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It’s no secret that removing particle contamination can greatly increase the life of the components in any hydraulic system. The truth is, is that there are always some particles present, even in brand new hydraulic fluid. How much contamination can be accepted is going to vary depending on the hydraulic system being used.
So how do you go about removing particles in a hydraulic system? If the system is large, then it’s recommended to add filters wherever you feel it’s going to give you the most advantage.
The strange thing about hydraulic system filters is that although they are there to help keep your machine running well, they can sometimes cause damage and shorten the life of your system.
Hydraulic filters run on a rating system that defers to the micron size of the particles being removed. To get the best results, take a sample of your hydraulic fluid so that you have some idea of where you’re starting from and which filters you’ll need. Ideally you’ll flush the fluid before adding new filters so that their main job will be to maintain its cleanliness rather than to clean it up.
No matter where you start from, it’s important to keep in mind that the filters that you use will affect pressure and it could drop it by providing restriction. If this occurs, then it’s possible that the bypass valve on the filter will open and therefore filtering will not be implemented.
If you’re an engineer who has the responsibility of deciding where to put the filters, it’s critical to do this with a mind to prevent any harm from occurring. There’s no point in delivering cure that is worse than the disease.
Here’s our take on what you need to know about where to fit filters on hydraulic systems:
Pressure line. Many components that are located downstream will benefit from positioning the filter in the pressure line. It’s possible to capture as small as 2 microns or less as the pressure will help as it forces the fluid through the filter. However, it’s possible that the filter will be reduced by the high flow velocities which can loosen up trapped particles. In the long term its pressure filtration that costs the most to maintain and the most to get going with.
Return line. The most effective principle to apply to this is that if you start out with a reservoir full of clean fluid, then keeping fluid clean by filtering it will keep it that way. Another benefit of using the return line to add a filter is that you’ll gain advantage from the higher pressure. It’s possible to gain good filtering results at a relatively low cost as there won’t be filter or housing design complications with a pressure of that measure. In actual fact, the lower flow will deliver a filtration that is efficient and low cost. The only disadvantage of this method is that there is a chance of the back pressure causing some issues.
Off line. Filtering your fluid using the off-line method will provide you with a continuous filtration that is multi-pass. The results are excellent filtering efficiency where you can pull out particles that are 2 microns or less in size. It’s also possible to extract water and even heat in order to give your fluid total conditioning. However, there is a cost disadvantage when starting out with this method, but this can usually be reclaimed over the life of the machine.
Suction. Positioning a filter next to the pump intake can provide advantages gained by not being in a high pressure area, however, it does have potential to cause some issues with regards to pump life.
Filtering is important in any hydraulic system and it’s not just about what you use, it’s about where you put it.
Electrostatic charge builds when there are two bodies moving and creating friction. The fact is that this also occurs in hydraulic systems from the friction caused by system components with moving fluid.
Although we haven’t had a lot of situations that have involved electrostatic discharge, it is still something that every engineer should be aware of.
When an electrostatic discharge occurs, there is a clicking noise as charge increases and is then released. This is something that will often occur in a filter – leaving burn marks and potentially other damage.
With the increasing preference of using non-metallic additives in hydraulic oils the electrostatic charge could be on the increase. Those hydraulic oils that contain anti-wear additives that are zinc-based have considerably high conductivity.
Conductivity in hydraulic oils helps when it comes to moving electrostatic charge around the system. Although zinc-based additives will rarely collect enough charge to cause a big problem, synthetic oils can. This is because they have less conductivity and therefore will potentially accumulate more charge before discharging it.
Another change that could lead to an increase in electrostatic discharge is that there has been a change made to the materials that filter elements are made of. In order to make them easier to dispose of them in an eco-friendly way, they have more non-metallic material in the design, which lowers conductivity and therefore increases the capacitance.
The manufacturers of hydraulic filters are aware of these issues, and are looking into how they can minimise or even eliminate these issues.
However, if you come across a situation where there is electrostatic discharge in the meantime, then consider this:
By adding larger filter elements you can reduce flow density and therefore the amount of charge that is being generated. You might also want to consider increasing the tank size so that the time between charge generations increases.
