• 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.

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.