what causes hydraulic pump cavitation factory

The second leading cause of hydraulic pump failure, behind contamination, is cavitation. Cavitation is a condition that can also potentially damage or compromise your hydraulic system. For this reason, understanding cavitation, its symptoms, and methods of prevention are critical to the efficiency and overall health of not just your hydraulic pump, but your hydraulic system as a whole.

The product of excessive vacuum conditions created at the hydraulic pump’s inlet (supply side), cavitation is the formation, and collapse of vapors within a hydraulic pump. High vacuum creates vapor bubbles within the oil, which are carried to the discharge (pressure) side. These bubbles then collapse, thus cavitation.

This type of hydraulic pump failure is caused by poor plumbing, flow restrictions, or high oil viscosity; however, the leading cause of cavitation is poor plumbing. Poor plumbing is the result of incorrectly sized hose or fittings and or an indirect (not straight or vertical) path from the pump to the reservoir. Flow restrictions, for example, include buildup in the strainer or the use of an incorrect length of hose or a valve that is not fully open. Lastly, high oil viscosity—or oil that is too viscous—will not flow easily to the pump. Oil viscosity must be appropriate for the climate and application in which the hydraulic pump is being used.

The greatest damage caused by cavitation results from the excessive heat generated as the vapor bubbles collapse under the pressure at the pump outlet or discharge side. On the discharge side, these vapor bubbles collapse as the pressure causes the gases to return to a liquid state. The collapses of these bubbles result in violent implosions, drawing surrounding material, or debris, into the collapse. The temperature at the point of implosion can exceed 5,000° F. Keep in mind that in order for these implosions to happen, there must be high vacuum at the inlet and high pressure at the outlet.

Cavitation is usually recognized by sound. The pump will either produce a “whining” sound (more mild conditions) or a “rattling” sound (from intense implosions) that can sound like marbles in a can. If you’re hearing either of these sounds, you first need to determine the source. Just because you hear one of these two sounds doesn’t guarantee that your hydraulic pump is the culprit.

To isolate the pump from the power take-off (PTO) to confirm the source, remove the bolts that connect the two components and detach the pump from the PTO. Next, run the PTO with no pump and see if the sound is still present. If not, it is safe to assume your hydraulic pump is the problem.

Another sign you may be experiencing cavitation is physical evidence. As part of your general maintenance, you should be inspecting and replacing the hydraulic oil filter"s elements at regular intervals based on the duty cycle of the application and how often it is used. If at any time during the inspection and replacement of these elements you find metallic debris, it could be a sign that you’re experiencing cavitation in the pump.

The easiest way to determine the health of your complete hydraulic circuit is to check the filter. Every system should have a hydraulic oil filter somewhere in-line. Return line filters should be plumbed in the, you guessed it, return line from the actuator back to tank—as close to the tank as possible. As mentioned earlier, this filter will have elements that should be replaced at regular intervals. If you find metallic debris, your pump could be experiencing cavitation. You’ll then need to flush the entire system and remove the pump for inspection.

Conversely, if you’ve already determined the pump to be damaged, you should remove the filter element, cut it open, and inspect it. If you find a lot of metal, you’ll need to flush the entire system and keep an eye on the other components that may be compromised as a result.

Once cavitation has been detected within the hydraulic pump, you’ll need to determine the exact cause of cavitation. If you don’t, cavitation can result in pump failure and compromise additional components—potentially costing you your system.

Since the pump is fed via gravity and atmospheric pressure, the path between the reservoir and the pump should be as vertical and straight as possible. This means that the pump should be located as close to the reservoir as is practical with no 90-degree fittings or unnecessary bends in the supply hose. Whenever possible, be sure to locate the reservoir above the pump and have the largest supply ports in the reservoir as well. And don"t forget, ensure the reservoir has a proper breather cap or is pressurized (3–5 PSI), either with an air system or pressure breather cap.

Be sure the supply line shut-off valve (if equipped) is fully open with no restrictions. This should be a “full-flow” ball valve with the same inside diameter (i.d.) as the supply hose. If feasible, locate a vacuum gauge that can be T’d into the supply line and plumb it at the pump inlet port. Activate the PTO and operate a hydraulic function while monitoring the gauge. If it reads >5 in. Hg, shut it off, and resume your inspection.

A hose with an inner bladder vulcanized to a heavy spiral is designed to withstand vacuum conditions as opposed to outward pressure. The layline will also denote the size of the hose (i.d.). You can use Muncie Power’s PPC-1 hydraulic hose calculator to determine the optimal diameter for your particular application based on operating flows.

Another consideration, in regards to the inlet plumbing, is laminar flow. To reduce noise and turbulence at the pump inlet, the length of the supply hose should be at least 10 times its diameter. This means that any type of shut-off valve or strainer at the reservoir should be at least 10 diameters from the pump inlet. A flared, flange-style fitting at the pump inlet can also reduce pump noise by at least 50 percent compared to a SAE, JIC, or NPT fitting.

Selecting the proper viscosity of hydraulic fluid for your climate and application is also critical. Oil that is too viscous will not flow as easily to the pump. Consult your local hydraulic oil supplier for help selecting the optimal fluid viscosity.

By maintaining a regular maintenance schedule, remaining vigilant for any signs or symptoms, and taking preventative measures, the good news is that you should be able to prevent cavitation and experience efficient operation for the duration of your pump’s lifespan.

Poor plumbing is the leading cause of cavitation and can be prevented by selecting a properly sized hose, choosing the appropriate fittings, ensuring the most direct, straight routing from the pump to the reservoir, etc.

Although cavitation can occur anywhere in a hydraulic system, it commonly occurs within the suction line of a pump. This will cause excessive noise in the pump – generally a high pitched “whining” sound. However, this excessive noise is only the tip of the iceberg! The real result of this phenomenon is severe pump damage and a decrease in pump life. I have personally seen many instances where a customer was replacing pumps frequently, thinking they were receiving defective pumps from their vendor. In reality, the pump failures were not due to poor pump quality – the failures were occurring because of cavitation.

