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Buffalo State College
Mechanical Engineering Technology
Senior Design Project Proposal
Spring 2015
Centrifugal Compressor Impeller
Tip-to-Shroud Measurement System
Sponsor: FS-Elliott Co. LLC
Date Presented: March 10, 2015
Design Team:
Morgan Allis _________________________
Chuck Mantell _________________________
Bradley Brooks _________________________
Jeff Villont _________________________
FS-Elliot Design Proposal Buffalo State College
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ABSTRACT
This project will introduce sensor technology to measure impeller-tip-to-shroud clearance
into FS-Elliott’s Polaris+ centrifugal air compressor model P300. FS-Elliott Co., LLC, is a
leading centrifugal air compressor manufacturer with manufacturing locations around the world.
FS-Elliott is seeking solutions to maximize efficiency of their compressors by controlling and
monitoring distances between impeller blade tip and compressor shroud; the first step to this is
installing a sensor to measure impeller tip clearance. This project will explore different sensing
options to measure blade tip to shroud clearance. FS-Elliott will receive a package including
optimal sensing recommendations, product installation into their P300 compressor, and cost
analysis.
FS-Elliot Design Proposal Buffalo State College
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TABLE OF CONTENTS
Section Title Page Number
LIST OF FIGURES 1.0 4
LIST OF TABLES 2.0 5
INTRODUCTION 3.0 6
P300 COMPRESSOR 4.0 8
GUIDELINES 5.0 12
SENSOR TECHNOLOGY 6.0 13
CAPACITIVE 6.1 13
INDUCTIVE 6.2 15
LASER DOPPLER 6.3 16
EDDY-CURRENT 6.4 18
FIBER OPTICAL 6.5 20
VARIABLE RELUCTANCE SENSOR (VR) AND HALL EFFECT 6.6 22
Variable Reluctance 22
Hall Effect 23
PHOTOELECTRIC SENSOR 6.7 25
MICROWAVE DISTANCE SENSOR 6.8 27
RESTRICTIONS 7.0 28
CHOSEN DESIGN 8.0 28
CAUSE AND EFFECT MATRIX 8.1 29
PROJECT MANAGEMENT 9.0 30
APPENDIX 10.0 31
REFERENCES 11.0 32
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LIST OF FIGURES 1.0
FIGURE 3.1. A CENTRIFUGAL COMPRESSOR IN THE MANUFACTURING PROCESS 6
FIGURE 3.2. AN INTERNAL VIEW OF A COMPRESSOR BEING ASSEMBLED AT FS-ELLIOTT’S MANUFACTURING
FACILITY 7
FIGURE 4.1. FS-ELLIOLTT’S P300 COMPRESSOR SHOWING OUTBOARD (A.) AND INBOARD (B.) VIEWS 8
FIGURE 4.2. A LOOK AT INTERNAL COMPONENTS OF FS-ELLIOTT’S P300 COMPRESSOR 9
FIGURE 4.3. TYPICAL OPERATION FLOW-DIAGRAM OF FSE’S P300 COMPRESSOR 10
FIGURE 5.1. SENSOR PROBE INSTALLATION ANGLE 12
FIGURE 6.1.1. POSSIBLE CAPACITIVE SENSOR PROBES 14
FIGURE 6.2.1. INDUCTIVE SENSOR PROBE 15
FIGURE 6.3.1. FIBER OPTIC LASER DOPPLER SYSTEM 17
FIGURE 6.4.1. TWO BODY STYLE EDDY-CURRENT PROBES 19
FIGURE 6.5.1. PHOTO OF MONARCH OPTICAL SENSOR 21
FIGURE 6.6.1. IN A VR SENSOR, THE RESULTING ANALOG SIGNAL MUST BE FILTERED AND
THRESHOLDED TO YIELD A USEFUL PULSE OUTPUT 22
FIGURE 6.6.2. IN A HALL-EFFECT SENSOR, ALL LOW-LEVEL SIGNAL PROCESSING IS PERFORMED ON
THE SILICON CHIP HOLDING THE TRANSDUCER 23
FIGURE 6.6.3. WAYS THAT SENSORS TAKE THEIR READING FROM ROTATING OBJECT 24
FIGURE 6.7.1 PHOTO OF PHOTOELECTRIC SENSOR 26
FIGURE 6.8.1 MICROWAVE SENSOR DIAGRAM 27
FIGURE 8.1. SIMPLIFIED SCHEMATIC OF HB-01 30
FS-Elliot Design Proposal Buffalo State College
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LIST OF TABLES 2.0
Table 8.1 Cause and Effect Matrix 29
Table 9.1. Gantt chart for project management section of our design 31
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INTRODUCTION 3.0
FS-Elliott Co., LLC is a leading manufacturer of oil-free centrifugal air and gas
compressors with sales, service, and manufacturing locations around the world. They
manufacture a variety of air compressors that range from 450 HP (335 kW) to 2500 HP (1864
kW) and 2090 CFM (987 l/sec) to 11500 CFM (5430 l/sec), As shown in FS-Elliott Co., LLC
Polaris+ (2014). Markets and industries they serve include air separation,
chemical/petrochemical, electronics, general industry, medical, mining, oil and gas,
pharmaceutical, and refining.
