Mex 21305

Engineering Encyclopedia
Saudi Aramco DeskTop Standards
Evaluating Installation Of Vibration

Monitoring Equipment For Steam Turbines
Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees.
Any material contained in this document which is not already in the public
domain may not be copied, reproduced, sold, given, or disclosed to third
parties, or otherwise used in whole, or in part, without the written permission
of the Vice President, Engineering Services, Saudi Aramco.
Chapter : Rotating Equipment For additional information on this subject, contact

File Reference: MEX21305 Darryl Turner on 873-4120
Engineering Encyclopedia Rotating Equipment
Evaluating Installation of Vibration
Monitoring Equipment for Steam Turbines
Content Page
INTRODUCTION................................................................................................................ 1
VIBRATION MONITORING EQUIPMENT....................................................................... 2

Vibration Monitoring................................................................................................. 2
Basic Vibration .............................................................................................. 2
Transducers for Vibration Variables............................................................... 8
Seismic Probes..............................................................................................13
Temperature Monitoring Instruments .......................................................................16
Resistance Temperature Detector..................................................................16
Thermocouples .............................................................................................18
TYPICAL VIBRATION MONITORING ARRANGEMENTS ...........................................21
General-Purpose Steam Turbines..............................................................................21
Vibration Monitoring ....................................................................................22
Temperature Monitoring...............................................................................29
Special-Purpose Steam Turbines...............................................................................31
Vibration Monitoring ....................................................................................32
Temperature Monitoring...............................................................................32
GLOSSARY........................................................................................................................34

Table of Figures Page
Figure 1. Example of a Bode and Polar Plot.............................................................. 4

Figure 2. Basic Relationship of Measured Parameters with a Simple Harmonic
Motion..................................................................................................... 6
Figure 3. Formation of a Complex Harmonic Signal.................................................. 6
Figure 4. Views from the Time and Frequency Domain............................................. 7
Figure 5. Advantages, Disadvantages, and Useful Ranges of Transducer Types ........ 9
Figure 6. Range and Limitations on Machinery Vibration Analysis Systems and
Transducers ............................................................................................10
Figure 7. Eddy Current Proximity Probe..................................................................13
Figure 8. Velocity Transducer .................................................................................14
Figure 9. Piezoelectric Accelerometer......................................................................15
Figure 10. Noncontact Eddy Current Probe Orientation...........................................23
Figure 11. API 670 Axial Position Probe Installation for a Shaft with an Integral
Thrust Collar...........................................................................................25
Figure 12. API 670 Standard Axial Position Probe Installation Arrangement............26
Figure 13. Typical Vibration and Axial Position System Arrangement for a
Turbine ...................................................................................................28
Figure 14. Temperature Detector Installation for a Tilting Pad Thrust Bearing.........29
Figure 15. Typical Thermocouple Installation in Line and Thrust Bearings................30
Figure 16. Oil Drain Line Thermocouple Installation................................................31

INTRODUCTION
A vibration, axial position, and bearing temperature monitoring system consists of probes,
accelerometers, and temperature sensors; signal conditioning devices, interconnecting cables,
power supplies, monitors, and communication devices. As defined by Saudi Aramco Engineering
Standard SAES-J-604, “Vibration, Axial Position and Bearing Temperature Monitoring System”
will be referred to as the “Vibration Monitoring System.”
Vibration and axial position information is acquired by transducers and proximity probes
positioned at optimal locations on a steam turbine. Transducers convert mechanical responses to
electric signals that are conditioned and processed by electronic instruments.
Steam turbine bearing temperature information is acquired by temperature detectors positioned at

the bearings.
The vibration monitoring system provides the information that is necessary to monitor steam
turbine condition, to verify performance, and to diagnose faults. Vibration monitoring systems
provide the electrical signals to the Rotating Machinery Protection System (RMPS) and the
condition monitoring system. The RMPS automatically sends shutdown commands to the turbine
control system if a turbine vibration, axial position, or monitored temperature exceeds a specified
limit. The condition monitoring system is a computer-based data collection system that
communicates directly to the vibration monitoring system. The condition monitoring system will
also accept process data from communication links to the Distributed Control System (DCS) or
directly from process instruments. The condition monitoring system collects, stores, processes,
displays and prints the steam turbine operating data in a variety of formats. The condition
monitoring system data will typically be used for historical trending, machinery diagnostics, and
predictive maintenance purposes but not for shutdown protection.
This module describes the types of vibration monitoring system equipment for general-purpose
and special-purpose steam turbines, and it also describes the installation arrangements that are
used at Saudi Aramco installations.
Saudi Aramco DeskTop Standards 1

VIBRATION MONITORING EQUIPMENT
This section of the manual describes the following processes and the equipment that are used for
condition monitoring:
• Vibration Monitoring
• Temperature Monitoring
Vibration Monitoring
Vibration monitoring is a monitoring method and process. Vibration monitoring measures the
condition of the machine from the initial vibration signature after installation and then at periodic
intervals throughout the machine’s life. This monitoring method and process enables an accurate
accrual or trend of information by which equipment may be diagnosed before any problems occur.
Because vibration is the most sensitive and accurate of the indicators that are used for monitoring
machinery condition, vibration sensors are typically used to prevent unscheduled downtime and/or
equipment failure. Saudi Aramco requires automatic vibration shutdown at pre-set levels on all
critical equipment. Vibration sensors can identify a machinery defect earlier than other types of
sensors, and they can also be used to pinpoint the specific source or machinery component that is
defective; therefore, vibration analysis is frequently used in predictive-maintenance programs to
provide the basic guidance for performance of maintenance and overhauls.
Basic Vibration
Vibration is the back and forth motion across a point of equilibrium. Rotating equipment
vibration is usually periodic, i.e., it is related in some manner to the action of the rotating element.
At times, there are non-periodic vibrations in rotating equipment, but such vibrations are normally
from external sources. The vibration motion is described by the variables of frequency,
displacement, velocity, and acceleration.
The terms and expressions that are used in this discussion of vibration monitoring are presented in
the text that follows.
Vibration is defined as the oscillation of an object about its position of rest. When the mass of an
object is set in motion, it will move back and forth between some upper and lower limits. This
movement of the mass through all of its positions and back to the point where it is ready to repeat
the motion is defined as one cycle of vibration. The time that it takes to complete this cycle is the
period of vibration.

Frequency is the number of cycles in a given period. Frequency is occasionally stated in cycles
per minute (cpm) or cycles per second (cps) and is also referred to as “hertz” (Hz). More
frequently though, frequency is expressed in multiples of rotative speed of the machine because of
the tendency of machine vibration frequencies to occur at direct multiples or sub-multiples of the
rotative speed of the machine. Frequency of vibration is expressed in terms as one times rpm, two
times rpm, or 49 to 53% of rpm, rather than expressing all vibrations in cycles-per-minute or
hertz. Frequency is one of the basic characteristics that is used to measure and describe vibration.
The force that causes the vibration is the first event that occurs in time. The responses to these
forces are the other basic characteristics or movements, such as displacement, velocity, and
acceleration. The magnitude of each of these characteristics describes the severity of vibration.
The magnitude of severity is described by the amplitude of the movement. Amplitude of vibration
on most machinery is expressed in peak-to-peak mils. Vibration probes that are mounted near
bearings with hydrodynamic bearings or on casings can sense the maximum excursion (amplitude)
of the shaft or the high frequency casing vibrations. A normal operating machine will generally
have a stable amplitude reading of an acceptably low level less than 1.0 mil (25 microns). Any
change in this amplitude reading indicates a change of the machine condition. Increases or
decreases in amplitude should be considered justification for further investigation of the particular
machine condition.
Phase, or phase angle, is another characteristic of vibration that is important to diagnose and
correct machinery problems. Phase angle is used to compare the motion of a vibrating part to a
fixed reference, or to compare two parts of a machine structure that vibrate at the same
frequency. Phase angle can be defined as the angular difference at a given instant between two
parts with respect to a complete vibration cycle. Phase angle is usually expressed in degrees. The
phase angle measurement is a means of describing the location of the rotor at a particular instant
in time. Phase angle is also valuable in determining the rpm location of the natural rotor balance
resonance or critical speeds. Furthermore, a good phase angle measuring system will define the
location of a high spot on the rotor at each transducer location relative to some fixed point on the
machine train. Through determination of these high spot locations on the rotor, the amount and
the locations of the residual unbalances on a rotor can be determined. Changes in the balance
condition of a rotor will be shown as changes in phase angle. Accurate phase angle measurements
are important in the balancing of rotors, and they can be extremely important in the analysis of a
particular machine malfunction. Determination of phase angle requires use of either portable
analysis or the computer-based condition monitoring systems.
Information from portable analysis or computer-based condition monitoring equipment can be