Simply put, cavitation is the formation of vapor cavities in the hydraulic oil. In hydraulic pumps, cavitation will occur any time the pump is attempting to deliver more oil than it receives into the suction (inlet) line. This is commonly referred to as “starvation” and results from a partial vacuum in the suction line. To fully illustrate what is happening when this occurs, we need to discuss vapor pressure. Vapor pressure is the pressure below which a liquid at a given temperature will become a gas, and this pressure varies significantly depending on the liquid. Generally, as the temperature of a liquid rises, the vapor pressure will proportionally increase. Likewise, as the temperature decreases, the vapor pressure will decrease. Most of us know that water will boil (turn to vapor) at 212°F (100°C) at 14.7 PSI (atmospheric pressure at sea level). In other words, the vapor pressure of water at 212°F is 14.7 PSI. If the pressure is reduced, the temperature at which the water boils will be reduced. If the temperature is lowered, the vapor pressure will decrease. In fact, water will boil at room temperature if the pressure is sufficiently reduced! The same principle applies to hydraulic oil, although the vapor pressure will be somewhat different than that of water. The vapor pressure of hydraulic oil is somewhere between 2 and 3 PSI at normal temperatures. In ideal conditions, the pressure in the suction line of the pump will be around 14.7 PSI at sea level. Of course, this pressure decreases with altitude, but sufficient pressure will normally be maintained in the suction line to prevent cavitation of the oil. However, if the pressure in the suction line of the pump is sufficiently reduced to the vapor pressure of the oil, vapor cavities will form. As the oil passes from the suction line to the outlet of the pump, the pressure will increase and the vapor cavities will implode violently. These extremely powerful implosions will cause erosion and premature failure of the pump components. In fact, a brand-new pump can be destroyed in a matter of minutes if the cavitation is severe enough. The picture below shows a rotor and cam ring from a vane pump that had failed due to severe cavitation.

In my 35-plus years of troubleshooting hydraulic components, this is the worst case of cavitation damage I have ever seen. In addition to the usual erosion of the parts, the vanes were actually fused to the rotor slots! Although this is an extreme example, it shows the potential damage to a pump due to cavitation. The good news is that cavitation is preventable and we will look at several conditions that can trigger this phenomenon.

A variety of factors within the system could produce such a vacuum. When fluid enters the hydraulic pump and is compressed, the small air bubbles implode on a molecular level. Each implosion is extremely powerful and can remove material from the inside of the pump until it is no longer functional. Cavitation can destroy brand new pumps in a matter of minutes, leaving signs of physical damage including specific wear patterns. The process of cavitation destroying a hydraulic pump also has a distinctly audible sound similar to a growl.

The good news is that cavitation need not be a common problem in hydraulic systems. A few design flaws are largely responsible for causing cavitation: improper configuration of pump suction lines and the use of suction-line filters or strainers. To prevent these causes of cavitation and ensure the creation of a quality hydraulic system with a long, productive life, seven design elements must be properly executed:

In addition to improper pump suction-line configurations, suction-line filters or strainers can be a leading cause of cavitation. These filters are often placed under the oil reservoir, and thus are rarely serviced properly due to their inconvenient location. With this configuration, the entire reservoir has to be drained and disassembled in order to the reach the filter, so this necessary task is often neglected. As the filter becomes increasingly full of debris over time due to a lack of regular maintenance, not enough fluid will flow to the pump, and cavitation will occur.

Such causes of cavitation can be prevented using a series of correct design practices based on the specific needs and functions of a hydraulic system. Many systems are unique, so an experienced engineer with a firm grasp on each of these concepts must ensure the proper installation and maintenance of a hydraulic system.

Air bubbles in hydraulic fluid first originate is in the reservoir. New oil being introduced into the reservoir can cause turbulent flow, stirring up the oil and introducing air into the fluid, which can lead to cavitation. A correctly designed reservoir tank will prevent this issue.

The size of the tank and the amount of fluid that needs to rest before being extracted depends on the amount of system flow. However, a minimum 4 to 1 tank capacity to flow rate ratio is recommended — four times the oil available in the reservoir at any given time than is needed for extraction to send to the pump. This ensures that the pump will receive clean oil and the oil spends enough time in the reservoir for air bubbles and impurities to work their way out.

Beyond properly designing the reservoir itself, it’s important to include the correct accessories to ensure proper functionality. The breather filter is perhaps the most important accessory for maintaining the correct conditions for the hydraulic fluid in the tank.

When fluid is drawn from the reservoir by the pump, and an equal amount isn"t returned, the oil level will drop. To regulate the pressure and prevent forming a vacuum, air needs to be introduced to the tank to occupy the extra volume created upon removal of the oil. A breather filter performs this function, which helps avoid cavitation.

Incorrect design and configuration of suction lines is the primary cause of cavitation in hydraulic systems. For this reason, it’s crucial to use correct design practices when designing the suction lines, such as using the proper line size, minimizing fittings on the line, and properly sizing the ball valve to handle the amount of flow through the line.

The size of a suction line should be large enough for the liquid’s area to flow through at the correct rate and in the correct amount. Because the pump needs to be constantly supplied with oil, it becomes obvious how a line that’s too small could prevent this essential function. The exact specifications of a suction line in terms of length and width can’t be determined in a general sense — it requires a skilled engineer with a firm grasp of the process to make the correct decision on this specification.

Another best practice to consider when configuring suction lines is to include a lock on the suction ball valve, preventing it from being accidentally closed or left partially closed during the pump’s operation. Shutting off the flow of a suction line during pump operation will have cataclysmic effects on the system.

For example, the oil can be filtered upon entering the reservoir tank rather than when leaving the tank. Or a off-line (kidney loop) filtration system can be used to pull the oil out of the tank, filter it, and reinsert it before it’s extracted and sent to a hydraulic pump. These solutions allow for greater ease of maintenance and lower the chance of system failure.