Turbo machinery is widely used with countless applications. Efficient aerodynamics
makes centrifugal compression ideal for many industrial applications. Centrifugal compressors
produce pressure by transferring energy from a rotating impeller to the air. Centrifugal
compressors are efficient, low maintenance with minimum wearing parts, low vibration, and
have excellent reliability over extended periods of time. FS-Elliott Co., LLC Polaris+ (2014).
Figure 3.1. A centrifugal compressor in the manufacturing
process
at FS-Elliott’s manufacturing facility in Export,
PA
This project will specifically deal with FS-
Elliott’s Polaris+ P300 centrifugal compressor. This
compressor is a medium to small multi stage compressor with three stages of compression. P300
FS-Elliot Design Proposal Buffalo State College
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compressors range from 250-450 hp (186-335 kW) and flow rates of 900-2090 CFM (424-986
l/sec), and a discharge pressure of 45-150 psi (310-1034 kPa). FS-Elliott Co., LLC Polaris+
(2014).
This project involves introduction of a sensor that will measure impeller tip-to-shroud
clearance on FS-Elliott’s P300 centrifugal compressor. Numerous sensing technologies will be
analyzed based on characteristics including cost, size, resolution, temperature rating, surface
area, installation, electronics, durability, life expectancy, sensitivity to contamination, and need
for calibration. FS-Elliott will receive design recommendations based on six-sigma matrix, 3D
model of design, and project management analysis for the project. Successful introduction of
sensor technology into FS-Elliott’s centrifugal compressors could lead to further control of
impeller tip-to-shroud distances by means of actuation.
Figure 3.2. An Internal view of a compressor being assembled at FS-
Elliott’s manufacturing facility
1. Driving bull gear for centrifugal
compressor
2. Driven pinion gear in which compressor impeller will be mounted
P300 COMPRESSOR 4.0
FS-Elliot Design Proposal Buffalo State College
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FS-Elliott’s P300 compressor is a medium to small size, oil free, multi stage compressor,
with three stages of compression. The P300 is the smallest centrifugal compressor that FS-Elliott
manufactures and also has the highest rotational impeller speed as shown in FS-Elliott Co., LLC
P-300 (2008).
Rotational impeller speeds range from 51,310 rpm at low speed and 68,414 rpm at high
speed. The P300 compressor has 15 full and 15 splitter blades in its 1st stage of compression, and
17 blades in its 2nd and 3rd stages of compression. It operates at or below a temperature of 350°F
(177°C). FS-Elliott Co., LLC P-300 (2008).
Figure 4.1. FS-Ellioltt’s P300 compressor showing Outboard (a.) and Inboard (b.) views, as shown by Smith, D.,
Tursky, M., Wellek, R. (2013).
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Figure 4.2. A look at internal components of FS-Elliott’s P300 compressor, as shown, in Smith, D., Tursky, M.,
Wellek, R. (2013).
1. Pinion (driven gear), attached to the impeller.
2. Bull gear (driving gear), connected to engine output, and drives impeller pinion.
3. Impeller, consisting of 15 or 17 blades, moves air in a rotational direction to
diffuser plates.
4. Diffuser plates, causing a decrease in velocity allowing for pressure to increase.
5. Shroud of compressor, component concerned with impeller tip clearance.
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Operation of the P300 compressor consists of ambient air entering an inlet control into its
first stage of compression. A centrifugal impeller accelerates air and then velocity is slowed and
pressure in increased, by means of a diffuser. Air then enters a bundle tube heat exchanger for
cooling. The following diagram shown in (FSE P300 brochure), demonstrates the typical
operation of FS-Elliott’s P300.
Figure 4.3. Typical operation
flow-diagram of FSE’s P300 compressor,
as shown in FS-Elliott Co., LLC P-300
(2008).
1. Ambient air entering inlet control
2. Air accelerated by first impeller. Temperature and velocity rise, before a radial
diffuser plate slows velocity and creates pressure.
3. Hot air enters first stage of intercooling. Air passes over water filled tubes with fins.
4. Air makes two 90-degree turns begins to flow upward, allowing separation of
condensed moisture from cooled air.