This is one of the reasons why you shouldn’t skimp on tank size or on filter capacity.
Filters are a small but vital part of hydraulic machinery, although serving no mechanical purpose they are integral to the proper functioning of the machine and to prolonging the active life of the equipment and the hydraulic fluid which drives it. The role of a filter is to remove and/or contain particles of contamination and keep the fluid that flows round the machine clear. Any contamination in the oil can cause damage to moving parts further downstream, and this can lead to seizing up of moving parts, corrosion of other parts and costly replacements. When we consider the consequences of not using a filter, the importance of these little items becomes more significant.
There are two main styles of hydraulic oil filter – surface and depth filters. The surface filter, as the name suggests, removes contamination from the surface of the oil and may be useful in applications where gravity feeds the oil through a space suitable for such a filter during operation. A depth filter can be submerged in a reservoir or chamber and will remove particles from the entirety of the body of fluid. Depth filters therefore, are more effective and will retain a larger amount of contamination and unwanted particles before they need to be cleaned or replaced.
The materials used to make hydraulic filters varies, as does the cost accordingly. Glass filters are more expensive but they are more efficient, especially when glass fibres are used in a depth filter. Glass is also non-reactive, meaning it can be used with any type of hydraulic fluid. Metal filters are also reasonably efficient, but they cannot be used with all types of hydraulic fluid, due to incompatibilities between certain hydraulic oils and some metals. Where it is appropriate to use a metal filter there is the option of a magnetic filter system, which works as a depth filter and uses a magnetic charge to attract metal particles that may have entered the fluid system. If the hydraulic equipment is used in metal working and fabrication then the chance of potential contamination being from metallic particles is high; magnetic filters will deal with this contamination while another type of filter can be used to deal with other types of pollution.
Cellulose or paper can also be used to make a hydraulic fluid filter, but these have a short life expectancy and need frequent replacement. They are cheaper than the other types, but could end up costing more in the long run, due to regular downtime for replacement and the potential damage to machinery if they are not replaced often enough.
It may seem like glass is the best choice; despite costing a little more it does the best job and will not need replacing too often. The drawback of a glass filter is actually its strength as well; the superior filtering ability can actually lead to a drop in pressure, in the system, which is usually undesirable. It is especially important, therefore, to check how a drop in hydraulic system pressure will affect productivity and performance if you are considering switching to glass filters from another material type. Consider also whether this drop in pressure is likely to lead to unnecessary adjustments of pressure relief valves, leading to potentially dangerous build ups of pressure elsewhere.
The options of hydraulic oil filters do not end after material type and construction, as there is still the issue of ratings and specifications to contend with. All filters have an ISO 4406 rating, and the lower the code of any given filter, the better it is at removing contamination. Hydraulic filters may also have a beta ratio, which is the ratio of particles found upstream of the filter divided by the number found downstream. For beta ratios, the higher it is the better, and this can also be used to give a percentage for effectiveness.
It is also important to check that the flow rate of the filter is compatible with the flow rate of the machinery it is to be used in; too fast and the filter will not be able to effectively remove contamination, too slow and it could become clogged quickly. The operational pressure of the hydraulic system is also important, as the filter must be able to withstand that force for a prolonged amount of time. The final thing to check is that the filter can be connected to the equipment, so check the port size of the filter and ensure it is compatible with the machinery in the location it is to be fitted.
There is a lot to consider here, but choosing the most suitable hydraulic fluid filter is an exercise that it is worth spending some time on, as it can make a big difference to the performance and lifespan of your hydraulic equipment and save on downtime and replacement costs in the long run.
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.
Changing the filter element in your hydraulic machinery is a very important part of routine maintenance and for this reason it is often tacked onto a schedule and done at a set time, usually determined by the number of hours it has been in service for. Changing filter elements, wherever they are situated in the hydraulic loop, needs to be done in order to keep the fluid as clean as possible to therefore prolong the life of the components in the system, however, getting the timing right to ensure you are making the most of your expensive filter elements is a more intricate art than simply totting up the service hours and basing a filter change schedule on that data alone.
