Cavitation and aeration in the hydraulic system are both related to bubbles. If the judgment is incorrect, it could cause damage to your hydraulic system and result in great losses. This article will introduce the difference between cavitation and aeration to help you quickly identify faults. It can also reduce risks and improve the reliability during the hydraulic system design stage.
What is Cavitation?
When the fluid pressure in a local area of the hydraulic system drops below the saturated vapor pressure of the hydraulic oil, the oil will vaporize by itself and form steam bubbles. After these vapor bubbles follow the oil into the high-pressure area, they will suddenly burst in a very short period of time. This entire process is known as cavitation.
What is Aeration?
Aeration refers to the phenomenon where external air enters the system through leakage points such as seals, pipe joints, and oil suction pipelines, and mixes with hydraulic oil to form a large number of bubbles and foams. These foams flow with the hydraulic oil in the low-pressure area and burst due to compression after entering the high-pressure area.
Cavitation vs Aeration: Damages
Whether it is cavitation or aeration, they will seriously shorten the service life of components such as hydraulic oil, hydraulic pumps, valves and other components. Once a serious equipment failure caused by cavitation or aeration occurs, the cost of replacing and repairing hydraulic components is relatively high.
At the same time, the downtime caused by this will also affect the production pace and result in certain production capacity losses for you.
Damages of Cavitation
- The damage of cavitation to hydraulic systems mainly comes from the impact generated when bubbles burst. Long-term repeated impacts can cause irreversible damage to the metal surface.
- Metal debris is generated and enters the system with hydraulic oil, causing secondary wear and expanding the scope of damage and shortening the service life of your equipment.
- Meanwhile, the precise clearance between the valve core and valve seat is disrupted, leading to an increase in internal leakage. Then, faults such as abnormal pressure regulation and disorderly flow control may occur.
- The generation and burst of bubbles disrupt the normal fluid transport process, leading to a decrease in pump volumetric efficiency and weakened power output capacity.
- The impact and uneven force caused by cavitation can also cause abnormal vibrations in pumps, pipelines, etc. Long-term vibration can cause loose connections and fatigue damage to pipelines.
Damages of Aeration
- After aeration occurs in the hydraulic system, it will directly affect the performance of the hydraulic oil. Mixing air into hydraulic oil will reduce its lubrication ability and damage the stable lubricating oil film on the surface of metal components. This will increase direct contact between components, leading to increased wear and shortened lifespan.
- The air will also accelerate the oxidation of the hydraulic oil. The anti-foaming additives in the hydraulic oil will be rapidly depleted and lose their effectiveness, and their lifespan will be shortened.
- Air has a strong compressibility. It will interfere with the pressure transmission efficiency within the oil circuit. The actuator may exhibit phenomena such as slow response, weak movement, and positioning deviation. The operational accuracy and stability of your equipment will decrease.
- The air in the system is constantly being compressed and expanded, generating additional heat which exceeds the cooling capacity of the cooling system. The high temperature further accelerates the deterioration of the hydraulic oil, creating a vicious cycle.
Cavitation vs Aeration: Symptoms
Whether it is cavitation or aeration, you need to make a comprehensive judgment based on multiple symptoms to prevent incorrect judgments from leading to more serious failures.
How to Identify Cavitation
Abnormal Noise
The most typical symptom of cavitation is abnormal noise. When cavitation occurs in the hydraulic system, you will hear a sharp knocking sound similar to gravel hitting metal coming from the hydraulic pump area.
This type of noise will become more pronounced as the device load increases. The sound source is concentrated inside the pump body and will not weaken or disappear with the oil exhaust operation.
Hydraulic System Vibration
The repeated generation and rupture of foams can generate unstable forces inside the pump. You will find that the device experiences continuous or intermittent vibrations.
System Performance Degradation
The continuous generation and bursting of bubbles will affect the fluid transport state inside the pump, causing a continuous decrease in the pump’s volumetric efficiency. The actual output flow of the device does not meet the rated standard. You will find that the hydraulic actuator has insufficient power and slow movement.
Metal Surface Damage
During the shutdown inspection or maintenance process, you may see damage on the metal surfaces of pumps, valves, and other components. When the bubbles burst near the metal surface, the resulting micro water hammer impact repeatedly strikes the metal surface, causing pitting, spots, and even honeycomb-shaped erosion pits.
How to Identify Aeration
Appearance Changes of Hydraulic Oil
The most obvious symptom of aeration in hydraulic systems usually occurs in the appearance of hydraulic oil. As air mixes with the oil and forms bubbles, an obvious foam layer appears on the surface of the hydraulic oil in the tank, which is milky white or bubbly. In the oil level window, you will easily observe this milky white appearance.
Abnormal Noise
Unlike cavitation, the noise produced by aeration is usually characterized by irregular humming, gurgling sounds or similar gas flow noises. This is because the air is not evenly distributed within the system.