5. Air exiting heat exchanger and flowing through second inlet control device for
second stage of compression.
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6. Air discharging form second stage of compression, enters second intercooler,
identical to the first.
7. Air then moves into third stage impeller, diffuser and scroll casing
8. Air finally discharges from final intercooler into the air system
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GUIDELINES 5.0
Design requirements that must be met include: temperature tolerance, sensing
resolution/accuracy, target size, target frequency and probe size. The sensor must be able to
function in temperature up to 350° F. The sensor must also be small enough so that it can easily
be installed inside the compressor housing. It should be noted that because there is very little
area (0.09”) of the impeller that could be perpendicularly accessed by a sensor, the face of the
sensor would be unable to be flush with the inside of the shroud and therefore the sensor would
have to sit back inside the casing; this would lead to calibration at installation. Most importantly,
the sensor must have a relatively low target area to sensor tip ratio and be able to work with an
uninform target area, or in other words, a target area that is constantly entering and exiting the
target region with some frequency. The required operating frequency is 8500-20,000 Hz.
Because it is important that the shroud-
to-tip distance measured is from the furthest
outmost point on the impeller, the sensor probe
will be installed in a diagonal fashion to the
shroud and will be placed is such a fashion that it
will detect the furthest most impeller radius. The
arrow in figure 4.1 demonstrates the approximate
angle at which the probe will be placed. The tip
of the arrow signifies what part of the sensor will
be detected.
Figure 5.1. Sensor probe installation angle, as
shown in Smith, D., Tursky, M., Wellek, R. (2013).
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SENSOR TECHNOLOGY 6.0
Sensors that have been used to measure impeller-tip-to-shroud clearance include: x-ray,
capacitive, inductive, optical, eddy-current, microwave, acoustic, and fiber optic laser Doppler.
Capacitive 6.1
Capacitive sensors are arguably the most common sensing technology used to measure
turbo machinery impeller-tip-to-shroud clearance. Noncontact capacitive sensors work by
measuring changes in capacitance As a general rule, the target should be 30-50% larger than the
capacitance sensor; the further the probe is from the target, the larger the minimum target size. A
conductive target area is necessary for a conductive sensor, but provided that the target is
conductive the specific target material does not affect capacitive sensors. Because the sensing
electric field stops at the surface of the conductor, target thickness does not affect the
measurement. Additionally, they can measure high frequency motions because no part of the
sensor needs to stay in contact with the object.
Pros: As with any sensing technology capacitive sensors have advantages and
disadvantages. A significant strength of capacitive sensors is their ability to resolve
measurements below 0.001 in., at a fraction of the cost of other high performance sensing
technologies. Their simple design allows for use in extreme environments while still
maintaining accuracy. Furthermore, they are a versatile sensor- able to be used in various
applications. They are immune to target composition and work equally well on all
conductive targets, unlike eddy current probes. Lastly, they are immune to ultrasonic
noises, lighting conditions, humidity and temperature for the most part.
Cons: A major disadvantage of capacitance sensors is that the probe must be mounted
close to the target, not a problem relevant to the application at hand. Furthermore,
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capacitive sensors must be kept in a mostly sterile environment, free of dirt and debris,
and therefore frequent cleaning may be necessary. These sensors are usually not used
where fluids, or splatter of fluids is present.
Probe Cost: $100.00-$300.00
Cost of Electronics: TBD
Size: 0.125”-0.50”
Total Cost: TBD
Target Frequency: Up to 250,000 rpm
Target Area: 33% of probe head size
Temp. Range: -70°-1500° F
Figure 6.1.1. Possible Capacitive Sensor Probes, Lion Precison, (2014)
Inductive 6.2
Inductive sensors are non-contact detection devices that require a metallic target at ranges
under 2 in. The sensor emits an alternating electro-magnetic sensing field which when disrupted
by a magnetic target passing through, eddy currents are induced, which in turn reduces the signal
amplitude and triggering a change of state at the sensor output- thus measuring displacement.
Pros: Inductive sensors are not affected by water, oil, dirt, and non-metallic particles. They are
insensitive to target color or target surface finish. Also, the sensors are very short-circuit
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resistant. Furthermore, inductive sensors have the ability to withstand high shock and vibration
environments.
Probe Cost: TBD
Cost of Electronics TBD
Total Cost: TBD
Size: 0.125”-0.50”
Target Frequency: Electronics Dependent
Target Area: Equal to or greater than the sensor face
Temp. Range: -40°- 446° F
Figure 6.2.1. Inductive Sensor Probe, Lion Precision (2015)
Laser Doppler 6.3
Capacitive and inductive probes are usually employed for tip clearance measurements
because they are robust and cheap, but when accuracies above 0.001” are required fiber optic
laser Doppler sensing technology can be applied. Unfortunately, fiber optic Doppler distance
sensing technology is not common practice and therefore is an in-house assembled system.