used to generate a bode and polar plot. Figure 1 shows an example of a bode and polar plot for a
compressor.

Figure 1. Example of a Bode and Polar Plot
Note: This plot is for a compressor, not a steam turbine.

In measurements of radial vibration, amplitude of displacement is labeled “peak-to-peak”
displacement and is measured in units of “mils peak to peak.”

Velocity indicates the speed at which the object is vibrating. It is highest where the object passes
through its position of rest, and it is zero at the upper and lower maximum displacement limits of
a harmonic vibration. The maximum value is usually the value that is recorded when
measurements are taken. Velocity is measured in units of “inches per second peak.” Velocity is
usually the best parameter for machinery-vibration analysis, particularly where important
frequencies lie in the 10- to 1000-Hz range. Velocity is always used to monitor anti-friction (ball
and roller) bearing systems. Velocity is also the best method for detecting a wide variety of
different machinery defects that occur at low, mid, and high frequencies. Displacement primarily
senses low-frequency problems, and acceleration primarily senses high-frequency defects.
The acceleration of the object is related to the forces that cause the vibration. Acceleration
reaches a maximum value as the object reaches its maximum limits of displacement, or when it
begins to move in the opposite direction. The maximum or “peak” acceleration that is measured
is usually the recorded value. Acceleration is measured in units of “g peak” (1 g = 386 in/sec2).
Acceleration monitors are typically used to monitor anti-friction (ball and roller) bearing systems;
however, because of their large range, acceleration monitors can be used to measure other
sources of vibration.
Simple harmonic motion provides an illustration of the relationship between displacement,

velocity, and acceleration. In simple harmonic motion, vibration occurs at a single frequency,
with acceleration being proportional to displacement and occurring in a direction opposite to
displacement. Simple harmonic motion can be represented by a sine wave, and it can be
illustrated as the linear vertical motion of a weight that is suspended or supported on a coiled
spring. The displacement of the weight below and above its point of rest and the return to the
point of rest, as a function of time, is the frequency variable. The change in the amount of
displacement as a function of time is the velocity variable. During a single cycle, this velocity
constantly changes from a value of zero at the peak displacement above and below the rest or
equilibrium point to a maximum velocity value as the weight passes through the equilibrium point
at zero displacement. The rate of change in the velocity is the acceleration variable. The
acceleration variable is a negative value as the velocity slows down and the displacement
approaches maximum.
The phase relationships between the variables for vibration measurement (displacement, velocity,
and acceleration) are shown on a simple sine wave in Figure 2.

Figure 2. Basic Relationship of Measured Parameters with a Simple Harmonic Motion
Typical vibration signatures are not as simple as a single sine wave. Most machinery vibration
consists of complex harmonic signals. A complex harmonic signal can be described as many sine
waves mixed together. Figure 3 shows a basic example of a complex harmonic signal that
consists of two pure sine waves. The upper sine wave is four times the frequency and one-fourth
the amplitude of the lower sine wave. The resulting complex harmonic signal results when the
two sine waves are mixed together.
Figure 3. Formation of a Complex Harmonic Signal

The vibration signals shown in Figures 2 and 3 are shown as amplitude verses time, which is also
known as the “time domain.” Amplitude is on the vertical axis, and time is on the horizontal axis.
If a vibration transducer is connected to an oscilloscope, the oscilloscope display is in the time
domain. Another method to view vibration signals is to plot the amplitude verses the frequency,
which is called the frequency domain. Figure 4 shows the same two sine waves previously shown
in Figure 3, but it shows them in a three-dimensional plot illustrating the views from the time and
frequency domain.
Figure 4. Views from the Time and Frequency Domain
The French mathematician, Jean Babtiste Fourier, discovered that, by using a mathematical
method, all complex harmonic signals can be broken down into a series of simple sine waves. The
mathematical method can be used to break down periodic signals into discrete waves (sine waves,
square waves, and triangular waves) as long as the waves repeat themselves. An FFT spectrum
analyzer takes a complex waveform from a vibration transducer, calculates, using Fouriers
mathematical method, the discrete waves that form that signal, and displays the individual waves
in the frequency domain. Using digital technology, the process has been made “fast”, leading to
the term fast Fourier transformation or FFT.

Besides sine waves, which are pure tones, there are random vibrations. Random vibrations look
similar to a complex vibration signal except that the vibrations do not repeat regularly or on a
cycle. It is difficult to assign a frequency to random vibrations. Random vibrations can occur in
steam turbines when the moving steam encounters stationary objects, such as stage nozzles or
throttle valve seats, creating vortices and turbulence. For example, the high-velocity steam will
produce nozzle passing frequencies that can excite turbine blade natural frequencies. Friction can
also cause random vibrations.
Mechanical sources of vibration in rotating equipment, such as rotor unbalance, misalignment,

critical speeds, associated gearing, and looseness in parts, are only partially responsible for any
vibration. Process-type sources that also contribute to vibration may come from the high velocity
and turbulence of the steam supply and vibration sources from the driven equipment.
Transducers for Vibration Variables
There are two general applications for vibration sensors that are used on rotating equipment.
Both applications are used by Saudi Aramco.
One application is used to detect the actual vibrations of the rotating shaft within a hydrodynamic
radial bearing and to provide a signal to the appropriate monitoring equipment. Saudi Aramco
uses a noncontacting proximity sensor for the detection part of the vibration system in this type of
application.
The second application is used to detect the effects of the rotating element vibrations on the static
equipment casing and/or bearing housings. The seismic-type sensor is used in this application and
is directly mounted on the surface of the body to be monitored. When anti-friction bearings are
used in a machine, the seismic sensor gives a good indication of rotor motion because anti-friction
bearings have essentially zero clearance and the dynamic force of rotor vibration is directly
transmitted to the bearing bracket through the bearings.
Vibration information is acquired through the use of transducers that are strategically located in
various positions on the steam turbine or the auxiliary equipment. The vibration transducers
convert the mechanical motion of the equipment to an electrical signal that is sent to a
monitoring/control unit. The table in Figure 5 describes the advantages, the disadvantages, and
the useful ranges of the transducer types. The selection and positioning of the proper transducers
are discussed later in various parts of this module.