Weak Actuator Action
Due to the compressibility of air, when it is mixed into the hydraulic oil, it will weaken the system’s ability to transmit pressure. There will be obvious jittering, unevenness or delay in the movement of the actuator.
Hydraulic System Overheating
During the repeated compression and expansion of the air, additional energy is generated. This energy is converted into heat, raising the oil temperature. The original cooling system is unable to promptly disperse the excess heat.
In the absence of any abnormal external load, you will notice that the temperature of the hydraulic system will continuously exceed the normal operating range.
Cavitation vs Aeration: Causes
Causes of Cavitation
Insufficient Net Positive Suction Head
When the NPSHA provided by the hydraulic system cannot meet the operating requirements of the hydraulic pump, a continuous negative pressure environment is formed in the suction chamber of the hydraulic pump. The oil pressure drops rapidly.
When the oil pressure drops below the saturated vapor pressure of the hydraulic oil, the hydraulic oil vaporizes and cavitation occurs.
Excessive Flow Resistance on Suction Side
Excessive resistance in the oil suction side oil circuit is one of the most frequent causes on site. For example:
- The oil suction filter screen is clogged due to a long-term lack of maintenance.
- The diameter of the oil suction pipe is too small.
- Pipeline too long.
- Too many bent joints.
- The negative pressure section hose collapses under pressure.
These factors will all exacerbate the loss of oil flow, causing severe pressure loss in the oil before it reaches the pump body. This will subsequently create a low-pressure vacuum area, leading to vaporization.
Blockage of the Fuel Tank Respirator
When the air respirator on the fuel tank gets clogged, the internal pressure of the fuel tank cannot maintain sufficient pressure to pump the oil. When the pump is pumping oil, a negative pressure is formed inside the tank, forcing the pump to work even harder to extract the oil. This indirectly leads to insufficient inlet pressure, resulting in cavitation.
Low Oil Level in the Fuel Tank
When the oil level in the tank is too low, the pump may not be able to obtain sufficient hydraulic oil and may even directly suck in air from the tank. This will lead to increased cavitation.
Defects in the Internal Structure of the Fuel Tank
If the internal partition arrangement of the oil tank is improper or the oil stays in the tank for too short a time, tiny air bubbles will not be able to be expelled from the oil in time. These remaining bubbles, when entering the pump suction port along with the oil, will increase the risk of cavitation.
Abnormal Hydraulic Oil Condition
In a low-temperature startup environment, it may cause the viscosity of the hydraulic oil to be too high and its fluidity to decrease. The hydraulic oil cannot enter the pump suction port at a fast enough speed, and the pump will be forced to form a local vacuum, resulting in cavitation.
Continuously high hydraulic oil temperature will reduce its viscosity. At the same time, the saturation vapor pressure of the hydraulic oil is increased. This means that the hydraulic oil can vaporize without having to be subjected to a very low pressure, and then form bubbles which cause cavitation.
Excessive Operating Speed of Hydraulic Pump
When the operating speed of the hydraulic pump exceeds its designed limit, the instantaneous flow demand will rapidly increase. At this point, the conveying capacity of the oil suction side of the system cannot meet the oil suction demand of the pump body.
The suction inlet pressure continues to drop and eventually reaches the critical value for oil vaporization, causing cavitation.
Pressure Fluctuations Caused by Sudden Load Changes
During the operation of the equipment, sudden changes in load and frequent rapid opening and closing of control valves can cause severe pressure fluctuations in the oil circuit. The system will constantly experience brief moments of instantaneous low pressure, where the oil quickly vaporizes and generates bubbles.
These bubbles will subsequently enter the high-pressure area and burst, thereby causing intermittent cavitation faults.
Causes of Aeration
Leakage on the Oil Suction Side
The pressure inside the suction side of the pump is lower than atmospheric pressure. Therefore, the leakage here will not cause hydraulic oil to leak out, but will instead suck external air into the system. The pump shaft seals, pipe joints, flange connections, as well as the connection points between flexible hoses and hard pipes are all typical leakage areas.
Unreasonable Layout of Oil Return Pipeline
When the outlet of the return oil pipeline is above the oil level of the oil tank, the high-pressure return oil directly impacts the liquid and causes violent splashing. This process will entrain a large amount of air into the hydraulic oil and form bubbles and foams.
The foams enter the oil circuit and then form aeration. In addition, excessive return oil flow rate or a narrow return oil pipeline can also exacerbate oil turbulence and bubble generation.
Low Oil Level Inside the Oil Tank
When the hydraulic oil in the tank is insufficient, the inlet of the suction pipe cannot be completely submerged below the oil level. The hydraulic pump will directly suck in air above the oil surface while extracting oil, thereby generating aeration.
Decreased Hydraulic Oil Performance
After long-term use, the anti-foaming additives contained in the hydraulic oil will gradually be depleted and lose their effectiveness. The oil’s ability to separate from the entrained bubbles decreases. A large number of tiny bubbles cannot rise and burst quickly, and remain mixed in the oil for a long time, intensifying the aeration phenomenon.