Because the system is not a manufactured package it is a costly. The laser Doppler
measurement sensor consists of an optical head, a light-source unit, and a detection unit (see
figure 1). While this technology meets all the design requirements it is very costly in contrast
to other sensor options.
Probe Cost: TBD
Cost of Electronics: TBD
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Total Cost: Greater than $2000.00
Size: TBD
Target Frequency: TBD
Target Area: TBD
Temp. Range: -50°-1500° F
Figure 6.3.1. Fiber Optic Laser Doppler System, Pfister Thorsten (2008)
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Eddy-Current 6.4
Eddy-current sensing technology is well established and therefore one of the cheapest
sensing technologies. Eddy-current sensors are noncontact devices capable of high-resolution
distance measurement. Tolerance to heat and dirty environments is very high. With eddy-current
sensors the spot size (the area that will be detected) must be 33% larger than the sensing element
diameter. This results in a cone shaped cylinder from the point of the end of the sensor to the
inside surface of the shroud. Compared to other noncontact sensing technologies eddy-current
sensors have some distinct advantages: high tolerance to dirty environments, higher temperature
tolerances, not sensitive to material in the gab between the probe and target, and they are less
expensive than most other sensors. A major drawback to eddy-current sensing technology is that
the target area must have infinite frequency (be solid). The only way it would be able to be used
in the specific impeller tip clearance application is for it to measure the back of the impeller and
to be calibrated based on the static shroud to sensor displacement- this is a cumbersome
approach.
Probe Cost: $100.00-$300.00
Cost of Electronics: TBD
Total Cost: TBD
Size: 0.125”-0.50”
Target Frequency: Infinite
Target Area: 33% larger than probe face
Temp. Range: -70°-400° F
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Figure 6.4.1. Two Body Style Eddy-Current Probes, Lion Precision (2015)
Fiber Optical 6.5
A fiber optic sensor works by converting light rays into electronic signals. It measures the
physical quantity of light and then translates it into a form that is readable by an instrument. An
optical sensor is generally part of a larger system that integrates a source of light, a measuring
device and the optical sensor. This is often connected to an electrical trigger. The trigger reacts to
a change in the signal within the light sensor. An optical sensor can measure the changes from
one or several light beams. When a change occurs, the light sensor operates as a photoelectric
trigger and therefore either increases or decreases the electrical output. Optical switches are
optoelectronic devices which can be integrated with integrated or discrete microelectronic
circuits.
In general, the optical sensors have the best resolution using triangulation method and they
also have the highest system bandwidth. But optical sensing technology, compared with other
technologies, is relatively complex, expensive, bulky, and are sensitive to contamination. Also,
another downfall could be the lack of light that’s needed to take these measurements.
Probe Cost: $151.90 for the probe
Cost of Electronics: TBD
Total Cost: TBD
Size: 0.256” D
Target Speed: 1-250,000 RPM
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Target Area:
Resolution:
Temp. Range: -10 to 250 F
Figure 6.5.1 Photo of Monarch optical sensor shown in, Remote Optical Sensor (2015).
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Variable Reluctance Sensor (VR) and Hall Effect 6.6
Many sensor technologies are out there on the market to measure speed of a rotating
object and can also be used to measure distance away. For many applications especially those
that operate in extreme environments the choice often comes down to either Hall-effect or
variable-reluctance (VR) speed sensors as said by Ed Ramsden (2000). Sensors based on either
of these technologies can be applied in both conditions, with both extreme heat and extreme
cold. These sensors can maintain function even in the presence of dirt and debris. For these
reasons our group has found these two sensors to be extremely important to thoroughly research
in our continuation of sensor research.
VR sensors
In its most basic form, a VR sensor consists of a coil of wire wrapped around a magnet as
shown in Fig5.7.1. As gear teeth (or other target features) pass by the face of the magnet, they
cause the amount of magnetic flux passing through the magnet and consequently the coil to vary.
Figure 6.6.1. In a VR sensor, the resulting analog signal must be filtered and thresholded to yield a
useful pulse output Ed Ramsden (2000).
When a target feature such as an impeller blade is moved close to the sensor, the flux is at
a maximum. When the target is further away, the flux drops off. The moving target results in a
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time-varying flux that induces a proportional voltage in the coil. Electronics are then used to
massage this signal to get a digital waveform that can be more readily counted and timed. One
area in which VR sensors stands out, is in high-temperature applications which we are somewhat
dealing with. With appropriate construction VR sensors can be made to operate at temperatures
up to and possibly higher than 300°C. Ed Ramsden (2000).