Transducer Useful Frequency Measure- Advantages Disadvantages

Type Range ment
Radial Shaft 0-10 kHz Displacement Sensor Observes Senses surface
Vibration Shaft Directly imperfections
Transducer
Conductive parts only
Mounting difficulty
Frequency limits
Velocity pickup 1-10 kHz Velocity Self-generating Moving parts
Good indicator of Large
machine
Senses EMFs
condition
Frequency limits
Hand-held
Accelerometer With acceleration Acceleration High frequencies Temperature limits
output = 10 - 100 kHz
Rugged
With velocity output =
Small size
2.5 - 100 kHz
Hand-held
Figure 5. Advantages, Disadvantages, and Useful Ranges of Transducer Types
Figure 6 shows the range and the limitations on machinery vibration analysis systems and
transducers. The acceleration line shows that the signal strength (vibration amplitude) is low at
low frequencies. The displacement line shows that displacement probes have a low signal
strength at high frequencies, but its frequency response is flat at frequencies where signal strength
is good. The velocity sensor line indicates that the signal strength is good throughout a range of
frequencies, but frequency response rolls off at high or low frequencies.

Figure 6. Range and Limitations on Machinery Vibration Analysis Systems and

Transducers
Displacement Probes - Displacement is generally the best parameter to use for very low
frequency measurements (i.e., less that 600 cpm) in which velocity and acceleration amplitudes
are extremely low. Displacement is traditionally used for machinery balancing at speeds up to
10,000 or 20,000 rpm, and it should also be used where stress levels or clearances are the
important criteria. Displacement probes are available for a variety of applications and are
sometimes referred to as transducers. Saudi Aramco uses noncontacting proximity systems for
displacement probes.

The noncontacting proximity systems, as used by Saudi Aramco, have the following basic
applications that are related to the proximity probe installations: radial to the rotating shaft, axial
to the rotating shaft, shaft speed, and phase reference. Regardless of the application, the same
types of proximity systems are used. Each type consists of the noncontact proximity probe that is
connected with a precise impedance cable to an oscillator/demodulator unit, which is also known
as a proximitor. Typically, the outputs from the proximitors that are mounted on a single piece of
equipment are instrument-wired to a common plug-in module installed in a rack that houses plug-
in modules for one or more than one machine train.
Noncontacting Proximity Sensor Probes - Noncontacting proximity sensor probes do not

contact the rotating element; however, they are rigidly positioned so that the probe tip is in close
proximity to the rotating surface. The sensor measures the gap between the probe tip and the
surface. Such measurement makes the sensor very suitable to detect and to measure the radial
displacement of the shaft with its radial bearing. A number of different types of proximity probes
are made that operate on different principles to achieve basically the same result. The following
are types of proximitors:
• Light Proximity Probe

• Inductance Proximity Probe
• Capacitance Proximity Probe
• Eddy Current Proximity Probe
Although Saudi Aramco only uses the eddy current-type probes and some light proximity probes,
a brief description of each type is presented below.
The light proximity probe consists of a light source, a two-way light-conducting fiber-optic lead
and probe, and a photo-electric sensor. Light is conducted to the probe tip through use of half of
a fiber-optic bundle. This light is directed at the surface of the rotating element. Light that is
reflected back by this surface is conducted to the photo-electric sensor by the other half of the
fiber-optic bundle, and it is converted to a voltage. The light intensity at the photo-sensor is
proportional to the gap between the sensed surface and the probe tip.
This system has high sensitivity, resolution, and frequency response, and the system can be used
to observe any type of surface that is reflective or that can be made reflective. However,
industrial application is limited by two problems:
• Circumferential variations in surface finish and reflectivity of most shafts causes

significant noise and errors when observing rotating shafts.
• Oil mist or process-fluid vapors may distort the light in the probe-to-shaft gap and
cause noise and errors due to the variations in gap transmittance.

Due to the erratic responses, the light proximity probe is only used as a phase reference
transducer by Saudi Aramco.
The inductance proximity probe consists of a ferromagnetic core inside a coil of wire. A high
frequency alternating current is supplied to the coil, which establishes an alternating magnetic field
at the tip of the probe. The proximity of a metallic surface near the probe tip varies the strength
of the magnetic field, changing the probe inductance, which modulates the amplitude of the high
frequency alternating current.
The rotating element under the inductance probe tip does not have to be made of a magnetic
material, but it must be conductive and magnetically permeable. The probe will not sense non-
conducting materials; therefore, if the conducting material has a non-conducting coating applied
to it, the probe will only respond to the underlying metal. Any defects or eccentricity of the
underlying surface will cause noise and erratic false readings even though the actual finished shaft
surface is running true. Because the probe calibration curves are relatively non-linear and vary
with different materials, the inductance proximity probe is not satisfactory for use on Saudi
Aramco rotating equipment.
The capacitance probe is basically only one plate of a capacitor. The rotating element forms the
other plate, and the air in the gap is the dielectric material. The variable capacitance of the probe
is generally placed in the feedback loop of an operational amplifier with a high frequency ac
excitation signal. Variations in the probe-to-shaft gap size vary the capacitance of this circuit
element, which changes the excitation signal. The readout circuitry transforms this signal to a dc
voltage that is proportional to the instantaneous gap.
Of all the proximity systems, the capacitance system offers the greatest accuracy, linearity, and
freedom from drift and temperature effects. However, the capacitance system is not applicable
for many industrial uses because the type of material in the probe-to-shaft gap affects the output
signal. Water vapor that passes through the probe tip gap will change the dielectric and output
signal or will short-circuit the output completely. When the rotating shaft is coated with dielectric
materials, such as plasma-sprayed ceramics, the probe senses only the metallic substrate.
The eddy current probe consists of a small coil, which is usually of a flat “pancake” shape, at the
tip of the probe. A high-frequency ac (in the frequency range for radio transmission) is applied to
this coil from an oscillator circuit. The proximity probe sets up a magnetic field in the gap
between the end of the probe and the rotating shaft. In turn, the magnetic flux induces eddy
current in the portion of the shaft that is exposed to this flux. Loss of energy in the returning
signal is detected through use of the proximitor. Relative distance or displacement is measured
between the probe tip and the surface by sensing the change in the gap. The eddy current probe is
useful for gaps from about 10 to 70 mils, which is the approximate linear range of the eddy
current probe. The sensitivity of most eddy current probes is 200 mV/1 mil. The demodulator
circuit in the proximitor converts the amplitude-modulated ac to a varying dc signal (along a scale
of 0 to -24V).

The eddy current type of noncontact proximity probe is shown in Figure 7.
Figure 7. Eddy Current Proximity Probe
The eddy current system is not affected by water vapor in the probe tip gap. The output signal
provides an indication (in mV) of the varying gap between the sensor and the “observed” shaft
surface.
The impedance of the probe to proximitor system is a critical item as the proximitors are “tuned”
to a matching impedance in the connecting wire cable. Impedance matching prevents errors in
measurement. Tuning is controlled through the use of only certain equivalent electrical lengths of
cable that match the required impedance. During field installation, this cable length must never be
cut to make an attractive installation. The excess cable should be rolled and neatly installed. If
the cable length is changed, the system will require recalibration. If the system is ever replaced, it
should be with a cable of the same impedance or equivalent electrical length.
Seismic Probes
Seismic (mass-spring) transducers use the response of a mass-spring system to measure vibration.
The seismic transducer consists of a mass that is suspended from the transducer case through the
use of a spring of specific stiffness. The motion of the mass within the case may be damped by a
viscous fluid, a spring, or an electric current. When the transducer case is contact-mounted to the
moving part, the transducer may be used to measure velocity or acceleration, depending on the
frequency range of interest.
Velocity transducers are no longer being used by Saudi Aramco. Velocity measurements (usually
required for all structural vibrations with the exception of high frequency gear mesh vibrations)
are obtained through the use of an accelerometer with signal integration to velocity. This type of
transducer configuration is sometimes called a piezoelectric velocity transducer.