A key aspect of a hydraulic system is a pump that’s properly sized to handle the flow rate and amount of fluid in the system. Again, this decision must be made by an experienced engineer with a good understanding of the entire process. A pump’s size can be determined by incorporating several variables of the process into a standard equation while also considering unique application conditions.

Another key element within a hydraulic system is to maintain the proper fluid temperature. If the hydraulic fluid gets too cold, it can become too viscous, increasing pressure drop in fluid lines and eventual cavitation in the pump. On the other hand, overheated hydraulic fluid can become too thin, compromising its ability to lubricate the hydraulic pump.

To regulate the temperature of the fluid, electric heating elements can be placed in the reservoir to keep the fluid at the ideal temperature of 110°F. Hydraulic systems often heat themselves naturally, so it’s also important to monitor for temperatures in excess of 110° and provide a heat exchanger or operate the system at reduced capacity.

Most systems use a flooded suction design, meaning that the pump is placed below the oil level to achieve net positive suction. The oil comes out of the reservoir above the location of the pump, which means gravity is used to assist in creating pressure into the pump and suction line. This represents the ideal configuration for a pump in a hydraulic system.

The alternative to this layout is non-flooded suction, in which the pump is placed on top of the tank. This configuration is often used to save space in a system with a limited footprint, but results in several disadvantages. For instance, the pump has to perform the extra work of pulling the oil up against gravity to create a vacuum and then pump the fluid out, which inherently creates restrictions by working against gravity. Also, certain types of pumps will function poorly in a non-flooded suction layout. In these cases, a charge pump can be used to provide positive pressure in the pump suction line.

If each of these design elements is carefully considered while engineering a hydraulic system, the risk of cavitation damaging or destroying hydraulic pumps should decrease significantly. Latest from Valin"s Blog

Hydraulic pumps are used in various industries to pump liquid, fluid, and gas. Although this equipment features robust construction, it may fail at times due to various issues. Cavitation is one of the serious issues faced by this equipment. Like all other technical issues, right planning as well as troubleshooting will help avoid this issue to a large extent. What is pump cavitation and how to troubleshoot these it?

It is seen that many times, Strong cavitation that occurs at the impeller inlet may lead to pump failure. Pump cavitation usually affects centrifugal pumps, which may experience several working troubles. At times, submersible pumps may also be affected by pump cavitation.

Non-inertial Cavitation: This type of cavitation is initiated when a bubble in a fluid undergoes shape alterations due to an acoustic field or some other type of energy input.

Suction Cavitation: This cavitation is brought by high vacuum or low-pressure conditions that may affect the flow. These conditions will reduce the flow, and bubbles will be formed near the impeller eye. As these bubbles move towards the pump’s discharge end, they are compressed into liquid, and they will implode against the edge of the impeller.

Discharge Cavitation: Here, cavitation occurs when the pump’s discharge pressure becomes abnormally high, which in turn affects its efficiency. High discharge pressure will alter the flow of fluid, which leads to its recirculation inside the pump. The liquid will get stuck in a pattern between the housing, as well as the impeller, thereby creating a vacuum. This vacuum creates air bubbles, which will collapse and damage the impeller.

Sound: The pump affected by cavitation will produce a marble, rock, or gravel type of sound when in motion. The sound will begin as a small disturbance and its intensity will increase as the material slowly chips away from the surface of the pump.

Metallic Debris: If during the maintenance, you find metallic debris on the filter of the hydraulic pump then it may be a symptom of cavitation. One of the easiest ways to confirm it is to check the filter. If any debris is found, you should clean the entire system, and thoroughly inspect the pump.

Damage: This is one of the most obvious symptoms of cavitation. If you already know that the pump is damaged, you need to remove its filter, open, and inspect it thoroughly. If you find a lot of metal inside the filter, then flush the entire system, and check for damages in other parts, too.

If you notice any of the above-discussed symptoms, the next step would be to identify the causes, and rectify the changes in industrial pumps, otherwise, it may affect other components, too.

Avoid using suction strainers: These are designed to inhibit the ingestion of grime and dirt. However, these strainers do not succeed in their purpose, because they are not designed to entrap large particles. These large particles may get deposited in the flow path, thereby affecting the flow of fluid. The deposition also creates pressure, and produces bubbles, which may lead to cavitation.

Clean the reservoir: A dirty reservoir is one of the most common causes of cavitation. Various types of small and large objects may block the suction tube, and create pressure, thereby causing cavitation.

Use properly sized components: This is one of the important factors of cavitation prevention. If the inlet plumbing is too large, there will be too much liquid flow, which may trigger cavitation. Hence, check with the pump manufacturer to ensure that properly sized components are being used in the pump.

In addition to these preventive steps, you must source hydraulic pumps from a trusted manufacturer or supplier. JM Industrial is one of the industry-leading provider of unused and used industrial process equipment from industry-leading brands. These pumps can be availed at cost-effective prices.

Many maintenance technicians confuse cavitation and aeration. In fact, aeration is sometimes referred to as pseudo cavitation. While these two conditions have similar symptoms, their causes are entirely different.

Cavitation is the formation and collapse of air cavities in liquid. When hydraulic fluid is pumped from a 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 but rather pushed into it by atmospheric pressure, as shown in the left illustration below.

The movement of the rotating gears leads to 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. As 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 create problems. Whenever the pump cannot get as much fluid as it is trying to deliver, cavitation occurs, as shown in the right illustration below.

Hydraulic oil contains approximately 9 percent dissolved air. When a pump does not get enough oil, air is pulled out of the oil. These air bubbles travel into the pump and eventually collapse and implode when they reach an area of relatively high pressure. The ensuing shockwaves produce a steady, high-pitched whining sound and damage to the inside of the pump. In the early stages, the sound goes undetected in loud plants unless an ultrasonic sensor is employed. These sensors can listen to super high frequencies emitted by the pumps, detecting cavitation before it does too much damage.

Any increase in fluid velocity can lead to cavitation. Fluid velocity is inversely proportional to the size of the hydraulic line. Most pumps have a suction line that is larger 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 result in the pump cavitating. A contaminated suction strainer is the most common cause of cavitation simply because it is underneath the oil level in the reservoir.

