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To the hydraulic system maintenance engineer, it can be both normal and alarming that hydraulic oil contains air.
Nonetheless, it’s part of hydraulic fluid’s natural composition to contain between 6% - 12% of dissolved air. The trouble starts if this air transforms into being a form that is not dissolved, and it can lead to serious trouble!
If the air makes its way into the pump intake, then air can then transform into entrained air. The problem with this is that it can spoil the stiffness of the fluid, which will decrease its efficiency. It can also increase the levels of noise. But that’s not all.
Supposing the entrained oil reaches the pump outlet and gets compressed, you can then expect very high peak temperatures to develop. Air mixing with the oil film will oxidize it. The cracking noise is an indication of oxidisation. As we all know, it’s oxidisation that degrades hydraulic fluid. The air bubbles will then start to pop after smashing themselves against the valve plate of the pump and any other areas that they touch. This is when cavitation takes place and there’s erosive wear going on.
The only way to stop this occurring is to prevent it. Air will come out of the solution in certain conditions. For example, when the temperature of the hydraulic oil increases or there is a decrease in static pressure, then the solubility of the air is reduced and that’s when the fluid can host the formation of bubbles.
The release of the air is referred to as gaseous cavitation. It can come about from:
· Suction strainers or inlet filters becoming clogged
· The intake-line isolation valves causing turbulence
· An inlet that has been poorly designed such as it being too long, having multiple bends or the diameter of it being too small
· The intake line being restricted or collapsed
· There being too much lift between the minimum fluid level and the pump intake
· The reservoir being clogged or undersized
The bottom line is that although you will have air in your hydraulic fluid, it’s key to keep an eye on what condition it is in. it can be a serious and costly error to allow air to contaminate a hydraulic system.
This article focuses on the common problems associated with hydraulic systems and how these issues can affect individual components as well as the systems themselves. It gives an in-depth insight into how problems initially develop, any knock on effects and how failures can happen if the issue isn’t diagnosed correctly.
Abnormally high fluid temperatures
High fluid temperatures are usually caused when the system struggles to correctly dissipate heat and can lead to increased heat load. When fluid temperatures exceed 180°F (82°C), this can lead to damage to the seals and in turn start to degrade the fluid itself.
When viscosity levels drop below optimal values for the system components, the fluid temperature is judged to be too high and the reservoir should be checked for any obstructions or blockages.
The heat exchanger is another component that should be checked making sure the core is not blocked. In order for the heat exchanger to successfully dissipate heat, the flow rate of both the hydraulic fluid and cooling air/water should be at the correct levels.
When fluid circulates to areas of differing pressures without correct pressure correction, excess heat can be generated and any areas that show signs of internal leakage can increase the heat load on the system. This includes anything from a leaking cylinder to an incorrectly adjusted relief valve.
High fluid temperatures can also have an effect of components as they go through a thinning process which affects the oil film, otherwise known as a low viscosity which leads to inadequate lubrication. This issue can be tackled by setting up a fluid temperature alarm to warn of dangerously high temperatures.
When a machine starts to show signs of reduced performance and functionality this points to a problem with the hydraulic system. A loss of speed in the system is usually caused by a poor flow rate and can be noted when the system takes longer to cycle or is slow in its general operation.
Leakage is a common cause of slow operating speeds as flow can escape from hydraulic circuits. This leakage can be either internal or external with typical candidates being burst or degraded hoses or leakage from pumps, valves and actuators.
A useful tool to measure leakages and pressure drops is an infrared thermometer which can identify components with internal leakage. So going by the manufacturers limits for correct temperatures, the thermometer can pinpoint potential problems. Incorrect hydraulic oil out of manufacturer specification can also lead to pressure changes and a decrease in system performance. A successful infrared test will also pick this issue up.
Sometimes, under extreme operation, overworked machines start to overheat. Using the correct oil viscosity will help to alleviate this issue from occurring in the first place and avoid costly failures by lubricating the system effectively. It is also important to let the system prime itself before use thus ensuring all important components are lubricated correctly and to manufacturer standards.