Velocity Transducers - The velocity transducer is an adaptation from a voice coil in a speaker,
and it is shown in Figure 8. There are two configurations of velocity transducers: stationary
magnet/moving coil and stationary coil/moving magnet. Figure 8 represents a stationary
magnet/moving coil configuration. The velocity transducer consists of an internal mass (in the
form of a permanent magnet or coil) that is suspended on springs. A damping fluid, usually oil,
surrounds the mass. A coil of wire or magnet is attached to the pickup case. The case is held
against the vibrating object. The pickup case moves with the vibrating object while the internal
mass remains stationary and suspended on the springs. The relative motion between the
permanent magnet and the coil generates a voltage that is proportional to the velocity of motion.
Figure 8. Velocity Transducer
The velocity transducer is self-generating, and it produces an output that can be fed to the
monitoring system channel without any further signal conditioning. The raw (unfiltered) output
signal from a velocity transducer can be transmitted to an oscilloscope or other analyzer
instrument. The measurement processed from a velocity transducer’s output is a seismic
measurement (referenced to inertial space). For this reason, a velocity transducer is also called a
seismic transducer.
The velocity transducer has an internal natural frequency (referred to as mounted resonance) of
about 8 Hz (those sizes that are used for machine monitoring). This natural frequency is simply
the resonance of the single degree of freedom of the internal mass suspended on springs. The
response at resonance is highly damped because of the internal fluid. This transducer produces a
linear output only above this resonant frequency.

Accelerometers - The most common acceleration transducer is the piezoelectric accelerometer,

as shown in Figure 9. The piezoelectric accelerometer consists of piezoelectric disks that are
made of a quartz crystal (or barium titanate, which is an industrial ceramic) with a mass bolted on
top and a spring that compresses the quartz. A piezoelectric material generates an electric charge
(voltage) output when it is compressed.
Figure 9. Piezoelectric Accelerometer
In operation, the accelerometer base is contact-mounted to the vibrating object, and the mass is
stationary in space. With the mass stationary and the base moving with the vibration, the
piezoelectric disks get compressed and relaxed. In the most typically used compression-type
models, the seismic mass and the base alternately exert compression in the piezoelectric discs.
The piezoelectric disks generate a charge (voltage) output that alternates between positive and
negative as the disks are alternately compressed tighter and relaxed. The charge output follows
the motion of the surface in the direction of the accelerometer’s sensitive axis. The immediate
millivolt output of this transducer is proportional to the acceleration of the vibrating subject; if the
acceleration level is high, the force transmitted from the shaft to its supporting radial bearing is
high. This force is the cause of excessive wear and premature failure in a radial bearing.
The measurement processed from an accelerometer’s output signal is seismic (absolute motion
relative to inertial space). Unlike the velocity pickup, it is practically unaffected by external
electrical or magnetic fields. Accelerometers are as sensitive to ground loops as are other
pickups. Ground loops can be easily eliminated by providing ground isolating washers at the
accelerometer base.

As specified in API Standard 670, the accelerometer channel accuracy for measuring casing
vibration must be within ±5 percent of 100 millivolts per g (mV/g) over a minimum peak range of
0.1 g to 75 g, and over the frequency range of 10 Hz to 10 kHz. The electrical impedance of the
cable linking the accelerometer to the signal conditioner and to the channel plug-in module is
matched to the electrical impedance of the accelerometer case in order to avoid problems from
noise and cable whip and to minimize error in measurement.
The accelerometer has a very high mounted resonance, typically 25,000 kHz, because it has no
moving parts. The response is linear for the first third of the accelerometer’s range and it is used
below its mounted resonance. The range is 5 to about 10,000 kHz, depending on its size. Small
accelerometers have low sensitivities but higher operating frequencies. Some small
accelerometers are useful above 50,000 kHz. Large accelerometers have high sensitivities but
lower high-frequency limits (800 to 1000 kHz).
Temperature Monitoring Instruments
Temperature monitoring instruments are used to monitor bearing conditions on steam turbines,
and they are occasionally used to measure turbine stage temperatures.
Resistance Temperature Detector
A Resistance Temperature Detector (RTD) is a general term for any device that senses
temperature through a measurement of the change in resistance of a material. All metals produce
a positive change in resistance for a positive change in temperature. RTDs are available in many
forms; however, they usually appear in sheathed form. An RTD probe is an assembly that
consists of a resistance deterrent, a sheath, a lead wire, and a termination connection. The sheath,
which is a closed end probe that immobilizes the element, protects the element against moisture
and the measured environment. The sheath also provides protection and stability to the transition
lead wires from the fragile element wires. Some RTD probes can be combined with thermowells
for additional protection. In this type of application, the thermowell will also isolate the system
gas from the RTD.
When the nominal value of the RTD resistance is large, system error is minimized. To obtain a
high RTD resistance, a metal wire with high resistivity must be chosen. Platinum has the highest
resistivity of the selected metals that are commonly used for RTD construction.
RTDs can be constructed of several different types of metal. Gold and silver are rarely used as
RTD elements because of their lower resistivities. Tungsten has a relatively high resistivity, but it
is reserved for very high temperature applications because it is extremely brittle and difficult to
work. Tungsten would also suffer in an oxidizing environment because of the high reaction rates.
Copper is occasionally used as an RTD element. Copper’s low resistivity forces the element to be
longer than a platinum element, but its linearity and very low cost make it an economical
alternative. Copper RTDs have an upper temperature limit of 120°C (248°F).

The most common RTDs are made of platinum, nickel, or nickel alloys. The economical nickel
derivative wires are used over a limited temperature range. Nickel wire output is non-linear and
tends to drift with time. For the best measurement integrity, platinum is the metal of choice.
Platinum is used at the primary element in all high-accuracy resistance thermometers. Platinum is
especially suited for widely varying degrees because it can withstand high temperatures while
maintaining excellent stability. As a noble metal, platinum shows limited susceptibility to
contamination. Saudi Aramco practice recommends three-wire, platinum RTDs calibrated to 100
ohm at 0°C (32°F).
Although the RTD is an accurate temperature measurement device, some errors may develop.
The RTD is a passive resistance element, and a current must be applied to the RTD to develop an
output signal. This current generates heat, which becomes objectionable when the heat is
sufficient to significantly change the temperature to be measured. This self-heating effect causes
minor errors. A limited amount of power used to produce the output signal should minimize the
error.
Another error that may affect the accuracy of the temperature measurement can be caused by the
lead wire. The copper lead wire for connection of the RTD to the transducer, although a
satisfactory trade-off between cost and resistance, represents a resistance in series with the RTD
and thus is a source of inaccuracy. For long transmission distances, ambient temperature effects
can cause appreciable errors; however, these errors can be compensated for by designing the RTD
as a three- or four-terminal device.
Lack of standardization among manufacturers concerning the relationships between resistance and
temperature may cause an accuracy problem. Errors can occur when RTDs of several
manufacturers are used in a single system, or when the element of one manufacturer is replaced
with the element of another manufacturer. These errors can be avoided by not mixing RTDs with
different temperature versus resistance curves.
Inaccuracy of an RTD may also result from slow dynamic response. Slow response may be
caused by the RTD construction; the RTD sensing element consists of an encapsulated wire that is
cut to a length that provides a predetermined resistance at 0°C. The temperature-sensitive
portion of the probe, which depends on the length of the sensing element, is from 0.5 to 2.5 in.
The RTD is thus considered to be an area-sensitive device, and it has a significantly slower
dynamic response than point-sensitive devices like thermocouples. Because RTDs are invariably
installed in thermowells, the thermowells represent a much larger contribution to the slowing of
the dynamic response; therefore, the error is of little significance.

