turbine supervisory guide turbine supervisory guide

turbine supervisory guide turbine supervisory guide
TURBINE
SUPERVISORY
GUIDE
TECHNIQUES FOR THE
MONITORING & PROTECTION OF
POWER PLANT EQUIPMENT
25
YEARS
1978 -2003
CONTENTS
03
Introduction
04
Overview
TSE Application Diagram
06
Transducers and Sensors
Accelerometer
Velocity Transducer
Eddy Current Proximity Probe
LVDT/RVDT
Choosing the right transducer
11
Measurement Techniques
Absolute Vibration
Eccentricity & Shaft Vibration
Rotor Differential Expansion
Shaft Position
Speed Monitoring
Casing & Cylinder Expansion
Valve Position Monitoring
18
Auxiliary Plant Monitoring
19
Protection Systems
Introduction to API Standard 670
20
Special Techniques
Rod Drop
Rundown Monitoring
Orbit Analysis
24
System Equipment
Bracketry
Cubicle Panels
26
Seismic Monitoring and
Protection Equipment
Site References
27
Quick Product Selection Guide
INTRODUCTION
SENSONICS
PRODUCTS & COMPETENCES
Turbine Supervisory Systems
For nearly 30 years Sensonics has been supplying
Turbine Condition Monitoring solutions to the
power generation industry worldwide. Involved in
measurement definition through to supply and
final system commissioning, our experience within
the power sector is second to none.
Standalone Monitoring Solutions
Accelerometer, Displacement &
Seismic Transducers
Nuclear Infrastructure Protection
We have produced this guide to capture the
essence of that experience and to explain the
basics of vibration and expansion measurement
techniques relating to turbine and auxiliary plant
equipment.
Structural Monitoring Solutions
Turn-Key Design, Manufacture and
Project Support
ATEX & IEC61508 Capability
The guide starts with an introduction to the basic
transducers available for plant mounting with
associated options, and details the various
measurement techniques used as standard
throughout the power industry. This is followed by
typical equipment protection configurations for
safe plant shutdown. In the final part of this guide
the system components are introduced and
special measurement regimes discussed.
Installation & Commissioning
The guide aims to provide a balance of basic
theory and practical advice but obviously cannot
cover all measurement scenarios. For a detailed
discussion on any measurement issues you may
have, please feel free to contact Sensonics.
3
OVERVIEW
Turbine supervision is an essential part of the day-to-day
running of any power plant. There are many potential
faults such as cracked rotors and damaged shafts, which
result from vibration and expansion.When this expansion
and vibration is apparent in its early stages the problem
can usually be resolved without any of the disruption
caused when a turbine has to be shut down. By
appropriate trending of the various measurement points
and the identification of excessive vibration or
movement, scheduled equipment stoppages or outages
can often be utilised to investigate and resolve the failure
mechanism.
It is for this predictive maintenance market that Sensonics
produces a wide range of sensors and systems specifically
for the power generation industry. With flexible and
configurable equipment, we can tailor our supervisory
equipment to your needs. In this brochure we aim to
give a brief explanation of why turbine supervision is so
essential and how Sensonics can provide the right
solution to protect your turbine.
The diagram on page 5 illustrates a generic configuration
of a set of Turbine Supervisory equipment. The steam
turbine shown is fairly standard with an HP (high
pressure) stage followed by a single LP (low pressure)
rotor section; different turbine configurations depending
on power rating, may have an intermediate (IP) section in
addition to a number of LP’s which finally drive the
turbine generator.This type of configuration is illustrated
in the adjacent picture. Although the equipment
configuration does vary, the measurement techniques
remain the same, with each turbine installation generating
its own unique set of measurements. Typical
measurement techniques include:-
Absolute vibration of bearing pedestals
Shaft vibration relative to bearing
Shaft eccentricity
Differential expansion or shaft movement
Valve position on steam inlet
Casing expansion, both inner and outer
Speed, including overspeed and zero speed
Temperature
Structural & foundation vibration monitoring
Each of the measurement techniques are used to monitor
the turbine during its operating cycle, some
measurements may be configured to provide warning
alarms as well as automated shutdown, although these
systems tend to operate on a voted principle to ensure
maximum system integrity.
4
5
ATYPICALTURBINE SUPERVISORY EQUIPMENT APPLICATION
TRANSDUCERS & SENSORS
The Accelerometer
The accelerometer is based on the electrical properties
of piezoelectric crystal. In operation, the crystal is
stressed by the inertia of a mass. The variable force
exerted by the mass on the crystal produces an electrical
output proportional to acceleration. Two common
methods of constructing the device to generate a residual
force are compression mode and shear mode
respectively. A residual force is of course required to
enable the crystal to generate the appropriate response,
moving in either direction on a single axis. A shear mode
construction is illustrated below.
An accelerometer operates below its first natural
frequency. The rapid rise in sensitivity approaching
resonance is characteristic of an accelerometer, which is
an un-damped single-degree-of-freedom spring mass
system. Generally speaking, the sensitivity of an
accelerometer and the ratio between its electrical output
and the input acceleration is acceptably constant to
approximately 1/5 to 1/3 of its natural frequency. For this
reason, natural frequencies above 30KHz tend to be used.
The frequency response curve can be influenced by a
number of factors, mainly the mass, the stiffness and the
degree of system damping. The resonant peak of the
accelerometer can be eliminated by increasing the
damping. However, increasing the damping introduces a
phase shift in the linear range whereas un-damped
accelerometers have very little phase shift until near the
natural frequency. It is therefore usual to have un-damped
accelerometers with very high natural frequencies so that
the linear range is extended as far as possible. Typical
damping ratios are 0.01 to 0.05.
Shear mode construction
Shear mode devices which apply a shear force to the
inner and outer surfaces of a ring of crystal (as opposed
to a perpendicular force to a disk of crystal), offer a
distinct advantage over standard compression techniques.
When mounting the device to the plant, normally through
a stud & screw arrangement, the mechanical stresses
within the transducer assembly change. Compression
mode devices are particularly affected by these stresses,
which produce low frequency effects, compounded if
further integration is carried out. Sensonics shear mode
range of transducers are unaffected by base strain and
offer a true low frequency performance down to 0.4Hz.
This resonant frequency in combination with the
appropriate damping can be utilised to monitor bearing
impact. Several manufacturers, including Sensonics have
developed transducers that utilise the ‘ringing’ of the
transducer to mechanical impulses to measure the health
of roller bearings. This technique analyses the high
frequency response of the transducer (at resonance) to
determine an ‘impact factor’ normally in dB, which is
directly proportional to the quantity and force of metal
on metal impacts. This factor is normalised to an overall
health measurement by consideration of bearing
dimensions and rotational speed.
Although the piezoelectric accelerometer is a selfgenerating device, its output is at a very high impedance
and is therefore unsuited for direct use with most display,
analysis, or monitoring equipment.Thus, electronics must
be utilised to convert the high impedance crystal output
to a low impedance capable of driving such devices. The
impedance conversion electronics may be located within
the accelerometer, outside of but near the accelerometer,
or in the monitoring or analysis device itself.
Accelerometers with internal electronics are convenient
and can use inexpensive conventional plugs and cable but
they are limited to temperatures of typically 120ºC.
Locating the electronics in a cool location away from the
accelerometer allows the transducer to tolerate higher
temperatures.
Typical accelerometer frequency response
6
For practical purposes, a typical velocity pick-up is limited
to frequencies between approximately 10 and 1500 Hz.
This has an advantage in certain applications where high
frequency vibration (generated from steam noise for
example) can saturate standard accelerometers with a
built-in integration function.
