what causes hydraulic pump cavitation free sample

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.

Bubbles might not seem very powerful, but the types of bubbles in pumping systems are nothing like the ones you make by waving a wand around with little kids. Tiny bubbles created by changes in pressure inside pumps collapse and create shock waves that occur over and over and the repeated shocks erode the components.

Pumps are designed to work with a full flowing water supply, but in some cases a flooded inlet is not enough to maintain pressure required to prevent cavitation. The inlet, or suction side of a pump is the point of lowest pressure in a given pump.For positive displacement pumps, the lowest pressure occurs just prior to rotor meshing; for centrifugal pumps, lowest pressure is near the eye of the impeller.

Cavitation is possible in all pump types and since its principles are essentially the same, we will focus on centrifugal pumps. The eye is where fluid is drawn into the impeller and where the rotation of the impeller begins to act on the fluid. When pressure acting on the liquid (Net Positive Suction Head Available) is too low, bubbles form, and as the liquid accelerates because of impeller rotation, pressure increases and the bubbles collapse.

Under normal atmospheric pressure conditions, fluids have predictable vapor pressure. As the pressure inside the pump falls below the liquid"s vapor pressure, bubbles form. The bubbles collapse when they reach areas of the liquid where the pressure is above the vapor pressure. In the case of cavitation, this formation and collapse is both rapid and violent.Disrupted or poorly executed processing lines can cause suction or discharge pressure to fall, which leads to cavitation.

At extremely high discharge pressure, some fluid circulates inside the pump instead of discharging. Fluid trapped between impeller and housing at very high velocity cause a drop in pressure, creating the same conditions as for suction cavitation.

Cavitation sounds like marbles or gravel circulating through the pump, pipes, or hoses. The effects of prolonged cavitation are visible on the pump impeller and other components.

Start by identifying the cause of the pressure drop. In many cases moving the pump closer to the fluid source and removing as many bends and valves as possible corrects the problem because each component causes additional pressure drop. When suction lift is too high to maintain pressure, move the pump closer to the fluid source or move the fluid source closer to the pump.

Enlarging suction lines can also be effective. In some obvious cases, a blockage occurs in piping or hoses near the pump. Clear those blockages to resolve the issue.Clean suction lines by clearing debris. Avoid blowing the debris back toward the fluid source because it’s likely to create a blockage again.

Don’t exceed your pump manufacturer’s performance guidelines. Pump curves tell you how much net positive suction head the pump requires, so check your pump’s performance curve to ensure it has the right specifications for your application.

The best way to prevent cavitation is to select the right pump for the application. Cavitation increases as pump head falls or as capacity increases, so selecting the correct pump to maintain a positive margin of NPSHa above NPSHr is the best first move.

NPHS at the inlet depends on atmospheric pressure, friction losses in the suction piping, and flow velocity. A good rule of thumb is for pressure at the pump inlet to be 10% greater than the pump"s specified NPSHr. For example, if NPSHr is 10 feet, NPSHa should be at least 11 feet.

Discharge cavitation occurs when pressure at the discharge end of the pump is too high. High discharge pressure limits the volume of fluid flowing out of the pump, causing high-velocity fluid to recirculate between pump impeller and housing, causing cavitation.

Check filters and strainers. Dirty or blocked filters and strainers generate pressure buildup inside the pump. Setting a maintenance schedule ensures that systems are in place to keep the pump system flowing at capacity.

Evaluate the curve. Consider the job pressure demands and then consider the pump data to see if it fits the application. From there you determine if the pump fits the needed flow rate.

The best ounce of cavitation prevention is pump selection and system design for maintaining pressure and flow. The goal of installation is therefore to maintain net positive suction head available (NPSHa) at greater than net positive suction head required (NPSHr) by considering four key variables:

Physically install the pump so the water flows into the pump suction inlet smoothly.Make sure that the suction lines leading to the inlet of the pump are adequately sloped to ensure that the pump housing is flooded.

Placing the pump at a point that is lower than the water level in the tank from which it pumps, for example, uses the force of gravity to maintain flooded suction, which in many cases prevents cavitation.