Thermocouples
Thermocouples are another reliable source for electrical temperature measurement.

Thermocouples function very differently from RTDs, but they generally appear in the same
configuration. Thermocouples are usually sheathed, and they can be used in conjunction with a
thermowell. Thermocouple-type instruments have a range of -280 to +2750°C (-440 to +5000°F)
and an accuracy of 0.1°C (0.2°F).
The thermocouple (T/C) consists of two dissimilar metal or alloy wires that are joined at one end,
the so-called measuring (or “hot”) junction. The free ends of the two wires are connected to the
measuring instrument to form a closed path in which current can flow. The point at which the
T/C wires connect to the measuring instrument is designated as the “reference” (or “cold”)
junction.
Application of heat to the measuring junction causes a small electromotive force (EMF or
voltage) to be generated at the reference junction. When a readout device is employed, it
converts the EMF that is produced by the temperature difference between the measuring and the
reference junctions to record or otherwise display the temperature of the measuring junction.
When the reference temperature is known (usually 0°C), and when the measuring junction is
exposed to an unknown temperature, the EMF that is developed will vary directly with changes in
the unknown temperature.
The noble metal T/C, Types B, R, and S, are all platinum or platinum-rhodium T/C and share
many of the same characteristics. Platinum wire T/C should only be used inside a non-metallic
sheath, such as high-purity alumina, due to metallic vapor diffusion at high temperatures that can
readily change the platinum wire calibration. The only other acceptable sheath would be one
made from platinum, which would rather expensive.
The platinum-based T/C is the most stable of all the common T/C. Type S is so stable that it is
specified as the standard for temperature calibration between the antimony point
(630.74°C/1167.33°F) and the gold point (1064.43°C/1947.97°F). Type R is similar to the type
S; the only difference is that the rhodium makes up 10% instead of 13% of the wire.
The Type B T/C is the only common thermocouple that exhibits a double-valued ambiguity. Due
to the double-valued curve, Type B is not used below 50°C (122°F). Because the output is nearly
zero from 0°C (32°F) to 42°C (107.6°F), Type B has the unique advantage that the reference
junction temperature is almost immaterial when it is between 0°C (32°F) and 40°C (104°F).
However, the measuring junction temperature is typically very high.

The Type E T/C positive element is made from nickel-chromium metal. Saudi Aramco practice
recommends Type E chromel-constantan thermocouples (ISA Type E), grounded junction,
manufactured in accordance with ANSI MC 96.1. The Type E is ideally suited for low
temperature measurements because of their low thermal conductivity and high corrosion
resistance. The Type E thermocouple is useful for detecting small temperature changes.
Saudi Aramco practice recommends ISA Type J or Type E unless an existing monitoring system
requires a different type. The Type E chromel (nickel-chrome/constantan vs. copper-nickel)
thermocouple is specified for thrust and journal bearing temperature sensors instead of the Type J
because of the larger Type E temperature range and higher EMF output.
The Type K T/C is similar to the type E with the exception that the negative element is made from
nickel instead of constantan.
Iron is the positive element in a Type J T/C. Iron is an inexpensive metal and is rarely
manufactured in pure form, which contributes to the poor conformance characteristics. Although
the impurities in the iron are high, the Type J T/C is popular because of its low price. The Type J
T/C has a more restrictive temperature limitation than most T/C. At 760°C (1400°F), an abrupt
magnetic transformation occurs that can cause decalibration even when the T/C is returned to
lower temperatures.
The measuring instrument usually is located away from the point at which the temperature is
measured; therefore, an extension is needed. Because the temperature-sensing resistor for
maintaining a constant reference junction EMF can be most conveniently located in the instrument
as a part of its circuit, the reference junction itself must be located in the instrument. Therefore,
the thermoelectric circuit must be extended from the measuring junction, at the point where the
temperature measurement is desired, to the reference junction in the instrument. This is done
through the use of extension wires.
Extension wires theoretically extend the T/C to the reference junction in the instrument. This
wire is generally furnished in the form of a matched pair of conductors. The simplest procedure is
to use the same types of wire from which the T/C itself is made. However, in installations with
noble-metal T/C where several hundred feet of extension wire must be used, or where numerous
T/C are employed, such a procedure may become too expensive. In such cases, alternative lower-
cost materials with similar characteristics at lower temperatures are available.
Thermocouples, much like RTDs, suffer from errors in their measurement. Static electrical noise
may be introduced into T/C circuits by adjacent wires carrying ac power or rapidly varying
(pulsating) dc. Static electrical noise may also be introduced if the T/C extension wires are
capacitively coupled to an electric field. These noises can be minimized or avoided by shielding
each pair of extension wires and by grounding the wire shields. T/C wires must never run in the
same conduit with electric power wires.

Magnetic noise may be induced into a T/C circuit any time the extension wires are subjected to a
magnetic field, and a current is produced to oppose the magnetic field. This can be minimized by
twisting each pair of T/C extension wires. Crosstalk noise between adjacent wire pairs in the
same conduit may also occur. Crosstalk can be avoided by shielding each pair of extension wires.
Common-mode noise in the circuit between the measuring junction and the transducer may occur
when the circuit is grounded in more than one place, or when different grounding potentials exist
along the wire path. Three different approaches can avoid these problems: the noise can be
minimized by proper grounding (T/C circuits are usually grounded at the measuring junction), by
shielding each pair of extension wires and ground the shields at the T/C only, or by using
differential input measuring devices.
The monitor/control unit should be the same as the general control instrumentation. Monitors
must consist of a separate alarm unit for each point and a single, time-shared temperature
indicator. The alarm units must have dual setpoints and outputs, and they must accept the signal
directly from the element. The alarm units must be suitable for back-of-panel rack mounting, or
for mounting at a remote location. The alarms must be displayed on a separate annunciator.
The monitor must provide a fault alarm for open or short circuits in the control wiring between
the detector and the monitor. Monitor relays that are used for pre-alarm and shutdown output
functions must be the hermetically sealed, plug-in type. The trip settings must be in accordance
with the recommendations of the turbine manufacturer.
In accordance with SAES-J-601, Recommended Temperature Alarms and Input Shutdown

Devices, 100-ohm platinum RTDs or Type E or K thermocouples that are wired directly into a
Triple Modular Redundant Emergency Shutdown (TMR ESD) system, or analog 4-20 mA dc, or
digital signals from ambient temperature-compensated temperature transmitters/transducer, are
recommended for measuring and inputting ESD temperature signals. Capillary or bimetallic type,
direct process actuated temperature switches with an associated indicating gauge, must not be
used unless thermocouple or RTD measurements are not practical or feasible.

TYPICAL VIBRATION MONITORING ARRANGEMENTS
Vibration monitoring arrangements describe the type of monitoring instruments and the locations
of the monitoring instruments on a turbine. API 670 requires that a monitoring arrangement plan
be furnished for each machinery train. Monitoring arrangement plans typically illustrate the
following:
• The position of each monitoring probe in relation to the turbine bearings.