Accelerometers are available in various output
configurations. The industry standard drive utilises a 2wire current source interface operating at a nominal bias
voltage of +12Vdc typ., with an output sensitivity of
100mV/g. This configuration limits the dynamic range to
around ± 70g, which is suitable for most applications.
Larger dynamic ranges can be achieved through lowering
the sensitivity at the expense of signal to noise ratio,
10mV/g as an example, or utilising current output devices,
which can provide ± 1000g depending on sensitivity
requirements (10pC/g is typical).
It is possible to obtain moving coil type transducers
which operate in any axis.The high specification units
tend to be only available for operation in either the
vertical or horizontal plane due to the arrangement of
the sprung mechanism and orientation during factory
calibration.
Standard accelerometers are also available with velocity
outputs in either metric or imperial format, although for
these configurations the required measurement range
must be understood if the correct sensitivity is to be
selected (0-20mm/s, 0-50mm/s etc.). Since the integration
function to convert the acceleration to velocity is carried
out within the accelerometer, the low frequency
performance is typically limited to around a few hertz.
The limited complexity of the conditioning circuitry that
can be included within the accelerometer combined with
the integrated noise tend to be the limiting factors.
The ATEX Directive
The ATEX directive 94/9/EC defines the specification
requirements of equipment intended for use in potentially
explosive atmospheres. Equipments supplied to meet the
directive are approved by an authorized external body,
the manufacturer must also maintain a quality system to
meet with the standard. Sensonics produce a wide range
of ATEX approved accelerometers and eddy current
proximity probes for various intrinsically safe applications.
For direct integration with SCADA and PLC based
systems, most manufacturers offer direct 4-20mA outputs
covering a factory set range of either acceleration or
velocity vibration. This can be an extremely cost effective
solution (provided the required measurement range is
again well understood) since no signal conditioning unit is
required to drive the transducer or process the resulting
measurement. The resulting current loop output is
typically either a peak or an RMS representation of the
vibration signal and therefore signal frequency analysis is
not possible.
The accelerometer is of particular concern when
operating within a potentially explosive atmosphere
because of the self-generating nature of piezoelectric
devices and the high potential voltages that can be
generated under shock conditions. For this reason the
ATEX directive specifies complete encapsulation of the
inner transducer body and limited capacitive capability
within the electrical interface to minimize this effect. The
inner crystal construction is voltage limited through the
addition of diodes, as is the electrical interface. The
construction of the device and the internal features will
be specific to the approved temperature range and zone
of operation.
Velocity Transducer
The velocity transducer is inherently different to the
accelerometer with a conditioned velocity output. This
device operates on the spring-mass-damper principle, is
usually of low natural frequency and actually operates
above its natural frequency. The transducing element is
either a moving coil with a stationary magnet, or a
stationary coil with a moving magnet. A voltage is
produced in a conductor when the conductor cuts a
magnetic field and the voltage is proportional to the rate
at which the magnetic lines are cut. Thus, a voltage is
developed across the coil, which is proportional to
velocity.
This type of transducer can provide sensitivities of up to
20mV/mm/s and is convenient because it generates a
signal without an external power supply and the signal
usually does not require further amplification.
The sensitivity vs. frequency response curve of a velocity
pickup is limited at low frequencies by the optimum
damping of the first natural frequency; at high frequencies
its response is limited by the amount of motion necessary
to overcome the inertia of the system, as well as by the
presence of higher order natural frequencies.
The ATEX approved PZS4
7
THE EDDY CURRENT PROXIMITY PROBE
The principle of operation, as the name implies, depends
upon the eddy currents set up in the surface of the target
material - shaft, collar, etc. adjacent to the probe tip.
In rotating plant, the variations in shaft/bearing distance
created by vibration, eccentricity, ovality etc. can thus be
measured by probes mounted radially to the shaft. When
the target is stationary the measured voltage can be used
to set the probe/target static distance. Shaft speed can
also be measured by placing the probe viewing a
machined slot or a toothed wheel.
The Eddy probe tip is made of a dielectric material and
the probe coil is encapsulated within the tip. The coil is
supplied with a constant RF current from a separate Eddy
Probe Driver connected via a cable, which sets up an
electromagnetic field between the tip and the observed
surface.
Any electrically conductive material within this
electromagnetic field, i.e. the target material, will have
eddy currents induced in its surface. The energy
absorbed from the electromagnetic field to produce these
eddy currents will vary the strength of the field and
hence the energising current, in proportion to the probetarget distance. Such changes are sensed in the driver
where they are converted to a varying voltage signal.
The whole probe, extension cable and driver system
relies for its operation on being a tuned circuit and as
such is dependent on the system’s natural frequency.
Thus each system is set up for a fixed electrical/cable
length. Eddy probe systems are usually supplied with 2, 5,
9 or 14 metre total cable lengths.
The probe types available are generally according to the
API670 standard (see later discussion). Three main
variants, straight mount, reverse mount and disc type
probes make up the Sensonics range. The main difference
between the straight and reverse mount is the location of
the thread on the probe body and the fixing nut. Reverse
mount tend to be used exclusively with probe holders,
while straight mount are the more common and are used
on simple bracketry or mounting threads where
adjustment to the target is achieved through use of the
thread on the probe body in conjunction with a moveable
lock nut. The maximum measurement range available on
this type of probe is typically 8mm.
The disc probe mounts the encapsulated coil on a metal
plate with fixed mounting holes, making a very low profile
assembly with a side exit cable. Larger coils can be
mounted on this plate; for example, the 50mm diameter
tip probe can provide a measurement range of beyond
25mm. However, care must be taken to ensure the target
area is sufficient to obtain the required linear response.
Note the relationship opposite – between linear range,
probe tip and target area.
System overview
Eddy current probe empirical relationships
T
ypical straight probe with driver
8
THE LVDT
The LVDT is an electromechanical device that produces
an electrical signal whose amplitude is proportional to
the displacement of the transducer core.
The LVDT can be operated where there is no contact
between the core and extension rod assembly with the
main body of the LVDT housing the transformer coils.
This makes it ideal for measurements where friction
loading cannot be tolerated but the addition of a low
mass core can. Examples of this are fluid level detection
with the core mounted on a float and creep tests on
elastic materials. This frictionless movement also benefits
the mechanical life of the transducer, making the LVDT
particularly valuable in applications such as fatigue life
testing of materials or structures.This is a distinct
advantage over potentiometers which are prone to wear
and vibration.
The LVDT consists of a primary coil and two secondary
coils symmetrically spaced on a cylindrical former.
The principle of operation of the LVDT, based on mutual
inductance between primary and secondary coils,
provides the characteristic of infinite resolution. The
limitations lie within the signal processing circuitry in
combination with the background noise.
The principles of operation of the LVDT enables the
transducer to be configured in a variety of housings
depending on the degree of mechanical protection
required. The use of rod end bearings, linear rolling
element bearings and flexible conduit, helps the LVDT to
survive even the most severe environments.
Schematic of an LVDT
A magnetic core inside the coil assembly provides a path
for the magnetic flux linking the coils. The electrical
circuit is configured as above with the secondary coils in
series opposition.
The LVDT principle can also be applied to the
measurement of angular position; an RVDT (Rotary
Variable Differential Transformer) converts the rotation of
a shaft into a proportional electrical output signal.
Although the transducer is capable of continuous
rotation, a plot of angular rotation against magnitude and
phase of the output signal over 360º would result in a
complete sinusoidal wavelength response and therefore
two null positions. To avoid ambiguity, just one of the null
positions is chosen during calibration, providing a typical
measurement range of ± 60º.
When an alternating voltage is introduced into the
primary coil and the core is centrally located, then an
alternating voltage is mutually induced in both secondary
coils. The resultant output is zero, as the voltages are
equal in amplitude and in 180º opposition to each other.
