Resources
F.A.Q.
Do you need more information on the effects system conditions have on your products & processes? You may find the answer in this section!
We recommend when measuring particulate contaminants that steps be taken to ensure that the product is not exposed to aerated fluid. The following shortlist should be considered for new and existing installations if aerated fluid is to be avoided.
- Significant or sudden pressure drop
- Hydraulic shock as a result of sudden operation of valves and pumps
- Inadequate operating conditions for various pump types
- Inadequate diffusion of the fluid at the return tank
Extra care should be taken when replacing system components. Where necessary/possible, pre-fill components with filtered new oil before placing them on the system. This will reduce the amount of air being placed into the system.
There are a number of different ways to remove air from a system, but the following are probably the most simple and commonly used.
- Reservoir air bleed valves
- Baffled Reservoir tanks
- System maintenance procedures
- Adequate diffusion back to tank
- Reservoir level
Reservoir bleed valves are good, and are readily available, however their use is limited to the reservoir only. They may not detect air in other parts of the system. It is certainly good practise to use these devices on closed systems.
On systems with open tanks, often baffles are used to allow any air to naturally diffuse into the surrounding atmosphere. When using this type of system, it is imperative to keep tank levels high enough so that the baffles can be effective. When returning back to tank, also make sure that the flow is diffused so that sudden and violent discharge does not occur. This can drag air into the system, and it certainly won’t help to remove any entrained air. Try to keep the flow rate back to the tank low and discharge the fluid towards the surface of the tank to encourage its escape
To make an evaluation of how much air is in your system, measure the reservoir level pressurized and un-pressurized. If the fluid level in the tank is lower when pressurised, this could be a indication of air present in the system, and signal that some maintenance may be required.
Aerated fluid can result in other problems of varying severity. These should be avoided where possible to keep personnel safe and maintenance costs low. The following is a short list of symptoms which can be associated with entrained air.
- Increased fluid temperatures
- Reduced lubricity
- Cavitation and system component erosion
- Noise
- Dampening and poor system control
In addition to this, automatic particle counters which operate on the light extinction principle can suffer sensitivity issues when air is entrained in a system.
Hydraulic fluids should be kept in sealed containers until ready for use. Lids should be checked routinely and tightly secured, with drums being kept in relatively dry environments. Moisture from rainfall or humidity, can cause the ingress of water into the container and as some fluids are hygroscopic (absorb moisture) they require extra measures to be taken to reduce the chance of contamination.
In addition, good housekeeping and practises relating to food and drink will also reduce the risk of contaminants entering a system.
Always use lids on tanks, and if necessary use a hygroscopic breather to further reduce the risk of moisture entering your system.
Various de-humidifier products are available on the market and are quite effective at removing entrained water. Good quality controls and working practises are the real key to eliminating water all together. Consider the working environment. Try and reduce large temperature fluctuations and be mindful of the weather outside, especially the dew point temperature.
Water is a chemical contaminant in oils and at 100% relative humidity will exist as bubbles in hydraulic systems. Where water is present in a system, the effects can be dangerous if not monitored and controlled properly.
When free water exists in an oil, the bubbles it forms can interfere with particle counts, and therefore effect the desired outputs. Typically excess water will give a dirtier reading than actual system cleanliness.
At MP Filtri we offer water sensor options with our products which can monitor levels of water in your hydraulic oil allowing you to keep your system in good condition. Poor water monitoring and control can lead to some or all of the following symptoms:
- Shorter component life
- Wire erosion and vaporous cavitation
- Hydrogen embrittlement
- Oxidation
- Component wear
All of these can exist to varying degrees of severity depending on individual system designs, however the results can be catastrophic for both equipment and personnel. At MP Filtri, we recommend that you set a maximum alarm setting for water content in your oil, and where possible aim to operate well within this limit so that water never becomes a problem.