Hall-effect sensors
In a Hall-effect speed sensor the Hall-effect transducer element detects target-induced
flux changes from where it is situated, between the magnet and the target being measured.
Unlike a VR sensor, however, a Hall sensor is sensitive to the magnitude of flux, not its rate of
change.
Figure 6.6.2. In a Hall-effect sensor, all low-level signal processing is performed on the silicon chip
holding the transducer from Ed Ramsden (2000).
This feature of Hall-effect technology allows you to make speed sensors that can detect
targets moving at arbitrarily slow speeds, or even the presence or absence of nonmoving targets
as noted by Ed Ramsden (2000). This can be useful if we test the sensor when the machine is at
rest, or cold. An important feature of Hall-effect speed sensors is that the signal-processing
electronics are typically included into the same package as the transducer, providing several
tangible benefits. The most important is that little or no additional signal-processing circuitry is
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required--most Hall-effect speed sensors directly provide a digital output signal that is directly
compatible with digital logic, microcontrollers, and PLC.
Figure 6.6.3.
Ways that
sensors take their reading from rotating object as shown from www.daytona-
twintec.com/tech_sensors (2015).
Hall Effect Sensor Cost: $150-$250
Electronics Cost: TBD
Total Cost: TBD
Size: 0.472” D
Target Speed: TBD
Area: TBD
Temp. Range: -40-150oF
Variable Reluctance
Probe Cost: $100-$200 range
Electronics Cost: TBD
Total Cost: TBD
Size: 0.375” D
Target Speed: TBD
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Target Area: TBD
Resolution TBD
Temp. Range: Can handle in excess of 300oC
Photoelectric Sensor 6.7
Proximity photoelectric sensors use the target as the reflector to measure displacement
which can be useful in our application due to wanting to find how far away the impeller is.
Presence is detected when any portion of the reflected signal bounces back from the detected
object. Reflective properties of the target must be evaluated for correct placement, as these
sensors can be affected by target material and surface properties as noted in the article by IHS
Engineering 360 (2015). Convergent proximity photoelectric sensors are used to focus the
emitter beam at a fixed distance from the sensor which we can measure when it is broken and
how far back its path is broken. This allows for good sensitivity but limited depth of detection
which is okay for the system at hand. This sensor is small in size and does not require much
technology to obtain readouts.
A major downfall to the photoelectric sensor is that it more likely than not can be used
while the compressor is operational; which is an option FS-Elliot would like to have if possible.
The speed that the impellor will rotate at is much too great for what the sensors is rated for
reading at.
Cost: Unit in Figure 5.8.1 is $7.50
Size: 60 x 20.5mm/ 2.4" x 0.8"
Target Speed:
Target Area:
Temp. Range:
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Figure 6.7.1 Photo of photoelectric sensor from
Amazon (2015), seller is Lantee Limited Sales.
Microwave Distance Sensor 6.8
A microwave measure system can be defined as systems that are coupled together with a
transmission line having a uniform cross section. The concept of traveling electromagnetic
waves on that transmission line is fundamental to the understanding of microwave
measurements. The particular sensor being considered for this project is the 300 kHz Ultrasonic
microwave distance sensor, model TA030013. Some of the technical aspects of this device are
that it has a sensibility factor of -70db, a bandwidth of 25 kHz, and has an operating temperature
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between -40 to +80 degrees Celsius.
Figure 6.8.1 Microwave sensor diagram as shown from 300Khz Ultrasonic Microwave Distance Sensor(2015).
Strengths A. Small size and light weight
B. Low power consumption
C. High Reliability
Weakness
A. Bad temperature range
B. Not really a probe may be hard to correctly assemble inside the compressor
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CHOSEN DESIGN 8.0
Table 8.1 Cause and Effect Matrix
Rating of Importance to
Customer
7 7 7 8 8 10 10
Sensor Type Pro
be
Co
st
Elec
tro
nic
s C
ost
Tota
l Co
st
Res
olu
tio
n
Targ
et A
rea
Max
imu
m
Tem
per
atu
re
Req
uir
ed
Targ
et
Freq
uen
cy
Weighted Totals
1 Capacitive 5 5 5 3 5 5 5 269
2 Inductive 5 5 5 3 5 5 1 229
3 Optical 3 3 3 5 4 5 5 235
4 Eddy-
Current 5 5 5 3 2 5 5
245
5 Microwave 3 3 3 3 4 5 5 219
6
Fiber Optic Laser
Doppler 1 1 1 5 4 5 5
193
7 VRS 4 4 4 3 5 5 5 248
Using the cause and affect matrix it is determined that capacitance sensor technology will
be used (See the appendix for the weighted scales used in the matrix). Two suppliers will be used
in conjunction with each other, Aerogage Inc. and Capacitec, Inc. Aerogage will supply all
necessary electronics while Capacitec will supply the probes. Aerogage will supply the model
HB-01 hi-band amplifier, an amplifier that has been used for similar applications. The HB-
01amplifier is designed to be coupled with capacitor sensor. The HB-01 contains a low-noise,
high-efficiency switching power supply, which generates a dc voltage of approximately 100
volts for biasing the capacitive probe. The output stage of the HB-01 is capable of driving 50-
ohm cables up to 1000 feet or more in length. The bandwidth of the system is about 5 MHz,
which covers most applications between 150,000-200,000 rpm.