When the core is moved away from the null position the
voltage in the coil, toward which the core is moved,
increases due to the greater flux linkage and the voltage
in the other primary coil decreases due to the lesser flux
linkage. The net result is that a differential voltage is
produced across the secondary tappings, which varies
linearly with change in core position. An equal effect is
produced when the core is moved from a null in the
other direction but the voltage is 180º different in phase.
Some key characteristics of the Sensonics LVDT range
are as follows.
Range
Core type
Signal connection
Mechanical conn
Operating temp
Specials
Core displacement characteristics
:2.5mm to 600mm
:Sprung, guided or free
:End or side exit connector
or cable with option of
conduit.
:Rolling, ball or rod end
: -40ºC to +220ºC
:Submersible (.100m depth)
:Short body to stroke ratio
A range of Sensonics LVDT devices
9
CHOOSING THE RIGHT TRANSDUCER
Each type of transducer has an area of measurement in
which it is particularly useful. In the figure below, the
relationship is clearly shown. Note that the velocity
curve is constant. Acceleration (the first derivative of
velocity) increases with frequency, while displacement
(the first integral of velocity) decreases.
Eddy Probe are used for low frequency measurements
(typically dc to 1kHz), and the velocity pick-ups cover the
mid-frequency band (10Hz to 2kHz). Accelerometers
generally have the widest frequency range and certainly
the highest (1Hz to 10kHz typically).
In practice, combinations of transducers are employed to
supply different types of information simultaneously. For
example, an Eddy Probe would be required to measure
shaft vibration relative to a bearing housing, while an
accelerometer (or velocity pick up) would measure the
vibration of the housing itself and the bearing.
At 2000Hz (or 120000rpm) this example shows that
measuring displacement would be impractical: the signal
would be too low. A velocity pick up would be adequate,
since the 0.3in/sec velocity produces a substantial voltage.
However, the highest voltage level would be produced by
an accelerometer.
TRANSDUCER
ADVANTAGES
DISADVANTAGES
Eddy Current probes
Static and dynamic displacement measurements
Immune to non-conductive materials such as
plastic, wood oil and water
Large linear ranges
Non-contacting
Requires power supply
Calibrated cable length between
probe and driver unit
Velocity Transducer
Self-Generating
Doesn't require an amplifier
Can provide displacement data via integrator
Good general purpose vibration transducer
Frequency response limits its lower
frequency to 5-10Hz
Moving parts make it prone to wear
Phase response varies with frequency
Accelerometer
Wide frequency range
High operating temperature (external electronics)
No moving parts
Can provide acceleration, velocity or
displacement data
Requires charge amplifier
Displacement information normally
restricted to greater than 10Hz due
to problems of double integration
LVDT
Infinite resolution
Robust
High temperature rating
Adjustable zero and gain controls
Contacting
Limited frequency response
10
MEASUREMENT TECHNIQUES
Absolute vibration
Absolute vibration monitoring is perhaps the primary
method of machine health monitoring on steam turbines.
The type of transducer used is seismic (ie vibration of
turbine relative to earth) and can either be a velocity
transducer or an accelerometer.
The choice of transducer has been the subject of debate
for many years and often the final decision is purely
subjective. A number of factors however should be
taken into account.
The steam turbine is a fairly simple machine when
considering vibration signatures.The frequencies of
interest are normally from one-half to five times running
speed (broadly 25 to 300Hz).The unique high frequency
detection capability of the accelerometer is not often
used.
Pedestal vibration monitoring
Accelerometer with external electronics for high temperature applications
Vibration monitoring is nearly always in terms of velocity
or displacement and can therefore be obtained by an
accelerometer or a velocity transducer. Particular care
needs to be taken when double integrating an
accelerometer signal to provide a displacement
measurement. Problems usually occur below 10Hz when
double integrating and 5Hz when single integrating. In
the frequency ranges normally monitored on steam
turbines this is not a problem. These measurement
issues can be reduced by integrating the signal at source
rather than after running the signal through long cables
(ie having picked up noise on route). Accelerometers
with built-in stages of integration are available to perform
this task as discussed in the previous section.
Difficulties can be encountered when monitoring the HP
turbine pedestals using accelerometers. The high
frequencies generated by steam noise can saturate the
amplifier electronics. Filtering the signal prior to the
charge amplifier will eliminate the problem but this must
be incorporated into the amplifier circuit of an
accelerometer with built in electronics.
Pedestal vibration is normally measured in the two axes
perpendicular to the shaft direction where the bearing is
under load, providing complete measurement coverage.
In some instances the thrust direction is also monitored
depending on turbine configuration.
The velocity transducer has the advantage over the
accelerometer of being self generating and not requiring
any power supply. On the other hand, the accelerometer
has no moving parts and should not require frequent
calibration.
In summary, the velocity transducer is simple and easy to
fit to a turbine but has limited frequency and phase
response (not a problem in the range 10 to 1000Hz) and
requires periodic maintenance.The accelerometer on the
other hand requires more careful installation but can then
be left without maintenance.
Gas turbines demand high temperature transducers for
absolute vibration monitoring (>400º typ). For this
reason, a separate charge amplifier is normally utilised,
located away from the high temperature environment.
11
ECCENTRICITY & SHAFT VIBRATION
Eccentricity monitoring can be subdivided into shaft
vibration and bent shaft monitoring. Bent shafts normally
result when the turbine is stationary and thermal arching
or bowing of the shaft occurs or the shaft sags under its
own weight. The Turbine is rotated slowly (barring) to
prevent this happening or to straighten the shaft after it
has occurred.
The graphs illustrate that misalignment effects are readily
ignored through utilising the peak to peak measurements
of eccentricity. When higher frequency components are
present (e.g hammer marks) the RMS value is more
representative.
Sensonics have developed a dual path eccentricity module
to specifically eliminate the effects of marks and dents on
the shaft. The module utilises a speed signal derived from
a second probe to actively tune the low pass elliptical
filter response of the eccentricity unit to remove the high
frequency components of the eccentricity waveform.This
provides accurate peak measurements, particularly at low
barring speeds and is effective through the full speed
range.
The accurate monitoring of the shaft, both when at full
speed and when on barring is therefore vitally important
but often requires two techniques to monitor them
effectively. At shaft speeds above 300rpm, conventional
detection circuits are used but below this speed, analogue
meters and recorder traces fluctuate at the frequency
being measured. Recorder traces become a blur of ink as
the pen ‘bands’ at the frequency of the barring speed.
To ensure that all the shaft vibration data is captured,
probes mounted in the X and Y axis are normally used.
Probes are invariably mounted at the 0º and 90º points
or at the 315º and 45º points.
Detection circuits can be employed that hold the peak
value of eccentricity so that a continuous line trace is
obtained. In order to monitor gradually decreasing
eccentricities, the peak hold function is discharged by 1%
per rev using a tacho signal.
The eddy current probe measures displacement in the
plane of its own axis only. Displacement vibration
perpendicular to the probe axis is not measured.
One of the difficulties encountered when using shaft
displacement transducers be they the eddy current probe
or the older inductive probes, is the problem of “runout”.
Runout is the error signal generated by mechanical,
electrical or metallurgical irregularities of the shaft
surface.These error signals are generally of a low
magnitude in comparison to the vibration signal and are
often at a much higher frequency.
Typical probe mounting configuration
The 315º and 45º points are used to avoid the half joints
of the bearings and to ensure that when bearings are
removed the probes are removed along with them. This
moves the probes away from possible mechanical damage
when the turbine is being worked on.