Pumps, and especially centrifugal pumps, work most efficiently when the fluid travels in a smooth, laminar flow, and turbulence of any kind reduces pump efficiency, so positioning the pump as close as possible to the fluid source makes sense.

In general, you want 12 cm of straight pipe for every centimeter of pump suction diameter.To maintain laminar flow, connect 5-10 pipe diameters of straight piping to the pump inlet. Do not include elbows, reducers, valves, or strainerswithin the final length of pipework. Connecting an elbow directly to the pump flange, for example, draws fluid towards the outer curve of the elbow instead of directly into the eye of the impeller.

Also, the piping arrangement must not cause strain on the pump casing, so pumps can never support the piping for suction or discharge. Use hangers and supports instead.

As fluids pass through a pipe, friction between the fluid and the inside surface of the pipe causes turbulence, which slows the fluid and results in a pressure drop. Pipe length, diameter, and flow rate all affect friction loss.

Good piping layouts prevent cavitation by helping to maintain constant velocity. Obstructions in piping layouts affect flow velocity, which changes fluid pressure, which can cause cavitation.

While it’s true that some cavitation can have positive purposes, such as for surgical equipment sterilization or to break down pollutants in water systems, it’s not something you want in your processing system, so time spent preventing cavitation is time well spent.

When your pump, pipe, or hose sounds like marbles or gravel are circulating, you"re witnessing cavitation and need to take immediate action or risk severe damage to components. When cavitation does occur, you need a trusted partner that can diagnose the cause, provide a long-term solution, and repair or replace parts that are damaged.

CSI"s pump service and maintenance program is designed to take the irritation and guesswork out of dealing with pump cavitation. Each audit and repair that CSI completes include an evaluation by an OEM-trained pump technician, a report on the findings, and all the materials required to perform the service. Call us today to schedule your next system audit or pump repair!

Central States Industrial Equipment (CSI) is a leader in distribution of hygienic pipe, valves, fittings, pumps, heat exchangers, and MRO supplies for hygienic industrial processors, with four distribution facilities across the U.S. CSI also provides detail design and execution for hygienic process systems in the food, dairy, beverage, pharmaceutical, biotechnology, and personal care industries. Specializing in process piping, system start-ups, and cleaning systems, CSI leverages technology, intellectual property, and industry expertise to deliver solutions to processing problems. More information can be found at www.csidesigns.com.

This guide is intended for engineers, production managers, or anyone concerned with proper pump selection for pharmaceutical, biotechnology, and other ultra-clean applications.

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.

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 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.

Pumps are designed to produce enough flow and pressure to move a liquid from one location to the next. Sounds simple enough, but the internal system of a pump is complex. When you combine the complexity of the pump system with the wide-ranging properties of various fluids, the surrounding environment, and other factors, cavitation can occur.

Pump cavitation is a potentially damaging problem in pumps that are not properly configured or being used for their intended application. Here, we’ll explore what causes cavitation in pumps, the signs to look for, and ways to prevent it from happening.

Pumps are designed to pump liquids but, when the combined flow rate and pressure are inadequate or not conducive to the type of liquid being pumped, pockets or cavities can form, resulting in cavitation.

Some describe pump cavitation as the creation and collapse of the air bubbles in a fluid. While they may appear to look like air, those bubbles are technically a cavity, gaseous vapor, or vacuum. While cavitation is possible in all pump types, it is more common in centrifugal pumps where the bubbles quickly develop around the impeller’s axis. When the bubbles pass from the middle of the impeller to the outer edges, the centrifugal force creates higher pressure, causing those bubbles to quickly collapse or implode with great force.

Pump cavitation is the rapid succession and released energy of the implosions of gaseous cavities which cause an intense rattling sensation that can damage a pump.

Various conditions can make a pump more prone to cavitation. A clogged filter or strainer, for example, can easily restrict flow and lead to pump cavitation. Similarly, a restricted or flimsy inlet hose might collapse and cause issues. Fluid viscosity is a major contributor, too, especially when combined with the wrong hose. Imagine drinking a milkshake through a thin straw: the combination of the thick viscosity, pressure, and weak structure of the straw causes it to collapse and restricts the flow. Or consider if that straw has a pinhole in it; the leak would also affect flow and pressure. Similar phenomena can occur in pump systems.