Abnormal and irregular operative noise
Abnormal or irregular sounds while the system is operating are good warning signs of a potential problem either currently occurring or about to occur.
There are two main candidates that attribute to this problem – aeration and cavitation. Aeration is a condition where too much air enters the system and contaminates the hydraulic fluid within. This commonly leads to a loud banging or knocking noise from the system when it compresses and decompresses whilst the fluid circulates through the system. Actuator movement can also become erratic and the fluid degradation can eventually lead to damage to seals through overheating.
Cavitation commonly occurs when a hydraulic circuit demands too high a level of fluid than is being supplied at any given time. This in turn causes circuit pressure to fall below the level of vapour in the hydraulic fluid. The knocking sound that comes from this is caused as the vapour cavities implode during compression.
Both these issues can cause component or system damage with extreme cases of cavitation being known to cause metal erosion and failure of system components.
Hydraulic filtration is a vital component of keeping a system running smoothly.
For example, did you know that up to 75% of failures with fluid power can be attributed to contamination? With the use of hydraulic filters, contamination damage can be significantly lowered which can not only cut down on expense but lower that 75% drastically.
If you’re looking to save costs from less downtime then it’s also time you looked into what a difference hydraulics filtration can make for extending the life of your equipment. Running your system optimally is essential when it comes to cost saving, but protecting its longevity is also a critical element in running any business efficiently.
Muck and dust can destroy a hydraulic system, that’s why it’s essential to make the best use of hydraulic filters. You wouldn’t even be able to remove that dirt yourself, as it’s likely to be dust that is so fine that you won’t be able to see it without the use of a microscope. Dirt has the same detrimental effect as sandpaper or gravel and not only will generally deteriorate the system, but it could even destroy it.
However, through the use of a hydraulic filter system you will be able to maintain control over the level of contamination and by doing so reduce the failure of systems by as much as 75% just be removing that dirt.
Hydraulic parts are expensive. Combine that with down time and having to keep engineers on hand to fix worn components and that’s a lot of expense to deal with. Putting filters into place can even save costs by increasing how long the hydraulic fluid will last.
Degradation of fluid – hydraulic fluid that contains fine metallic particles can degrade rapidly through chemical breakdown. Without protecting against this, there could be issues such as slippage, internal leakage, corrosion or sticking parts.
Scoring of surfaces – this can occur when particles get trapped between surfaces of seals
There’s no doubt about it, but …
· System performance is affected by dirt levels
· Hydraulic filters can control levels of dirt. Without using this management method, the system will get dirtier and dirtier until it fails.
In fact, hydraulic filters are the only way to control how much dirt is in fluid. Without them you will be forced to change out the hydraulic fluid regularly, which can be a time consuming and costly event.
Hydraulic system dirt particles are incredibly small. In fact, they are so small that they cannot be seen by the human eye – and 98% of hydraulic fluid has some dirt in it.
Engineers have found that when it comes to size of particles in samples taken from operating systems, the smaller the particles, the more dirt there is in the system.
So where do these particles come from that we have to work so hard to deal with?
In order to have an idea of what goes on inside the closed system, let’s examine where these particles come from.
Instead of enjoying the typical 20 gpm that is the measurement of a pumped flow from a 2000 psi system, you can expect to see something in the region of just 10 gpm. Although your pump will still produce for you, you’ll discover that the degradation results in just 50% efficiency and you should als be prepared to experience extra heat and other unwanted issues.
As with any hydraulic system, there is an optimum level of cleanliness, but there is a point where you cannot get any better performance out of the system by improving the quality of the fluid. However, with the use of hydraulic filters you should be well set to extend the life of your machinery.
As we sympathise regularly with our readers, running hydraulic systems can be very costly. Not only can costs build rapidly from replacing damaged or worn components, but there is also system downtime to consider and to add to the expense.
If there is one deadly enemy for hydraulics, it’s contamination. In fact, contaminated fluids can be connected to more than 80% of all hydraulic failures. This includes all the related failures that can result including those of hoses, fittings, pumps and valves.