Measuring eccentricity with runout using RMS
or peak to peak detection
Measuring lower frequency signals using RMS or
peak detection
Eddy current probes monitoring vibration at 45º and 315º Points
12
ROTOR DIFFERENTIAL EXPANSION & SHAFT POSITION
The eddy current probe, as well as providing ac vibratory
information, also provides dc information of the probe to
target gap. This makes it ideal for measuring rotor to
casing differential expansion via a non-contact method.
The eddy current probe and the measurement of
differential expansion are governed by a series of
empirical relationships.
The linear measurement range of an eddy current probe
is approximately one third of its coil diameter as shown
earlier. The ideal flat target area for an eddy current
probe to “observe” is twice the coil diameter. Therefore
the ideal target size for an eddy current probe is six
times the linear measurement range.
Probes monitoring differential expansion by observing a tapered collar
For differential measurement ranges of 25mm a target of
100mm is therefore required. This large target size is
often impractical to fit. It is also often the case when
retrofitting differential expansion systems that the existing
collar is much smaller than that ideally required.
One option to overcome this limitation is to reduce the
size of the probe and therefore obtain a more linear
output against the fixed target area, in combination with
measuring both sides of the collar. However, this push –
pull technique does require some simple arithmetic
within the signal conditioning units to generate the
correct expansion measurement.
The illustrations opposite show the effect of a less than
ideal target on the output of an eddy current probe.
To overcome this problem and obtain a linear output the
probe electronics can either be calibrated in-situ or
supplied pre-calibrated with a non-linear output. This
non-linear output becomes linear when the probe is
fitted in-situ. Eddy probe drivers are normally precalibrated to give a linear output when observing an ideal
target.
The diagram opposite illustrates a disc type eddy current
probe measuring movement against a flat collar, the
limitations in terms of target area can clearly be seen.
13
Another technique utilised in measuring differential
expansion is to use tapered rather than flat collars.The
use of tapered collars fitted to the turbine shaft enables
longer linear ranges to be obtained. A 1 in 10 taper
enables an axial expansion of 10 times the normal range
of the probe to be measured.
It is also possible to measure differential expansion or
axial movement with a small range probe using a mark
space technique. This principle operates on detecting
movement in special plates attached to the turbine shaft.
The shaft target pattern consists of a number of pairs of
‘teeth’ and ‘slots’ surrounding the shaft and rotating with
it. Each pair of teeth are tapered axially such that
alternate teeth taper in the opposite directions, the
narrow parallel slot between the teeth being at an angle
to the shaft axis. There is a wider parallel slot between
each pair of teeth to allow the system to identify each
pair.
A problem arises however if there is any radial movement
eg if the shaft moves 100 micrometers within the
bearings, this is incorrectly seen as 10 x 100 micrometers
(ie 1mm) of differential expansion. To overcome this, two
eddy probes are fitted. Thus two unknowns can be easily
solved by two simultaneous equations through software
manipulation.
When the shaft rotates, the voltage pulses produced by
the proximity probe and driver, have a tooth to slot pulse
width ratio dependant upon the axial relationship
between the shaft pattern and the probe position. The
probe is mounted on a fixed part of the machine so
variations in pulse width ratio are a measure of shaft axial
position. The shaft pattern is illustrated below.
Travel = Normalised Range x T3+T4
+ Offset
T1+T2 +T3+ T4
Two probes monitoring expansion by observing a tapered collar
A further complication arises when the casing holding the
eddy probes is subjected to twisting as can happen if
slides start to stick (see below). A further two eddy
current probes are then required to give a correct
reading of differential expansion.
‘Normalised range’ is the total travel range divided by the pulse
width ratio range determined from each travel extreme.
The Sensonics Sentry machine protection MO8612
module is suitable for this type of monitoring. The
module exhibits a self-tracking threshold level, which
ensures that the width of the signal pulses are measured
at the optimum position within the pulse height. The unit
is pre-programmed with specific plate patterns that can
be selected to suit applications.The number of plates on
the mark-space wheel is also an important parameter;
when correctly set up this enables the module to
minimise ‘plate wobble’ through the implementation of
averaging algorithms. Customised patterns can also be
entered into the module.
Since this technique measures axial movement based
upon the ratio between detected pulses, it is immune to
shaft movement in any other direction. This is a distinct
advantage over the other techniques detailed in this
section. A large expansion range can also be measured
with a low cost probe through the fitting of the
appropriate plate pattern, several centimetres if
necessary, which would be impossible to achieve with a
shaft collar.
Four probes monitoring expansion by observing a tapered collar
14
SPEED - OVERSPEED - ZERO SPEED MONITORING
The eddy current probe as well as being used for shaft
vibration and differential expansion can also be used as a
speed monitoring transducer. The eddy current probe
gives a large voltage output, which is independent of shaft
speed.
The speed at which the turbine trips is obtained by a
display freeze on external contact closure.This contact
closure is obtained by a micro-switch on the governor
valves or some similar method.
The maximum turbine speed is obtained by a sample and
hold circuit within the speed monitor. It is imperative
however during the screen freezing or peak holding, that
the recorder output follows the speed uninterrupted.
A pulsed signal is obtained by observing a projection, a
slot or series of slots. The size of the slot need only be
16mm wide by less than 0.25mm deep.
A zero speed facility can also be incorporated in a speed
monitoring system or by a separate system entirely. A
zero speed facility works by measuring the period
between two pulses. When the period exceeds the
selected zero speed time limit an alarm is initiated. Zero
speed periods range from 1 second (60 rpm) to 300
seconds (0.2 rpm).
Alarm voting within speed trip systems is a common
technique. Typically 2 out of 3 or 2 out of 4 voting
methods are used, where multiple measurement heads
around a single toothed wheel, generate individual trip
alarms, which are further processed to generate a plant
trip if any two signals are simultaneously valid. These type
of systems also incorporate self test facilities that enable
signal injection on individual speed channels for trip
verification as well as testing the admissible trip
combinations; A+C, A+D, B+C and B+D as an example for
a 2 out of 4 system.
Effect of slot width on eddy probe output
The number of slots used is dependent on the minimum
speed required. Most phase locked loops have a
minimum operating frequency. This lower frequency is
approximately 4Hz. In order to measure a minimum
speed of say 1Hz ie 60 rpm, 4 pulses are required and at
30 rpm, 8 pulses are required. For monitoring of speeds
above 240 rpm one pulse/rev is sufficient.
The test rack provides key switch facilities to enable on
load testing. This provides an interlock mechanism, which
prevents two channels from being selected at the same
time. A typical 3-channel system is illustrated below:-
The eddy current probe can work using specially fitted
speed wheels or by observing an already fitted gear
wheel. Where gear teeth are less than 16mm thick then
a lower voltage swing output signal is obtained as shown
above.
Most speed monitors have a conventional 4 or 5 digit
display and have a number of alarm set points.The update
time of the display should be such as to give a steady
reading but respond fast enough to speed changes.
An update time of 1 second is sufficient for on load and
run-ups but is usually insufficient for overspeed testing. A
speed monitor that has a faster update time when in
overspeed mode is an advantage: typically 100 milliseconds.
When carrying out overspeed testing, to set the
emergency stop valves, two speed readings are required:a) The speed at which the turbine trips
b) The maximum turbine speed reached.
3-channel overspeed trip system
Turbine speed profile
15
CASING & CYLINDER EXPANSION
These techniques require a larger measurement range
than can be offered through standard proximity probe
equipment, the necessary probe target is also not easy to
achieve. This is where LVDTs are used to provide the
expansion measurements required. A total range of
50mm usually suffices and the various mounting options
available with LVDTs makes installation straightforward.
