of energy, similar to the potential energy stored in a compressed spring. Like a compressed spring, compressed fluid has the ability to do work.
If decompression is not controlled, the stored energy dissipates instantaneously. This sudden release of energy accelerates the fluid, which does work on anything in its path.
Uncontrolled decompression stresses hydraulic hose, pipe and fittings. It creates noise and can cause pressure transients that can damage hydraulic components.
Decompression is an inherent problem in hydraulic presses for example, due to the large volume cylinders operating at high pressures.
Although hydrocarbon-based hydraulic fluids compress 0.4% – 0.5% by volume per 1,000 PSI, in actual application it is wise to calculate compression at 1% per 1,000 PSI. This compensates for the elasticity of the cylinder and conductors and a possible increase in the volume of air entrained in the fluid.
For example, if the combined captive volume of the hydraulic cylinder and conductors on a press was 10 gallons and operating pressure was 5,000 PSI, the volume of compressed fluid would be 0.5 gallons (10 x 0.01 x 5).
This equates to potential energy of around 33,000 watt-seconds. If the release of this amount of energy is not controlled, you can expect to hear a bang!
Decompression is controlled by converting the potential energy of the compressed fluid into heat. This is achieved by metering the compressed volume of fluid across an orifice.