If a filter element is changed early it will still have plenty of dirt holding capacity left, and to replace one with life left in it is a false economy – yes, you will not run the risk of leaving it too late and potentially allowing contaminants into the oil, but it is also a waste of resources. Leaving an element change too late means that dirt can enter the system and cause damage to the components, leading to machine failure and spiralling associated repair costs. If we cannot use service hours to inform the timing of a filter change, how can we tell when is the right time for a switch?
The location of the filter can make a difference to the regularity of the element changes, as pressure filtration systems work a lot harder than off line or return filter systems. They are often higher in initial cost as well as in ongoing maintenance costs, but offer a fine level of cleansing as they can trap the smallest particles, thanks to the pressure forcing the fluid through the filter. The same pressure can dislodge trapped dirt, sending it back into the machinery and causing damage, so there are also downsides to using this location for hydraulic fluid filtration. Off line filtration systems are also very expensive but are the most effective at filtering, as they run continuously and therefore offer the best extension of machinery life. They also require regular replacement of the filter elements, as they are active for much longer than a filter that is only operational while the machinery is running. Return filtration is the most popular and most economical location for filtering hydraulic fluid within a system and also offers the opportunity to filter new oil in via the same part of the loop.
The best way of knowing when a filter element needs to be changed is to monitor the pressure drop across the filter, with a marked drop in pressure on the downstream side indicating that the dirt holding capacity is almost used up. A clogging indicator is one way of measuring the pressure drop and this can be used when it is suspected that the time may be approaching. Clogging indicators can be visual or electronic and set to go off before the pressure for the filter bypass valve is reached. Not all hydraulic systems have a filter bypass, but those that do can link the associated pressure point to the point at which a clogging indicator is activated.
A more sophisticated system involves continually monitoring the pressure drop across the filter, wherever it is located, and the resulting data can be used to inform a more reasonable and reliable filter element change schedule, as it shows the true lifespan of the filter elements. The data also acts as an early warning system of system or component failure. Although an advance continuous monitoring system costs more, in the long run it will save money on unnecessary filter changes, warn of expensive system failure before it happens and provide valuable information about the operating capacity of your hydraulic equipment that could prove a lot more valuable in the long run, and just think - all these benefits can arise from simply trying to work out a proper filter element replacement schedule!
Spring is finally here - in its full beautiful entirety with sunshine and plenty of tabloid coverage telling us that it’s going to be hotter than Ibiza. All part of the glorious British summertime!
This week we’ve got five tips for you on giving your hydraulic equipment a spring maintenance session.
Take stock – As the old Chinese saying goes, ‘He who fails to prepare, prepares to fail’. That’s definitely something to consider when it comes to hydraulic systems. It’s an often forgotten task to prepare for failures by ordering spare parts upfront so that they’re on hand for a speedy change out.
It’s a recommend practice to keep your paperwork up to date of what you’ve purchased on each machine so that you can cross reference your parts including valves, pumps and cylinders with inventory to ensure that you have what you need on hand and ready to use.
You’ll no doubt already be aware of the lead times on some parts. It’s a smart move to stock the parts you may need that are critical to operation. By revisiting what’s required for each machine, you can either opt to order parts or you can remove what you don’t need from the list.
Revise your schematics. Nobody can troubleshoot successfully with an out of date schematic, in particular if your machine is particularly large or of a complex design. Drawings need to be accurate and in line with the current inventory.
Oftentimes machines are upgraded or modified to be in line with safety regulations and overall safety considerations. If the schematic wasn’t revised to show the new layout of a machine, then engineers and technicians can waste time trying to fix a circuit they have no idea about. Although it’s best practice to change schematics at the time of the change, confirming that this has been done is a smart proactive move.
Check out your fluid. It’s important to investigate the condition of your fluid at least 4 times a year, and spring should be one of those times. Take a sample of your active fluid in to a clean sample bottle, so that you’re able to judge what true condition it’s in. Ideally your sample will be from the centre of a reservoir or from a return line so that you can get a fair representation of the fluid moving around the hydraulic equipment circuit.