Rotor to casing expansion
It is also possible by a combination of two different
techniques to monitor the rotor to casing expansion by
measuring the rotor and casing movement separately
with respect to the bearing pedestal and adding or
subtracting to achieve the rotor to casing displacement.
This is particularly relevant when the rotor differential
expansion, carried out typically by a proximity probe,
cannot be referenced to the casing through fixed
mechanical connection. Sensonics Sentry range provides
a solution through the use of an LVDT module for the
casing expansion and a displacement module for the
rotor expansion. An additional process module is
implemented to calculate the relative displacement. This
configuration is illustrated below.
The movement of the turbine pedestals on the cylinder
sole plates is a relatively easy measurement to make
requiring an LVDT mounted on the turbine and the
extension rod fixed or sprung onto the slides. The
environment is not hostile although care must be taken
to prevent mechanical damage to the transducer.
Turbine cylinder crabbing, movement of the cylinder in
the horizontal plane on the cylinder sole plates, is
monitored at the cylinder front and rear by two pairs of
LVDT transducers mounted so that one pair is to the
cylinder front outer corners and one pair to the rear
corners. The outputs of a pair of LVDT transducers are
summated and conditioned by an arithmetic unit to
determine movement and to give a single output, which is
displayed in the control room as cylinder crabbing.
LVDT monitoring cylinder casing expansion
The inner to outer cylinder measurement requires a
much higher degree of sealing within the transducer
against moisture as the measurement is made inside each
outer cylinder. The transducer body is mounted on a
bracket.This is adjustable for initial setting up and
calibration against the inside structure of the outer
cylinder in such a position that the spring loaded core is
held at approximately mid travel against a point on the
inner cylinder. Expansion of either inner or outer
cylinder with respect to the other, changes the relative
position of the LVDT core, giving a change in output to
the position monitoring instruments.
Understanding the relative positions of the LP rotor and
casing is extremely important, as contact of these parts
can be catastrophic. This is most critical on run-up and
run-down, where the expansion between the shaft and
casing is occurring at different rates.
16
VALVE POSITION MONITORING
The Linear Variable Differential Transformer (LVDT) is
ideally suited for valve position monitoring. In this type of
application the LVDT is used to provide positional
feedback to the governor control system to effect a
closed loop system. The electrical properties of the
LVDT therefore play a key role in determining the system
response. Linearity is obviously key as well as a robust
construction with flexible mounting options. AC type
devices (as opposed to DC) are exclusively used for this
type of application, which permit long cable runs and
offset adjustment with gain control.The picture below
shows a heavy industrial LVDT utilised in a valve position
application.
The LVDT is usually driven by a 3kHz oscillator which
enables frequencies up to 300Hz to be monitored,
permitting a fast closed loop response within the control
system.
Drive electronics can also be combined with signal
conditioning functions to provide various forms of
positional information in addition to alarm triggering and
process outputs. A twelve channel Sensonics Aegis
system is illustrated below.
Aegis valve position monitoring system
The Aegis system offers a high integrity low cost multichannel monitoring solution in a compact 3U, 19” rack
format. The system utilises a common display to view
positional information as well as to set up the dual level
alarm trip facilities.
Each module excites an LVDT with a sinusoidal 3.0kHz
waveform of fixed magnitude. The construction of the
LVDT provides a feedback signal to the module via
inductance in to the secondary windings from the
primary dependant on stroke position. The centre of the
stroke range is normally the null point. The module
performs rectification of the secondary induced signal
converting to relative displacement.The module is usually
configured to show a percentage of the total stroke
length required.
Heavy industrial LVDT monitoring valve position
The usual LVDT’s for this application have universal joints
at each fixing point. This feature allows for any lateral
movement as well as being a positive fixing.
Calibration is achieved once the LVDT transducer is
installed and the zero displacement point can be
determined; this is then set on the module. The LVDT is
then stroked to its maximum travel and the unit adjusted
to display 100% displacement. It is also possible to display
actual distance through the addition of a scaling factor.
Other options include rod end types, which tend to be
spring-loaded but not suitable for this type of application
since spring loaded LVDT’s are subject to bouncing when
the valve is “hunting” or oscillating.
Mounting of the LVDT is also critical to the application
custom bracketry may be necessary to position the
transducer, away from locations where high temperature
steam leaks or vents occur. Although the Sensonics range
of devices operate up to 180ºC the life of the LVDT can
be extended by minimising these temperature extremes.
The module also has a calibrate function which can be
enabled remotely. Once in calibration mode the
transducer signal is disconnected and replaced with the
calibration signal which is common to all units within the
rack. The calibration signal is normally set to be 50% of
the stroke length, therefore the standard analogue
outputs available (4-20mA, 0-5V) can be confirmed as mid
range.
The LVDT drive electronics would normally be mounted
locally (up to 100 metres) either in a weatherproof
housing or in a modular rack assembly.The use of
external electronics has a number of advantages over
internal electronics.
The rack also has a common reset facility to clear all
latched alarms, which can be operated remotely in
conjunction with the calibration function for alarm trip
testing. As well as two sets of relay contacts per channel
for the dual level alarms, (configurable for both positive
and negative going alarms) each module also has a
transducer integrity relay. This will indicate a fault if
either the primary or secondary windings exhibit a
continuity problem or short to earth.
1. LVDT operating temperature increased to 220ºC
2. Setting up is made easier as zero and gain controls are
accessible.
3. Local 4-20mA output can be made available.
17
AUXILIARY PLANT MONITORING
Fan & Pump Monitoring
of teeth, as well as harmonics of this. Although in some
cases gear hunting problems can generate frequencies
1/10th to 1/20th shaft speed.
The majority of rotary fan and pump systems can be
broken down in to three categories; direct coupled,
gearbox coupled and belt coupled. Each of these systems
will exhibit a unique set of failure modes between the
motor and shaft interface. Faults at the blade or driven
unit interface are common across the various
configurations.
It is also important on gearbox monitoring to
differentiate between tooth faults and shaft/bearing faults.
For better diagnostics, a once per shaft revolution pulse is
sometimes required.
Unbalanced Fan or Pump
Worn / Loose belts
This is likely to be caused by a build up of debris on the
blades over time and can put stresses on to other system
components. Depending on the amount of imbalance, this
generally results in high levels of vibration and can be
detected using accelerometers fitted on both the
fan/pump bearing and the motor bearing.The typical
vibration frequency of the unbalance would be at the
shaft rotating frequency. If the problem affects all blades
then the frequency would be the shaft rotational speed
multiplied by the number of blades.
Eccentric Pulleys/Sheaves &
Misalignment
Generates vibration frequencies of 2x, 3x & 4x the belt
frequency, normally 2x being the dominant peak and
amplitudes are usually erratic.
These problems all cause high levels of vibration at the
rotating frequency. If misalignment is the problem, the
highest vibration is usually at the fan/pump end and in the
axial direction (eccentric vibration is normally in line with
the belt).
Journal Bearing Defects
Like all bearings, sealed bearings will fail over time.
Premature failure can be the result of insufficient or
contaminated lubrication, as well as excessive operating
temperatures. The failure will cause elevated levels of
vibration at the shaft frequency and the vibration
spectrum signature will normally be high in shaft
frequency harmonics. The bearing will typically run at a
higher temperature during the early stages of failure.
Fitting a sensor offering both vibration and temperature
detection is the ideal.Vibration can also be measured by
monitoring the shaft transverse movement with proximity
probes, if access to the bearing is difficult.
Roller or Ball Bearing Defects
As above, the same causes of failure apply, although for
roller or ball bearings, manufacturing defects do play a
larger role in premature failure mechanisms.Vibration
monitoring again is the best method of detection,
although the exact signature of failure is dependent not
only on the rotational speed but also the bearing race
dimensions and configuration. Usually these defects are
best detected by using shock transducers mounted
directly on the bearing housings.