That said, the inlet supply hose can’t be too rigid either. If you use a solid metal hose or a system is plumbed with hard PVC or copper piping, it could cause water hammering. Finding the right combination of dampening with a strong yet soft inlet hose or flexible PVC is ideal. Here at Pumptec, we use pulse hoses with metal springs inside them to add strength and rigidity. They’re less likely to collapse yet still offer the right level of flexibility.

The position of the reservoir tank also makes a difference. If the tank is positioned below the pump, the pump will need to decrease pressure to draw the fluid vertically through the inlet piping. The longer the inlet hose and the farther the vertical distance between the tank and the pump, the greater the chance of creating a vacuum and cavitation.

Heated liquids are a major contributor to cavitation, too, especially as the hot fluid approaches the boiling point and creates additional vapor pressure. In this instance, the liquid needs to be fed/pushed through a pump rather than drawn/pulled through it. The easiest way to achieve this is to have the tank containing the heated fluid elevated above the pump so that it is gravity fed into the pump system.

In all these instances, the flow is being disrupted or poorly executed, causing the discharge pressure to fall. The pump is basically being starved of fluid, resulting in cavitation.

Obvious signs of pump cavitation are excessive noise and vibration. The implosions and released energy within the system produce a loud growling sound, or it may sound and feel like gravel is circulating through the system.

Cavitation can damage seals, O-rings, and bearings, resulting in leaks and loss of pressure. On a centrifugal pump, the constant force of the implosions can erode the impeller and pump housing. Because of the added strain, the pump will also consume more power than it should.

On a positive displacement plunger pump, cavitation can erode seals and the metal around the poppet and seat on the check valves. If the pump body is made of anodized aluminum, which appears as a black coating, the black anodization will wear away upstream from the outlet check valve. In extreme cases, wear will occur outside of the main seal. If that happens, the pump will start sucking air, and bubbles — sometimes called washout — will form on the outlet.

Many of these issues can be prevented with proper maintenance and by selecting a pump that is designed for the intended use, accommodates the viscosity of the fluid being pumped, and has the proper configuration.

Some people think they just need to increase the size of the inlet hose. While it’s true that a hose that’s too small could cause cavitation, having a hose that’s too large could cause priming problems.

Centrifugal pumps have such high flow rates that it’s difficult to control their output, plus they have a lot of back pressure downstream that affects the amount of flow. Because of their lower flow rates, plunger pumps typically have fewer cavitation issues.

All in all, the pump pressure must be maintained above the liquid"s vapor pressure to avoid cavitation. In the end, the best tip for reducing cavitation is to work with a pump manufacturer to determine the proper use and placement for your application.

Contact the pump experts at Pumptec to discuss your pump challenges and how to overcome them with a solution that’s customized to your application. Get a head start by checking out our

As pressures and temperatures fluctuate in a hydraulic system, the fluid will absorb gas or release dissolved gases to main this equilibrium. The release process is gaseous cavitation.

It’s important to understand that the bubbles are not a result of a phase change. Unlike vaporous cavitation, there is no final gas-to-liquid phase change. Thus, the material erosion caused when vapor bubbles violently collapse does not occur with gaseous cavitation.

It is important to understand that aeration and cavitation are not the same thing, but a relationship exists between them. Aeration simply refers to a presence of air in a liquid.

Vaporous cavitation is not related to aeration. Again, the bubbles created by this process are simply a liquid-to-vapor phase change—they do not contain any air.

If free air in the crankcase becomes bound to the oil (by agitation, for example), the resulting entrained air bubbles may be unable to escape before reaching the suction inlet of the oil pump. These air bubbles will diffuse into the oil due to the higher pressure on the discharge outlet. As the pressure drops through the oil galleries and return to the sump, the air bubbles will re-emerge through the process of gaseous cavitation.

When a hydraulic system fails, finding the source of the problem can be a challenge. Though hydraulic systems primarily consist of a sump, motor, pump, valves, actuators and hydraulic fluid, any of these parts could be the source of failure. That"s not to mention the additional potential for failure through human error and faulty maintenance practices. If your system fails, you need to know why it fails, how to find the failure and how to keep it running smoothly in the future, all while keeping personnel safe.

