Cavitation or aeration? You can hear the difference!
Many maintenance technicians confuse cavitation and aeration. Aeration is sometimes referred to as “pseudo cavitation”. While the two conditions do have similar symptoms, their causes are entirely different.
Cavitation is the formation and collapse of air cavities in the liquid. When the fluid is pumped from the reservoir, a low pressure drop occurs in the suction side of the pump. Despite what many people believe, the fluid is not “sucked” into the pump. It is pushed into the pump by atmospheric pressure, as shown in the left illustration of Figure 1. The movement of the rotating gears causes a drop in pressure at the suction line. The resulting pressure difference between the reservoir and the pump inlet causes the fluid to move from the higher pressure to the lower pressure. So long as the pressure difference is sufficient and the flow path is clear, the operation goes smoothly, but anything that reduces the inlet flow can cause problems. Whenever the pump cannot get as much fluid as it is trying to deliver, cavitation occurs, as shown in the right illustration of Figure 1. Hydraulic oil contains a small amount of dissolved air. When the pump cannot get the oil it is calling for, pressure in the suction line drops very low. The pressure drops so low the air is extracted from the oil. The air bubbles (or “cavities”, hence the name “cavitation”) travel into the pump and, when they are delivered to the pressure side of the pump, they collapse and implode. The resulting shock waves cause a steady, high-pitched whining sound and damage to the insides of the pump.
Causes of cavitation can include:
- Any increase in fluid velocity. Fluid velocity is inversely proportional to the size of the hydraulic line. You may have noticed that most pumps have a larger suction line than the pressure line. This is to keep inlet velocity low, making it very easy for oil to enter the pump. Any blockage, such as a plugged suction strainer or filter, can cause the pump to cavitate. A contaminated suction strainer is the number one cause of cavitation simply because it is underneath the oil level in the reservoir.
One of our consultants was called to a plant in Georgia recently that had changed 5 pumps within the last week on a machine. The first thing that was noticed was that a high pitched whining sound was heard every 20 – 30 seconds. The millwrights had changed the suction line and said, although a suction strainer was shown on the schematic, none was found in the line. The machine was then shut down and the reservoir drained to be cleaned. Guess what was found in the reservoir? The suction strainer, which had been floating around in the oil, occasionally blocking the suction pipe to the pump.
- A plugged breather cap can cause the pressure in the reservoir to drop. Suction pressure at the pump must drop very low to compensate for this and vapor cavities form.
At a plywood plant in Oregon, a hose ruptured on the lathe which caused a loss of 150 gallons of oil in the reservoir. After the hose was changed, the lubrication technician removed one of the breather caps to refill the reservoir. While filling the tank, a shift change occurred and the second shift lube tech took over. Once the reservoir was refilled, the lube tech installed a pipe plug on the threads where the breather cap was originally located. The result was that one of the pumps on the unit failed due to cavitation within a few hours after start up. After losing 2 pumps in 24 hours, the pipe plug on the breather opening was discovered.
- High oil temperature causes vapor cavities to form with less of a pressure drop and can cause or contribute to cavitation.
- Low oil temperature increases the oil viscosity making it harder for the oil to get into the pump. Most hydraulic systems should not be started up with the oil any colder than 40°F and should not be put under load until the oil is at least 70°F.
- If the drive motor RPM is too high for the pump, the pump tries to deliver more oil than it can get into its suction port.
- If the pump is located such that the fluid has to be lifted a long way from the reservoir, atmospheric pressure may be insufficient to deliver enough fluid to the pump inlet and it can cavitate.
- If the system is located at a very high altitude, the available atmospheric pressure may be insufficient. It is for this reason that aeronautic hydraulics must use pressurized reservoirs.
Aeration occurs whenever outside air enters the suction side of the pump. This produces a sound that is much more erratic than that of cavitation. The whining sound may be augmented by a sound similar to marbles or gravel rattling around inside the pump. If you can see the oil in the reservoir, you may see foaming. Air in the oil can cause a system to perform sluggishly and can damage the pump and other system components. Aeration can be caused by:
- An air leak in the suction line. Pressure in the suction line is below that of atmospheric pressure so if there is any leak in the suction line, oil won’t leak out; air will leak in. If you suspect an air leak, squirt oil on all the fittings and connections in the suction line. If the sound of aeration stops briefly, you have found your leak. An ultrasonic gun can also be used to detect leaks.
One of our consultants was asked to diagnose several pump failures on a system at an automotive manufacturing plant. When he arrived at the unit, he heard an erratic high pitched sound. He also noticed that there were several fittings in the suction line. (Figure 2). He had one of the millwrights fill a bottle with oil (figure 3) and squirt it around all the fittings in the suction line. When squirted on the fitting marked “A” in the Figure 2, the pump momentarily quieted down. This fitting had vibrated loose after 12 years on the machine.
- A bad shaft seal on a fixed displacement pump. If you suspect a bad shaft seal, spray some shaving cream around the seal. If it is bad, holes in the shaving cream will develop as air enters the pump.
I was called to a papermill recently where foam came out of the log kicker reservoir shortly after the fixed displacement pump was started. After performing the shaving cream test, I knew the shaft seal was badly worn. Upon further inspection, I found the pump elastomeric coupling was worn causing wear on the shaft seal.
- Improperly tightened or aligned fittings in the suction line. Check all of the fittings and make sure they are torqued and aligned according to specifications.
- Incorrect shaft rotation. This may not be an issue with all pumps, but some will aerate if they are turned backwards. Most all pumps have a direction of rotation stamped or located on a sticker on the pump housing. Many times when a pump is rebuilt, the sticker is removed. Always check the part number of the new pump to be installed with the old pump. There is usually a number or letter designation that indicates whether it is a right hand or left hand rotation. If you are unsure, remove the outlet line of the pump and secure it into a container. Never hold this line as this could be a hazardous situation. Momentarily jog the electric motor. If the pump is rotating in the correct direction oil will flow out of the outlet port.
- Low fluid level. The oil level should never drop more than 2 inches above the suction line. If so, a vortex can form, much like when draining a bathtub. This allows air in the suction line resulting in aeration of the pump.
When troubleshooting hydraulic pump problems, make the visual and sound checks first as these are the easiest to do. Remember that aeration and cavitation produce different sounds. Many times you can determine the cause of the problem before the first wrench is turned.