One of our consultants was recently called to a plant in Georgia that had changed five pumps on a machine within a week. The first thing that was noticed was a high-pitched whining sound, which was heard every 20 to 30 seconds. The millwrights had changed the suction line, and 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, was occasionally blocking the suction pipe to the pump. Had there been early detection of cavitation, the plant could have saved a decent chunk of change on all those pumps.

A plugged breather cap is another common cause of cavitation. It can lead to falling pressure in the reservoir. Suction pressure at the pump must drop very low to compensate for this, creating vapor cavities.

At a plywood plant in Oregon, a hose ruptured on the lathe, which resulted in 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 within a few hours after startup due to cavitation. After losing two pumps in 24 hours, the pipe plug on the breather opening was discovered.

Extreme oil temperatures can also cause cavitation. High temperatures allow vapor cavities to form with less of a pressure drop, while low temperatures increase the oil’s 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 degrees F or put under load until at least 70 degrees F.

In addition, cavitation may result if the drive speed is too high for the pump, as the pump tries to deliver more oil than it can get into its suction port. If the pump is positioned so fluid must be lifted a long way from the reservoir, atmospheric pressure may be insufficient to deliver enough fluid to the pump inlet, which can cavitate.

Systems at high altitudes are also susceptible to cavitation, as the available atmospheric pressure may be inadequate. 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 more erratic than that of cavitation. The whining noise may be augmented by a sound similar to marbles or gravel rattling around inside the pump. If the oil in the reservoir is visible, you may see foaming. Air in the oil can lead to sluggish system performance and even damage the pump and other components.

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. He had one of the millwrights fill a bottle with oil and squirt it around all the fittings. When oil was applied to one fitting, 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 is another common cause of aeration. 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 once called to a paper mill 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, which resulted in wear on the shaft seal.

Incorrect shaft rotation may not be an issue with all pumps, but some will aerate if they are turned backward. Most pumps have a direction of rotation stamped or located on a sticker on the pump housing. Many times when a pump is rebuilt, this sticker is removed. Always check the part number of the new pump to be installed with the old pump. Often a number or letter will indicate whether it is a right-hand or left-hand rotation. If you are unsure, remove the pump’s outlet line and secure it into a container. Never hold this line, as it 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.

Aeration may also result from a 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, leading to aeration of the pump.

When troubleshooting hydraulic pump issues, make the visual and sound checks first, as these are the easiest to perform. Remember, aeration and cavitation produce different sounds. Usually you can determine the cause of the problem before the first wrench is turned.

Al Smiley is the president of GPM Hydraulic Consulting Inc., located in Monroe, Georgia. Since 1994, GPM has provided hydraulic training, consulting and reliability assessments to companies in t...

The phenomenon of cavitation consists in the disruption of continuity in the liquid where there is considerable local reduction of pressure. The formation of bubbles within liquids (cavitation) begins even in the presence of positive pressures that are equal to or close to the pressure of saturated vapor of the fluid at the given temperature.

Various liquids have different degrees of resistance to cavitation because they depend, to a considerable degree, upon the concentration of gas and foreign particles in the liquid.

The mechanism of cavitation can be described as follows: Any liquid will contain either gaseous or vaporous bubbles, which serve as the cavitation nuclei. When the pressure is reduced to a certain level, bubbles become the repository of vapor or of dissolved gases.

This process of condensation takes place fairly quickly, accompanied by local hydraulic shocks, the emission of sound, the destruction of material bonds and other undesirable phenomena. It is believed that reduction in volumetric stability in most liquids is associated with the contents of various admixtures, such as solid unwetted particles and gas-vapor bubbles, particularly those on a submicroscopic level, which serve as cavitation nuclei.

A critical aspect of the cavitation wear process is surface destruction and material displacement caused by high relative motions between a surface and the exposed fluid. As a result of such motions, the local pressure of the fluid is reduced, which allows the temperature of the fluid to reach the boiling point and small vapor cavities to form.

When the pressure returns to normal (which is higher than the vapor pressure of the fluid), implosions occur causing the cavity or vapor bubbles to collapse. This collapse of bubbles generates shock waves that produce high impact forces on adjacent metal surfaces and cause work hardening, fatigue and cavitation pits.

Thus, cavitation is the name given to a mechanism in which vapor bubbles (or cavities) in a fluid grow and collapse due to local pressure fluctuations. These fluctuations can produce a low pressure, in the form of vapor pressure of the fluid. This vaporous cavitation process occurs at approximately constant temperature conditions.

Vaporous cavitation is an ebullition process that takes place if the bubble grows explosively in an unbounded manner as liquid rapidly changes into vapor. This situation occurs when the pressure level goes below the vapor pressure of the liquid.

Gaseous cavitation is a diffusion process that occurs whenever the pressure falls below the saturation pressure of the noncondensable gas dissolved in the liquid. While vaporous cavitation is extremely rapid, occurring in microseconds, gaseous cavitation is much slower; the time it takes depends upon the degree of convection (fluid circulation) present.

Cavitation wear occurs only under vaporous cavitation conditions - where the shock waves and microjets can erode the surfaces. Gaseous cavitation does not cause surface material to erode.

It only creates noise, generates high (even molecular level cracking) temperatures and degrades the chemical composition of the fluid through oxidation. Cavitation wear is also known as cavitation erosion, vaporous cavitation, cavitation pitting, cavitation fatigue, liquid impact erosion and wire-drawing.

Cavitation wear is a fluid-to-surface type of wear that occurs when a portion of the fluid is first exposed to tensile stresses that cause the fluid to boil, then exposed to compressive stresses that cause the vapor bubbles to collapse (implode).

This collapse produces a mechanical shock and causes microjets to impinge against the surfaces, unifying the fluid. Any system that can repeat this tensile and compressive stress pattern is subject to cavitation wear and all the horrors accompanying such destructive activity.