• The direction of active thrust for the turbine.
• The direction of turbine rotation as viewed from the high pressure side of the
turbine.
• A complete description of the monitoring system, including:
1. The number, type, and position of probes
2. The type of bearings.
3. The radial clock position of the probes, with degrees referenced to the vertical
top dead center (TDC) as zero.
4. The location of axial position probes.
5. The arrangement of the turbine/oscillator-demodulator box.
6. The layout of the radial shaft vibration, axial position, casing vibration, and
bearing temperature monitors and all signal locations on the monitor.
The following section will describe the monitoring equipment requirements and arrangements for
general-purpose and special-purpose turbines as defined by industry and Saudi Aramco standards.
General-Purpose Steam Turbines
In accordance with 32-SAMSS-009, vibration and temperature monitoring systems, when

specified on the turbine data sheets, must be installed in accordance with the following standards:
• API 611, General-Purpose Steam Turbines For Refinery Service

• API 670, Vibration, Axial Position, and Bearing Temperature Monitoring Systems
• 34-SAMSS-625, Vibration, Axial Position, and Bearing Temperature Monitoring
Systems
Additional requirements for installation of vibration and temperature monitoring systems for
general-purpose turbines are located in the following Saudi Aramco Standard SAES-J-604,
Protective Instrumentation for Rotating Machinery

Standards associated with general-purpose steam turbines specify the requirements for the
installation of non-contacting proximity probes and seismic probes.
Proximity Probes - As mentioned previously, the noncontact proximity systems as used by Saudi
Aramco have the following applications related to the proximity probe installations: radial to the
rotating shaft, axial to the rotating shaft, rotative speed, and phase relationship.
A noncontact proximity probe is usually permanently mounted in a bearing housing to analyze the
surface of a rotating shaft. Noncontact proximity probes can also be clamped to the bearing
housing, in which case the mounted resonance of the fitting must be taken into consideration.
The probe must be calibrated for the specific shaft material, and the material must be electrically
conductive for the proximity probe to properly set up a magnetic field to sense any gaps.
The proximity probe senses shaft surface defects, such as scratches, dents, thermal growth, and
variations in conductivity and permeability. The proximity probe also senses electrical and
mechanical runout but has difficulty distinguishing vibration from runout. Electrical runout can be
described as an electrical signal from a proximity probe due to the effect of irregular shaft
conductivity and magnetic permeability in the shaft material. Mechanical runout can be described
as the measurements of shaft surface imperfections. Shaft surface imperfections are always
present. A proximity probe cannot readily distinguish shaft runout (mechanical runout) from
vibration. A slow roll may be performed, however, to allow the electronic circuit to memorize
all of the shaft imperfections, which include the runout, and subtract the imperfections from the
signal that the proximity probe reports at running speed. Slow roll is low rpm (200 to 600 rpm)
that occurs during the turbine startup or coastdown. A digital vector filter (used to obtain the
Bode plot) must be “zero nulled” so the runout will not be a factor during the slow roll. The
acceptable shaft vibration limit, excluding electrical runout, can be determined by the following
equation:
12 ,000
Allowable shaft vibration in mils peak-to-peak = or 1.0 mil, whichever is less.
rpm
The measurement of radial vibration is accomplished by monitoring the dc output of a

displacement probe that is associated with the radial vibration at the bearings. Under normal
operation and with no internal or external pre-loads on the shaft, the shaft of most machine
designs will ride on the oil pressure dam. However, as soon as the machine receives some
external or internal type pre-load (steady-state force), the radial position of the shaft in the journal
bearing can be anywhere. The radial position measurement can be an excellent indicator of
bearing wear and heavy pre-load conditions, such as misalignment. In installations in which only
single-plane monitoring is present, radial position must be measured on a periodic basis.

Radial displacement should be closely monitored during turbine startup or coastdown. During a
turbine startup (with hydrodynamic radial bearings), the shaft would be expected to rise from the
bottom of the bearing to some place toward the horizontal centerline of the bearing. This
movement is fundamentally due to the oil flowing under the shaft, which causes the shaft to rise in
the bearing. It is generally believed that the oil film is about one mil in thickness.
Because of the ability of the radial position to change under varying conditions of machinery load
and alignment, the proximity probe transducer system must have a sufficiently long linear range to
allow for the large radial position changes. A long linear range is required in large machines in
which large bearing clearances are normally present.
For a radial vibration transducer, Saudi Aramco requires that two noncontact proximity probes be
mounted to or in each radial hydrodynamic bearing of a turbine rotor. Unless the rotating
equipment construction prevents access to the bearings, this requirement should be strictly
adhered to. As shown in Figure 10, the two probes should be installed with as close to 90
degrees of radial separation as feasible. The probes must be in the same radial plane to the shaft,
so that a true representation of the shaft movement can be monitored. Also, the probes must be
installed so that each probe is offset by 45 degrees from the top dead center of the bearing. The
probes should be identified as X and Y, not horizontal and vertical, and oriented to rotation as
shown in Figure 10. The position of the X and Y probes is defined by Saudi Aramco convention.
The positions of the X and Y probes is determined by standing outboard, facing the turbine in the
direction of the steam flow.
Figure 10. Noncontact Eddy Current Probe Orientation

As specified in API Standard 670, the noncontact proximity probe for a phase reference
transducer must be installed so that its radial axis of observation is along a plane other than the
plane for the radial axes that are observed by the probes for a radial vibration transducer.
A phase reference transducer also serves as a noncontact proximity probe. Only a single probe is
required to be radially mounted on an equipment train with the same rotation and speed. If part
of the train has a different rotation or speed, a separate probe should be provided.
The phase reference transducer detects, once each revolution of the shaft, a phase reference mark
on the shaft. This mark may be a keyway, a key, a hole, a slot, or a projection on the shaft. Any
of these marks will cause a radical change in the probe tip gap and thus provide a signal change to
the proximitor on each revolution.
An oscilloscope references the output signal from a phase reference transducer to a filtered output
signal from a radial vibration transducer. On the oscilloscope display, the detection of the phase
reference mark appears as a pulse on the radial vibration waveform. Phase angle is the number of
degrees (along the x axis of the X/Y plot) from a pulse mark to the first positive peak in the
waveform.
Axial displacement measurements are typically used to monitor the condition of thrust bearings in
rotating machinery. Axially mounted noncontact proximity probes are used to detect the axial
movement of the rotating element during operation. All rotating elements have some axial
movement in response to external forces, such as forces that are imposed through couplings from
other equipment in the train or from the coupling itself, and in response to internal forces in the
rotating equipment, such as changes in process conditions and thermal changes. All
hydrodynamic machines have sufficient axial clearance that allows relatively large gaps to be set
for alarm and trip setpoints. The typical setpoint for alarm is 5 mils into the surface (wear) of the
bearing babbitt. The typical setpoint of machine trip is 10 mils into the surface (wear) of the
bearing babbitt. At least two axial thrust position probes should be mounted to provide axial
thrust position protection. Under the normal operating conditions of a steam turbine, thrust
position can vary with the load of the driven machine, so a variation in thrust position
measurements under differential loads and conditions of a machine are not uncommon.
The axial shaft movements are normally constrained within allowable limits by the design of the
equipment. Axial shaft movement constraints are commonly thrust bearings or thrust shoulders,
both of which interact between the rotating and stationary parts of the equipment.
During normal operation, rotating equipment will have a thrust load in one direction. The
direction of the thrust load depends on the direction of steam flow through the steam turbine, the
types of stages, the external loading, and the design of the balance drum. The design of the thrust
bearing compensates for any residual axial thrust force. The rotating element must be protected
from excessive axial movement that is caused by normal thrust bearing wear, balance drum seal
wear, or thrust bearing failure that would then permit internal rotating element wear and
catastrophic failure.