It"s often easy to tell when a hydraulic system fails — symptoms can include high temperatures, low pressure readings and slow or erratic operation are glaring problems. But what are the most common causes of hydraulic systems failures? We can trace most hydraulic issues back to a few common causes, listed below.

Air and water contamination are the leading causes of hydraulic failure, accounting for 80 to 90% of hydraulic failures. Faulty pumps, system breaches or temperature issues often cause both types of contamination.

Air contamination is the entrance of air into a hydraulic system and consists of two types — aeration and cavitation. Both can cause severe damage to the hydraulic system over time by wearing down the pump and surrounding components, contaminating hydraulic fluids and even overheating the system. Although we are not pump manufacturers, we know it is essential to be aware of these types of contamination and how to identify their symptoms.

Cavitation:Hydraulic oil consists of about 9% dissolved air, which the pump can pull out and implode, causing pump problems and damage to the pump and to other components in a hydraulic system over time. You can identify this problem if your hydraulic pump is making a whining noise.

Aeration:Aeration occurs when air enters the pump cavity from an outside source. Usually, loose connections or leaks in the system cause this issue. Aeration also creates a sound when the pump is running, which sounds like knocking.

Water contamination is also a common problem in hydraulic systems, often caused by system leaks or condensation due to temperature changes. Water can degrade hydraulic components over time through oxidation and freeze damage. A milky appearance in hydraulic fluid can help you identify water contamination.

Fluid oxidization: Extreme heat can cause hydraulic fluid to oxidize and thicken. This fluid thickening can cause buildups in the system that restrict flow, but can also further reduce the ability of the system to dissipate heat.

Fluid thickening:Low temperatures increase the viscosity of hydraulic oil, making it harder for the oil to reach the pump. Putting systems under load before the oil reaches 70 degrees or more can damage the system through cavitation.

Fluid levels and quality can affect hydraulic system performance. Low fluid levels and inappropriate filtration can result in air contamination, while fluid contamination can cause temperature problems. Leaks can further exacerbate both issues.

Using the correct type of fluid is also essential, as certain hydraulic oils are compatible with specific applications. There are even oil options that offer higher resistance to temperature-related problems. Some oils even offer anti-wear and anti-foam additives to help prevent against wear and air contamination, respectively.

Human error is the base cause of many hydraulic system problems. Some of the most common errors that may result in your hydraulic pump not building pressure include the following.

Faulty installations: Improper installation of any component in a hydraulic system can result in severe errors. For example, the pump shaft may be rotating in the wrong direction, negatively affecting pressure buildup, or pipes may be incorrectly fitted, resulting in leaks.

Incompatible parts: An inexperienced installer may put mismatched components together, resulting in functional failures. For example, a pump may have a motor that runs beyond its maximum drive speed.

Improper maintenance or usage:Using systems outside their operational capabilities or failing to perform regular maintenance are some of the most common causes of hydraulic system damage, but are easy to rectify through updated maintenance policies and training.

The sources of system failures can be tricky to identify, but some hydraulic troubleshooting steps can help narrow down the options. So how do you troubleshoot a hydraulic system? Here are some of the fundamentals.

Check the pump: Take the pump assembly apart and assess all parts to ensure that they are functional and installed correctly. The most common problem areas include the pump shaft, coupling and filter.

Check the fluids:Check the level, color and viscosity of the hydraulic oil to ensure it meets specifications and has not become contaminated. Low hydraulic fluid symptoms include pressure or power loss. When in doubt, drain and replace the fluids.

Check the seals: Look for evidence of any fluid leakage around your hydraulic system"s seals, especially the shaft seal. Leakage can indicate worn-out or blown seals that can cause malfunctions with pumps, motors and control valves.

Check the filters: Ensure filters are clear of plugs and blockages. Common clogged hydraulic filter symptoms include sluggish operation and noisy operation.

Hydraulic system issues are inevitable at some point. However, simple steps can help you avoid these issues and increase the longevity of your hydraulic system. On top of effective troubleshooting, you can prevent hydraulic system failure by taking the following steps.