Cavitation wear is similar to surface fatigue wear; materials that resist surface fatigue (hard but not brittle substances) also resist cavitation damage.

Liquid is the medium that causes cavitation wear. Cavitation wear does not require a second surface; it requires only that high relative motion exists between the surface and the fluid. Such motion reduces the local pressure in the fluid. When the liquid reaches its boiling point and ebullition occurs, vapor bubbles form, which produces cavitation.

Each vapor cavity lasts a short time because almost any increase in pressure causes the vapor in the bubble to condense instantaneously and the bubble to collapse and produce a shock wave. This shock wave then impinges on adjacent metal surfaces and destroys the material bonds.

Figure 1 depicts the collapse of a vapor bubble and the birth of a microjet. Cavitation is generally found where a hydrodynamic condition, characterized by a sudden and gross change in hydrostatic pressure, exists. Because ebullition can occur the instant pressure drops, vapor bubbles form and collapse frequently and quickly.

Entrained air and dust particles in the fluid serve as nucleation sites for the formation of vapor cavities. These nuclei can be small gas-filled pockets in the crevices of the container or simply gas pockets on contaminant particles moving freely in the flow stream. Therefore, all confined fluids may contain sufficient impurities to produce cavitation.

Small voids near the surface or flow field, where minimum pressure exists, indicate that cavitation has begun. Once initiated, bubbles continue to grow as long as they remain in low-pressure regions. As the bubbles travel into high-pressure regions, they collapse, producing intense pressures and eroding any solid surfaces in the vicinity.

Equipment users can detect cavitation audibly, visually, by acoustical instrumentation, by machine vibration sensors, through sonoluminescence measurement or by a decrease or change in performance from that produced under single-phase flow conditions (for example, loss of flow, rigidity and response).

Under cavitating flow conditions, the wear rate can be many times greater than that caused by erosion and corrosion alone. Cavitation wear can destroy the strongest of materials - tool steels, stellites, etc. Such damage can occur rapidly and extensively.

The amount of damage that cavitation causes depends on how much pressure and velocity the collapsed bubbles create. As a result of this pressure and velocity, the exposed surface undergoes a variety of widely varying intensities.

Each imposition lasts only a short time; the impulse magnitudes and collapse times are greater for larger bubbles at given collapsing pressure differentials. Thus, the greater the tensile stress on the fluid (the lower the static pressure), the larger the bubbles, the more intense the cavitation and the more serious the damage.

The impulses that result when vapor bubbles form and collapse cause individual symmetrical craters and permanent material deformations when the collapse occurs next to the surface. Consequently, cavitation damage, like fatigue failure, has several periods of activity:

So the region where damage occurs is often quite separate from the region in which cavities are created - often leading to an incorrect diagnosis of the problem. Cavitation wear is mechanical in nature and cannot occur without the application of the tensile and compressive stresses.

In leakage paths (across seals, valve seats and spool lands) where high velocities cause pressure levels to drop below the vapor pressure of the fluid (a cavitation condition often referred to as wire-drawing) and

Cavitation disturbs the normal operating conditions of fluid-type mechanical systems and destroys the surfaces of components. The process consists of cavities forming when pressures are low, the growth of subsequent bubbles as pressure stabilizes and finally the collapse of the bubbles when the cavities (gaseous or vaporous bubbles) are exposed to high-pressure.

Note that the pressure drop across the component is the driving force for cavitation wear. Figure 2 depicts the cavitation process that occurs in a gear pump and in a spool valve showing how cavities generate, grow and collapse in fluid-type components.

In cavitation wear, microcracks propagate to the point where the material can no longer withstand the impulse load that the imploding vapor bubbles impose. Therefore, particles finally break off and enter the system.

Therefore, a rough surface is prone to cavitation wear and because pittings and a rough profile characterize the cavitation damage, the damage increases as the surface becomes rougher.

The most basic means of combating cavitation wear is to minimize the tensile stress on the fluid. In other words, the equipment users must lower the level of refraction or vacuum conditions in zones of possible cavitation. In particular, the following steps may be appropriate:

In many cases, design engineers can minimize cavitation damage by properly selecting fabrication materials. For example, stainless steel may be selected instead of aluminum (Figure 3) and use hard facing with a cavitation-resistant alloy on the exposed surface. Rubber and other elastomeric coatings have also helped minimize cavitation wear. Despite their low resistance to cavitation, these surfaces reflect the shock wave without causing intense damage.

The size of the particles generated by cavitation wear is a function of the Brinell hardness of the exposed material. The largest particles occur during the accumulation period. The slopes of the cumulative particle size distribution curves increase as the strain energy of the material increases. The average size of the particles produced by cavitation decreases as the cavitation intensity increases.

When investigating a cavitation problem in a fluid system, you must identify all possible sources of low pressure (vacuum), high temperature (heat), and locations where air might be ingressing. The following list should serve as a guideline for identifying low pressure areas in a fluid system:

Reservoirs - sites where mechanical (agitation) type air entrainment occurs, swirling fluid exists, fluid impingement on liquid or solid surfaces, pressurized reservoir conditions, cyclonic flow at pump suction port, critical altitude (angled reservoir) occurring during operation that exposes the pump suction port to the atmosphere, jostling of the fluid due to movement over rough terrain and/or low reservoir fluid level that expose the pump suction port to the atmosphere.

Pump - small diameter conduits and/or ports, restrictive flow passages, flow diversions, and/or long suction line conditions, poor pump filling characteristics (restrictive internal flow passages, high pumping speed, overly large flow displacement); altitude too high to provide sufficient reservoir pressure to supply the pump at rated flow conditions; inadequate suction head to lift fluid to pump inlet level (that is, elevation between fluid level and pump intake too great), insufficient suction head to accelerate reservoir fluid to the rated flow conditions of the pump (non-responsive to the pump displacement demands).

Cavitation is a condition which destroys the effectiveness and efficiency of hydraulic pumps. The attached picture shows classic cavitation in a vane pump. The material of the port plate has literally been carried away downstream with the hydraulic fluid leaving a gap through which oil can leak from the pressure side back to the suction side on the port plate sealing surface. This obviously makes the pump ineffective and the only solution is to remove and replace the complete vane cartridge.