In fact, there is such a strong correlation between contamination of fluid and the lifespan of components that manufacturers of hydraulic and filtration products actually publish charts with the consequence predictions of not having inadequate filtration installed. Those systems that undergo rises in pressure will suffer from even more damage as contaminant particles make their way around the system.
Unfortunately the particles involved in hydraulic system contamination are usually far too small for the naked eye to see them. This is why it’s essential to use instruments specifically designed for contamination monitoring, otherwise a high system reliability cannot be expected to be maintained.
Although the operators and engineers who take care of industrial hydraulic systems are well aware of this problem, it’s only really coming to the fore of the mobile hydraulic system now. In this microcosm of the hydraulic world, there is still some time-based fluid maintenance going on. However, it’s becoming more apparent that this and spin-on filters are no longer enough to keep mobile hydraulic systems operating at their peak performance levels.
Quantifying contamination in hydraulic systems
Ideally every hydraulic system should have absolute filtration to capture both micro particles and those that are larger.
A Beta ratio of filtration will usually capture 99.5% of all particles that could contaminate a system. Alternatively the 1000 measurement will capture 99.9% of the particles. This will support the hydraulic system in enjoying a maximum service life. However, in addition to the Beta ratio, there are other considerations to ponder over when looking to keep the system clean.
How much dirt a filter can hold and how stable Beta ratio is will determine how well the filtration works out for the system. The best filters are usually cartridge-type that use a number of layers to help to maximise performance for all areas. Each layer will help the filter to either capture the dirt, hold it or to deliver the beta stability.
Another unexpected benefit of the cartridge-type filters is their ability to reduce how much loss of fluid there is when the filter is changed. This can keep go towards keeping costs down, whilst also lowering the impact on the environment. Although the cartridge type filter may cost more to buy, they deliver when it comes to protecting the system and cutting back on fluid loss.
With industrial hydraulic applications, cartridge filters are now considered to be the standard. They are also becoming more popular and widespread in the mobile market, which is becoming more sophisticated when it comes to components in addition to enduring rising costs.
Mobile Filtration Challenges and Solutions
Another area of concern with mobile hydraulic systems is that of space in the system to add filters and other components such as sampling valves. Quite often manufacturers will produce tank-top filters that can be integrated into the hydraulic reservoir, but sit out of the way. With global emission requirements becoming tighter, this trend is likely to accelerate in the coming years.
One issue that is unique to the mobile world is that of the cold start. It’s well known that any hydraulic fluid will thicken when sat at lower temperatures. This can increase the pressure drop for the filter element. The performance will take a downturn until the fluid begins to gain temperature and reaches the operating temperature level. Quite often the comment from an engineer will be ‘I started up and when I hit the level, nothing happened’.
Although it’s possible to install a large filter, it can add to the bulk and the cost of the system. Another work around is bypass the filter by adding in a pressure relief valve until the fluid is warmer. However, this can send contamination downstream. An approach that is less troublesome is to return the fluid to the reservoir as opposed to allowing it to circulate throughout the system.
In summary, as an engineer, the best move you can make is to identify and implement a fine filtration strategy that will enable your hydraulic system to run at its ultimate performance.
Cavitation was first discovered in 1917 when Lord Rayleigh (a British physicist) decided to investigate why fast-rotating ship propellers were eroding at such an incredibly rapid pace. He discovered that it was all down to a condition that came to be known as cavitation.
Cavitation is related to the word, cavity and has the Latin verb ‘cavitere’ at its root. It means to ‘hollow out’ and that’s exactly what cavitation does. Cavitation occurs when very hot small air and gas bubbles develop. As they reach high pressure areas they collapse and cause hot jets to hit the surface of any pipe or components that is reachable causing erosion.
When liquid passes through an hydraulic valve, it is under pressure due to the size of the valve. The speed of the flow will rise and then the speed drops again. This change in pressure will create a vapour and then small bubbles of gas will form. When the pressure gets higher than that of the vapour, the bubbles will collapse as the vapour liquefies again. The bubble collapse can send out the heated fluid jets which are very powerful and it’s this that causes so much damage to metal.