Two axially mounted noncontact proximity probes are installed to sense changes that occur in the
axial position of the shaft in any direction. The movement will be restricted to allowable values
for steam turbine alarm and shutdown functions.
In accordance with 34-SAMSS-625, Saudi Aramco uses axial position probe arrangements
specified in API Standard 670. There are two probe installation arrangements, an arrangement
for a shaft that is equipped with an integral thrust collar and an arrangement for a shaft without an
integral thrust collar. Figure 11 shows the axial position probe installation for a thrust bearing
with an integral thrust collar. One probe is mounted to measure the integral thrust collar, the
other probe is mounted to measure the end of the shaft.
Figure 11. API 670 Axial Position Probe Installation for a Shaft with an Integral Thrust
Collar
Figure 12 shows the axial position probe installation for a shaft without an integral thrust collar.
This configuration is referred to by the API Standard 670 as the standard axial position
arrangement. Both axial position probes are mounted to measure the end of the shaft.
Noncontact proximity probes must never be installed to observe a non-integral thrust collar. The
arrangement prevents incidental steam turbine shutdown or alarm in the event that a non-integral
thrust collar comes loose and allows the shaft to move axially.

Figure 12. API 670 Standard Axial Position Probe Installation Arrangement
In accordance with the requirements specified in API Standard 670, the axial position monitoring
system must use dual voting logic. In a dual voting logic system, the measurements processed
from the outputs of each transducer must equal or exceed the setpoint to activate the danger
alarm or shutdown (two out of two).
Axial vibration is not normally continuously monitored on centrifugal equipment, but it has
proven valuable in diagnosing some particular machinery malfunctioning conditions. If axial
vibration is monitored or used for diagnosis of a steam turbine, the monitored surface must be
relatively smooth (16 rms finish) and perpendicular to the centerline of the rotor. Monitoring a
smooth perpendicular surface will minimize any effect of mechanical runout on the dynamic
output of the probe, which provides accurate axial vibration readings. Axial vibration
measurements can be read from the same proximity probe that is used for axial thrust position
measurements. Probe-mounting locations must ensure minimum effect of thermal expansion of
the rotor and must minimize the effect of springiness of the thrust bearing assembly in the
accuracy of the reading.

Seismic Probes - Bearing housing seismic transducers are not required on steam turbines that are
rated for less than 1000 hp. A piezoelectric velocity or acceleration seismic transducer is required
to be mounted on the radial bearing housing for steam turbines that are rated for greater than
1000 hp. Piezoelectric velocity transducers must have a minimum linear operating range of 2500
Hz. Piezoelectric acceleration transducers must have a minimum linear operating range of 10
kHz. All piezoelectric transducers must be rated for temperatures above 120°C. Saudi Aramco
establishes some recommended practices for seismic transducers, and these practices are partially
based on whether a machine is horizontal or vertical. All vibration readings should be taken as
close as possible to the top bearing, perpendicular to the shaft, in four positions, 45° to each
other, and with one feeding in line with the piping. The acceptable reading level is 0.18 inches/sec
peak RMS.
The alarm level is set at one and a half times the acceptance level. The shutdown level is set at
two times the acceptance level.
Figure 13 shows a typical vibration and axial position system arrangement for a turbine. The
components shown in this arrangement applies to general-purpose steam turbines when specified
on the data sheets and are required for all special-purpose steam turbines.

Figure 13. Typical Vibration and Axial Position System Arrangement for a Turbine

Temperature Monitoring
General-purpose steam turbines require embedded dual element resistance detectors for each
radial bearing when specified on the turbine data sheet. When specified, radial bearing
temperature sensors, which are replaceable and embedded in the shoe, are to be in accordance
with FORM ISS 8020-415-ENG and FORM ISS 8020-416-ENG. Bearing metal temperature
detectors must be able to be removed without damaging the bearing shoe or pad in which the
detector is installed. When embedded elements are used for bearing temperature measurement,
extra elements shall be installed in the bearing oil throw-off lines. If the general-purpose steam
turbine is equipped with a hydrodynamic thrust bearing, then embedded dual element resistance
temperature detectors are required in the pads for both the active and inactive sides of the
hydrodynamic thrust bearing, as shown in Figure 14. The sensors are to be located in the lower
half of the bearing at 120 degrees apart.
Figure 14. Temperature Detector Installation for a Tilting Pad Thrust Bearing

Embedded Probes - An embedded temperature monitoring probe is typically a RTD or

thermocouple. Figure 15 shows a typical thermocouple installation in a line and in a thrust
bearing. Saudi Aramco does not permit the use of spring-loaded bayonet-type temperature
sensors that contact the outer shell of the bearing metal because experience has shown that a
consistently good contact for reliable and accurate readings is not obtained. In addition, through-
drilling and puddling of the babbitt is not permitted. The thermocouple is inserted through a
drilled hole in the bearing retainer, and its tip is made to firmly contact the backing metal, but not
in contact with babbitt. This installation method provides the most reliable results and can detect
a temperature change more quickly than if the thermocouples were measuring the temperature of
the oil stream. Measuring the backing material could be significant in the case of a sudden rapid
rise in bearing temperature, which might lead to severe bearing or turbine damage before the
turbine could be shut down.
Figure 15. Typical Thermocouple Installation in Line and Thrust Bearings.

Oil Drain Probes - Oil drain probes consist mainly of thermocouple-type temperature detectors
installed in the oil drain line, as shown in Figure 16. The thermocouple is installed in a thermowell
with the tip of the thermocouple in contact with the bottom of the thermowell. Oil drain
temperature is monitored to identify potential operational problems that may cause failure of a
bearing. When used with an embedded temperature sensor, the temperature of the bearing metal
is not to exceed 220°F with an oil inlet temperature of 140°F, while the oil drain return
temperature should not exceed 180°F.
Figure 16. Oil Drain Line Thermocouple Installation

Special-Purpose Steam Turbines
In accordance with 32-SAMSS-010, vibration and temperature monitoring systems must be

installed in accordance with the following standards:
• API 612, Special-Purpose Steam Turbines for Refinery Service
• API 670, Vibration, Axial Position, and Bearing Temperature Monitoring Systems
• 34-SAMSS-625, Vibration, Axial Position, and Bearing Temperature Monitoring
Systems
Additional requirements for installation of vibration and temperature monitoring systems for
special-purpose turbines are located in the following Saudi Aramco Standard SAES-J-604,
Protective Instrumentation for Rotating Machinery

All of the requirements for installation of vibration and temperature monitoring systems for
special-purpose turbines are the same as described for general-purpose turbines (when specified
by the data sheets) with a few exceptions. This section will describe the requirements that apply
specifically to special-purpose steam turbines.
For special-purpose steam turbines, the radial vibration probe sensing area locations must be
clearly shown on the rotor assembly drawing. In exception to API 612, 32-SAMSS-010 requires
that the total electrical and mechanical runout sensed by the radial vibration probes must not
exceed the tolerances as follows:
Equipment Speed Maximum Shaft Runout
1800 rpm or below 0.50 mil
1800 to 7200 rpm 450

rpm
7200 rpm and above 0.25 mil
Special-purpose turbines having a rated power of 224 kW (300 hp) and higher must have two
radial noncontact proximity probes mounted at each radial bearing, two axially mounted
noncontact proximity probes and a phase reference transducer. The probes must be installed as
described previously in the requirements for general-purpose turbines. Radial noncontact
proximity probes must be mounted with sufficient axial distance from the bearing journal to
prevent chrome plating repair of the bearing journal from interfering with the proper vibration
measurement.
All special-purpose steam turbines that are rated at 224 kW (300 hp) or higher must have a
minimum of one seismic velocity transducer mounted on the high pressure end bearing housing.
The seismic velocity transducer must have a minimum linear operating range of 2500 Hz and must
be rated for temperatures above 120°C.
Temperature Monitoring
Special-purpose steam turbines that operate with normal steam temperatures of 455°C (850°F) or
greater must have temperature probe pairs installed at the following locations for monitoring inner
and outer wall temperatures:

• Steam chest flange inner and outer surfaces

• Main pressure casing flange inner and outer wall, on the right and the left sides of
the casing, approximately inline with the first stage exit.
The temperature probes must be installed without thermowells and must not be subjected to
steam pressure. The probe temperature indications must be continuously monitored by a
temperature monitoring system.