In the interests of system life extension, oil should always remain relatively translucent. Where high concentrations of water are present, oil may turn cloudy or opaque and consideration should be made as to whether the oil should be replaced with new.
To remove the risk of water induced failures, the following list can be used for consideration.
- Oil management and handling
- Use of breathers or tank headspace protection
- Washing down of systems and protection during this operation
- IP rating of the equipment fitted to the system and its susceptibility to moisture ingress
- Formation of condensate on the surrounding area
- Secondary sealing for critical applications
- Store oil drums indoors
- Periodic draining of particularly susceptible systems
- Operator training
Good Housekeeping practises are essential. Below are a few steps you can take to make an immediate difference:
- no food and drink near your process
- pre-filter your oil before placing into or returning back to the tank
- use a dedicated funnel for that type of fluid for pouring into the tank
- have a dedicated fill point for the reservoir
- use a sloped or conical tank design with an outlet at the bottom so that contaminants captured by the first bank of filters
- after filling or topping up with new oil, let the system flow and filter, reaching a natural equilibrium point before using live in your process
The answer to this question varies from customer to customer, depending on their requirements and system conditions. What can be said is that the decision to control contamination is normally based on the sensitivity of the components within the process (e.g. servo valves, actuators). There is widely publicised data on the clearances in these types of component. It can also be found in our handbook here.
One of the main things which is overlooked in the industry is scale of cleanliness we are trying to control and measure. This is important to consider as it may change the way you choose to use your data to get a more realistic picture of system conditions over time. Below is a diagram showing the typical size particles we filter every day and measure with APC’s compared to common objects. It puts into perspective the challenge faced when designing a system. To eliminate all contaminants below a certain size is extremely difficult when you consider all the possible sources of contamination surrounding the system. Care should always be taken to select the right equipment and use suitable statistical methods when evaluating data, making decisions and taking action.
This can vary considerably depending on the type of system and installation, but below are some typical types of contamination. By looking at the certain types, conclusions can often be drawn as to where the contaminant may be entering the system. Steps can then be taken to reduce the effects of such a contaminant….
- metallic — both ferrous and non-ferrous
- silica (dirt, dust)
- silt
- filter fibers
- bacteria colonies
- water
Contamination can induce excessive stress on system components like pumps and valves as well as potentially clogging orifices, nozzles, and jets.
One of the main areas of degradation is the formation of oxygenated & heavy polymeric compounds. These compounds are often in-soluble and settle out of the fluid as a gel or sludge. The creation of such compounds is accelerated in the presence of water and metal and so care should be taken to remove these types of contaminant from your oil.
Typically when a fluid is contaminated its viscosity will increase, leading to higher than normal friction and subsequent temperature increases. This can reduce system efficiency, wear components and effect compression rates. In the worst-case contamination can lead to catastrophic failure.
Below is a list of common complaints associated with un-suitable fluid condition.
- mechanical wear
- clogging of nozzles, orifices and valves
- icing
- corrosion
- loss of protective coatings on components
- increased operating temperatures
- change in fluid compressibility
Unlike laboratory conditions real world applications are constantly changing. As a system operates, contamination is generated and needs to be controlled. As it is physically impossible to achieve 100% efficiency in any given system, some particles will always get through filtration. This is one source of variation.
More often than not it is assumed that downstream of any filtering and purification the fluid is “clean” however this may not be the case. As in most hydraulic systems, construction is mainly metal or elastomer/textile-based. Over time, and in reaction to changing fluid conditions such as temperature, pressure and chemical decomposition, these materials can become susceptible to corrosion and leach out contaminants into the system.
Homogeneity plays a significant part in accurately assessing contamination in a system. A homogenous solution is uniform in its composition and particles are evenly distributed within it. It is fair to conclude that the majority of real world systems are heterogeneous (un-evenly composed) and therefore when taking measurements this must be considered as a significant variable between tests.
Factors including but not exclusive to viscosity, temperature, electrical conductivity, surface tension can contribute negatively to the overall quality of your fluid.