Cavitation is no joke. It can be so severe that it can completely destroy hydraulic valves, pumps and piping to such a degree that the entire system can fail. In between, you may need to contend with leaking valves or even holes in pressure vessel walls.
Even cavitation that is at a low level can wreak havoc by eroding components until they need to be replaced. This is how engineers can recognise that their system is suffering from the effects of cavitation:
· Banging or knocking noise
· Chocked flow
· Fluid property changes
· Valve component erosion
· Control valve destruction
· Failure of plant leading to shutdown
Cavitation damage can often be identified visually with the use of either a microscope or a magnifying glass. If you are wary that cavitation is causing damage to your hydraulic system, check whether or not it is general wear and tear through corrosion. Some types of corrosion will actually mimic what you’d experience through cavitation. You can expect to find cavitation damage, if it’s in existence, downstream of the seating areas for the control valve. You could very occasionally witness cavitation bubbles further downstream from there.
Another easier way to identify whether your system is suffering from cavitation damage is to listen out for the noise of either crackling or popping. As pressure drop occurs, the cavitation will ramp up and you’ll hear rattling or hissing that increases in volume. If the cavitation is in full operation, it will sound more like you have a lot of small rocks or gravel passing through your system.
Cavitation is so powerful that there is currently no known material that can stay undamaged by it. The only way to deal with it is to eliminate it.
One of the simplest ways to eliminate cavitation is to reduce the operating temperature within the hydraulic system. The vapour pressure will be eliminated. If you have identified a valve that is experiencing cavitation, when you replace it, try to install it at the lowest possible elevation within your pipe system.
Another possible solution to cavitation issues is to introduce air or nitrogen into the area of the system where you are expecting to get it. It will need to be added through either the valve shaft or through a tap downstream on aside of the pipe as close as possible to the valve.
If for some reason you are unable to change the process conditions, then it’s recommended that you use valves with low recovery and treacherous flow path as opposed to high recovery valves such as the gate, ball or butterfly types.
When replacing hydraulic fluid, it is tempting to believe that the new oil will be clean and free of contaminants, and that it can be put straight into the reservoir without any problems. Unfortunately, this is not always the case. If you use hydraulic oil from a large drum, there is a high chance that it already contains some water and dirt particles; new hydraulic oil typically has a cleanliness level of ISO 4406 23/21/18, which is more than most hydraulic systems will tolerate. If the system has a rating of say, 20/18/15, then the new hydraulic oil is already too contaminated, as a single digit increase in any of those numbers is effectively a doubling of the contamination level for each micron size.
We can see, therefore, that it is a good idea to filter new hydraulic oil before it enters the system taking with it contamination that will potentially lead to problems with the system. Most hydraulic system failures can be traced back to contaminants in the oil causing friction, high temperatures and a loss or build-up of pressure that can cause serious damage to the components within. Avoidable problems should not be encouraged by cutting corners when replacing hydraulic fluid as it is a false economy.
If you usually replace the hydraulic oil straight into the reservoir, you can add a filtration cart or a kidney loop system to clean the fluid before it gets into the system itself. Even if you have a filter downstream it is a good idea to still keep a filtration system in the reservoir too, to ensure that the downstream filter does not have to work too hard and retains the lifespan it is expected to have, cutting down on element changes. A kidney loop system is ideal for filtering hydraulic fluid in the reservoir and runs independently of the equipment itself, meaning it can still be cleaning the oil even when the equipment is not being used. This means that the fluid can be filtered thoroughly before the machinery is switched back on and also offers a higher level of filtration throughout the life of both the hydraulic fluid and the equipment itself.
Dual filter elements are usually used in kidney loop systems to filter out particles of different sizes and ensure that the filter does not become clogged too early. This also allows for better element change schedules as they can both be done at the same time, rather than replacing the first, then the second, then the first again and so forth more frequently. The dual filter elements in a kidney loop system also perform better than in-system filters, as they are not exposed to any pressure and can retain contaminants more effectively.
