a feeling for what might fail
TRANSCRIPT
Feature3030
www.worldpumps.com
WORLD PUMPS April 2013
As a strategy, condition-based
predictive maintenance (CBPM)
provides the ideal compromises
between failure-based maintenance
(a.k.a. “run-to-fail”) and the potentially
equally expensive practice of conducting
maintenance on a periodic basis
irrespective of machine use.
Frequently, a rise, or in some cases a
drop, in vibration levels provides the
earliest indication that a bearing, gearbox,
motor or other electro-mechanical
component is in need of maintenance.
Moreover, analysis of the vibration signa-
ture can reveal the nature of the fault; for
example, a broken tooth on a gear or a
worn bearing.
During one inspection it was noticed that
the vibration levels at the impellor end of
one fan were higher than normal. The
data was analysed and it was found that
the overall RMS vibration amplitude had
more than quadrupled in the space of the
few weeks since the previous survey.
Mechanical looseness
The fan was displaying symptoms of
mechanical looseness; most likely some-
where along the shaft or a shaft-mounted
component judging by the vibration
signature. Plant staff agreed to take action
at the earliest possible opportunity.
However, that evening, the fan threw an
impellor blade which broke through the
fan housing and embedded itself into a
nearby control panel.
Had they their time again, those involved
in maintenance would not act in any
other way. Indeed, based on the informa-
tion to hand, the best course of action
had been taken; as the data captured
during the survey would not have
condemned the fan and led to an imme-
diate shutdown, impacting on
production.
In essence, whilst the vibration signature
provided a strong, and as it transpired
accurate, indication of what was failing
it was not possible to estimate the rate
of deterioration. Had vibration levels
started rising the day after the previous
survey or only a few hours before the
Vibration monitoring is of particular use
when it comes to protecting plant and
machinery that is not only expensive in
its own right but is also heavily depended
on to keep operations going. One ques-
tion though is: how often should vibra-
tion levels be measured as part of a
CBPM strategy?
For instance, a steel mill in the United
Kingdom, recognising the usefulness of
vibration monitoring, used to conduct
surveys on a monthly basis. These surveys
included the use of portable measuring
instruments to check the vibration levels
of, amongst other machinery, fans
supplying the furnace’s air/fuel mix and
also used to expel waste gases.
Condition-based predictive maintenance (CBPM) is increasingly
popular as a practical and cost-effective means of protecting pumps,
motors, gearboxes and other machinery. But of all parameters that
might be monitored as part of a CBPM strategy, vibration could be
the most useful – advises Monitran's Andy Anthony.
A feeling for what might fail
Operating
0262 1762/13 © 2013 Elsevier Ltd. All rights reserved
Figure 1. An MTN/1185 vibration sensor on a pump at the Adom desalination plant.
WOPU0413_Feat_Monitran 30 28-03-13 09:54:22
Feature 31
www.worldpumps.com
WORLD PUMPS April 2013
one conducted that day? Just where
were they on the failure curve?
Had the survey taken place a day later,
the impellor blade would of course
already have been thrown; and from a
seemingly healthy machine going by the
previous survey’s readings.
There is therefore much merit in
performing condition monitoring all of
the time and vibration sensors are increas-
ingly being employed as permanent
fi xtures to pumps, motors, gearboxes and
other assemblies to supply monitoring
and control systems with vibration data.
Accordingly, alarms can be raised the
moment vibration levels warrant investi-
gation and, worst case, machinery can be
automatically shut down if vibration levels
rise so quickly there is no time to
investigate.
There is also another reason for
monitoring vibration levels. Vibration is
the movement of something, and that
requires energy. Hence, if a powered
electromechanical assembly is vibrating
more than it should be then it will
undoubtedly also be less effi cient (in
terms of power in vs. work achieved) than
intended.
For example, Monitran’s vibration
sensors were recently used as part
of a valve monitoring system for a
compressor. Specifi cally, valve wear can
result in a high pressure gas leak into
the low pressure side of the system
– i.e. it does not physically escape –
so the compressor just has to work
harder to produce a given volume
of compressed gas.
The pressure is ON
One sector making very good use of
vibration monitoring is the water industry,
particularly seawater desalination plants
using a process called reverse osmosis
(RO) to make water for human consump-
tion and agricultural use. While salt can
be removed from seawater in a number
of ways – including distillation, freezing,
dehumidifi cation – RO is used extensively
in many countries for producing drinking
water.
The RO desalination process requires
passing seawater at very high pressure
(circa 1,000 psi) through a semi-perme-
able membrane. This requires a great deal
of energy and is why most RO desalina-
tion plants are located close to power
stations.
One country particularly reliant on
seawater desalination is Israel. It has three
fully operational plants and a further two
scheduled to come online shortly. The
ones in operation are: Adom, near
Ashkelon, which came online in 2005; Via
Maris near Palmachim, which came online
in 2007; and Omis-Water, near Hadera,
which came online in 2009.
Between them, the three plants have
several hundred high pressure pumps,
each of which is fi tted with at least one
vibration sensor. The majority of these
sensors are Monitran MTN/1185 general
purpose, top-entry transducers (Figure 1
shows a sensor in situ on a pump in the
Adom desalination plant).
