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.
When a hydraulic system sees a spike in pressure it won’t necessarily blow up with a bang. But damage can occur in a number of ways.
In fact, a single pressure spike of sufficient magnitude can render a piston pump or motor unserviceable.
In axial and bent axis piston pump and motor designs, the cylinder barrel is hydro-statically loaded against the valve plate.
To maintain full-film lubrication between the rotating cylinder barrel and the stationary valve plate, the hydro static force holding them in contact is offset by a hydro-static force acting to separate the parts.
The higher the operating pressure, the higher the hydro-static force holding the cylinder barrel in contact with the valve plate.
However, if operating pressure exceeds design limits, the cylinder barrel will separate from the valve plate.
Design geometry prevents a perfect alignment of the opposing hydro-static forces. This misalignment creates a twisting force (torque) on the cylinder barrel.
During normal operation, this torque is supported by the drive shaft – in axial piston designs or center pin in bent axis designs.
If operating pressure exceeds design limits, the magnitude of the torque created causes elastic deformation of the drive shaft or center pin. This allows the cylinder barrel to separate from the valve plate.
Once separation occurs, the lubricating oil film is lost and the resulting two-body abrasion damages (scores) the sliding surfaces of the cylinder barrel and valve plate.
Erosion of the kidney area of the valve plate can also occur as high-pressure fluid escapes into the case at high velocity. This surge of flow into the case can cause excessive case pressure, resulting in shaft seal failure.