I find a lot of confusion among maintenance professionals concerning aeration and cavitation. Many do not understand the difference between the two. This is not surprising - they can often sound similar. But they are in fact two entirely different problems - and with entirely different solutions.

Cavitation is the formulation and collapse of air cavities in the liquid.

Aeration is the result of outside air entering the suction side of the pump.

Cavitation results when the pump attempts to deliver more oil than it can get into its suction. Hydraulic oil has a significant amount of dissolved air - 6-12% by volume. A number of factors can determine how much air becomes dissolved - the oil temperature, additives, the level of other comtaminants and the oil pressure to name a few. When the inlet flow becomes lower than the flow the pump tries to deliver, suction pressure becomes very low and some of the air molecules are extracted from the oil. As this occurs, the air molecules implode inside the pump. The symptom is a very steady high-pitched whining sound. If this continues unchecked, the pump will, of course, deliver a reduced flow (obviously - since the pump can deliver no more flow than can enter) and will eventually destroy itself. This is one reason the suction port is typically larger than the pressure port, to keep suction velocity low so that it is easy for oil to enter. Anything that can restrict oil from entering the pump suction line can cause cavitation. If the oil is too cold, the viscosity will increase and the amount of oil that can enter the suction port will be decreased. In general, most machines should not be powered up unless the oil is at least 40^o F. It should not be placed under load until the oil temperature is at least 70^o F.

The most common cause of cavitation is a plugged suction strainer or filter. Suction strainers are inside the reservoir below the level of the oil. They are out of sight and out of mind. If it is not someone's job to periodically inspect and clean suction strainers, there is likely to be pump damage.

An undersized or clogged breather cap can cause a pump to cavitate. The reservoir needs to breathe because it requires more oil to extend a cylinder than to retract it. The oil level goes up and down throughout the cycle of the machine. If it's hard for air to get in, the pump has to work harder and suction pressure drops easily.

Turbulence caused by suction isolation valves can be another culprit as can an excessive vertical distance between the pump suction port and the minimum fluid level.

There are those who believe that, if a pump cavitates, it must be replaced. Often when the pump IS replaced, the cause of the cavitation is corrected, reinforcing this notion. The fact is, if a pump is strong enough to cavitate, it is probably strong enough to do its job. Of course, if it is allowed to cavitate for very long, it most certainly WILL need to be replaced.

Aeration has a much more erratic sound than cavitation. It is usually accompanied by a sound similar to marbles or gravel rattling around inside the pump. And if you can see the oil in the reservoir, you will often see foaming. Anything that can allow air into the suction line can cause aeration. When air enters the suction line, a number of problems are caused. Among them are spongy component response, reduced viscosity (and the subsequent compnent wear), increased temperature (because of the reduced thermal conductivity) and severe fluid degradation which will lead to damage from poor lubrication, overheating and burning of seals.

A leak in the suction line is one common cause of aeration. The pressure in the suction line is below that of atmospheric pressure so oil doesn't leak out - air leaks in. If you suspect an air leak, squirt oil up and down the suction line. If it stops aerating, you found your leak.

A bad shaft seal on a fixed displacement pump can cause aeration. To test the shaft seal, spray shaving cream around it. If the shaft seal is bad, holes will be drawn into the shaving cream as air enters the pump.

A low fluid level will cause a vortex to develop over the suction strainer - much the same as draining your bathtub. Both air and oil enter the suction line and aeration occurs.

Misaligned couplings also can let air into the suction line. Make sure couplings are properly aligned and torqued. Don't forget ball valves and gate valves in the suction line. They can leak, too. Prevention is always better than the cure and spotting aeration or cavitation early is simply a matter of paying attention to the sounds your machine makes.

• In-plant Troubleshooting
• Leakage Problems
• Pressure Settings
• Shock Problems
• Hydraulic Reliability Assessments
• Hydraulic Troubleshooting Manual Development
• Startup Consulting and Recommendations
• Heat Problems
• Repeated Component Failures
• Speed Problems

2 tablespoons freshly ground black pepper
2 tablespoons salt
1 tablespoon powdered garlic
1/2 teaspoon cayenne pepper
1/2 teaspoon curry powder
4 tablespoons dry mustard powder
Rub first with olive oil, then with the rub, getting the spice deeply into the meat. Not fancy, not exotic, just good. Leave out the cayenne pepper if you're a wuss.

2. Changing the Oil - What's wrong with changing the oil? New oil is always better than old oil, isn't it? Not exactly, no. Changing the oil when it is dirty is a waste of time and money. Even if you do drain the reservoir and replace the oil, what about the oil left in the machine? It's still dirty! Once the new oil picks up contaminants and mingles with the old oil, chances are that an analysis taken a couple of weeks after the oil change looks not much better than the one prompting the change in the first place. Add to that the possibility of contaminating the system by not properly filtering the new oil, stirring up sludge by not properly cleaning the reservoir and adding airborne contaminants to the system during the process and changing the oil could easily result in an outage that could have otherwise been avoided. A much better idea is to flush the system to reclaim the dirty oil. This addresses ALL of the oil in the system without adding contaminants. The only time oil must be changed is when the base oil is degraded (usually by overheating) or the additives have been depleted (I'll save the debate of replacing oil vs. using additive concentrates for a different article). Other than these two conditions, since oil is a mineral it doesn't wear out - it just gets dirty.

3. Overheating - Continuing to run a hydraulic machine when it is overheating is a guarantee of component failure. Remember that mineral oil based hydraulic oils will begin to break down once the system temperature reaces 140^oF. In extreme instances, the entire machine must be pickled and flushed after all or most of the components are replaced. Keep an eye on temperature and when it overheats, find out why and correct the problem.

4. Not Using the Best Oil for Your Machine - The oil is the lifeblood of the machine. It is the single most important component and its improper care is the result of over 96 percent of hydraulic failures. The oil performs 4 functions in the system:

5. Parts Changing instead of Troubleshooting - When a logical path to troubleshooting is not used, unnecessary components are replaced and the risk of system contamination increases. Even at some of the most well known and reputable companies, when a failure occurs the maintenance staff scrambles to start changing parts until the machine works again. This is not only an expensive, time-wasting practice, but leaves the distinct possibility of adding problems rather than solving them. The best five minutes ever spent troubleshooting a hydraulic machine are spent tracing the flow on the hydraulic schematic. Consider attending one of our Hydraulic Reliability and Troubleshooting workshops to hone your hydraulic troubleshooting skills.

6. Failing to Make the Proper Reliability Checks - It's best when the machine doesn't fail in the first place and a good reliability checklist can go a long way toward this end. There is a lot more to reliability than changing filters and checking the oil level. One of our Customized Hydraulic Reliability Assessments can provide you with all of the tools you need to get the best service from your machines. Regular system reliability checks are critical if your goal is to maximize uptime.