APPLICATION NOTE #213
Contamination Monitoring in the Hydraulics Industry
Maintaining the Balance
Operators of planes, trains, heavy
equipment, and other systems that rely on hydraulics live with the
constant battle of scheduling maintenance. The balance of when to
change the fluid, filters, or other parts to avoid down time and
excessive repair costs versus the cost of maintenance and replacement
is delicate. A wrong decision can result in unnecessary maintenance,
increasing operating costs; on the other hand, delaying maintenance can
result in a catastrophic failure, increasing ownership costs.
Over 90% of catastrophic failures are the result of
abrasive wear that causes the pumps, valves, cylinder rods,
differential drive, steering, clutch, or transmission systems to fail.
Part of the role oils and hydraulic fluids play is to coat these parts,
helping to reduce friction and abrasive wear. Yet these same
"protective" oils may be contributing to a catastrophic failure. Over
time, the oils themselves are contaminated with particles that can
contribute to wear on the machinery. These particles come from a wide
variety of sources, including particles left on new machinery or parts
by the factory, dirt and dust from the outside environment, wear and
tear on the machinery and parts, and even contamination in the oil
itself. Over time particles from all these sources build up in the oil,
and eventually can cause temporary or permanent equipment failure.
These particles can cause many types of failures:
1. The particles can clog small orifices that control critical
hydrostatic balances, causing a catastrophic failure.
2. Particle contamination can also cause moving parts to completely
lock.
3. The particles themselves may cause additional wear and tear on the
machinery and parts, creating more particles that contaminate the oil.
4. Each of these problems can cost thousands of
dollars in damage if not prevented.
One way to reduce the particle contamination level
in the oil is through the use of filters. The filter is placed before
the critical part to prevent the particles from reaching the part,
damaging it. Unfortunately, over time the filters become clogged, and
prevent the oil itself from reaching the part. Additionally, the
filters can develop holes, allowing the particles through to the part
they are intended to protect.
Monitoring the Contamination
Level
For many years operators have been trying to
develop a balanced maintenance schedule by using a regular oil analysis
program. One of the most common methods of analyzing the oil is
spectroscopy. Several types of spectroscopy are used, including atomic
absorption, ICP, and infrared.
Using spectroscopy, analysts can analyze the
composition of the oil; however, only limited information regarding
particle contamination can be determined. For instance, spectroscopy
can only analyze very small particles; the upper limit ranges from 5 to
15 µm. While it is important to know the composition of the oil, these
small particles are usually not the cause of equipment failures; larger
particles are often responsible for these failures.
Measuring Critical Particles
In the 1960's laboratories began using particle
counters to monitor particle contamination levels in hydraulic fluids
and oils. Recent changes in particle counting technology, especially
the introduction of portable particle counters, has increased the use
of particle counters throughout the industry.
Particle counters are typically used to measure
particles from l µm to 100 µm in size, so the number of the critical
larger particles is known. Therefore, the particle count information
gathered using particle counting complements the contamination
information gathered using spectroscopy.
Understanding Particle Counting
The basic particle counting system is composed of
three parts: the sampler, the sensor, and the counter. Additional
equipment, such as a software package, is often added to make the
system more efficient and useful. The sampler's primary purpose is to
assure the sensor receives a constant, even flow of sample. Many
samplers have additional features that allow the analyst to regulate
the flow, such as a vacuum regulator or pressure regulator.
The sensor measures the particles in the oil
received from the sampler. A light extinction sensor is commonly used
to test oils. As the sample moves through the flow cell it passes
through an area that is illuminated by a constant-intensity light from
a laser diode. The intensity of the light is monitored by a
photodetector on the opposite side of the flow cell. The intensity of
the light when no oil is passing through the flow cell is the base line
for subsequent measurements. As particles in the oil pass through the
light beam, they block part of the light, preventing it from reaching
the photodetector. The photodetector converts the decrease in light
intensity to an electrical signal and transfers the electrical signal
to the counter. The counter, in turn, converts this signal into a
particle size. A software package can be used to store, manipulate, and
view the data.