You can measure what the particle count is in addition to the water contamination levels. You can also check the additive content and how long the oil is likely to be able to stay in service, thus potentially preventing needless maintenance.
Unless you know exactly what you’re looking for. Use a graph with your baseline so that you can see if there’s a big change from typical conditions. You should then have enough time to identify any issues and fix them before a major breakdown occurs.
Change filters. If you don’t have an electronic or other indicator to warn when the filter is full, you will need to schedule in regular checks. Spring is a good time to put this into practice. In an ideal world you will have a sign that the filter is clogged, emanating either from a light, a pop-up or a switch.
Although it’s great to schedule in filter changes, the only true way of knowing if one needs a change is by manually and visually checking whether it’s clogged. You will also need to look at the component wear whilst you’re there.
Spring clean. This is the actual spring clean itself. Of course, getting a very clean machine is every engineer’s dream. In reality hydraulic equipment attracts an incredible amount of dirt!
Although it’s fine to have dirt on the outside of your machine, having it on the inside is another matter entirely. It’s smart to ensure that the grime from the outside cannot reach the inside. Keep entry points closed and reduce the chances of contamination to keep your machine operating happily.
It’s no secret that the use of hydraulic system filters is what makes all the difference when it comes to the expected longevity of your industrial machinery.
Keeping this in mind, it would make no sense to position your hydraulic system filters in a place where they could even produce results that reduce your machine’s life in service. That’s why it’s essential that you know where filters should be positioned so that you can avoid negatively impacting the system. Taking preventative action is far more favourable than addressing the resulting ailments.
With this at the forefront of our plans, let’s look at what you need to know about the range of potential suitable locations for your hydraulic filters:
Filters that are located in pressure lines deliver the ultimate in protection for fittings that are immediately downstream. It’s possible to use the pressure flow to your advantage and add filters with ratings of around 2 μm or less. If you have high flow velocities, you may experience a decrease in the effectiveness of the filter as there will be disturbance to any particles that are trapped. Pressure filtration usually proves to be the most expensive when it comes to installation and maintenance due to the need of them being high quality to withstand the pressure.
The philosophy behind using the return line for filtration addition is this: if both the reservoir and it’s fluid are clean, and anything that enters the reservoir has been filtered then it will continue to be clean. Fortunately the return line makes it possible to force fluid through fine filters such as those of 10 μm, but the pressure is not very high so will not interfere with design of the filter or housing and will make for an economical process.
Adding filters off-line clearly has both advantages and disadvantages, the latter being that it can have a high cost for installation. What it has going for it is that multi-pass filtration at controlled velocity flow delivers greater efficiency.
It’s no surprise that a filter located at the pump intake is in a great position. It becomes more efficient due to not having high fluid velocity nor high pressure. However, these benefits can be offset by adding a restriction to the intake line and what it can do to pump life.
Both cavitation and mechanical damage can result from having a pump inlet restriction. The fluid can be contaminated by cavitation and it may damage critical surfaces, with destruction occurring from vacuum induced forces on the pump.
The detriments of vacuum
In an axial-piston type pump, there is an issue with having a vacuum in the pumping chambers. It puts the piston ball and the slipper pad socket under tension that they’re not designed for and the slipper may even become detached from the piston. This can happen over time or even instantaneously.
The piston of the bent-axis pump usually fairs far better when it comes to withstanding a vacuum. It’s typically more indefatigable and a bolted retaining plate holds the piston ball in place in most cases. However, there can still be issues with the piston stem under high vacuum.
When it comes to vane pumps, it’s essential for the vanes to extend from their position of retraction in the rotor during inlet. If there is high vacuum at the inlet of the pump, then it will act on the base of the vanes. They may lose contact with the cam ring, and then get bashed around as the pressurized fluid enters. This can result in catastrophic failure.
In some cases, engineers regret trying to save their hydraulic system from suffering from fatigue and damage and rue the day they made changes. The best hydraulic systems have been designed to take filtration into consideration so that extensive changes don’t need to be made, and cavitation and mechanical damage are kept to a minimum.
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