The root cause is a restriction in the pump suction line. The normally entrapped air in the fluid is pulled out by the increased vacuum, forming bubbles in the suction line. When the air is passed from the low pressure side to the high pressure side of the pump. The air bubbles collapse and implode under the pressure, causing tiny explosions and intense heat which damages the metal port plate, sometimes taking the metal to the melting point. This continuous condition produces a high pitched whining noise from the pump and over time, with it’s integrity compromised, the metal is removed away with the hydraulic fluid leaving an empty pocket on the port plate. The constant repetition of this makes the hole grow and at some point renders the pump ineffective.

Aeration of the fluid can also cause cavitation, but in addition to the high pitched whining, there is also a more erratic noise, like a grinding or rumble. Aeration can be caused by a leak on a suction line allowing air to enter or it can also enter externally, like a return line not below the surface of the oil in the reservoir. With aeration you can visually see foam in the oil and the reservoir.

Repairing the pump is not the total solution. Any restrictions, kinked suction hose, plugged suction strainer, or leaks in the suction line or system allowing air in. Need to be remedied or the service life of the pump will be dramatically shortened again.

Hydraulic cylinder cavitation is a nightmare for any hydraulic system, leading to a cascading series of damaging issues that will eventually destroy critical (and costly) components within your hydraulic system. While most people associate cavitation issues with pumps and motors, cavitation can also be a problem for hydraulic cylinders. In order to minimize or eliminate cavitation damage, knowledge of how it works and what kind of damage it leads to are good starting points. However, the wisest approach to dealing with cavitation issues is to take preventative measures.

The simplest explanation of cavitation is the presence of gas entrapped within bubbles in a liquid. In the case of hydraulic systems and components like pumps or cylinders, the liquid involved is the hydraulic fluid transmitting pressure. In hydraulic cylinders, the pressure transmitted by the fluid is used for linear actuation that can move thousands of pounds at the press of a button. Obviously, very high pressures are involved.

When these bubbles collapse on themselves (or implode), the result is noise, heat, serious surface damage, and a negative impact on efficiency and productivity. Essentially, the bubbles generate a powerful mechanical shock that results in microjets impacting nearby metal. That implosion occurs when the bubbles reach an area of low pressure that causes the vapor within them to condense. The metal begins to wear away in a process known as cavitation erosion, cavitation wear, or cavitation pitting.

Cavitation can also occur when air is trapped within the hydraulic fluid (a phenomenon known as aeration). As pressure increases (e.g., as the rod closes in on the end cap in a hydraulic cylinder) the bubbles violently burst. In a hydraulic cylinder, cavitation most often occurs on the cap side of the piston when it is extending quickly, and especially when it is attempting to stop an over-running load. It also occurs when the rod is pointing downward while supporting a heavy tensile load on the end. Some experts would argue that this is not true cavitation, but its effects are the same.

One of the easiest ways to detect cavitation in a hydraulic cylinder is by sound: when cavitation bubbles implode, you can hear them in the form of an intense rattling sound. You will also notice pitting on the surface of parts affected by cavitation; in the case of hydraulic cylinders, it can be found on the surface of the rod and the inside surface of the cylinder.

Cavitation can lead to multiple problems within a hydraulic system as a whole, as well as hydraulic cylinders. These include accelerated wear of critical surfaces, seal failure, generation of unwanted heat, reduction in hydraulic oil quality, and lubrication issues.

One of the most serious problems caused by cavitation is surface wear, sometimes known as pitting. Keep in mind that the vapor bubbles implode at an almost molecular level, releasing a tremendous amount of force over an incredibly small area. That leads to high stresses — stresses high enough to remove metal from surfaces adjacent to those bubbles. The result is highly accelerated wear in the form of surface pitting and internally generated contamination. The metal particles removed from the surface will stay within the system and can cause the formation of even more cavitation bubbles.

Also, keep in mind that surface finish can be an important factor in many applications, and especially with hydraulic cylinders. When selecting an appropriate seal for a hydraulic cylinder, the surface finish of the rod is critical. If the rod is experiencing pitting due to cavitation, it will cause premature wear of the seal. As the seal begins to wear, hydraulic fluid can leak out (reducing performance) and environmental contaminants such as dust and moisture can make their way inside.

The bubble implosions also lead to high temperatures, sometimes reaching up to 5,000°F. In addition, as the surface is damaged, there will be increased friction as fluid flows over. That friction results in not only system losses and lower efficiency but also unwanted heat generation. This is one of the reasons why excessive heat can also be a strong indicator of cavitation.

Because of the high temperatures that can result from cavitation, the hydraulic fluid is very likely to suffer degradation (an overall reduction in fluid quality). Excessive heat and high temperatures will cause the fluid to age more rapidly than normal, seriously affect the additives present, and reduce the viscosity of the fluid.

The presence of bubbles within the hydraulic fluid can also cause a lack of lubrication, resulting in metal-to-metal contact and premature wear. This type of wear takes on a different form than that resulting directly from cavitation implosion and explosion: it will look like abrasions in a pattern consistent with the movement of the metal parts rather than pitting.

One of the means of preventing cavitation lies in temperature control. If the hydraulic fluid is too viscous, pressure drops will become more of a problem; this increase in viscosity can be the result of hydraulic fluid at a less than ideal temperature. In addition, high temperatures combined with low pressures can also cause cavitation, so care needs to be taken to ensure that the hydraulic fluid is as close as possible to an ideal operating temperature. In some instances, this can be as simple as insulating hydraulic pipes against direct sunlight.

Piping losses can contribute to cavitation and result from issues such as the use of too many fittings, a collapsed pipe liner or suction pipe, a gasket protruding into the pipes, a replacement pump that has too much capacity for the system, or the buildup of solids on the interior surface of the pipes.