Beta rating is the most commonly used rating in industry for filters. Its comes from the multi-pass method for evaluating filtration performance (ISO 16889:1999).
The Beta rating itself refers to filtration efficiency, however it should always be used in collaboration with the absolute rating to understand what contamination is likely to be seen in the system. See the table below for guidelines.
If you know how many particles you have upstream of your filter, from the ratings above you should be able to calculate how many particles appear on the downstream side.
For example, 1,000 particles of a given size upstream on a beta ratio of 20 (95% eff.) will means that 50 particles of that will not be caught by the filter.
A beta rating does not give any indication of dirt holding capacity, nor does it account for stability or performance over time. It should also be pointed out that the micron rating of a filter will not catch all particles greater than that size, mainly due to limitations such as metrology, materials technology & cost implications. The Beta ratio is the same for all particle size ranges.
Nominal ratings on filters are micron values given to filters by the manufacturer. They relate to the typical, or average micron rating for the filter. This does not mean that they will not let particles through which are far greater than nominal rating. There efficiency is less than that or an absolute filter, so you would expect a longer clean up time with this type of filter element.
Absolute ratings give the size of the largest particle that will pass through the filter. This is a much more reliable means of assessing a filter for an application as its performance is more repeatable.
There are however no standardized test methods to determine this value currently.
Beta ratios are still the most commonly used method for specifying and selecting filters.
The simple answer to this question is, many. Filter efficiency can be affected heavily by changes in viscosity, fluid homogeneity, electrical conductivity to name but a few. There is currently demand from industry for more standards and norms surrounding test methods. In recent years, due to the wide range of fluids available, some groups have created their own standards for test methods etc, such as automotive, drinking water & pharmaceutical.
When choosing filters, it is important to understand what the beta & absolute rating is, thereby understanding what the largest particle size should be in the system. Couple this with an APC and you can quantify the number of particles of a given size in the system, and start the process of quality control. Several filters in series will often improve the cleanliness of a system, as will exposure time.
Logic would dictate that you change your filters when the system cleanliness starts to increase above useable levels, and this is true to a degree but in actuality, most filters become more efficient as they become more full. The main driver for most people to change their filters will probably be flow.
As the filter becomes more blocked, the flow-rate through it reduces, and therefore the pressure differential increases. Most filters can be fitted with differential pressure indicators which help you to identify when to change them. For optimal performance, an automatic particle counter coupled with a flow-meter downstream of your filters will provide the greatest degree of accuracy.
Most commonly by the light extinction principle, but there are some other technologies on the market. Typically a beam of light is projected through the sample fluid, when a particle blocks the light, it results in a measurable electrical signal that can be proportioned to the size of the particle. Couple this with a known sensory volume and the quantity of each size can be determined.
As with all APCs, they rely on statistical analysis of a volume of fluid to derive an international standard format output. When an APC measures some fluid, typically it is only sampling a proportion of the total system volume, and there in lies a source of error per test result. Add to this the un-quantifiable variation in fluid homogeneity & other factors and quite quickly you realise that a more statistical approach is required. When conducting more than one test per day, we would recommend performing the tests at set equal intervals to paint as clear a picture as possible of how your process varies during the day, and perhaps over the course of a week or month.Most commonly by the light extinction principle, but there are some other technologies on the market. Typically a beam of light is projected through the sample fluid, when a particle blocks the light, it results in a measurable electrical signal that can be proportioned to the size of the particle. Couple this with a known sensory volume and the quantity of each size can be determined.
The biggest difference is the fact that you are removing fluid from a system rather than measuring in real time. On-line measurement means that you are seeing the true and real time behaviour of the system, whereas off-line sampling is exposed to a number of variables prior to the fluid passing through the APC. This can lead to error if care is not taken. From a practical point of view, sometime systems do not have test points attached to them, which can lead certain decisions about how to analyse fluid.