GLOSSARY
acceleration The rate at which velocity increases. Measured in g’s (the acceleration
produced by gravity at the earth’s surface; equal to 386.087 in/sec2).
accelerometer A transducer that responds to acceleration and that produces an

electrical output signal (in millivolts) that is directly proportional to
acceleration. Some accelerometers contain circuitry to integrate the
response to acceleration to an output signal proportional to velocity.
amplitude The magnitude of a variable that varies periodically at any instant during
a cycle (or period).
babbitt A soft lead/tin mixture used as a surface in bearings.
condition monitoring A process and a method of monitoring specific parameters on equipment

to determine the status of the condition of that parameter.
critical equipment Equipment that is considered to be vital to continued production and

that is usually non-spared.
displacement Movement of an object from a position of rest, equilibrium, or in

relation to a reference point.
electromotive force A rise in electrical potential energy.
frequency The number of cycles that a periodic variation completes in a given

period. Sometimes stated in cycles per minute (cpm) or cycles per
second (cps, Hertz, Hz). For vibration, frequency is also expressed as a
multiple (1×, 2×) of shaft rotative speed.
non-contact A sensor that detects the gap between its tip and a shaft surface.
proximity probe
non-critical Non-critical equipment is defined as equipment that is not critical to

equipment production and that can, therefore, be spared.
peak-to-peak In reference to a waveform that traces a periodic variation of

amplitude displacement, the maximum amplitude of displacement that occurs
during a complete cycle. On an X/Y graph, it is represented as the sum
of the vertical line from the zero reference line to the positive peak and
the vertical reference line to the negative peak.

phase angle An expression in degrees that defines the relationship between events
that occur as a rotating shaft vibrates. Typically, phase angle defines the
number of degrees that the unbalance mass (heavy spot) in a shaft has
rotated between the event in which a phase reference transducer detects
a phase reference mark and the event in which the heavy spot makes the
closest approach (high spot)to the sensor of a radial vibration
transducer.
phase reference A transducer that identifies a once-per-revolution event (phase reference

transducer mark) on the rotating shaft.
(keyphaser)
resistance A general term for any device that senses temperature by measuring the
temperature detector change in resistance of a material.
(RTD)
root mean square In reference to measurements of vibration, 71% (.707) of a zero-to-peak

(RMS) value for velocity or acceleration. Calculated algorithmically as follows:
a number of instantaneous values occurring during one cycle or during
several cycles are squared; the average of the squared values is taken;
and the square root of this average is then taken. In a vibration
monitoring system, velocity and acceleration are often measured in
terms of RMS values.
seismic transducer A transducer that is used to measure velocity or acceleration. The term
seismic indicates the measurement type: motion in relation to free space
or to a fixed point in free space. Seismic transducers include
accelerometers and velocity transducers, which measure structural
vibration.
thermocouple A junction of two dissimilar metals that has a voltage output that is
proportional to the difference in temperature between the hot junction
and the cold junction.
thermowell A closed-end tube that is designed to protect temperature sensors from

harsh environments, high pressure, and flows. Thermowells can be
installed into a system by pipe thread or welded flange, and they are
usually made of corrosion-resistant metal or ceramic material.
triple modular An emergency or safety shutdown system that employs a two-out-of-

redundant three voting scheme to determine the appropriate output action.
emergency shutdown
system

velocity The time rate at which an object is moving. For vibration, measured in
inches per second (in/sec).
velocity transducer A transducer that senses velocity of vibration and that produces an
electrical output signal (in mV) that is proportional to velocity.
vibration Motion in which an object undergoes periodically occurring

displacement. Vibration is measured in terms of its variables of
displacement (mils), velocity (in/sec), and acceleration (g’s). For
rotating machinery, vibration is assessed in terms of frequency, peak-to-
peak amplitudes of displacement, and either root mean square (RMS)
values or zero-to-peak values for velocity or acceleration.
zero-to-peak In reference to a waveform that traces a periodically varying quantity,

amplitude the maximum amplitude occurring during a half cycle. On an X/Y
graph, it is represented as a vertical line from the horizontal zero
reference line to either the positive or negative peak of the waveform.
Often used to quantify amplitudes of velocity and acceleration of
vibration.

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Engineering Encyclopedia

Saudi Aramco DeskTop Standards

Evaluating Installation Of Vibration

Chapter : Rotating Equipment For additional information on this subject, contact

VIBRATION MONITORING EQUIPMENT....................................................................... 2

Saudi Aramco DeskTop Standards

Table of Figures Page

Figure 1. Example of a Bode and Polar Plot.............................................................. 4

Saudi Aramco DeskTop Standards

Steam turbine bearing temperature information is acquired by temperature detectors positioned at

Saudi Aramco DeskTop Standards 1

VIBRATION MONITORING EQUIPMENT

Saudi Aramco DeskTop Standards 2

Information from portable analysis or computer-based condition monitoring equipment can be

Saudi Aramco DeskTop Standards 3

Figure 1. Example of a Bode and Polar Plot

Note: This plot is for a compressor, not a steam turbine.

Saudi Aramco DeskTop Standards 4

Simple harmonic motion provides an illustration of the relationship between displacement,

Saudi Aramco DeskTop Standards 5

Figure 2. Basic Relationship of Measured Parameters with a Simple Harmonic Motion

Figure 3. Formation of a Complex Harmonic Signal

Saudi Aramco DeskTop Standards 6

Figure 4. Views from the Time and Frequency Domain

Saudi Aramco DeskTop Standards 7

Mechanical sources of vibration in rotating equipment, such as rotor unbalance, misalignment,

Transducers for Vibration Variables

Saudi Aramco DeskTop Standards 8

Transducer Useful Frequency Measure- Advantages Disadvantages

Figure 5. Advantages, Disadvantages, and Useful Ranges of Transducer Types

Saudi Aramco DeskTop Standards 9

Figure 6. Range and Limitations on Machinery Vibration Analysis Systems and

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Noncontacting Proximity Sensor Probes - Noncontacting proximity sensor probes do not

• Light Proximity Probe

• Circumferential variations in surface finish and reflectivity of most shafts causes

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The eddy current type of noncontact proximity probe is shown in Figure 7.

Figure 7. Eddy Current Proximity Probe

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Figure 8. Velocity Transducer

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Accelerometers - The most common acceleration transducer is the piezoelectric accelerometer,

Figure 9. Piezoelectric Accelerometer

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Temperature Monitoring Instruments

Resistance Temperature Detector

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Thermocouples are another reliable source for electrical temperature measurement.

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In accordance with SAES-J-601, Recommended Temperature Alarms and Input Shutdown

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TYPICAL VIBRATION MONITORING ARRANGEMENTS

• The position of each monitoring probe in relation to the turbine bearings.

General-Purpose Steam Turbines