Another way to prevent cavitation that is especially applicable to hydraulic cylinders is to prevent air from entering the hydraulic system. For example, hydraulic systems and some components should be bled properly after they are filled (or refilled) to release the air trapped within the system. When additional fluid is added to the system, it should be introduced gently to prevent splashing and agitation — both of which can allow air to be entrained with the fluid. In addition, improperly designed hydraulic reservoirs can inadvertently encourage the entrapment of air within the hydraulic fluid.

The presence of contaminant particles within hydraulic fluid can serve as a starting point for cavitation bubbles to form, so keeping the hydraulic fluid clean and the system free of contaminants can be an excellent starting point. This involves changing hydraulic filters as needed, filtering any hydraulic fluid that enters the system, and addressing leaks as soon as they are detected.

When parts have experienced surface damage due to cavitation, repairs are often a lost cause. For parts with a circular geometry, such as rods and cylinders, the pitting damage is often too deep by the time it is discovered for polishing to work because rods and cylinders must meet very strict surface conditions and geometric tolerances. Unless the damage was discovered quickly, it would be wiser to replace the damaged components. For seals that have been damaged as a side-effect of cavitation, replacing the lip of the seal may be enough.

Cavitation causes so much damage: accelerated surface wear in the form of pitting, premature seal failure, generation of unwanted heat, compromised hydraulic fluid quality, and lack of lubrication leading to additional wear. The best way to deal with cavitation is to take measures to prevent it because once it wreaks havoc on the surfaces of equipment (including hydraulic cylinders), it can be extremely difficult if not impossible to repair. Preventative measures against cavitation are the wisest course of action and can often include basic maintenance principles (e.g., bleeding air out of the system, filtering hydraulic fluid). As soon as hydraulic cylinders start making rattling noises or become hotter than normal, it is time to get them checked out.

At MAC Hydraulics, our highly skilled team can maintain, troubleshoot, and repair hydraulic systems and components, including hydraulic cylinders. Our repair services include replacing seals, polishing rods, and honing tubes — and we also have a 24-hour resealing pick up and delivery service. MAC Hydraulics can manufacture custom cylinders, tubes, and rods with our state-of-the-art fabrication facilities and service any brand equipment from industries including construction, recycling, manufacturing, rental, aviation, and waste handling. If you are experiencing symptoms that are consistent with cavitation, we will help you track down the cause and keep it from happening again.

Frequently occurring in pumping applications, cavitation creates bubbles or vapor cavities in a liquid as a result of rapid changes in pressure. These liquid-free voids typically form in low-pressure zones and can burst when subjected to high pressures, sending powerful shockwaves throughout the entire application.

Manufacturers in the chemical processing, food processing, and petroleum industries must consider the risk of cavitation when designing machinery in order to avoid unwanted noise, vibration, and component damage.

Cavitation decreases an application’s efficiency over time and puts repeated stress on critical pump parts, shortening their overall lifespan. Shockwaves can cause significant damage to the pump, which in turn leads to premature valve failure, decreased flow pressure, and, ultimately, breakage. If you’re experiencing any of these issues with your pumping application, our in-house pump experts can help.

There are two types of cavitation that may occur in reciprocating positive displacement applications: suction and discharge. Suction cavitation occurs ahead of the suction stroke, when the pump is starved of flow, either from being in a high-vacuum or low-pressure environment. The opening of the valve is delayed by inertia, causing a lower flow rate on the suction side and resulting in expansion, pressure decrease, and the formation of bubbles close to the plunger.

Discharge cavitation occurs when the pump’s discharge pressure is too high. Under these conditions, it’s difficult for the fluid to flow out of the pump. Instead, it continues moving at high velocities inside the working chamber, forming bubbles in the process.

When working with pumping applications in a processing industry, cavitation should always be kept in mind; being able to recognize the warning signs and identify the root causes of cavitation in your machinery can significantly reduce the risk of long-term damages, saving both time and money.

Triangle Pump has nearly a century of experience assisting clients with pump issues such as cavitation. Our pump components are designed to preserve expensive parts such as crankshafts and power frames by transferring the majority of wear to less expensive, more expendable parts such valves, plungers, and packing.

While it’s common for people to think of a pump’s inlet as sucking in oil, in reality, it is atmospheric pressure doing the work. In essence, the weight of the atmosphere pushes the oil out of the reservoir and into a region of lower pressure—the inlet. Once the oil is forced from the reservoir and through the inlet, it then moves the volume of liquid into a region of decreasing volume to create flow. For this process to begin, there must be minimum pressure at the inlet of a hydraulic pump, as shown in the diagram below.

As you can see, the inlet of a pump plays a large role in how well it operates. Unfortunately, those designing and maintaining pump systems can become so focused on downstream flow that they overlook proper inlet maintenance. This can result in degradation of inlet function and serious problems such as cavitation.

Cavitation occurs when the absolute pressure on the inlet side of the pump is too low and air is drawn out of the solution, creating bubbles in the oil. As these bubbles get pushed around to the high-pressure outlet side of the pump, they collapse. This creates localized shock waves that blow bits of material out of the pump. It can also result in excessive heat and reduced lubrication that leads to friction and wear over time.

Cavitation can cause pump failure and it can damage other components of your system, which is why it is critical to examine the condition of the pump"s inlet on a regular basis.

PSI versus PSIA. What’s the difference? PSI, or pounds per square inch, is a unit of measurement for pressure used in the United States. PSIA describes the absolute pressure in psi, including the pressure of the atmosphere. Absolute pressure is also referred to as total pressure.

The energy it takes to lift oil through the suction line (including pressure drop due to flow). We refer to this action as Phase 1 pressure, because it represents the amount of energy it takes to accelerate the fluid through the pumps internal pathways and keep the pump full.

In order for a pump to function, atmospheric pressure must be greater than Phase 1 pressure + NPSH. Every pump has its own specifications regarding acceptable minimum/maximum inlet pressure, but we can use the example below to illustrate how to calculate it.

To begin, assume that you are maintaining an 18 GPM hydraulic pump. The NPSH is equal to 12 PSIA with standard hydraulic oil and 1800 RPM per manufacturer’s specifications.