Belt Resonance
This is normally detected as a small peak of vibration of
regular frequency and amplitude and should only be a
problem if the frequency coincides with the motor or
fan/pump rotating frequency.
Monitoring Solutions
A low cost, effective solution, for the protection and
monitoring of single or multiple units is to have vibration
transducers, temperature sensors and eddy current
probes (for shaft vibration, eccentricity or thrust wear)
feeding into a local unit with signal conditioning.The
Sensonics DN range of products is ideal for this type of
application
Din Rail Mountable
Buffered Sensor O/P
LCD Display
Alert & Danger Alarms
Integrity Alarm
4-20mA per channel
Misalignment
The DN2601 is capable of independently monitoring two
channels of vibration, allowing user configurable alarm
windows for the switching of internal contacts to raise a
warning or to trip the fan or pump. The outputs can be
used to log/view the pump/fan condition from a remote
location (either by analogue current/voltage outputs or by
a comms link).
This type of problem normally occurs following incorrect
assembly of the system components. eg misalignment
between the shaft, motor and driven unit.This is normally
detected through bearing vibration monitoring, ideally
utilising both horizontal and vertical mounted
transducers.The typical vibration frequency would be
1, 2, or 3 times the shaft speed depending on the exact
nature of the misalignment.
The DN2604 offers both vibration and temperature
monitoring in a single DIN rail mountable unit; this in
combination with Sensonics PZAT range of dual sensors
(vibration and temperature) forms a very cost effective
solution.
Gearbox Coupled Type
Apart from the bearing monitoring detailed above, gear
tooth faults can also be detected at an early stage, using
accelerometers mounted directly on the gearbox casing.
The typical vibration frequency of a tooth fault can be
detected at the shaft frequency multiplied by the number
Speed measurement with Overspeed protection is
available in the DN2608 module configuration.
18
INTRODUCTION TO API STANDARD 670
The American Petroleum Institute has developed a range
of standards to assist users in the procurement of
standardised systems of equipment for various
applications. One standard particularly relevant to the
monitoring of rotating equipment is the API Standard 670
covering Machinery Protection Systems, which has
become heavily adopted by industry. The standard details
the following measuring techniques:-
Sensonics ‘Sentry’ Machine Protection System conforms
to the API 670 standard. A typical Sentry rack
configuration is pictured below.
Casing Vibration
Radial Shaft Vibration
Shaft Axial Position
Shaft Rotational Speed
Piston Rod Drop
Phase Reference
Overspeed
Critical Machinery Temperatures
Sentry Machine Protection System
For each of the measurement options detailed opposite,
not only is the equipment specification important but also
the installation technique. The majority of the complexity
and difficulty involves the transducer mounting and
cabling. Each application is normally unique, the 670
standard details various standard approaches that can be
implemented. Important elements to consider include
access to the transducer for servicing or replacement,
termination to a suitable local junction box, the selection
of armoured cable with or without additional conduit for
either isolation or mechanical protection, cable
segregation, grounding techniques etc.
The standard also covers the transducer & monitoring
equipment requirements as well as installation,
commissioning and documentation. The majority of the
above techniques have been discussed in previous
sections.
The standard details a clear set of mechanical and
electrical properties for both accelerometers and eddy
current proximity probes. Adherence by manufacturers
to these specifications assist the user in finding several
sources for the same component (form, fit and function),
as well as a robust product designed for the monitoring
of heavy industrial machinery. Parameters detailed
include; sensitivity, dynamic range, operating temperature
range, accuracy, mechanical mounting options, connector
and cabling standards, immunity to shock, etc. The
Sensonics range of vibration transducers and eddy
current proximity probes both conform to the API 670
standard.
Protection Systems Incorporating Voting
To build on the good practice underpinned by the API 670
standard, some applications require extreme reliability and
also safe failure in the event of a fault. Where the system
has a direct impact on safety, IEC 61508 can be applied to
determine the required ‘Safety Integrity Level’ or SIL
rating. The rating can be between 1 and 4, with 4 being
the most stringent. The 61508 standard defines a
methodology for the protection system from concept
through to decommissioning, the complete life cycle. To
determine system suitability for such an application, the
backbone of the analysis is a failure mode and effect
analysis; carried out to determine the probability of a
failure on demand as well the percentage of safe failure
modes. This will determine the SIL rating.
In terms of the monitoring system philosophy, the focus is
of course on protection, detailing the appropriate
measurement technique and associated system elements
to form a multi-channel system with high integrity,
minimising the effects of single point failures. The
standard generally specifies that a single circuit board
failure shall not affect more than two channels of
measurement. The key concepts are as follows.
Voted systems can provide enhanced system reliability in
combination with a high degree of failure detection,
Integral display indicating measurement & status therefore quite suitable for SIL rated applications, which
may be a requirement because of a commercial impact
Monitoring resolution of 2% full scale
(ie unnecessary shutdown) rather than safety alone.
Adjustments for alarm set points and scaling
Channel fault monitoring
Two out of three majority voting has already been
Buffered transducer outputs
discussed with respect to turbine overspeed protection.
It is possible to implement more complex schemes
Digital output of measured variables and
through the logical combination of many measurement
channel configuration
parameters, such as a turbine high vibration trip system
Isolated / non isolated 4-20mA per channel
where pedestal bearing vibration alarms are processed to
Alert and danger setpoints for each channel
only trip under a specific combination, thus reducing the
effect of spurious events and transducer failures. Further
Relay for each setpoint with programmable
voting can be added to the alarm processing hardware to
delay
ensure complete system robustness, any voted decision is
Alarm defeat for system integrity testing
positive detection of a system failure.
Power supply protection
19
SPECIAL TECHNIQUES – ROD DROP
Rod Drop is an ingenious measurement technique for
increasing the productivity of industrial plant by providing
a reliable and accurate warning of rider band wear on
reciprocating compressors, thus eliminating the need to
shutdown the machine for inspection.
Research has shown that this is often not the case and in
fact each rod stroke cannot always be guaranteed to be
repeatable, which is often a cause of inaccuracy in the
calculated rod drop reading.
The Sensonics system uses a 4mm range probe (in the
standard 8mm body) and is therefore able to measure the
position of the rod throughout its 360º stroke (even
where the rod is coated with ceramic).This in turn
enables the monitor to calculate the true mean position
of the rod more accurately and give a truly reliable
measurement of the rider band wear. Due to the
increased measurement range, the unit can also provide a
peak to peak vibration measurement which can provide
additional information on the machinery health.
The Sensonics system consists of a special type of eddy
current proximity probe (non-contacting displacement
sensor) and a real time monitor module, which measures
the vertical position of the rod and calculates the wear
on the rider bands.
This measuring method is a well-proven technique;
equipment to do this has been available for many years.
The Sensonics system is different in that the probe used
has a dramatically increased measurement range, which
allows a different monitoring philosophy to be utilised.
The Sensonics system was first installed at a chemical
plant in the UK, alongside numerous ‘traditional’ rod drop
monitoring systems. Twelve months later the operating
staff reported that the new system was 100% reliable and
a vast improvement.They have since replaced their old
equipment with the new Sensonics system.
Traditionally, rod drop monitoring systems have been
confined to using standard eddy current probes with a
measuring range of just 2mm, which is often not sufficient
to cover the full range of lateral (or radial) movement of
the connecting rod on most types of machine.
The system is not just for monitoring rod drop; casing
vibration and valve temperature can also be incorporated
for early warning of other types of mechanical fault.