Using & analysing international reporting formats exclusively does not provide a true picture of whether a system is in control or out of control.
Although all of the international formats are based on a sound scaling method, they are all sensitive to a change of just one count at any concentration. For example, ISO 14 denotes that you have between 80 & 160 particles of a given size in your system. If the concentration in the system changes to 161, the APC will output a result of ISO 15. Conversely, if the count drops to 79, then the result will be ISO 13.
The question is, does a change of one particle count justify a decision to take action or not? What must be considered is at what point does cumulative effect make a difference to the function of the system?
Although it is easy to arbitrarily set limits, we need to understand how close we are to exceeding them. If ISO 14 is your upper limit of contamination, and at no costs should the system exceed ISO 14, then it would not be very responsible to be unknowingly operating at 99% or even 80% of your upper control limit (Certainly not for an extended period of time).Although international reporting formats are useful, and in a lot of cases are practically suitable, it is always good to understand the importance of detailed counts to paint a clearer picture of the situation and to set achievable control limits.
When it comes to contamination monitoring, entrained bubbles (commonly air or water) in a fluid can cause instability in output readings as the tiny bubbles can be “seen” by the sensor within the product. Where systems have large amounts of aeration, this can lead to a higher contamination reading than would normally be expected, and therefore confidence in system performance can be questionable. In addition, on-line and off-line sampling can also have an effect as the fluid is being removed from the system, and therefore it is always possible that by removing it you are altering its natural state.
It all depends how controlled you need your system to be. Dirtier systems can typically cope with a greater variability of result and as such are not as critical in the way they need to be controlled. Where possible we would always recommend analysing fluid straight from the system for the most representative data.
Automatic particle counters (APC’s) are instruments that quantify the size and quantity of particulate contamination in fluids. Some products have secondary functions such as the ability to measure temperature and moisture content. They normally output results in standard international formats (AS4059E, etc) and more often than not, the data from the units can be stored and retrieved for ongoing analysis of a system. They are currently broken down into two distinct categories, portable and in-line.
Automatic particle counters have existed since the 1960’s. The principle on which they operate has stayed close to their original concept, but over time they have been developed using methods such as lense & light source technology. Historically, particle counting was always done by a fairly rigorous and extended method such as optical microscopy which involved physically counting the particle concentration. As time, and demand has increased for this kind of data, a new technology was required to make analysis more practical and cost effective for the user. Automated particle analysis was more practical and cost effective.
Moisture sensors, RH% or ppm, operate typically on a capacitive method utilizing a dielectric sandwiched between two metal plates. Various substances, such as air, oil and water have specific dielectric values which allow for calibration of the sensor. For example, the dielectric value of water is 80. The dielectric value for the polymeric sensor is approximately 3. The change in the sensed dielectric, allows for a percentage figure to be arrived at. For example, if the dielectric is 45, then the RH% will be ~58%.
Importantly, all moisture sensors in oil are at risk of damage under prolonged exposed to free water. Currently there is no economic technology that exists specifically for moisture sensing in liquids. However, through testing and development, sensors designed for use in air can be adopted and applied. We recommend that suitable pre-control be applied when setting alarm limits for moisture content. This will benefit the sensor and the system. When analysing your system/process, take a few moisture readings and applying sound statistical methods to arrive at your system capability.
Relative humidity (RH%) describes the amount of water vapour in a hydraulic fluid. When the vapour content increases to a point where is condenses out of the fluid, this is termed “saturation” or “free water”. Whilst in the vapourized state, the water is dissolved and of little consequence to the system. Once it becomes saturated, the water exists as little droplets of water.
A saturated system will give an RH reading of 99%/100%. Generally speaking, an RH reading of 30% to 70% is typical of hydraulic system. The variation in reading more often than not is related to ambient temperature changes. You would expect to see higher RH readings in winter than in the summer months for example. There is no such thing as too little water in a hydraulic system. Always keep moisture levels as low as possible, and do not allow free water to exist in your processes!