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Because the blade and probe will be at an angle from each other the calibration will be
affected, but not the ability to measure the tip clearance. Because capacitance versus the gap
relationship is affected by the size of the blade and the size of the active sensing area of the
probe, a calibration process is required to determine the exact relationship; the angle of the probe
will be automatically included in this process. The resolution of the system is 1% of the target
area distance.
Figure 8.1. Simplified Schematic of HB-01
A capacitive sensor, shown as Cs in Figure 7.1, consists of a electrode plus the grounded
target, the compressor blade. The HB-01 manual states, “this structure can be modeled as a
grounded capacitor, whose capacitance changes (increases) when a blade passes in front of the
sensor. A coaxial cable connects the sensor capacitor, Cs, to the HB- 01 Hi-Band Preamp. The
center conductor of the coaxial cable is attached to the sensor electrode, and the shield of the
cable is attached to the engine casing, which is at ground potential. At the HB-01 end of the
cable, the center conductor is connected the input of the first stage amplifier and the shield of the
cable is connected to the analog ground of the circuit.”
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PROOF OF CONCEPT BENCH TEST 9.0
It is necessary to have a proof of concept bench test system before the final system is
implemented. As the name implies, the proof of concept system will prove that theoretically the
final system is sound and will work as expected once implemented in the actual compressor. The
system will be made of two main parts: (1) the mechanical system that will mimic the
compressor and impeller blades, and (2) the electronics that will allow the sensor measurement
to be read. Refer to table 9.1 for the complete bill of materials.
Table 9.1 Capacitive Sensor Bench Test Bill of Materials
Item Supplier Part # Item Price Quanity Total Price
Electric Motor McMaster 5990K47 $240.92 1 $240.92
Bearings McMaster 2722T34 $111.22 2 $222.44
3" Dia. Pulley (4L, A) McMaster 6245K67 $9.91 1 $9.91
5.25" Dia. Pulley (4L, A) McMaster 6245K93 $15.86 1 $15.86
V-belt (4L, A) McMaster 6191K16 $6.15 1 $6.15
Feeler Gauge McMaster 2334A2 $5.84 1 $5.84
Capacitive Sensor Capacitec, Inc. HPT-150I-A-L2-2-B $475.00 1 $475.00
Impeller FS-Elliott $0.00 1 $0.00
Oscilliscope Buffalo State College $0.00 1 $0.00
Electronics Buffalo State College $0.00 1 $0.00
Total: $976.12
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APPARATUS
Impeller Assembly
As seen in figure 9.1 the impeller assembly is made up of an impeller, an electric motor, a
motor speed controller (not pictured), a set of bearings, two pulleys, a v-belt, a sensor stand, and
a base that everything is mounted to.
Figure 9.1 Proof of Concept Impeller Assembly
The impeller was supplied by FS-Elliott. Their team balanced it by removing material
from the bolt and washer that fastens the impeller to the pinion shaft.
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The electric motor was purchased from Mc-Master. The motor is an AC motor and has a
maximum operating speed of 3450 rpm. An AC motor speed controller is used to ramp up the
system and to have variable operating speeds.
Two bearings purchased from Mc-Master are used to hold the pinion shaft. The bearings
are rated for a maximum of 9000 rpm. The test system operating speed will be approximately
6000 rpm, leaving a safety factor of 1.5.
Two pulleys with a velocity ratio of 1.75 will be used. This means that when the motor is
running at 3450 rpm the impeller will be rotating at 6038 rpm. A standard v-belt will be used to
transfer power from the motor to the pinion shaft.
A sensor stand will be used to hold the sensor in place and allow ample adjustment of
impeller tip to sensor tip distance as desired.
Electronics
A highly simplified circuit, as seen in figure 9.2, will be used in the bench test
system. Simply put, the circuit will take the change in capacitance from the probe and convert it
into an output voltage; this signal will be directly proportional to the distance from the impeller
tip to the sensor; the system will need calibration.