Follow specifications: We can trace the most common hydraulic system issues back to fundamental system problems like incompatible or improperly installed parts. For this reason, it"s essential to always double-check specifications to ensure your purchased parts can work together seamlessly.

Consult with professionals: When purchasing new equipment, consult with industry peers and professionals to discover what they recommend. While manufacturers can tell you how a product should work, industry professionals can provide concrete examples of how well the equipment works for their industry.

On top of these steps, look into hydraulic system products that are specifically designed to help prevent failures. One such product is Bear-Loc® by York Precision. This innovative locking actuator is a safe, reliable feature for hydraulic components, automatically locking when sleeve pressure is relieved, preventing movement if a hydraulic system fails. This way, your can protect your personnel from injuries related to hydraulic failures. Even better, York Precision offers in-house design, engineering expertise and machining and manufacturing capabilities to produce a hydraulic locking device that meets your exact specifications.

Regularly review hydraulic system maintenance, always following manufacturer recommendations and industry best practices. Also, consider the storage condition, external influences, working pressure and usage frequency of your system to tailor your maintenance schedule and procedures.

Daily tasks:Take care of a few simple daily checks to avoid issues. For example, personnel should check the oil levels, hoses and connections and listen to the pump for abnormal sounds.

Be mindful of location:Do not stand at endpoints while working on hydraulic systems. This safety measure can help prevent loss of limb and life, as there is a lot of pressure built up in these areas that can release and result in life-threatening situations.

The best safety measures, however, are to perform excellent maintenance and use high-quality parts. If you"re looking for a quality hydraulic component manufacturer, York Precision Machining & Hydraulics can help.

Cavitation is defined as the process of formation and disappearance of the vapor phase of a liquid when it is subjected to reduced and subsequently increased pressures at constant ambient temperatures. The formation of cavities is a process analogous to boiling in a liquid, although it is the result of pressure reduction rather than heat addition. Nonetheless, the basic physical and thermodynamic processes are the same in both cases. The entire process of cavitation is governed by fluid inertia (fluid dynamics) and heat and mass transfer (thermodynamics).

Clearly, from an engineering and design perspective there are two basic questions regarding cavitation. First, one has to answer the question whether cavitation will occur or not, and second, if cavitation is unavoidable, the question is whether a given design can still function properly. Economic or other operational considerations often necessitate operation with some cavitation, and under these circumstances it is particularly important to understand the (deleterious) effects of cavitation.

The potential for cavitation when dealing with pumps is typically evaluated by a parameter called Net Positive Suction Head (NPSH). This is defined as the total head of the fluid at pump suction (HS,T) above the vapor pressure of the fluid (in terms of head, HV): NPSH = HS,T – HV. It can be seen as a measure expressing the margin against vaporization of the fluid entering the pump.

Using a parameter like NPSH one can define various critical values corresponding to certain stages of cavitation or cavitation phenomena. Typically, this includes – but is not limited to – cavitation inception, head drop inception, percentage head drop, and performance breakdown.

The first appearance of cavitation when gradually lowering pump suction pressure from a level high enough to suppress cavitation is called cavitation inception. When pump suction pressure – or NPSH – is decreased further from this inception level, the region of cavitation enlarges, eventually starting to cause fluid-born noise (audible and/or non-audible), performance change (head loss), and finally full head breakdown.

By the time the inlet pressure is lowered enough to cause a certain percentage drop in pump head, cavitation is always fully established. Before reaching that stage there is already a significant amount of cavitation without the pump head being affected by it. This is illustrated by Figure 1, which shows so-called head drop curves. In a curve like this pump head is plotted against NPSH for constant fl ow rate (Q) and constant speed (N). For multi stage pumps one would plot the head of the suction stage.

NPSH required (NPSHR) constitutes the condition of having a specific amount of cavitation being developed inside the pump, or a specific cavitation criterion being established. This implies that a statement on NPSHR is meaningless without specifying the associated condition. NPSHR for a particular condition is a function of volumetric flow rate and speed of the pump.

Historically, the condition of three percent head drop is taken as the NPSHR for (rotodynamic) pumps. Unfortunately, this has led to a lot of misconception since many interpret NPSHR as the one needed to run free from cavitation, while in reality it constitutes the NPSH needed to have the pump cavitating to such an extent that it loses three percent of its head. Current practice of using NPSH3 remedies this misconception.