If used responsibly and with the correct approach to quality control, both ppm & RH% are excellent ways to measure moisture content in hydraulic fluids. At MP Filtri we have chosen to standardise on RH% as this provides us the greatest degree of flexibility and service to our customers.
To successfully use ppm on a wide range of fluids, you would need to test and validate a saturation curve for each particular oil. Given the sheer number of fluids available in the industry this can become an un-ending task in the laboratory. Take into account un-predictable error due to changes in fluid chemistry in the live environment and you have quite a complex problem to solve.
Outputting RH% on the other hand does not have this problem. Because it is a measure of the % of saturation, it does not need to be calibrated for specific fluids like parts per million. As long as you are measuring temperature at the same time (in built into the MP Filtri sensor technology) you can compare systems fairly, using the same datum position (saturation).
The saturation point of a brand new fluid sample in parts per million is validated in the laboratory as being 800ppm (100% RH). The engineer installs a moisture sensor to a system containing the same fluid, and sets an alarm limit of 640ppm (80% RH). The process is set in motion, and the initial sensor reading is 400ppm (50% RH). Everything is OK.
Lets now assume that real time changes in the chemical make up of the fluid due to wear and tear cause the saturation point to reduce to 420ppm but the system reading remains at 400ppm. The operator will continue as normal, and the upper control limit alarm (640ppm) has not been reached. What the operator doesn’t know is that the system is now running at 95% saturation which is perilously close to free water existing in the process and above the 80% threshold set in the alarm. Consider that for the alarm to signal, you would have to have free water in the system! This is a process out of control and the only way of making it capable would be to validate the saturation point of samples taken at set intervals throughout the systems life.
If the engineer had been using RH% from the start and given the example above, the alarm would have been raised when the saturation point of the fluid had reduced to 625ppm (100%). The alarm limit would remain at 80%RH, but the equivalent ppm value would now be 500.
For some customers their applications are critical to the point where they need to know the cleanliness at point of use. A larger number of tests and/or longer test times will ensure a more representative result (See “What factors can effect particle concentration and distribution in my system).
It is however important to monitor trend over time, and make a fair appraisal so that the right courses of action can be taken to keep system quality at the right level. If you require point of use results, this trending can be done as part of commissioning of your equipment so that you are informed from day one.
Normally pre-control charts are used to monitor information from systems. Acceptable cleanliness, or moisture level should be set at a limit well within the upper control limit (Red alarm) so that the system always performs the way it is intended. It is also recommended to use the detailed counts when analysing data, as this can be used more accurately and with greater flexibility.
When analysing your data we would recommend working to at least 4 standard deviations from the mean (Amber alarm) providing a 99.3% confidence interval when predicting the next result. If you are analysing data over the course of 1 day, try to take data points at set intervals throughout that working day to take into account any change in system distribution.
In statistics and probability theory, standard deviation shows how much variation or “dispersion” exists from the average. A low standard deviation indicates that the data points tend to be very close to the average, whereas high standard deviation indicates that the data points are spread out over a large range of values.
The standard deviation is the square root of its variance for a data set. A useful property of standard deviation is that, unlike variance, it is expressed in the same units as the data.
As discussed in previous sections of this knowledge centre, most systems are heterogeneous in nature. The contaminants in your process are not evenly distributed and therefore the data can vary from instrumentation one minute to the next.
Process capability is the ability for a system to maintain a set working level. In most instances, averages along with the standard deviation need to be taken into account to arrive at a repeatable and predictable result. Taking the average result in isolation can be a source of error.
Knowing the capability of your system can inform you about decisions to replace filters or add filters to your system. Depending on how efficient your system is, it may extend or shorten the time between element replacements. In addition to this, you can use this statistically sound data to help you in other areas of continual improvement and proposal justification. It also makes for more suitable warning alarm limit settings.