This has meant that monitoring system designers have had
to utilise a ‘snap shot’ measuring technique where the
vertical position of the rod is measured instantaneously at
the same point on each cycle of the machine. This
measurement is usually triggered by a pulse from a second
eddy current proximity probe looking at a single ‘phase
reference’ slot on the crankshaft.This measuring method
relies on the assumption that the rods position is identical
from one stroke to the next except for the gradual change
in position caused by rider band wear.
Fitted through the distance box of the compressor, the
small size of the probe means that the Sentry system can
be economically retrofitted to installed machines of any
make in the field with minimal modification.The whole
system can be installed and commissioned from scratch in
less time than it takes to strip the machine to take a
traditional rider band wear measurement.
As illustrated opposite, compressor systems are complex
machines and several techniques are normally utilised for
machine health monitoring. An effective additional
method to rod drop is to fit an impact sensor above the
piston housing to detect transient vibration events
generated from cracked parts, leaks or general machine
wear.
Conventional vibration monitoring is excellent for
detecting sinusoidal vibration events due to unbalance or
bearing wear. However for reciprocating compressors, the
transient shock and impact events are key, therefore
simple RMS evaluation techniques cannot determine
suitable trip levels for plant protection. The impact
sensor in combination with a suitable monitor can count
the number and magnitude of shock events providing a
more reasonable metric to enable effective plant
shutdown.
A typical compressor with rod drop
20
ROD DROP MONITORING SYSTEM
Typical Components of a Rod Drop system
Probe type AECP04 This is the sensing element of the system, with a measuring range of 4mm, 1 off is required for
each cylinder. Probes are supplied with 500mm unarmoured cable and integral connector.
Probe Holder type ECPH Aluminium body (standard) or ECPH stainless steel body (when required). 1 off is
required for each probe. This item is fitted through the distance box of the compressor and holds the probe at the
correct position over the surface of the rod. Adjustment of the probe position is possible from outside the distance box
by the mechanism of the probe holder. The length of the insertion piece can be from 30mm to 300mm max.
Extension Cable type AEXC Connects the probe to the driver unit. As a ‘tuned’ system, extension cables are
available in certain set lengths, choice of length is entirely dependent on the machine layout and positioning of the driver
housing.
Driver Unit type AD8 This unit conditions and amplifies the signal from the probe.
Driver Housing type DH Series Basically a junction box that holds the driver unit. If the layout of the compressor
allows, then 2 drivers can be housed in 1 box. Standard unit is painted steel, (SS an option) sealed to IP66 and supplied
with stuffing glands for probe cables.
Interconnecting Cable to connect the drivers signals to the monitors. We recommend BS5308 armoured individually
screened twisted pair cable. This is best supplied locally although Sensonics can provide it if required.
Zener Barriers for Intrinsically Safe applications. Note the barriers and monitors need to be mounted in
non-hazardous areas.
Monitor Module type M08604 provides a display, outputs and alarms for compressor rod drop. Modules are dual
channel and so one is required for every two probes or cylinders. Each module is programmed with a scaling factor for
a particular compressor.
Rack type RA8606 Standard 19” rack to hold up to 6 dual channel monitor modules. Can be panel mounted or to fit
standard 19” enclosures.
Blank Panels type MO8600 for spare positions in racks.
Sentry Set Up Software is used to configure the monitor modules (the most commonly used functions eg alarm
levels, etc can be adjusted from the front panel of the monitor module without the need for the set-up software).
A Communications Cable is also provided to connect a laptop PC to the module for set up purposes.
21
SPECIALTECHNIQUES – RUNDOWN MONITORING
Cracked rotor shafts have plagued nearly all turbine
manufacturers (and of course their users) to a greater or
lesser extent over the years, due to the costly and
disruptive problems they cause.Vibration monitoring via
pedestal vibration or eccentricity is not appropriate as it
is not good when it comes to detecting cracked shafts at
the early stage required.
When one of these harmonics coincides with a rotor
critical, as it is in the case of a run-up or run-down, the
vibration response at that frequency changes. A run-down
spectra of a cracked shaft will therefore show a change in
response at a frequency corresponding to one of the
known critical speeds when the rotor is actually spinning
at one-half, one-third etc of that critical speed.
In order for a crack to be detected by vibration
monitoring alone the size of the crack will be approaching
20 to 30% and by this point the shaft will have to be
scrapped. This late diagnosis can however be avoided so
that future planning can take into account predictions of
the shaft’s life-span.
This allows a crack to be spotted when it is a fraction of
the size of a crack spotted using vibration monitoring
alone and therefore long before the shaft has to be
replaced. Trending of vibration data is an extremely
valuable tool for machine health monitoring. Not only
does it indicate problems but it enables estimates to be
made as to when the problem will become serious
enough for the machine to be taken out of service.
A more sophisticated technique than simply monitoring
the overall or frequency components of shaft vibration
must be adopted to detect shaft cracks at such an early
stage.
Trending of vibration data however requires the machine
trend data to be taken when machine conditions are
identical.The diagrams below show that even when there
are relatively small changes in the turbine load, the
vibration spectra are markedly different.
When a crack appears in a shaft it affects the stiffness of
that shaft. The stiffness of the shaft is dependant upon
the width of the crack.When the crack is at the top, the
weight of the rotor forces the crack to close and when
the crack is at the bottom, the weight of the rotor causes
it to open.Therefore during one revolution of a shaft
with a crack in it, the stiffness varies according to the
crack’s position.
Trending also becomes difficult when the vibration
signature of the machine is constantly changing, due not
only to generator load but a number of factors including
steam conditions. Conditions between a number of rundowns do however remain constant so comparison
between rundown signatures provides valid trending data.
The varying stiffness as the shaft rotates means that the
deflections are not proportional to the forces causing
them.This varying stiffness/deflection causes harmonics of
the running speed to be generated.
Run-down plot from a turbine with cracked rotor
Typical run-down plot of vibration
spectum from steam turbine at two different loads
22
SPECIALTECHNIQUES – ORBIT ANALYSIS
By aligning the phase marker transducer and the phase
marker key way or projection, one can read off the high
spot of the shaft. Below the first critical, this high spot
indicates the position of unbalance and trial weight needs
to be added at 180º. Above the first critical this high spot
indicates the position at which the trial weight should be
added.
When two eddy probes are looking at a stationary shaft
at 90 degrees to each other and displayed on the X and Y
plates of an oscilloscope, an illuminated dot will appear
on the CRT. This dot represents the centre-line of the
shaft and will be seen to move in relation to the position
of the shaft. At slow roll the shaft centre-line will appear
as a discrete dot but as the speed is increased, the
centre-line of the shaft is represented as a continuous
circle known as a Lissajous figure. This is the actual shaft
movement within the bearings. If this Lissajous figure is
projected onto the screen with a phase reference on the
Z axis input then it is known as an orbit.
Shaft Pre-load (Misalignment Aerodynamic Forces - Elliptical
Bearings)
A pre-load is defined as a directional load or force
applied to the rotating shaft. The immediate result of preload is to force the shaft into one sector of a bearing and
results in non-linear restraint.This is where the spring
constant is much higher in the opposite direction to the
pre-load than in the perpendicular direction to the preload.This produces the classical twice per rev frequency
associated with misalignment. This can be seen below:-
The orbit and its resultant shape under various
conditions can be used to augment machinery
surveillance and analysis. Certain machine conditions
produce certain orbit shapes, thus a knowledge of the
orbit can lead to detection of machine conditions. As the
orbit is a direct display of the output from eddy probes
any run-out be it electrical or mechanical will also be
displayed. For meaningful orbit displays, then run-out
should be minimised. The conditions that cause classical
orbit shapes are as follows:-
Orbit from machine with misalignment
Oil Whirl
The oil film within a sleeve bearing normally flows around
with the journal surface to lubricate and cool the bearing.
