| Introduction PCS Potash New Brunswick
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| | continually being excited by vibrating
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| Division is a potash and salt mine near
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| | screens throughout the mill.
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| Sussex, N.B., a small farming community in
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| | High resolution spectra was taken at both
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| the southern part of the province. Potash
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| | the tower and vibrating screens to confirm
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| and salt are mined approximately 2000 feet
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| | that the exciting force at the towers is
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| below the surface. The salt requires
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| | coming from the vibrating screens in the
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| little processing and is used mainly for
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| | mill. The spectrum in Figure “2” taken
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| road salt. The potash is hoisted to
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| | at the cooling tower indicates a dominant
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| surface where it goes through a
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| | frequency of 197 c.p.m. By comparing this
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| concentrator.
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| | to the dominant frequency in Figure
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| There it has all the impurities removed
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| | “3” of 198 c.p.m., which represents
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| and is sized to the needs of customers all
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| | the output r.p.m. of the Vibrating
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| across the world. Potash is mainly used in
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| | Screens, confirms that the exciting force
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| the fertilizer industry, being one of the
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| | at the cooling towers is coming from the
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| three main ingredients.
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| | vibrating screens operating in the mill.
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| Cooling towers are required to remove heat
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| | Over the years, several false vibration
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| from the water that has been used to cool
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| | alarms have come in at the control center
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| many different systems throughout the
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| | showing severe vibration at the cooling
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| mill. The water travels in an endless
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| | towers, when in actual fact it was the
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| loop, from all the systems and then back
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| | building vibration that had increased.
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| to the tower where the heat is removed and
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| | Figure “4” is a trend comparing mill
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| back to the same systems. Basically, after
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| | vibration to actual fan vibration. The
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| the cooling water has been used to remove
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| | building continually vibrates at 197
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| any excess heat from any system, it is
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| | c.p.m. which is much higher than the 1x
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| pumped to the roof of the mill where the
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| | fan vibration.
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| cooling tower is situated. The water is
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| | Unfortunately, this causes many false
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| pumped into the top of the tower; an area
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| | alarms at the control room. This is a good
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| called a distribution basin (hot water
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| | example why band alarming is extremely
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| side). The basin has metering orifices
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| | useful in an online vibration monitoring
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| that allow even distribution of the water
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| | system. A highpass filter to filter out
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| over the fill.
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| | the low end background noise would also be
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| The fill acts like a radiator in a car.
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| | an asset. A filter in this application
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| On top of the cooling tower, there is a
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| | would also increase the dB range of the
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| large fan that draws the air through the
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| | system so that there would be less
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| fill, cooling the water and exhausting the
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| | attenuation at the high end of the FFT. In
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| hot air to atmosphere. After the water has
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| | the control room, we only alarm on
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| traveled through the fill, it reaches the
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| | velocity because displacement is too
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| cold water basin at the bottom of the
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| | sensitive to low frequency vibration.
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| tower where it drains by gravity back into
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| | However, there is a data collector
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| a holding tank in the mill. From there it
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| | interface at the signal conditioning rack
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| is pumped to all the different systems
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| | to plug into with a data collector. This
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| before heading back to the tower once
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| | allows the collection of spectrums and
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| again. The efficiency of the tower is
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| | waveforms while performing routine data
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| measured by the temperature differential
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| | collection or whenever there could be a
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| between the hot water and cold water
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| | problem arising. Having this ability
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| basin. Algae, dirt, scale, leaves and
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| | allowed us to head off at least a couple
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| organic growth are some of the solids if
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| | problems before a major failure may have
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| allowed to build up that can plug the
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| | occurred. The first problem was spotted
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| fill. The water is treated to help prevent
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| | after noting that the 1x vibration at fan
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| some of this from occurring.
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| | speed had increased (Fig. “5”).
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| Case History Cooling Towers are
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| | Although, a 1x vibration is usually the
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| categorized under critical equipment in
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| | first indication of unbalance, that was
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| our plant. A catastrophic failure of a
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| | not the problem. It was during the early
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| cooling tower has the potential to
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| | summer when the vibration level increased.
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| partially shut a major portion of the site
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| | The cooling tower was shut down to allow
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| down, costing thousands of dollars in lost
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| | us and the mechanics to do an inspection,
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| production.
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| | which revealed major cracks in the gearbox
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| It is very important that we know the
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| | base. The cracking was severe enough that
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| mechanical condition of our cooling towers
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| | the gearbox was flexing a fair amount,
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| at all times.
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| | especially at high speed. Severe corrosion
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| Unfortunately, that was not always the
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| | was the main cause of the problem. If
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| case. Monitoring Cooling Towers at the
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| | allowed to go undetected, this could very
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| site has changed drastically over the
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| | well have lead to a significant failure.
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| years. At the beginning, vibration
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| | After welding up the cracks, the amplitude
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| monitoring consisted of mechanical ball/
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| | at 1x returned very close to normal.
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| spring vibration cut-off switches mounted
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| | A second problem that showed up at 1x the
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| several feet from any actual drive
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| | input shaft speed was also detected.
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| equipment. This type of switch would not
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| | Through routine data collection, it was
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| detect any initial mechanical fault in the
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| | noted that the frequency at 1x input shaft
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| drive train until a potentially disastrous
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| | speed had increased dramatically in
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| failure was about to occur.
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| | amplitude with multiple harmonics (Fig.
