POWER January 2016 - 47

EQUIPMENT MONITORING
1. Shaft torsional telemetry system
components. Courtesy: EPRI
custom-fitted shaft collar or temporary belt is
eliminated. Developing a standard size rotating
telemetry transceiver is an important consideration,
so that it may be applied on various
shaft diameters without costly customization.
The transceivers contain a pair of semiconductor
strain gauges and a pair of accelerometers,
which are configured to simultaneously
measure shaft torsional vibration and lateral vibration.
This configuration improves mode frequency
detection regardless of the mode shape
at the selected transceiver location on the shaft.
The transceiver board transmits a 2.4 GHz
levels of torsional vibration associated with
modes having low elastic energy at measurement
locations. The sensor would have a low
" noise floor, " which would allow capture of
low-amplitude mode frequency indications
that can be hidden by traditional wireless instrumentation.
A high measurement sampling
rate would ensure sufficient frequency resolution
to verify that natural frequencies are
fully outside the avoidance band at normal
running conditions. In addition, the design
would not require custom-fitted components,
and it would take advantage of available lowcost
microelectronics.
EPRI Wireless Shaft Vibration
Monitoring System
Based on its analysis of current sensor hardware,
and with its objectives for improvement,
EPRI developed a wireless telemetry
system to meet the needs of the power generation
industry.
According to Chris Suprock, principal engineer
at Suprock Technologies, " We leveraged
advances in sensor technology, digital
microelectronics, and radio frequency (RF)
engineering. The design is built to withstand
the environmental conditions while providing
maintenance-free operation. The torsional
telemetry system represents a leap forward
from previous rotor sensing technologies.
Furthermore, the rotor installation process
provides a time savings and mechanical improvement
over traditional techniques. "
The major subsystems of the torsional vibration
monitoring system include: rotating telemetry
transceivers, stationary telemetry modules,
and a hub computer assembly (Figure 1).
Rotating Telemetry Transceivers
In the current sensor design, a wireless, battery-free,
rotating transceiver measures and
digitally transmits strain and acceleration data
obtained from the shaft surface (Figure 2).
The concept is developed from the need for a
very compact package with much lower mass
and power consumption than conventional
telemetry. By reducing the rotating telemetry
transceiver mass and size, the need for a
January 2016 | POWER
signal that contains the strain and acceleration
digital data stream. This is received and demodulated
by the stationary receiver. The resulting
data is archived on the system computer.
An epoxy-infused Kevlar band, or partial
band, attaches the rotating sensor board to
the shaft surface. The band adheres the sensor
to the shaft in a manner that can accommodate
expected temperatures and g-loading
due to rotation. The encapsulation also protects
the sensor board from environmental
and mechanical damage in a power plant environment.
Key to the design of the sensor
attachment and encapsulation system is the
ability to perform the encapsulation rapidly,
within critical path outage schedules. Equally
important is the long-term reliability of
the Kevlar infusion and the bond to the shaft.
A special vacuum impregnation process
has been developed to ensure an aerospacequality
epoxy bond produced within a timeconstrained
installation window.
Stationary Telemetry Modules
A set of stationary telemetry modules supplies
radio excitation and enables control communication
with the rotating telemetry transceivers.
The stationary telemetry module is attached to
adjacent turbine or generator housings within
approximately 1 meter of the transceiver location
on the shaft. Depending on the shaft
size and speed, anywhere from one to three
redundant transmitter/antenna assemblies are
positioned around the shaft. These power supplies
are controlled by the hub computer that
sets the data acquisition parameters and stores
time-stamped raw data.
Hub Computer Assembly. Each of the
several stationary telemetry modules communicates
digitally with a hub computer through
USB. Software on the hub computer manages
the received data, RF power, rotating telemetry
transceivers, and stationary telemetry module
redundancy. The hub computer is responsible
for data logging, configuring of the system,
and post-processing the data that has been received
from the rotating telemetry transceivers
through the stationary telemetry modules.
Data Received from Sensor. The archived
telemetry data can be analyzed using
www.powermag.com
2. Prototype. The telemetry receiver
components are positioned on a lightly abraded
shaft section prior to epoxy encapsulation.
Courtesy: EPRI
one of several data visualization methods:
■ Time-transient plots: These plots show
strain versus time for a selected strain
gauge or accelerometer. This is typically
used to capture a brief event associated
with a unit trip, grid disturbance, or synchronization
(Figure 3).
■ Averaged spectral plots: These plots are
used to accurately establish the natural frequencies
of the shaft system, indicated by
peaks in amplitude versus frequency plots.
Spectral plots document the variation in
shaft natural frequencies with operational
parameters such as unit load (Figure 4).
■ Spectrograms: Spectrograms consist of a
stacked series of spectral plots obtained
during non-steady machine operation-
for example, a rotor speed ramp following
unit trip. This data visualization format is
similar to a Campbell diagram when used
to detect trends in turbine blade frequencies
with speed (Figure 5).
Field Experience
To date, prototype EPRI torsional sensors
have been installed on three shaft locations
on two separate generating units. The accumulated
operating time represents a total of
11 unit-months of operation without failure
of the shaft sensors, circuit boards, or the stationary
antenna assemblies.
The data received from the prototype sensor
is considered high quality and sufficient
to meet the industry needs relative to shaft
frequency identification.
A Salt River Project (SRP) Navajo Generating
Station (NGS) spokesperson explained
47
http://www.powermag.com
POWER January 2016

Table of Contents for the Digital Edition of POWER January 2016