Alternatively, new hydraulic fluid can be filtered into the system via the return filter. If the application is very sensitive, it may be best to stick with a kidney loop filter, but if this is not possible due to the nature of the hydraulic equipment, the return filter route is a good option. A tee needs to be installed in the return line above the filter, and one branch connected to a drum pump discharge hose via a quick connector. When it is time to filter the new oil in the drum pump, it is attached to the return line and the oil gets pumped through the return filter and into the reservoir.
Not filtering new hydraulic oil into a system basically opens the door wide to dirt and water getting in, and undermines maintenance activities and careful user behaviour designed to keep the equipment in full running order. If you need to change how you replace the hydraulic fluid or add a filtration component into the system, the cost of doing so should be weighed against the savings in unnecessary maintenance and repairs due to contamination related damage.
Contaminated hydraulic oil is the biggest cause of system failure in hydraulic machinery and often it is entirely avoidable. Mistakes happen, and there is always room for improvement in maintenance and routine replacement activities which can help reduce contaminants in the system. Even when you have got everything right in that area, there are still extra tweaks you can make or things to avoid when refining your hydraulic machinery care process.
Using the correct weight and ISO rated hydraulic oil is essential operating practise with any type of hydraulic equipment. Using liquid that is too thin or one designed specifically for a different type of motor, can cause serious damage to the internal parts through overheating or having an unsuitable level of intrinsic contamination. However, it is possible to go one step further than simply using a dedicated oil; By checking the ISO rating of the standard oil and the rating that the machinery requires, and then using one with lower ratings, i.e. with a higher level of cleanliness it is possible to improve the lifespan of components operating at a higher than average pressure, speed or length of operation. These factors affect the suitability of the standard hydraulic oil for any particular system and by taking into account any higher than average operational requirements, it is possible to avoid premature component failure caused by contamination levels in the fluid.
When looking at whether a different rated hydraulic fluid would be more suitable for your system and deciding to opt for a lower rated one, it is important that this decision is made with the most sensitive component in mind. It may be a case of using the hydraulic fluid with that rating, or of installing added filtration systems before that part of the system, in order to clean the fluid as it passes through that part. They say an army marches at the pace of the slowest person and it is similar concept to choosing hydraulic oil and filtration systems, when there are different levels of capability and tolerance between the component parts.
As a guide, the typical cleanliness required of hydraulic fluid for different types of components is as follows:
Servo control valves
Vane and piston pumps
Direction and pressure control valves
Gear pumps and motors
Flow control valves and cylinders
An avoidable source of contamination in hydraulic fluid is paint flakes or rust in the system. Sometimes a decision will be made to paint the inside of a hydraulic reservoir to prevent rusting, and on the surface. This may seem like a sensible decision as tanks are not cheap to replace and when a piece of machinery is expected to last a long time, it is reasonable to take precautions against such problems. Rust in hydraulic reservoirs can be caused by condensation and settled water in the space above the oil level, but a simpler solution is to keep the reservoir topped up and using a hygroscopic breather to reduce the potential for any water or vapour to form. Painting the tank with a rust proof paint may not cause any problems, but the potential is definitely there and the risk is not worth taking.
Monitoring the cleanliness of hydraulic oil at all stages of its journey round the system is important for maintenance and replacement of filters and elements, but also for the daily operation of the machinery. When it is possible to check that everything is operating as it should, then the focus can remain on the job at hand. Monitoring also alerts users to a potential problem, as if the contamination level of the hydraulic fluid is too high at a particular point, an alarm or warning light can be deployed and the machinery switched off while the hydraulic fluid or filter element is replaced. Being aware and alert to these issues and resolving them before they cause damage to the parts, is preferable to continuing blindly and then incurring hefty costs down the line.
The steps outlined above go a lot further than simple best practice – these are next-level preventative activities, that can save time and money for companies already acting in a contamination-aware manner. There are always small improvements that can be made to the operation of hydraulic machinery and it is hard to implement them all, but at least the knowledge expansion can inform suitable changes to your operating practices.
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