Film flow occurs because of oil shear viscosity.The speed
of this oil film flow is slightly less than half the speed of
the journal surface. During normal stable rotor
conditions, the oil film separates at 180º from the
minimum oil thickness.
Shaft-Unbalance
Oil Whirl
Shaft Pre-load
Oil Whip
Shaft Bow
Shaft Rubbing
However, when Oil Whirl occurs there is an oil film
around the 360º of the bearing. The orbit in the case of
Oil Whirl is characterised below.The phase markers are
approximately 180º apart and because it is slightly less
than 180º the orbit is seen to slowly rotate in the
direction opposite to that of rotation.This counter
rotation is due to the sub-synchronous frequency of the
Whirl at 43% to 46% of rotational speed.
Shaft Unbalance
An orbit which is essentially circular is usually generated
by an unbalanced condition. It is the phase reference
marker that indicates the high spot at any particular time
and the dimensions of the orbit on the display that give
an indication of the magnitude of unbalance.
Shaft unbalance orbit showing
position of high spot
Orbit showing oil whirl
23
BRACKETRY
It is vital when mounting transducers that the bracket
itself and the fixing of the bracket is rigid. An
insufficiently stiff bracket will resonate in the frequency
range which is being measured and false readings of
vibration will result. A robust method of mounting eddy
current proximity probes is illustrated below.
An API670 probe holder
Note: A crude but effective test of the resonant
frequency of a transducer and bracket is to attach the
output of the transducer to a spectrum analyser and
monitor the output when struck with a steel hammer.
The mechanical configuration of disk probes permit large
measurement ranges to be achieved against a shaft collar
where mounting space is at a premium. However,
without appropriate adjustment, the gapping process is
virtually impossible. The disk probe bracket below
operates on a sliding plate principle in combination with a
thumb wheel to allow accurate setting up of the probe.
The above configuration of probe holder is used almost
exclusively with reverse mount probes with an 8mm tip
diameter and measurement range of up to 4mm. Since
the probe is threaded up in to the extension piece no
further adjustment at this point can be achieved,
therefore the definition of length ‘L’ is critical in offering
the probe to the shaft. The top section of this extension
piece is threaded to permit fine adjustment and therefore
appropriate gapping of the probe to the target.
The mounting thread for the holder can be either
standard imperial or NPT (tapered), which forms a tight
seal when tightened. If a standard thread is utilised and a
seal is required on the shaft housing an additional ‘O’ ring
should be fitted on a suitably prepared mounting face.
This type of holder permits robust signal cabling methods
to be applied to the installation. The probe extension
cable is routed from the driver unit and connects with
the probe in the main housing body. This allows flexible
conduit or armoured cable options to be utilised whilst
retaining an IP66 rating and protecting the probe RF
cables.
Right angled disk probe bracket
24
CUBICLE PANELS
Cubicles are an important part of the turbine supervisory system, forming a central location for the mounting of the
instrumentation for collective display as well as terminating the transducer signal cables in the appropriate manner,
earthing screens and centralising the communications hub to the DCS.
When drawing up a specification for a cubicle, consideration should be given to the following points:
Free standing or wall mounted?
Dimensions of cubicle?
Top or bottom cable entry?
Isolated or grounded gland plate?
Lifting bolts required?
Anti-vibration pads to be fitted?
Plinth required?
IP sealing required?
Number of spare terminals?
Ferrule system to be adopted?
Internal cable sizes?
Segregation of terminals required?
Cubicle earthing requirements?
Canopy for protection against running water to be
fitted?
Removable or fixed gland plate or gland box?
Paint colour internally and externally?
Glass or steel front door? (specify type of glass.)
Labelling requirements for:
Removable or fixed internal front panels?
Number of signal conditioning units to be
fitted?
Number of incoming and outgoing cables?
(specify number of cores)
Spring loaded, standard or terminals with
test points?
Isolation/distribution switch or fused mains
power supply?
Type of crimped terminals to be fitted?
How many electrical supplies to cubicle and
of what type?
Automatic mains changeover required?
Door operated or hand light switch?
Type and number of mains sockets inside
cubicle - if required?
Anti-condensation heater or cooling fans
required?
Automatic fire extinguisher required?
Terminal rails
Conditioning racks
Internal electrical fittings
Mimic diagram
Front of cubicle
Fuses
Rear of signal
25
SEISMIC MONITORING & PROTECTION EQUIPMENT
As well as our turbine supervisory equipment, we supply
seismic sensors for power plants. For many, particularly
nuclear stations, monitoring earth movement is as
important for safety reasons as monitoring the turbines
themselves.
Sensonics are the leading supplier of Seismic Monitoring
and Protection systems to the UK Nuclear Industry. We
offer a range of equipment, which can be scaled to meet
the most complex requirements. Integrity and robustness
is key to our system concept, offering protected power
supply configurations and custom indication panels as well
as alarm voting racks suitable for safety critical
applications. The systems provide a secure and extremely
dependable seismic alarm to the site operational staff,
allowing rapid shutdown of plant processes in a
controlled manner.
The key system components are as follows:-
facilities provide additional system robustness.
Calibrator – Activates individual transducer self test
facility for complete system integrity check.
UPS – Typically configured in a dual redundant PSU
configuration and available with various hold up times
dependant on system load.
Qualifications - Seismic qualification to IEEE Std 3441987, EMC approval to Def Stan 59-41.
Voting Rack - Offering multi-channel ‘two out of three’
alarm voting. Utilising either robust electromechanical
devices or triplicated programmable logic with self-fault
diagnosis.
Recorder – Robust PC with embedded operating
system, contains on board non-volatile memory for
program operation and event data storage.
Annunciator – For local alarm indication and reset
facilities. Key operated interlocks and alarm defeat
REFERENCE SITES
With over a thousand TSE systems installed within the power sector alone, Sensonics can provide references from
most of the major station operators worldwide. Please contact Sensonics for references regarding the monitoring and
protection of the following plant:-
Turbine & Generator
Absolute and Relative Vibration
Thrust, Casing & Differential Expansion
Eccentricity, Speed & Overspeed
Infrastructure Monitoring
Voted Protection Systems
Boiler Feed Pump
Absolute Vibration
Voted Protection
Speed, Overspeed & Reverse Rotation
Milling Plant
Absolute Vibration
Impact Monitoring
Fans, Motors & Pumps
Absolute and Relative Vibration
Thrust & Speed
Temperature
Impact
26
type
Spyder Net
Plant Scan
27
DN2600
8 Channel Vibration or process
inputs with internal trending
Sentry
Aegis
2/4/8 Channel Configurations
of Vibration and Temperature
Rack Mount
Din Rail Mt
4 – 20mA
Alarm Relays
TCP / IP Ethernet
RS232 / RS485
Bearing Element
Multiple Voting
LVDT
Rod drop
Differential Exp
Temperature
Speed
Thrust / Movement
Eccentricity
Relative Vibration
Absolute Vibration
Measurement
and Features
Sensor
Range
2-Wire
Constant Current
Broadband <0.5Hz
Conduit Version
Armoured Cable
Connector Version
Triaxial Version
ATEX Approved
Side Exit
Top Exit
Temperature 240ºC
Temperature 200ºC
Temperature 140ºC
Temperature 120ºC
Charge 100pC/g
Velocity 4 – 20mA
Vel up to 20mV/mm/s
Velocity 4mV/mm/s
Accel 10 – 500mV/g
Measurement
and Features
QUICK PRODUCT SELECTION GUIDE
Sensor selection matrix
PZS
PZV
PZDC
PZHT
VEL-G
PZP
Monitor selection matrix
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