Figure 2 - IS0 4406 Table
| NUMBER OF
PARTICLES PER MILILITRE (COUNTS/ML) |
SCALE NUMBER |
| MORE THAN |
UP TO AND INCLUDING |
|
| 2 500 000 |
|
>28 |
| 1 300
000 |
2 500
000 |
28 |
| 640 000 |
1 300
000 |
27 |
| 320 000 |
640 000 |
26 |
| 160 000 |
320 000 |
25 |
| 80 000 |
160 000 |
24 |
| 40 000 |
80 000 |
23 |
| 20 000 |
40 000 |
22 |
| 10 000 |
20 000 |
21 |
| 5 000 |
10 000 |
20 |
| 2 500 |
5 000 |
19 |
| 1 300 |
2 500 |
18 |
| 640 |
1 300 |
17 |
| 320 |
640 |
16 |
| 160 |
320 |
15 |
| 80 |
160 |
14 |
| 40 |
80 |
13 |
| 20 |
40 |
12 |
| 10 |
20 |
11 |
| 5 |
10 |
10 |
| 2.50 |
5 |
9 |
| 1.30 |
2.50 |
8 |
| 0.64 |
1.30 |
7 |
| 0.32 |
0.64 |
6 |
| 0.16 |
0.32 |
5 |
| 0.08 |
0.16 |
4 |
| 0.04 |
0.08 |
3 |
| 0.02 |
0.04 |
2 |
| 0.01 |
0.02 |
1 |
| 0.005 |
0.01 |
0 |
Using Particle Counting
in the Hydraulics Industry
To gain a complete understanding of the condition
of the oil, and the parts and machinery themselves, it is vital that
the oil be regularly monitored and the results recorded. Over time,
these results can be trended, and used to create effective maintenance
schedules. These schedules assure the operator that excess money is not
being wasted in disposing of used oil or performing unnecessary repairs
while eliminating worry that an unexpected failure is about to occur.
Particle counters can also be used to check the efficiency of the
filters and schedule filter changes.
In addition to aiding maintenance scheduling,
particle monitoring can identify a sudden increase in the number of
particles in a sample. This sudden increase can indicate an imminent
failure that might be avoided by unscheduled maintenance. In the long
run, performing this maintenance before the failure occurs will save
thousands of dollars in repair costs and unscheduled downtime on the
equipment.
Interpreting Particle Counting Results
For many years particle counters have been used to
monitor contamination levels in hydraulic fluids in military aircraft.
Because of this, many standards have been developed to judge test
results. One of the most commonly used standards is IS04406, which was
recently updated. Previously, this standard was a two-digit code
representing the cumulative particle counts/ml at 5 and 15 µm. These
two sizes were selected because it was felt that particles larger than
5 µm would settle and coat the surfaces of the parts, while particles
larger than 15 µm could cause excessive wear on machine parts. The new
update adds a third digit to the code. The use of this number,
representing particles larger than 2 µm, is not mandatory.
Once a sample has been analyzed the ISO code can be
applied in two ways. The first method, which is the most commonly used,
involves simply reading the code level off the table. First the code
for 5 µm is determined, then the code for 15 µm; the two numbers are
written separated by a slash. The second method involves plotting the
results at 5 and 15 µm on a graph, which has the ISO code overlayed on
it. While this method is slightly more involved, it provides more
information since it shows the actual distribution curve.
HIAC-ROYCO Particle Counting
Systems
HIAC Royco manufactures a complete line of particle
counters for use in testing hydraulic fluid and oils. The systems
described below were developed for specific aspects of the fluid power
industry. In addition to these systems, HIAC Royco manufactures a
complete line of sensors, samplers, and particle counters for all
applications.
8011 Hydraulic and Parts
Cleaning Particle Contamination Monitoring System
The 8011 Hydraulic and Parts Cleaning Particle Counting Monitoring
System was designed to analyze samples of hydraulic fluids, solvents,
and aqueous solutions in a laboratory setting. This versatile system
has many uses, including monitoring particle contamination levels in
mobile and industrial hydraulic systems, measuring roll-off cleanliness
of equipment, and testing cleanliness of parts cleaning systems.
BR8 Beta Ratiometer System
The BR8 Beta Ratiometer System was designed for beta ratio analysis to
monitor filter performance.
8012 High Viscosity Fluid
Particle Contamination
The 8012 High Viscosity Fluid Particle Contamination Monitoring System
was designed to analyze samples of gear compartment, transmission, and
hydraulic fluids in a laboratory setting. Each component of the system
meets the demanding requirements presented by dark, viscous, gear
compartment fluid. The system includes Particle Analysis and Reporting
Software. Developed specifically for use with the 8012, this software
allows the analyst to operate the system from the computer and view
results on the computer screen.
C600 Contamination Test
Center
The C600 Contamination Test Center was designed for the analysis of
hydraulic fluids in aerospace, fluid power, and heavy equipment
manufacturing plants, or any analytic lab where clean oil is needed.
The C600 works for mineral oil, Stoddard solvent, MIL-H-5606 hydraulic
fluid, MIL-H-6083 hydraulic fluid, and Shell Tellus #25-IOWT (or
equivalent).
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