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| Examples of these faults were motor
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| | 6). Again, the tower was shutdown to allow
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| bearing defects, bearing and gear defects
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| | for an inspection. A suggestion was made
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| in the gearbox, loose bolts, and drive
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| | to the mechanics to give the jackshaft a
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| shaft problems. The whole tower had to be
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| | close check because it was felt there was
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| in a spin before this type of vibration
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| | looseness showing up in the drive train
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| monitoring would detect a problem. Later,
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| | that was directly related to the input
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| walk around data collection was started,
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| | speed. This remark headed the mechanics in
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| but we were only able to access the motor
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| | the right direction, as they quickly noted
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| and not the gearbox. This type of
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| | that the jackshaft coupling inserts were
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| monitoring is okay for periodic checks on
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| | completely worn out. This was causing
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| the motor but not acceptable for
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| | looseness in the drive shaft. Most of the
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| continuous monitoring of the complete
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| | rubber was completely missing on a couple
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| power train. In the mill standard
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| | of the inserts. After replacing the
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| accelerometers are utilized for online
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| | inserts and doing a laser alignment, the
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| monitoring on the fans. These
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| | vibration levels returned to normal.
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| accelerometers were mounted on the
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| | We also experienced vibration problems
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| gearboxes of the cooling towers.
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| | after a major overhaul of the cooling
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| Cooling Tower fans have certain aspects
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| | towers. PCV honeycomb fill was replaced
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| that makes them stand apart from other
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| | and new style fan blades installed because
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| rotating equipment.
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| | they were not cooling to acceptable
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| 1. Combination of both slow turning and
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| | standards. The new blades are hollow, made
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| normal running speeds.
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| | from fiberglass and shaped very similar to
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| 2. Harsh environment.
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| | an airplane propeller.
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| 3. Structural rigidity problems.
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| | They are more efficient than the original
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| 4. Gearmesh defects.
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| | blades, as they move more air with fewer
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| It is important to use a transducer on the
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| | horsepower. After the job was completed,
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| gearbox that will effectively measure fan
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| | both tower fans had to be balanced. The
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| speed up through various gearmesh
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| | tower structure has weakened over the
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| frequencies. With this requirement and
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| | years because of normal aging and being
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| recommendations from the Cooling Tower
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| | weighted down with sludge in the fill.
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| Institute, velocity pickups were
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| | The tower swings back and forth with
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| installed. Unbalance should be the most
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| | little restriction because of less
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| common fault, which is usually measured in
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| | dampening in the system. Unfortunately,
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| displacement. We only wanted to integrate
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| | this causes the overall vibration level to
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| once (basic rule of vibration) so that is
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| | increase. It is necessary to have the
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| what tipped the scale towards the velocity
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| | unbalance force down to a minimum. Fig.
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| pickup. Since that decision was made a lot
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| | “7” is a multiple spectra display
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| of research and development has gone into
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| | showing the results of the balance job.
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| accelerometers.
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| | The unbalance force went from 11 mils to 2
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| The accelerometers on the market today are
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| | mils. At an operating speed of 324 rpm,
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| very capable of giving reliable data on
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| | this level of vibration is considered very
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| both slow and high-speed equipment.
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| | good.
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| Another advantage is the size of an
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| | Conclusions 1. If your online vibration
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| accelerometer compared to velocity
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| | monitoring systems are limited to only
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| transducers when mounting on smaller
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| | overall alarming, it would be beneficial
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| equipment.
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| | to look at a method to plug into it with
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| After mounting velocity transducers (IRD
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| | your vibration analyzers. Without the
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| 580) on the reducers and having the signal
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| | ability to collect spectra and waveforms,
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| sent directly to the control room for
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| | there is a good chance of missing certain
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| monitoring, everyone felt more comfortable
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| | faults in the drive train. This could lead
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| that they were getting closer to some real
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| | to unscheduled downtime and possibly a
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| online monitoring. The control room was
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| | catastrophic failure. There had been
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| able to get a velocity and displacement
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| | instances when fan blades have
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| overall readings from the velocity
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| | disintegrated due to blade fatigue,
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| transducers. One major restriction that we
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| | causing serious damage to the tower and
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| do have with our present online system is
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| | endangering lives. There are excellent
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| the inability to generate any spectrums or
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| | monitoring systems on the market today,
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| band alarming from our control room.
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| | but unfortunately industry has been slow
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| Again, all the control room receives is an
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| | adapting.
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| overall vibration signal. This is totally
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| | 2. Another PDM tool that should be used on
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| inadequate for truly effective machine
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| | monitoring cooling tower gearboxes is oil
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| condition monitoring. This, in our case
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| | analysis. Trending the different elements
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| can be very problematic because at times
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| | in the oil has proven to be an affective
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| the overall vibration level at the cooling
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| | way of giving early warning detection of
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| towers can be relatively high.
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| | bearing and gear failures in any oil
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| The tower is supported on a steel
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| | lubricated drive systems. Oil analysis
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| structure on top of the roof approximately
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| | integrates well with vibration analysis.
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| 225 feet off the ground, making it a
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| | 3. It should be noted that walk around
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| natural cantilever. The building sways
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| | data collection can be very effective but
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| back and forth at 198 c.p.m., (fig.
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| | you have to be aware of its limitations,
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| “2”) and the towers being on the top
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| | in certain areas it just cannot replace a
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| of the building are exposed to the bulk of
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| | good online system.
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| this vibration. The building vibration is
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