System Cavitation: Why Is Air Bad?

TECH TIP

SYSTEM CAVITATION: WHY IS AIR BAD?

AIR IS A CONTAMINANT!

Air is quite convenient for the support of all life forms; however, it can be extremely detrimental when allowed to infiltrate oil hydraulic systems. Air must therefore be considered a contaminant just like dirt, sludge and water. Air can cause problems such as:

Premature wear

Hydraulic components, particularly pumps, rely upon a specific lubricating film of fluid to separate moving parts. When this film is compromised by a percentage of air, the fluid film strength and therefore its lubricating qualities are diminished, resulting in metal to metal contact and premature wear.

Noise

Air bubbles traveling from inlet to outlet port of the pump are subjected to rapidly increasing pressures. Bubbles implode violently causing excessive noise. Bubbles under pressure will explode violently when moving from the pumps’ pressure zone to the suction zone also causing excessive noise. These implosions and explosions are capable of ripping hardened steel away from the pumping chambers.

System sponginess

Hydraulic systems transfer energy through a medium (oil) of a known density. If density, or the fluids’ bulk modulus is altered by air, system stiffness is negatively effected. Jerky, slow and sometimes uncontrolled motion may result.

Excessive heat

Keeping your hydraulic system from overheating is paramount to system and component life. As much as 30 to 50% of the horsepower input to a system can be released to the oil as heat. Temperature can be limited or even controlled by devices such as heat exchangers, chillers or by the radiant capabilities of the reservoir itself.

Valve instability

Hydraulic valves are sometimes mini hydraulic circuits in their own right. Air can affect their performance in all of the above ways as well.

WHAT IS CAVITATION?

Cavitation is the drawing of intermolecular and entrained gases out of solution.Oil is sometimes referred to as being non-compressible; however, this is not entirely true. The rule of thumb is that oil is typically compressible to one half of one percent per 1000 PSI. The compressed agents are intermolecular and entrained gases (sometimes referred to as air) dissolved within the oil.

These intermolecular and entrained gases tend to remain in solution at atmospheric pressure. These gases can be drawn out of solution by creating a vacuum on the fluid.

Atmospheric pressure is the weight of one square inch of Earths atmosphere from its’ highest point to mean sea level. This is generally considered to be 14.7 pounds per square inch (PSI) or approximately 29.92” of mercury. Hydraulic pump manufacturers generally rate the inlet characteristics of their pumping equipment as being capable of drawing a specific maximum amount of vacuum on their inlet ports. Frequently, this is expressed in inches of Mercury (In. Hg) at sea level, while operating on a specific type of fluid (i.e. oil). A hydraulic pump manufacturers’ rating of a maximum 6 In. Hg @ sea level on petroleum based fluid, is very common. In this case the manufacturer is saying their pump requires a net positive inlet pressure of at least 23.92 In. Hg, in part to avoid pulling the gases out of suspension.

Atmospheric pressure acts on the entire surface area of the fluid within the hydraulic reservoir. When a pump creates a vacuum at its’ inlet port, atmospheric pressure pushes the reservoir fluid into the pump. If the pump is creating a 6 in. Hg vacuum, the resultant net positive inlet pressure will be:

29.92 In. Hg (atmospheric pressure @ sea level) – 6 In. Hg = 23.92 In. Hg net positive inlet pressure. This is in line with the pump manufacturers specifications above.

If, however, the system is operating at an elevation of 5,000 ft. above sea level, the ball game changes. The weight of this column of air at 5,000 ft. will be less; approximately 11.7 PSI or 23.8” of mercury. This situation will result in less net positive inlet pressure available to the pump and will likely result in pump cavitation per below:

23.8 In. Hg (atmospheric pressure @ 5,000 ft.) – 6 In. Hg = 17.8 In. Hg net positive inlet pressure. This is far below the pump manufactures specifications as noted above.

Any restrictions in the pump inlet line will have an orificing effect; thereby reducing the amount of net positive inlet pressure available to the pump.

Any increase in elevation above mean sea level will reduce the amount of net positive inlet pressure available to the pump.

Any of the following will reduce the amount of net positive atmospheric supercharge pressure available at the pumps’ inlet port, potentially resulting in cavitation:

Restrictions in the pumps’ inlet line
Higher than sea level pump elevation
Operating on fluids with a specific gravity greater than that at which the pumps’ inlet rating was based upon (especially high water content and water glycol fluids)
Pump inlet line being too small in diameter
Pump inlet line being too long in length
Excessive number of elbows in the pump inlet line
Inadequately sized inlet strainers or filters
Dirty inlet strainers or filters
High viscosity fluid
Lack of sufficient supercharge pressure

What is Air Infiltration?

Air infiltration is the drawing of atmospheric air into the system (most frequently a pumps’ inlet line).

The following situations can cause air infiltration:

Loose fittings in the pumps’ inlet line(s)
Leaky pump shaft seal
Deteriorating or missing seals (O-rings) in the pumps’ inlet connections
Low reservoir fluid level

Air infiltration often sounds like marbles are passing through the pump.

RECOMMENDATIONS

There are many ways to address the problems of cavitation and air infiltration in the design of the modern hydraulic system:

Construct pump inlet lines at least the same size as the pump inlet connection.
Do not use suction strainers on pump inlets
Use few if any elbows in the pump inlet line
Use few if any pipe unions in the pump inlet line
Keep inlet lines as short as possible
Make certain pump inlet line connections are tight and leak free
Maintain adequate fluid level within the reservoir
Place the pump below the reservoir fluid level (overhead or “L” shaped reservoir). This is an absolute necessity at elevations at or above 5000 ft. above sea level on oil or 2,500 ft. above sea level on water content fluids

CONCLUSION

Whether they come from cavitation or air infiltration, air (or gasses) can be extremely detrimental to pump life. While contamination is not uncommon, many of the specific causes can be avoided by good system design and fabrication practices. However, preventative maintenance is also routinely required.

When in doubt, consult your local fluid power professional.

Note: “Tech Tips” offered by Flodraulic Group or its companies are presented as a convenience to those who may wish to use them and are not presented as an alternative to formal fluid power education or professional system design assistance.

Experts in fluid power, electrical and mechanical technologies.

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