Incomplete Air Exhaust After System Maintenance
If you fail to completely evacuate the air after inspecting and disassembling the hydraulic system or the pipeline, air will remain. The air trapped in the valve group, pump body and internal pipelines will continuously circulate and diffuse along with the oil, eventually causing foaming.
Cavitation vs Aeration: Solutions
Solutions of Cavitation
The key to solving cavitation is to make sure that the local pressure at the inlet of the hydraulic pump is always higher than the saturated vapor pressure of the hydraulic oil. You need to reduce the flow resistance of the oil suction pipeline.
For example, you should regularly clean or replace the oil suction filter, and check whether the air respirator on the fuel tank is unobstructed. Maintain the oil level in the fuel tank within the standard range during daily operation.
When designing hydraulic systems, you can try to shorten the length of the oil suction pipeline, reduce bends, and use larger pipe diameters. This can reduce flow resistance and pressure drop, ensuring that the liquid can enter the pump smoothly.
For systems with large suction, you should consider lowering the installation height of the pump or using a pressurized fuel tank.
During operation, you need to avoid operating the system in conditions that can easily generate low pressure, such as excessive flow rate or pump speed.
In addition, it is also important to make adjustments for specific operating environments. If your system needs to start in a low-temperature environment, you need to increase the oil temperature through a preheating device or choose low-viscosity hydraulic oil.
Solutions of Aeration
The key to solving the aeration problem in hydraulic systems is to prevent air from entering the system and to discharge the air that has already entered.
Firstly, you need to conduct a comprehensive inspection of the suction side, with a focus on checking the pipeline joints and seals. Tighten all loose pipeline interfaces and replace aging hoses and seals. This can block the path of external air entering the system, reducing the entry of air from the source.
Replenish hydraulic oil during daily operation to stabilize the oil level in the tank within the specified operating range of the equipment. Make sure that the inlet of the oil suction pipe is completely immersed in the oil to avoid the pump body directly sucking air above the oil surface.
The oil outlet end of the pipeline should also extend below the oil surface of the tank to avoid the backflow of oil impacting the oil surface, causing the oil to carry air.
After completing the disassembly and maintenance of the pipeline, you need to exhaust the hydraulic system according to the standard procedures. You can also install an automatic exhaust valve at the high point of the pipeline. It can automatically detect and discharge the accumulated air during the operation of the system.
FAQs
Will Cavitation and Aeration Occur Simultaneously in the Hydraulic System?
Yes. When air enters the system, aeration will occur. At the same time, the air may alter the flow and pressure conditions of the hydraulic oil, causing the oil to vaporize and leading to cavitation. In complex systems, they may exist simultaneously and interact with each other, thereby increasing the difficulty of your troubleshooting process.
If My Multiple Hydraulic Devices Share the Same Oil Tank, Which Kind of Malfunction might be Exacerbated?
Sharing fuel tanks will exacerbate fluid disturbance and air mixing. Large-scale oil surface rolling will continue to draw in air, exacerbating aeration issues.
At the same time, centralized fuel supply is prone to causing uneven flow distribution and abnormal flow velocities in certain areas. This will indirectly increase the probability of cavitation in some branch pipelines.
Is Cavitation More Likely to Occur in High-Altitude Environments?
Yes. The atmospheric pressure in high-altitude areas is lower. The ability of oil to be pushed into the pump inlet is weakened. This makes the system more prone to having insufficient pressure at the inlet, leading to cavitation.
Therefore, for equipment in high-altitude areas, you need to increase the diameter of the suction pipe or use a top-mounted oil tank or other additional compensatory measures.
Why are Vane Pumps more Vulnerable to Cavitation than Gear Pumps?
The vane pump relies on multiple sliding blades to reciprocate in the rotor slot and continuously adhere to the inner surface of the stator to form a sealed cavity. Once cavitation occurs, the impact of bubble rupture will directly affect the blade edge and stator surface. This can easily cause local wear or peeling, thereby rapidly breaking the sealed state.
Gear pumps use gear meshing to transport liquids. Its internal structure is simpler and more rigid, with higher tolerance to impact and unstable flow. The main load-bearing components of gear pumps are gears and pump bodies, which can disperse the local impact caused by cavitation.
Will Water in Hydraulic Oil Induce More Severe Cavitation?
Yes. When there is free water or water exceeding the solubility limit in hydraulic oil, this water is more likely to turn into bubbles or steam in low-pressure areas. Once these bubbles enter the high-pressure area, they burst rapidly and cause cavitation.
Final Thoughts
Having a comprehensive understanding of cavitation and aeration is not just about avoiding malfunctions. It can also keep the long-term stable operation of your device and continuously create value for your business. If you are troubled by cavitation or aeration, please feel free to contact us. We are happy to solve your problems and provide customized solutions for your hydraulic system.
