One of our newsletter readers wrote to me recently regarding the following problem:
"I have a problem with a large garbage compactor. Each and every time the valve cycles the
cylinder, there is a loud bang. What are the possible causes?"
Assuming this noise is being generated by the hydraulics, i.e. it is not a symptom of a
mechanical problem, the likely cause is uncontrolled decompression.
Bulk modulus and decompression
This problem arises because hydraulic fluid is not perfectly rigid. 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).
The formula for calculating the volume change of a hydraulic fluid under pressure using its
bulk modulus is available here.
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 conductors, creates noise and
can cause pressure transients that damage system components.
Troubleshooting decompression problems
Decompression is an inherent problem in press applications, which have large volume cylinders
operating at high pressures (a garbage compactor is effectively a press). 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.
So, if the combined captive volume of the cylinder and conductors on our garbage compactor 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!
Controlling decompression
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. A simple
decompression control circuit, which will eliminate the bang from our garbage compactor, is
shown here.
When the directional control valve (1) is activated to extend the cylinder (5), fluid enters the
cylinder via the check valve (2). Pressurization of the cylinder during extension closes
the pressure reducing valve (3), so that when the directional control valve (1) is activated
to retract the cylinder (5) the compressed volume of fluid is metered across the orifice (4).
When pressure upstream of the orifice (4) falls below the setting of the pressure reducing
valve (3) the remaining fluid in the cylinder flows back to tank across the pressure reducing
valve (3).
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"This book has the potential to save many
organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and
technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more
Alexander (Sandy) Dunn
Plant Maintenance Resource Center
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