Procedures and Specifications - Vibration Analysis
The following is a detailed
specification of testing procedures, equipment, and reporting methods used in
our vibration analysis program. It is fundamentally based upon the testing
methods established by DLI Engineering, as well as our own experience. Such
testing and reporting procedures have evolved over many years based upon our
response to the needs and requests of our customers. Changes in our procedures
will be reflected in this specification.
Preliminary Investigation
Prior to the actual field testing, East
Coast Industries, Inc. shall conduct a walk through of the equipment
location(s), and review all pieces of rotating equipment such as fans,
chillers, generators, condenser water, chilled water, hot water and radiation
pumps, etc. in use. Operating parameters such as previous mechanical
maintenance history shall be noted.
Based upon the number of equipment to be
tested, a general plan of action shall be proposed, and testing fees
quoted.
Test Equipment
Field testing shall be performed using
the DLI Watchman® DCX™ XRT which is equipped with a triaxial
accelerometer. The DLI Watchman® DCX™ XRT is capable of many
analytical functions including time synchronous averaging and demodulation for
analysis of gears and bearings. Other analysis functions include log or linear
amplitude scales, live harmonic markers, and single or multiple channel
displays for both time and spectrum.
To determine machine health and identify
problems, vibration data from each measurement point will be taken in three
directions. This is accomplished by using a triaxial accelerometer. A triaxial
accelerometer has three transducers in one housing, and they are oriented in
the three orthogonal directions called Axial, Radial, and Tangential.
Axial is the direction parallel to the
centerline of a shaft or turning axis of a rotating part. Radial is that
direction toward the center of rotation of a shaft or rotor. The Tangential
measurement is that measurement that is tangent or perpendicular to the radial
transducer.
Machine Preparation
In a machinery monitoring program, it is
important that the data is collected in exactly the same manner each time a
measurement is taken. For that reason ECI installs a cylindrical mounting block
or "pad" at each bearing point on the machine. A pad is a bronze disc
with a center tapped hole and has a knoch at the edge that receives an indexing
pin that is located on the accelerometer.
The direction that is sensitive along the
axis of the mounting screw is channel number one. The axis in the direction of
the key way is channel number two, and the axis perpendicular to this is
channel number three. The "pad" is attached to the machine using an
adhesive which consists of two parts to permanently assure that the data is
repeatable and can be trended over time. The installation of the mounting
blocks is sometimes referred to as "blocking" a machine.
When blocking a machine it is important
to locate the test pad as close as possible to the bearing ensuring solid metal
contact between the bearing and the accelerometer. It is important to keep in
mind that thin metal is a poor conductor of vibration energy. The test point
locations will be chosen so that there is no metal-to-metal joint between the
bearings and accelerometer. An example of this would be the joint between the
end bell and stator housing and fan housings on the ends of motors.
A typical motor driving a pump or fan
usually consists of four test points. Typically having one test point located
at the motor free end. The second located at the drive end of the motor. The
third test point located on the driven end of the pump or fan, and the fourth
point located on the free end of the pump or fan.
Software Database Setup
Once the machine preparation is
completed and all the information is collected, the information can then be
programed into the DLI Watchman® DCX™ XRT software. The software used
by the DLI Watchman® DCX™ XRT is called Expert Alert. This software
allows you to setup a database in a hierarchy of plant, area, and machine. A
plant is the physical site where the company preforms normal everyday
operation. Areas are used to group machines together in a specific region of
the plant. The final grouping is done by machine. This allows you to use a name
or number to relate to a specific machine in an area.
To get the best results from the Expert
Alert software it is important to have a good average database. The average
database tells the system what the vibration levels should look like, what
vibration levels are normal, and how much vibration variation to expect. There
should be at least five averages to make up a good average database. There is
only one average recorded per "MID". A "MID"is a grouping of
identical machines running under like conditions. A unique machine will have
its own "MID".
Testing Procedure
After the machine is blocked and the
Expert Alert software is configured to the equipment to be tested, we are ready
to began testing. The triaxial accelerometer is then attached to the proper
test point "pad"on the machine. It is attached by lining up the indexing
pin, and tightening the center screw into the center tapped hole in the
"pad".
While the machine is running, the DLI
Watchman® DCX™ XRT will collect the vibration data. All data collected
is then stored on the DCX's built in hard drive in wave form spectra. This step
is then repeated until all test points for a particular machine have been
tested. If we are testing a machine with a "VFD" or variable frequency
drive it is recommended that the run speed be set to 50 Htz. so the test data
can be repeatable.
Once all data is collected it is brought
back to the office and normalized on a desktop computer. After the data is
normalized it is run through the software's "expert system". The expert
system is an option in the software that evaluates the vibration results for
possible machine problems. The software also gives you a detailed diagnoses as
well as a severity level for all faults present.
Severity levels are based on a
combination of vibration levels and there accedences of average plus one sigma.
Other frequencies and groups of vibration peaks are addressed by a templet.
Each specific mechanical fault is represented by a diagnostic templet. The
expert system uses approximately three hundred and fifty different templets to
determine faults, harmonics, sidebands, and elevated noise floors.
Reporting Procedure
A final written report shall be produced
following the completion of the field testing. The report will contain
information for all equipment tested, and will be made up of five sections.
The first section of the report will be
an introduction to vibration analysis. This section of the report will describe
detailed information about vibration analysis, and will cover a history as well
as overall information about the procedures and equipment used by East Coast
Industries. This section will also contain a list of basic definitions used in
the vibration field.
Section two of the report is for properly
functioning pieces of rotating equipment. This section will list all properly
functioning machines tested and each machine will have a picture and basic
information gathered off of the machine tag. All machines listed will have a
maximum level Vdb which was recorded during testing. This section also contains
machines with slight diagnoses and no recommendations required.
The third section of the report will be
for malfunctioning pieces of equipment. This section of the report will also
contain a picture and basic information gathered from the machine tag along
with recommendations, and detailed diagnoses for all machine faults. Each
machine will have an illustrated graph which will show an increase in amplitude
in the vibration spectra. There will also be a trend plot if the problem was
present in previous tests. If there are any machines not tested during our
survey they will be listed in section three A of the report.
Section four of the report is a repair
wooksheet. This section of the report lists all machines with faults as well as
there test status and there previous test status.
The final section is conclusions and
recommendations. This is an overview of the report and lists all the basic
findings of our analysis.
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