Other typical and popular NPSHR criteria include: NPSHi for cavitation inception, NPSH0 for the starting of head drop and NPSH1 for one percent head drop. NPSH0 is sometimes termed “incipient NPSH”, which should not be confused with NPSHi.

NPSH should always be reference against a datum plane (elevation). Typical NPSH datum planes include: i) horizontal plane thru impeller (shaft) centerline; ii) top of foundation; iii) impeller eye for vertical pumps or horizontal plane thru suction flange centerline for vertical can pumps. When changing from one NPSH datum plane to another the NPSH needs to be corrected for the static height and any head loss between the two datum planes (Figure 2).

Figure 3 shows typical NPSH characteristics that can be identified for centrifugal and axial-flow pumps. In this figure NPSHA is the NPSH available as provided by the system. Beyond the so-called shockless-entry capacity—or best cavitation point (BCP)—the NPSH3 is seen to follow the steep rise of the inception curve (NPSHi). Below QBCP the inception curve shows an absolutely striking departure from the shape of the conventional NPSH3 curve. Instead of decreasing with decreasing capacity, it rises until a local maximum is reached. This local maximum is of particular importance since it identifies the onset of suction recirculation. It can further be seen that a cavitation free region of operation will exist for those capacities where NPSHA > NPSHi. Outside this region the pump will cavitate and the extent of cavitation will depend on the operating capacity of the pump.

Figure 4 shows an example of an impeller with cavitation damage. Cavitation damage starts somewhere beyond inception and will disappear near head break-off , with maximum erosion rate occurring somewhere in between, and often marginally before NPSH3 (Figure 5). A more accurate description is difficult to give since many parameters influence bubble geometry and its potential for causing damage. For instance, impeller material, air content, NPSH available, vane geometry, inlet geometry, type of cavity, fluid density, and fluid temperature, to name a few, can be contributors or inhibitors of cavitation damage. Furthermore, with maximum erosion rate occurring marginally before NPSH3 one need to be cautious about specifying the NPSH margin (i.e., NPSHA-NPSH3). Without given it proper thought, the NPSH margins suggested in literature may result in running the pump exactly near maximum erosion rate, if cavitation erosion actually occurs.

In order to have cavitation erosion the cavitation vapor pockets need to implode on or near a material surface, and the energy release must exceed the cavitation resistance of the surface material. Since the latter is hard to quantify many experimental and semi-empirical studies have attempted to correlate between cavity shape and damage potential, and research in this field is still ongoing. The only certainty is that the absence of visible cavities means that cavitation damage will not be an issue. This fact is used in some conservative designs, such as liquid sodium pumps, and some water injection applications, where the NPSH available is high enough to suppress cavitation. The occurrence of cavitation erosion is also less of a worry when pumping hydrocarbon mixtures due to the absence of a vapor pressure. The vapor-liquid region of such mixtures, enclosed by the bubble and dew pressure, prevents any violent (high energy) cavity implosions.

Suction specific speed (S or NSS) is often seen as the key indicator determining the susceptibility to cavitation. This, however, causes a lot of misunderstanding and misinterpretation of the concept of suction specific speed. To understand this, one most realize that it is a quantity that originates from scaling model pumps and prototypes, and that is has a clear definition: suction specific speed is the running speed (RPM) of a geometrically identical impeller/pump scaled at a (suction) specific diameter, such that it will have an NPSH of 1 m [1 ft] at a flow rate of 1 m3/h [1 USGPM]; as such (by similarity) the well-known equation to calculate suction specific speed is readily found: NSS = N Q1/2 / NPSH3/4.

Suction specific speed is a relative index number that should be used and judged with extreme caution. In order to have some consistency the widespread rule is that it should be evaluated at pump peak efficiency capacity, with maximum diameter impeller fitted in the pump. Although this leads to a workable definition in practice, it has some serious drawbacks: i) The volute or diff user characteristic will greatly determine the peak efficiency capacity of the pump, and ii) for multi stage pumps the series stages may have a peak efficiency capacity quite different from the first stage. This makes S or NSS very sensitive to the construction of the entire pump whereas it should only reflect the suction capabilities. A way to overcome this objection is to evaluate S or NSS at the so-called shockless entry capacity of the (suction) impeller.