Figure 9.2 Bench Test Circuitry
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The capacitive sensor shown in figure 9.2 as Cs, is made up of a spark-plug type
electrode plus a grounded target, in this case the impeller blades. A coaxial cable connects the
probe to the high voltage power source (usually 100 V), which enters through R1 and supplies
the voltage to the capacitive probe. The dc voltage of the sensor is blocked by capacitor C2, and
any change in voltage at the sensor is than applied to the negative terminal of op-amp U1 from
C2. Amplifier U1 has a feedback capacitor, C1, which causes the amplifier to act as a charge
amplifier. This setup will cause U1 to try and keep the voltage on Cs constant. The result is an
output voltage from U1, which will be the change in charge required to keep the capacitive probe
at a constant voltage. If the capacitance of the sensor changes due to blade passing in front of the
sensor the output charge of U1 will change as well. This relationship can be described by:
∆𝑉 =(∆𝐶𝑠)(𝑉𝑜)
𝐶1
Where: ∆𝑉 is the change in output voltage at the output of U1
∆𝐶𝑠 is the change in capacitance of the sensor
C2 is the value of the feedback capacitor
A typical value for 𝑉𝑜 is 100 volts; this value drives the amount of gain to the amplifiers
output- the higher the value the higher the output voltage. C1 is typically either 10 pF, or 47 pF;
we used 47 pF.
Concerns
There were significant concerns with regards to the proof of concept test system.
The system was highly sensitive to “electrical noise” and this had a very significant impact on
the results of the system. The noise of the system was many times greater than the output signal,
which did not allow for precise measurement.
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Calibration
Every unique system must be calibrated because there are variances system to system.
The reason for this is because each system target will give a unique capacitance signal. In theory
calibration is very simple. The system is run at various clearances and the corresponding change
in voltage is observed, this represent a data point (distance vs. voltage). A significant amount of
data is then observed and then plotted. A linear trend will emerge and a function is created based
on this trend. The function should have a R-squared value no smaller than .999. A hypothetical
data set is shown in figure 9.2. The equation 𝛿 = 0.001(∆𝑉) − 0.001, would be used to
determine impeller clearance, where 𝛿 is equal to the clearance and ∆𝑉 is equal to change in
voltage.
Figure 9.2 Capacitive Sensor Calibration
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Proof of Concept Conclusions
The validity of an impeller tip clearance system is supported with the proof of concept
system. With this said, the degree of accuracy that is necessary is not supported by the bench
test. The system would need significant improvements in order to support a final installation of
the system into the P300 centrifugal compressor. The general theory behind the system was
indeed proven- a change in capacitance output from a capacitance sensor can indeed be
converted into a voltage waveform, and this waveform is directly proportional to the clearance.
To eliminate noise an expanded circuit would be necessary and this circuit would need to be
encapsulated in a “Faraday cage” of sorts.
CONCLUSIONS 10.0
It is clear that measuring impeller clearance is not an easy, inexpensive thing to do. This
is mainly due to tight tolerances, high target frequencies, and most importantly, the need for very
advanced electronics. Because of the above mentioned factors it quickly becomes clear why the
leading centrifugal compressor companies have not implemented impeller sensing technology in
their products.
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1/26 2/2 2/9 2/16 2/23 3/2 3/9 3/16 3/23 3/30 4/6 4/13 4/20 4/27
Initiating
Centrigugal compressor information
Different types of sensors available
Restrictions and Design Guidelines
Planning
C&E Matrix
Plant and manufacturing facility visit
Final sensor probe and electonics
Position of sensor in compressor
Design costs
Executing
implementing sensor set-up
Monitoring & Control
Sensor calibration
Accuracy of sensor measuring blade tip clearance
Closing
Design improvements
PROJECT MANAGEMENT 9.0
Project points of completion are set to have an outlook on the project, from beginning to
finish. A Gantt chart is used to keep track of all planned project dates throughout the entire
project. As unexpected problems or events occur in our project, Gantt charts can be modified
easily to keep everyone on track to finish the project by our final goal of April, 30th 2015.
Table 9.1. Gantt chart for project management section of our design
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APPENDIX 10.0 Cause and Effect Matrix Key
Cost
1. Greater than $1,251.00
2. $1,001.00-$1,250.00
3. $751.00-$1000.00
4. $501.00-$750.00
5. Less than $500.00
Probe to Target Area Ratio
1. Greater than 1:4 (Greater than 25%)
2. 1:3.1-1:4 (32%-25%)
3. 1:2.1-1:3 (48%-33%)
4. 1:1.1-1:2 (100%-50%) 5. Less than 1:1 (Less than 100%)
Sensor Tolerance (in.)
