How Cleanliness Your Hyd Oil

How Clean Is Your Hydraulic Fluid?

Of all the oil-lubricated assets found in industrial plants, hydraulic systems are by far

the most sensitive to contamination. Put simply: kept clean, they should run reliably;

allowed to get dirty, and it’s likely that problems will occur. The reason for this

sensitivity stems from the unique components used in hydraulic systems. From pumps

to valves, cylinders to motors, clearances are tight, making even the smallest particle

or water droplet a potential problem.

The design of hydraulic systems is, of course, many and varied and beyond the scope

of this article, but they all share some common features with respect to the hydraulic

fluid power system. First is the reservoir. Designed well, the hydraulic reservoir can aid

in contamination control by allowing contaminants to either drop to the bottom of the

tank or be removed by supplemental kidney-loop filtration that may be installed. By

contrast, poor design, which might include too small a tank for the required fluid flow or

suction, and return lines that are too close without adequate baffling between them,

can cause problems to occur.

Next up is the pump. For lower-pressure systems where gear pumps are

commonplace, contamination control is not as much of an issue since most gear

pumps are reasonably forgiving with respect to contamination. By contrast, vane and

piston pumps—particularly where variable volume pumping is required—have very

tight clearances and by inference a much lower tolerance to contaminants.

Finally, we need to consider the flow control valves. Again, the sensitivity of valves to

contamination can vary widely. Simpler systems where check valves or directional

valves are used are typically far less prone to contamination-induced failure compared

to more complex servo-controlled systems, which are very sensitive to contamination,

particularly where valve dwell times are long.

Developing a Contamination Control Strategy

For any plant that relies on hydraulics, developing a comprehensive contamination

control strategy should be high on the priority list. This is a fairly simple three-step

process:

1. Develop contamination control targets based on system design.

2. Take action to meet or exceed contamination control targets.

3. Use oil analysis to make sure target cleanliness levels are maintained.

Let’s look at the each of the three steps in more detail.

1. Developing Cleanliness Targets

For the purpose of this article, we’ll consider the two primary contaminants found in

most plants: particles and moisture. However, a similar three-step approach can and

should be used for other contaminations, such as air or heat, both of which can have a

deleterious effect on hydraulic systems. For particle contamination, our primary

concern needs to be silt-sized particles in the 1-micron to 10-micron size range. While

small in nature—less that 1/10th the thickness of a human hair—3-micron silt-sized

particles, which are no bigger than a red blood cell, are as much as five to ten times

more likely to induce a failure. The reason for this lies in the fact that many filters are

not designed to remove such small particles, coupled with the fact that dynamic

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clearances (the separation between moving parts under operating load, speed, and

temperature) in pumps and valves are typically in the 1-micron to 5-micron size range.

Particle contamination is usually expressed according to the ISO 4406:99 standard.

This standard reports particle concentrations in hydraulic fluids in three size ranges:

particle >4 microns, particles >6 microns, and particles >14 microns. For those

unfamiliar with this standard, a number of excellent articles are available online that

explain the standard.1

Based on the ISO 4406:99 standard, Table 1 shows recommended target cleanliness

levels for different types of hydraulic systems. For highly critical systems, the target

cleanliness levels detailed in Table 1 should be lowered by one to two ISO codes (i.e.

for a highly critical servo-controlled system running at 3,000 psi, we would lower the

target from ISO 15/13/10 as shown in Table 1 to ISO 14/12/9).

Aside from particle contamination, water is the second most insidious contaminant

found in hydraulics. Present in most fluids even in the most pristine environments,

water can increase failure rates 10-20 fold depending on circumstance. Water causes

problems in a number of ways: first, any iron or steel surface in contact with water will

start to rust. This can induce premature failure due to corrosion, as well as introduce

rust particles into the fluid. Second, water is very different to most hydraulic fluids in

that changes in pressure and temperature can readily induce a phase change. While

water may be a liquid under atmospheric pressure inside the reservoir, on the suction

side of a hydraulic pump the lower pressure can cause water to vaporize even at

relatively low temperatures. These vapor-filled bubbles will continue to grow until they

reach an area of high pressure (e.g. on the discharge side of the pump) when the

bubble suddenly and violently collapses. The intense pressures generated by such

microscopic implosion events can cause damage to pumps and valves—an effect

referred to as “vaporous cavitation.”2 Water also helps pull oil degradation byproducts

out of solution, which can cause sticky-resinous deposits to form. When these

deposits accumulate in the clearances of valves, they can cause small particles to

become trapped, further increasing the system’s sensitivity to particle contamination.