If we apply the same numbers to a variable-volume pump, the result will be less acceptable. Here’s why: Imagine the pump is not in demand and is therefore being held off stroke, meaning there is no flow. When the pump is suddenly needed, it will come on stroke, requiring the column of oil in the suction line to accelerate. This sudden change in demand requires the pump pressure to accelerate from static to a pressure that is strong enough to move the oil and prevent cavitation.

Assume the pump strokes on in 70 milliseconds (msec). The volume of liquid that has to accelerate is 1.5in^2 x 18.1” = 27.1 cubic inch (cu. in.). Note: Since the entire column of oil in the pipe has to accelerate, we used a measurement of 18.1” instead of 12.1”.

These calculations reveal that the system is fine, because the pump requires a minimum 12 PSIA at its inlet to operate effectively. However, if this system was installed at 2,300 feet above sea level (13.4 PSIA), the pump would cavitate whenever it came on stroke.

The above example assumes no losses due to other plumbing, however, it is not uncommon to see elbow fittings on pump ports, which could add to losses in the inlet line.

In addition to maintaining good inlet conditions, any small leak on the inlet will entrain air, which is also bad for the pump. Small leaks will cause a pump to lose prime whenever the system is shut off, which means it will start dry and run dry until prime is re-established. For this reason, it is never a good idea to install hydraulic pumps above the fluid level. Rather, hydraulic systems designs should ensure that the pump inlet is flooded, i.e. the oil level is above the pump inlet. A ball valve can be used to isolate the pump from the reservoir in case it needs service, and a limit switch can be used on the ball valve to prevent the system from running if the ball valve isn’t fully open.

Most modern hydraulic pumps are reliable, robust pieces of equipment that will withstand years of constant service. However, any hydraulic pump can suffer from mechanical issues. Aeration and cavitation are two serious problems that can affect a hydraulic pump, and either problem can cause serious damage if the issue is ignored.

However, it can be difficult to figure out if a malfunctioning pump is suffering from aeration or cavitation, as the two problems tend to produce similar symptoms. Despite these similarities, aeration and cavitation have different causes and require different solutions, so knowing the difference between the two problems is vitally important knowledge for any hydraulic pump user.

All hydraulic pumps contain a small amount of air, which provides space for the pump"s hydraulic fluid to expand into as it heats up. However, excessive amounts of air in a hydraulic system can cause serious problems.

Aeration in hydraulic pumps occurs when air is drawn into the pump"s hydraulic fluid supply. Air can infiltrate your system through perished pump seals, damaged pipe connectors, leaking suction lines, and other forms of damage. This unwanted air mixes with and dissolves into the hydraulic fluid supply.

Hydraulic fluid that has been contaminated by exposure to air loses many of its functional properties. For example, hydraulic fluid containing excess air cannot conduct heat as efficiently, so aeration can cause rapid overheating in affected pumps and other hydraulic systems. Serious aeration can also impede the pump"s stroke cycle, causing a significant drop in usable power.

For a hydraulic pump to function properly, the flow of fluid from the pump"s reservoir to the pump itself must remain constant and unimpeded. If this flow speeds up or slows down for any reason (such as improper pressure settings or a block in the suction line), this can lead to a potentially catastrophic problem known as cavitation.

When a running hydraulic pump does not receive hydraulic fluid at the proper rate, fluid pressure in the pump"s suction line can drop dramatically. This causes the formation of bubbles inside the hydraulic fluid, which contain air and vaporized hydraulic fluid. These bubbles subsequently collapse, creating an incredible amount of force and heat.

The shockwaves created by these imploding bubbles can do an incredible amount of damage to a hydraulic pump in a short space of time. The pump"s impellers and interior surfaces tend to be particularly badly affected by cavitation, and a pump that has suffered from extensive cavitation may need a complete rebuild before it can be used again.

While cavitation and aeration have different causes, they can produce similar problems, making proper diagnosis difficult. High hydraulic fluid temperatures, lowered pumping rates, and visibly damaged valve seals can all be caused by either cavitation or aeration.

Generally, the easiest way to tell the difference is by listening to the malfunctioning pump, as both types of problem usually produce distinct noises. A cavitating pump tends to produce a steady, rhythmic pattern of knocking or whining noises. Unusual noises produced by aeration are more erratic and random and tend to sound more like rattling than knocking.

However, it is important to note that cavitation and aeration are not mutually exclusive problems, and a poorly maintained hydraulic pump may suffer from both problems simultaneously. Combined aeration and cavitation can lead to a particularly damaging form of cavitation, known as gaseous cavitation.

If your hydraulic pump is suffering from any signs of cavitation or aeration, calling in a professional hydraulic repair service is usually the fastest way to get to the bottom of the problem. These services have access to specialized testing tools and equipment that can quickly detect cavitation and aeration and will be able to provide rapid and effective solutions.

If your hydraulic pumping system is not functioning as it should, and you think cavitation or aeration may be the cause, contact the hydraulic experts at Quad Fluid Dynamics, Inc., for professional advice.

Two leading causes why hydraulic pumps usually fail are: (1) contamination and (2) cavitation. In order to prevent any potential damage to your entire hydraulic system, it’s imperative to understand cavitation, the indications or symptoms from your system it is occurring, as well as the preventive measures.

How does cavitation happen exactly? It starts when vapor bubbles in the oil are created due to high vacuum. When these vapor bubbles are carried and collapsed on the pump outlet (discharge side), cavitation happens.

Make Sure Oil flow Paths are Straight – Hydraulic pumps are being supplied via atmospheric pressure and gravity, so it’s ideal to place the reservoir above it. Make sure that the path is as straight and vertical as possible. Keep an eye on bent or twisted supply hose.

Check Laminar Flow – If you’re hearing turbulence or noise in pump inlet, make sure that the supply hose length is the correct ratio to its diameter. A flange-style, flared fitting in the pump inlet can also help in eliminating pump noise.

Check Proper Viscosity – It"s important to c