Jan, 12 2010

Hydraulic

Can hydraulic fluid be compressed? In this edition of Newsletters that Teach, we're going to have a look at unique issues around large volumes of fluid under pressure. Hydraulic circuits are often used in applications with a press function. These circuits typically feature a large hydraulic ram that compacts or reshapes material in some way, or a number of large cylinders in parallel that provide a clamping function.

Some examples include:

• Pulp mill bale or slab press
• Plywood press
• Forage feed compactor
• Residue filter press
• Aluminum profile extrusion press
• Cardboard baler
• Garbage compactor

Decompression

At the end of a press cycle, with the pressed material pushing back against the ram, the fluid in the ram is still under great pressure. Correctly controlling the rate of decompression before retracting the ram is a serious issue that is common to most hydraulic presses. If you have always heard and believed that hydraulic oil is not compressible, you’ll have to open your mind to some new teaching. It's true that hydraulic oil is not very compressible when we compare it to a gas such as air (used in factory air compressors), but that's not the end of the story.

Without compression, the available 400 gal volume is filled with 400 gal of hydraulic fluid.

Most petroleum-based hydraulic oils compress by approximately ½ of one percent for every 1000 PSI of hydraulic pressure. This means that at 1000 PSI you can fit 502.5 gallons into a 500 gallon ram. At 5000 PSI you can fit 512.5 gallons into that same 500 gallon ram.

Under compression, 512.5 gal have been crammed into the 500 gal space.

A typical industrial press function such as an aluminum extrusion press or a pulp bale press features a very large cylinder (or ram) that may have several gallons (perhaps 10 litres) or more of fully compressed fluid at maximum pressure when fully extended. This is an enormous amount of potential energy.

The trapped and pressurized (5000 PSI) 512.5 gallon volume of fluid inside a 500 gallon press ram will expand very rapidly when a valve port opens to expose the fluid to atmospheric pressure.

The compression of the hydraulic fluid was carried out in a gradual and controlled fashion as the ram was moved against the load (the pressing function). The release of that compressed fluid must also be very carefully controlled. The decompression is usually handled by opening a very small orifice valve. Once the pressure has dropped to atmospheric level, a larger valve is opened for a quick and efficient retract cycle.

Decompression is initiated by opening the decompression poppet, allowing a controlled release of the extra hydraulic fluid.

If the fluid is released at an uncontrolled rate, as would happen through a large valve port that has immediately moved to its maximum position, the fluid will move at a very high velocity. This sudden burst of energy against the walls of hoses and tubing that are essentially undersized during this moment can cause a pressure build-up that may rupture the line. A flexible return hose may bulge diametrically during a sudden ram decompression, and in doing so may contract in length and break free from a fitting, or tear out a portion of the reservoir wall surrounding the return line connection.

Smaller diameter press cylinders such as those used in grocery store garbage compactors may not feature a decompression control. These cylinders with their smaller volumes and lower pressures may only have a small amount of compressed fluid inside at the end of the stroke. The inexpensive hydraulic components typically only include a bang-bang style of directional valve that fully releases the fluid from the cylinder’s blind end port at the end of stroke. The effects of uncontrolled decompression, however, are still often heard and felt as the shock wave is amplified by the machine’s entire metal enclosure. Even these small volume shocks can have a destructive effect over time.

In the next edition of Newsletters That Teach, we'll look at some of the specific valving at work for decompression control on vertical and horizontal press machines.