1. Greater than 0.0031”
2. 0.002”-0.003”
3. 0.001”-0.0021
4. 0.00075”-.00011
5. Less than 0.00075”
Resolution (% of target distance)
1. Less than 1%
2.
3. 1%
4.
5. Greater than 1%
Maximum Temperature (°F)
1. Below 350°
2.
3.
4.
5. Above 350°
Required Target Frequency (Hz)
1. 17,001-Infinite Hz
2. 14,001-17,000 Hz
3. 11,001-14,000 Hz
4. 8,001-11,000 Hz
5. 5,000-8,000 Hz
Ease of Installation
1. Greater than 5 hours
2. 4-5 hours
3. 3-4 hours
4. 2-3 hours
5. Less than 2 hours
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4 1 AP-04-01 Four-Channel Blade Tip-Clearance/Timing Measurement System Configured as Single-Channel System: Signal Processors: AP-04 four-channel signal processor chassis with one AP-01 Analog Processor card. Includes power supply and interconnect cable for power distribution (for upgrade to three-channel system, Item 5). Output signals for each channel include Blade Sync, Blade Tip Clearance, Minimum Tip Clearance, Maximum Tip Clearance, Average Tip Clearance.
$6,440.00 $6,440.00
5 1 AP-04-03 Upgrade
Upgrade of Single-Channel System (item 4) to Three-Channel System: Upgrade cost, including testing:
$5,376.00
$500.00
$5,376.00
$500.00
6 1 AP-04-03 Four-Channel Blade Tip-Clearance/Timing Measurement System Configured as Three-Channel System: Signal Processors: AP-04 four-channel signal processor chassis with three AP-01 Analog Processor cards. Includes power supply and interconnect cable for power distribution. Output signals for each channel include Blade Sync, Blade Tip Clearance, Minimum Tip Clearance, Maximum Tip Clearance, Average Tip Clearance. Note: Item 6 would be in lieu of items 4 and 5.
$11,816.00 $11,816.00
7 3 Probes Capacitec Probe. HPT-75 series. 0.075”
sensor tip, 400°F operation, threaded case,
0.250” diameter, Probe length: 1 inch, Cable length: 6 feet, Connector: BNC
$935.00
(estimated)
$2,805.00
(estimated)
8 1 Calibration
Probe
Capacitec Probe. HPT-75 series, 0.075”
sensor tip, 300°F operation, threaded case,
Probe length: 1 inch, Cable length: 2 inches, Connector: BNC
$890.00
(estimated)
$890.00
(estimated)
9 1 CP-01 Calibration system for probes Capacitance Preamp without probe. Includes Excel spreadsheet to facilitate calculation of probe coefficients for data acquisition system. Connector: BNC for attaching Calibration Probe (Item 8).
$1,050.00 $1,050.00
10 1 Calibration services and/or on-site technical support (optional). Billed on time and materials basis. Labor billed at $150/hour. Customer to pay for travel and lodging expenses for on-site activities.
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REFERENCES 11.0
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FS-Elliott Co., LLC. (2014) Polaris+ Air Compressor [brochure]. Export, PA
FS-Elliott Co., LLC. (2008) Polaris P-300 Air Compressor [brochure]. Export, PA
IHS Engineering 360. Photoelectric Sensors Information. (n.d.). Retrieved March 1, 2015, from
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Lion Precision: High-Performance Noncontact Sensors and Experts to Help You Use Them.
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Pfister, Thorsten, Lars Büttner, and Jürgen Czarske. "Fiber Optic Laser Doppler Distance Sensor
for In-situ Tip Clearance and Vibration Monitoring of Turbo Machines." Applications of
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P51-200-G-B-I36-5V-000-000. (n.d.). Retrieved March 1, 2015, from http://www.daytona-
twintec.com/tech_sensors.html
Ramsden, E. (n.d.). Hall vs. VR: Which speed sensor is wrong for you? Retrieved February 28,
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http://www.electronicproducts.com/Electromechanical_Components/Hall_vs_VR_Which
_speed_sensor_is_wrong_for_you.aspx
Remote Optical Sensor. (n.d.). Retrieved March 4, 2015, from
http://www.zoro.com/i/G0967486/?utm_source=google_shopping&utm_medium=cpc&u
tm_campaign=Google_Shopping_Feed&gclid=Cj0KEQjwifWnBRCB5PT57KSVw-
kBEiQASV7aRBmuW1xlS2JNCEojJ6L63S-y3i8yJTyN_RBepHlbiOcaAkaM8P8HAQ
Smith, D., Tursky, M., Wellek, R. (2013). Compressor Assembly 3 stage, 60hz. P300. [Drawing].
FS-Elliott Co., LLC. Export, PA