Lastly, it is paramount to understand that suction specific speed is not an indicator to warrant healthy pump selection or operation in relation to NPSH. Unlike common belief it is not a limiter. At best it can be a caution sign if it is outside the ballpark.

Frank Visser is Principal Engineer at Flowserve, Global Engineering Services, in Etten-Leur, The Netherlands. He joined Flowserve in 1995 and has held several positions in research, development, and (product) engineering. His key expertise and interests relate to fluid mechanics, CFD and thermodynamics of (centrifugal) pumps and hydraulic turbines. He has authored & co-authored multiple technical papers in journals and proceedings, and lectured at various symposia. He has a BS and MS in Mechanical Engineering, and a PhD in Technical Sciences. He has received the ASME 2017 Sankaraiyer Gopalakrishnan-Flowserve Pump Technology Award, is a member of the Industrial Advisory Board for the J.M. Burgers-centrum (JMBC), National Research School for Fluid Mechanics in the Netherlands, and a former Associate Editor for ASME Journal of Fluids Engineering.

Cavitation is a phenomenon in which the static pressure of a liquid reduces to below the liquid"s vapour pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal, causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation.

The process in which a void or bubble in a liquid rapidly collapses, producing a shock wave, is called inertial cavitation. Inertial cavitation occurs in nature in the strikes of mantis shrimp and pistol shrimp, as well as in the vascular tissues of plants. In artificial objects, it can occur in control valves, pumps, propellers and impellers.

Non-inertial cavitation is the process in which a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers, etc.

Since the shock waves formed by collapse of the voids are strong enough to cause significant damage to parts, cavitation is typically an undesirable phenomenon in machinery (although desirable if intentionally used, for example, to sterilize contaminated surgical instruments, break down pollutants in water purification systems, emulsify tissue for cataract surgery or kidney stone lithotripsy, or homogenize fluids). It is very often specifically prevented in the design of machines such as turbines or propellers, and eliminating cavitation is a major field in the study of fluid dynamics. However, it is sometimes useful and does not cause damage when the bubbles collapse away from machinery, such as in supercavitation.

Inertial cavitation was first observed in the late 19th century, considering the collapse of a spherical void within a liquid. When a volume of liquid is subjected to a sufficiently low pressure, it may rupture and form a cavity. This phenomenon is coined cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating in the liquid with sufficient amplitude and acceleration. A fast-flowing river can cause cavitation on rock surfaces, particularly when there is a drop-off, such as on a waterfall.

Other ways of generating cavitation voids involve the local deposition of energy, such as an intense focused laser pulse (optic cavitation) or with an electrical discharge through a spark. Vapor gases evaporate into the cavity from the surrounding medium; thus, the cavity is not a vacuum at all, but rather a low-pressure vapor (gas) bubble. Once the conditions which caused the bubble to form are no longer present, such as when the bubble moves downstream, the surrounding liquid begins to implode due its higher pressure, building up inertia as it moves inward. As the bubble finally collapses, the inward inertia of the surrounding liquid causes a sharp increase of pressure and temperature of the vapor within. The bubble eventually collapses to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism which releases a significant amount of energy in the form of an acoustic shock wave and as visible light. At the point of total collapse, the temperature of the vapor within the bubble may be several thousand kelvin, and the pressure several hundred atmospheres.

Inertial cavitation can also occur in the presence of an acoustic field. Microscopic gas bubbles that are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size and then rapidly collapse. Hence, inertial cavitation can occur even if the rarefaction in the liquid is insufficient for a Rayleigh-like void to occur. High-power ultrasonics usually utilize the inertial cavitation of microscopic vacuum bubbles for treatment of surfaces, liquids, and slurries.

The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapor. Boiling occurs when the local temperature of the liquid reaches the saturation temperature, and further heat is supplied to allow the liquid to sufficiently phase change into a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature.

In order for cavitation inception to occur, the cavitation "bubbles" generally need a surface on which they can nucleate. This surface can be provided by the sides of a container, by impurities