So how much water is too much? To a large extent the answer depends on the type,

the age of the fluid, and the operating temperature. The reason for this lies in the form

that water takes in lubricants and hydraulic fluids. Most fluids will hold a certain

amount of water in the dissolved phase. For the most part, as long as the water

remains dissolved, cavitation and corrosion will not occur. However, as soon the water

comes out of solution and becomes free or emulsified, water becomes a very real

concern. While highly temperature dependent, the saturation point of most

conventional hydraulic fluids—the point at which water starts to come out of

solution—is in the 100-200 ppm range (0.01-0.02%). Below these levels, most

hydraulic systems should be relatively free of water-induced failures.

2. Excluding Particles and Moisture

Once cleanliness targets have been established, the next step is to take measures to

meet the targets. To do this we need to focus on two areas: contamination exclusions

and contamination removal. Contamination exclusion focuses on making sure that

particles and moisture never make it into the system in the first place, while removal

requires the use of filters and other systems to remove them from within the system.

Our efforts should always start with exclusion since it costs as much as 10 to 15 times

more to remove contaminants than to keep them out from the start.

Contamination exclusion requires a holistic focus on all steps of the process of

lubrication—from receipt, storage, handling, dispensing, and finally use of the lubricant

within the system. Perhaps the first place to start is with the storage, handling, and



dispensing of new oils and recognition that most new oils coming into the plant are too

dirty for immediate use without pre-filtration. Even new oil in a barrel that has yet to be

opened will show particle concentrations in the 18/16/13-191/17/14 range and as

much as 400-500 ppm of water, which is too dirty and wet for most hydraulic

applications.

Because of this, the rule of thumb is that all new hydraulic fluids should be pre-filtered

at least five times prior to use. For most applications, using a 3-micron filter complete

with a permanent or portable kidney-loop filtration system is sufficient. For highly

critical applications where lower particle counts are required, we may even need to go

down to a 1-micron filter.

To remove moisture, we may also need to use a polymeric water-removing element.

These elements use a water-absorbent polymer (similar to baby diapers) that absorbs

water and retains it within the filter. Both water and particle removal can be achieved

simultaneously with two filter heads in series—first using a water-removal element

followed by a particle-removal element (Fig. 2).

The next step is to transfer oil from storage to the system. To do this, common

practice is to remove the oil fill port on the reservoir and transfer the fluid using a

transfer pump. However, in doing so we expose the system to airborne contaminants,

which can enter through the open fill port. A better approach is to install quick

connects so the system can be filled non-intrusively using the same filter transfer cart

shown in Fig. 2.

Once inside the reservoir, our job is not done yet. All systems breathe, so even if there

is no net flow of oil into or out of the reservoir, changes in ambient and operating

temperatures ensure that there will always be air exchange from the outside in and

vice-versa. For many hydraulic systems, of course, there’s a large exchange of air with

every cycle: as fluid leaves the reservoir—for example, as a rod extends out from the

cylinder—air needs to flow into the system to compensate for the volumetric change in

fluid level in the tank. When this occurs, many hydraulic systems suck in vast

quantities of dirty, contaminated plant air. Despite this, many hydraulic systems are

still designed with inadequate combination breather and filler caps, such as that shown

in Fig. 3. Within this fill cap/vent, particle exclusion is by means of a wire mesh, steel

wool, or foam—none of which exclude silt-sized particles—while water exclusion is

non-existent. Where present, standard breather filler caps should be replaced with a

combination manifold (Fig. 3), which permits the use of a high-efficiency particle and

desiccant (water-removal) element, as well as quick connects for oil fill and sample

valve for oil sampling. When selecting a desiccant breather, care must be exercised to

ensure that airflow rates through the breathers match the maximum anticipated oil flow

rate, but this can easily be achieved for even the largest systems.

3. Measuring contamination levels

The final step in the process requires the use of oil analysis to validate that our

contamination control measures are having the desired effect. For hydraulics,

measuring the degree of contamination in the oil should be commonplace. For particle

contamination, particle counting using the ISO 4406:99 standard outlined above

should be used, while for water content, % or ppm of water should be reported using

the Karl Fischer water test (ASTM D6304). However, oil analysis is only as good as

the sample that’s taken. Wherever possible, samples should be taken on the return

line from actuators. In some cases, this may mean multiple samples from the same

system where separate return lines are in place. For hydraulic systems without

adequate return-line flow, samples can be taken from the reservoir but will be less

indicative of what’s happening in the rest of the system.

Summary



Hydraulic system reliability is inextricably linked to contamination levels. Kept clean

and dry, well-designed hydraulic systems should be relatively trouble-free. Allowed to

get dirty, they can become unreliable and troublesome. Contamination control is

possible, even in the harshest environments. With just a few basic concepts and a

simple three-step process, controlling contaminants can be as easy as 1-2-3!

REFERENCES

1How Important is the ISO Cleanliness Code in Oil Analysis? Matt Spurlock,

Machinery Lubrication Magazine May-June 2012

2Proactive Maintenance for Mechanical Systems, E. C. Fitch, FES Inc., 1992



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How Cleanliness Your Hyd Oil