March 9, 2004 

'Inside Hydraulics' Newsletter

1. Safety first - always
2. Compression and decompression of hydraulic fluid
3. Recommended reading
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Safety first - always

The previous issue of Inside Hydraulics carried a case study outlining the root cause of a leaking rod seal on a hydraulic ram and its solution. If you missed this issue, or for a recap, it's available here. In response to this article, one of our readers wrote the following:

  "I love your newsletter and wish it came out more frequently, however I'd like to comment on the article Hydraulic ram leak caused by operator error where you implicitly suggested leaving implements in the raised position to maintain pressure and prevent leaks (or use different types of seals)... My concern is the safety issue of leaving equipment unattended with implements raised. Kids love equipment, and love it more when they can make it move, as they can if the implements are left raised... I have heard stories of children being trapped under implements in just such a situation. If someone wishes to leave implements raised, they should do so only a couple inches off the ground to prevent children being able to get underneath."

In the case study in question, the machine was in an industrial application, secure from public interference. However as pointed out above, leaving loads suspended on hydraulic equipment can pose a safety hazard in certain situations. For this reason, it is recommended that a safety risk assessment be carried out on a case-by-case basis before adopting this practice. And keep in mind that in the majority of hydraulic circuits, leaving load on the hydraulics is more likely to maintain any leaks rather than prevent them.

Safety should always be priority number one when carrying out maintenance, repairs or adjustments to hydraulic equipment. Remember that high-pressure fluid is present in operational hydraulic systems. Fluids under pressure are dangerous and can cause serious injury or death.

Always consider gravity before carrying out any work on a hydraulic machine. Any part of a hydraulic circuit that is supporting a load can remain pressurized after the system has been shut down. Attempting to remove a hydraulic line or component that is supporting a load can result in sudden release of high-pressure fluid and/or uncontrolled movement, causing serious personal injury and/or property damage.

Never make modifications, repairs or adjustments to any hydraulic system unless you are competent or are working under competent supervision. If in doubt, consult a qualified technician or engineer.

2.   Compression and decompression of hydraulic fluid

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).

"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

3.   Recommended reading

If you're involved or interested in the maintenance of hydraulic or any other type of machinery, Machinery Lubrication magazine is highly recommended reading. Keep up to date with machinery maintenance and reliabilty issues with a complimentary subscription to this informative magazine.

4. Content for your web site or e-zine

Need some fresh content for your web site or e-zine? You now have permission to reprint these 'Inside Hydraulics' articles on your web site or in your e-zine, provided:

1. Each article is printed in its full form with no changes.

2. You send an e-mail to to advise us where you'll be publishing them.

3. You include the following acknowledgement at the end of each article:
About the Author: Brendan Casey has more than 16 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his web site:

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