These sensors output a direct current
(DC) in the range 4-20mA, proportional
to velocity. The outputs then feed into
PLCs which control the pumps, and in
which two pre-determined alarm
levels have been set for each sensor.
If vibration levels reach the fi rst level,
an alarm is raised. If vibration levels reach
the second and higher level, the PLC
automatically shuts down the relevant
pump.
Advice
Above, we have cited the use of general
purpose sensors with DC outputs. Sensors
have a number of characteristics which
should be considered during the selection
process. In addition to top-entry (as per
Figure 1), sensors are available where the
cable enters from the side. Choosing
between top- and side-entry will be
governed by how you plan to run the
cables. Also, the connection to the sensor
can be integral (as per Figure 1) or via a
connector.
Should you wish to go down the route of
monitoring vibration levels at set intervals
(i.e. a temporary installation) then sensors
are available with magnetic bases. It is
essential to use the same sensor in
exactly the same position for each
reading and you may wish to consider
using paired ID tags/labels.
Next, the presence or risk of water, high
humidity levels or hazardous atmospheres
will steer you toward certain sensor
sealing characteristics (IP67, 68...) or
intrinsically safe (ATEX, IECEx) devices.
Temperature needs to be considered too,
as output drift (with temperature) may be
an issue; but only if you intend to operate
Vibration sensors are typically electronic
devices, employing either piezoelectric or
piezoresistive technology. Of these two
technologies, the former is more
prevalent in monitoring applications.
In the diagram, the mass is bonded to a
piezoelectric crystal which, when placed
under tension or compression, generates
an electric charge proportional to the
acceleration of the mass; hence why
vibration sensors are also referred to as
accelerometers.
The electric charge is conditioned by the
circuitry (PCB) to produce a more useful
output; typically a direct current (DC) in
the range 4-20 mA for ease or integrating
the sensor into a monitoring system or a
Programmable Logic Controller (PLC).
However, sensors with AC outputs are
also popular.
The output (proportional to acceleration
in mm/s/s) may also be conditioned to
represent velocity (mm/s) or displacement
(mm), and sensors are also available with
multiple outputs (e.g. acceleration and
velocity).
With no moving parts, a piezoelectric
sensor off ers long-term reliability and
stability. Typical frequency responses
range from about 1 Hz to 10 kHz or
more. As for the operating temperature
range, even a general purpose sensor will
cover -25 to 90°C.
The vibration sensor
WOPU0413_Feat_Monitran 31 28-03-13 09:54:22
Feature3232
www.worldpumps.com
WORLD PUMPS April 2013
the sensor close to its declared operating
limits. See box ‘Taking Measure.’
Summary
CBPM helps plant/facility owners walk
that fi ne line between “run-to-fail” and
over servicing machinery, and there is
much to be said for monitoring 24/7.
Also, of all parameters that could be moni-
tored, vibration is perhaps the most useful,
as the analysis of vibration signatures can
provide an indication of which components
are failing or have already failed. With such
knowledge you can better schedule mainte-
nance routines and make sure replacement
parts/components are delivered in time.
There are several diff erent types of vibration
sensors available on the market, but selec-
tion need not be a daunting process if you
follow the above advice regarding charac-
teristics and measure normal operating
vibration levels to determine the sensor’s
sensitivity; thus enabling you to determine
the limits at which to raise an alarm or
perform an emergency shutdown.
www.monitran.com
For any monitoring application, it
is important to make optimum
use of the vibration sensor’s
output range, without sacrifi cing
sensitivity. For those interested in
the maths, it is worth noting that
many sensors are guaranteed to
operate across the range ±80 g,
though they can typically
experience far greater forces
without fear of damage.
Assuming a sensor’s sensitivity is
100 mV/g the output will swing
between -8 and 8 V across its
declared operating range.
However, if the sensor is likely to
be exposed to no more than ±8 g
then it would be wiser to select a
sensor with a sensitivity of 1 V/g
as the output voltage swing will
provide better resolution (10x in
fact) of the vibration levels.
Accordingly, if you are considering
mounting vibration sensors onto a pump
or motor as part of your CBPM strategy it
is always worth measuring the vibration
levels fi rst. For this purpose a hand-held
instrument like the MTN/VM220 will
certainly fi t the bill.
Conforming to ISO10816-3, the MTN/
VM220 can measure and store readings
for vibration (as acceleration, velocity or
displacement) and temperature against a
real-time clock. It is supplied with a probe
into which is integrated a dual-output,
general purpose sensor, the MTN/2200T
(with default sensitivities of 100 mV/g and
10 mV/°C) and to which can be attached
a magnetic base or spike.
Such a hand-held instrument is therefore
very useful for establishing the normal
operating conditions of machinery before
installing vibration sensors on a
permanent basis or for maintenance
engineers taking readings on a scheduled
basis, as part of their CBPM processes.
Taking measure
Contact
Andy Anthony
Monitran Ltd.
Monitor House Hazlemere Road
High Wycombe, Buckinghamshire HP10 8AD
United Kingdom
WOPU0413_Feat_Monitran 32 28-03-13 09:54:22