Efficient Plant February 2020 - 15

feature | lubrication strategies

Breather life % (RH)

Particle count

Particle size (µm)

is the nature of the result obtained.
Whether it is a vibration analyst making
Fig. 1: Real-time particle counting during system flushing
a reading or a lubrication technician
10,000
25
acquiring a sample, both technologies
9,000
are taking an instantaneous snapshot of
20
8,000
what is happening with the machine at
7,000
the moment of the reading or sampling.
15
6,000
In situations where machine operation
5,000
10
4,000
is constant, with the same load, duty
3,000
cycle, and operating parameters, this
5
2,000
may seem like a subtle point. However,
1,000
for assets where operational conditions
0
0
are constantly changing, the results can
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Sample dates/times
render either technologies less effective.
Temperature
4-µm
6-µm
14-µm
4-µm particle
By way of illustration, consider a
particle
particle count
particle count
count (linear
count
trend)
vibration analyst taking a reading from
an electric motor driving a centrifugal
Data in this graph show the results of flushing a hydraulic system using a bypass filtration unit
during normal operation.
pump. While the load on the pump and
motor might be constant, if the pump is
operating close to a vibratory screen that
runs intermittently, the reading from the motor may differ dependFig. 2: Hydraulic system water ingression
ing on whether the screen is running or idle. In vibration analysis,
Normal
a good analyst is trained to account for operational variants. In
oil analysis, since the analysis is one step removed from sampling,
operational changes may go unnoticed.
Abnormal
In oil analysis, the data is also an instantaneous snapshot of how
Routine oil
a machine is operating at the exact moment the sample is acquired.
sampling
indicates
This can have a profound effect on the results. Consider the example
low water
shown in Fig. 1. The data shows the results of flushing a hydraulic
content
(30 ppm)
system using a bypass filtration unit during normal operation. To
Next scheduled
oil sampler
insure that filtration is having the desired effect, the overall cleanliness of the oil is being monitored using real-time particle counting in the 4-, 6-, and 14-μm particle size range per ISO 4406:99.
Time
While the overall trend in particle contamination is decreasing, as
Graph shows hydraulic system water ingression identified using a "smart"desiccant breather capable of measuring changes in headspace humidity.
measured by the linear 4-μm trend, the shot-to-shot variation in
all three particle size ranges is significant, corresponding to when
pumps, valve, and actuators are starting and stopping.
Now, imagine taking a bottle sample sometime during the trend
breather is monitoring the relative humidity in the headspace of the
shown in Figure 1. Depending on the machine-operation phase, it
reservoir of a hydraulic system. Based on a bottle oil sample taken
is highly possible that the sample will by taken at either a peak or
during normal operation, the water content in the oil was measured
trough in the particle-count trends. As such, the reported particle
as 30 ppm (0.003% v/v), well below the limit for this application.
count may appear erroneously high or low since the bottle snap
However, two days after the sample was taken, the operations crew
is simply an instantaneous snapshot of what is happening in the
elected to clean the system using pressurized water. Based on the
system. Only by monitoring particle count in real time can the true
real-time data from the sensor, the headspace, and hence the breatheffectiveness of the bypass system be determined.
er, saturated within 24 hr. This alerted the maintenance team, in real
Figure 2 shows a similar example. In this case, a "smart"-desiccant
time, about a potential catastrophic situation that could have result-

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Efficient Plant February 2020

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