By: Reggie Fett, Motor Circuit Analyst

Synopsis

Summary of Action

Supporting Data

Conclusion

By: T.J. Garten, IR/MCA Specialist

Bean Conditioner – Horizontal Rotary Vessel

Synopsis

Fault

Summary of Action

Supporting Data

Figure 1: Pre-repair Demodulation Spectrum

Figure 2: Pre-repair Historical Data

Figure 3: Pinion Wear

Figure 4: Broken Welds on Pinion Gear Pillow Block Pedestal

Figure 5: Post-repair Demodulation Data

Conclusion

The corrective action taken during the repair of the failed trunnion bearing resulted in an altered elevation of the rotary vessel. The attempts to adjust for this were inadequate and resulted in a changed mating pattern between the pinion gear and bull gear assemblies. Over time, the increased impacting caused the pinion shaft to deflect, breaking the welds on the pedestals. This deflection also caused an angular misalignment between the pinion shaft and the gearbox output shaft. The level of misalignment may also have been affected by utilizing dial indicators for precision shaft alignment rather than the on-site laser alignment equipment. As the trunnion rollers wear, the clearance between the pinion gear and bull gear’s high spot decreases, resulting in increased shaft deflection. The increased shaft deflection results in transmitted vibration to the feed screw platform and torsional stress on the gearbox output coupling.Future corrections need to take into account all clearances and their effect on correlating components. Monitoring of adjusted components should be within days of adjustments and load changes. Installation of remote accelerometers on the pinion gear shaft pillow blocks would allow for quicker analysis of shaft misalignment, bearing condition, and pinion gear mesh frequencies.

By: Aubrey Green, Lead Analyst, Allied Reliability

Case Study Information:

While working as a Predictive Maintenance Technician in a paper mill, an analyst received a telephone call from the Pulping area. Their samples of white liquor were showing particles (turbidity). These particles were affecting the quality of pulp that was being produced. When these sand-like particles were processed in the paper machine, they produced a large mark. This mark lowered the quality of the paper. The department wanted to know if the analyst had any information on the process that could be causing the problem. They were particularly interested in pump vibration that would show the origins of the particles.

A quick look through the vibration data showed no issues that could be noted. The analyst asked for a simple piping diagram of the route of the white liquor from the clarifier to the digester. Armed with this information, the analyst decided to start at the beginning of the system and work to the end.

The system began with white liquor clarifiers started the system. Samples of liquor at the clarifier stage showed a few expected particles in the system. There was not enough turbidity to cause the problem the Pulping Department was experiencing. The analyst continued to study the diagram, looking for potential problem areas.

A holding tank was located before the digester. Between the clarifier and the tank there were only pipes and pumps, and the analyst decided to focus his attention on the tank. He knew he needed to find an accurate way to see what was in the tank. White liquor is extremely caustic, so dropping a weight into the tank would be dangerous. In addition, there were only one or two places to access the tank from the top.

Based on past experience, the analyst knew that the tank level could be determined with an Infrared (IR) camera. The temperature differential between the tank contents and the gas above the contents can be enough to show the tank level. The analyst had used this procedure many times in the past to confirm actual tank levels versus instrument readings. In this situation, he thought that if there was material in the bottom of the tank, it may have a temperature differential also.

Since the tank was not insulated, the image was not difficult to acquire. Using a thermal imager, the analyst took an image of the tank (Figure 1). The level of the tank was quickly noticeable. In addition, there was a wedge of material in the bottom of the tank, which was evident because the tank was known to have a flat bottom. This deposit was the result of years of particles from the clarifier settling in this tank. The inlet piping shown on the left of the image was actually acting as an agitator, mixing the particles with the white liquor before sending it to the digester.

Figure 1: Image of White Liquor Tank Showing Deposit

The analyst showed the image to the operator, who remarked, “Now we have an x-ray machine to see inside our tanks.” In this case, his comment was true.

An immediate resolution to the problem was needed, but the tank could not be taken out of service. The concentration of particles increased as the tank level lowered. Therefore, the tactical plan was to keep the tank level high enough to keep the concentration of particles at an acceptable level. The strategic plan was to clean the tank when it could be taken out of service.

Conclusion:

The deposits were removed from the White Liquor Tank during the next mill outage. Following that, the issues with turbidity were determined to be eliminated.

Dealing with hazardous chemicals in tanks and discovering problems within a containment may seem to be a daunting issue. Using the right technology in the right manner can give you insight to internal conditions and help you solve the problems in an efficient and effective manner.

Allied Reliability Analyst Keith Smith shares this case study conducted on a vertically mounted belt driven grinding mill with faulty bearings.  The bearings have failed several times over a period of two (2) years. Before this time, the mill had no recorded bearing failures (as confirmed by the CMMS).   Read on to see how Keith solves this mystery two years in the making.

Grinding Mill Bearing Study

Allied Reliability Analyst Tim Greismer offers this case study on the motor of a Coal Pulverizer which was failing and creating operational problems for the coal plant.  Vibration analysis failed to isolate the problem, Tim and his team use Motor Circut Analysis to identify the root cause of the issue.  In this study he walks you through his process.  Read it for yourself here:

MCA Detects Where Vibration Analysis Could Not

Chris Colson and Timothy Weilbaker GPAllied subject matter experts explore our current economic situation and the implications of a reactions as published in  ‘uptime’ Magazine’s April /May 2010 issue.  Read it for yourself here:

Is Your Company Running Scared?  Fight or Flight Might Not Be Your Best Strategy

John Schultz, with co-authors Rick Baldridge and Tim Goshert, wrote an artilce on Reliability and Healthy Assets. Read it for yourself here:

“Nourishing Reliability and Healthy Assets”

Ron Moore, John Schultz and Jose Wilkins wrote an artilce on Process Reliability Analysis that is featured in the August/September Edition of “uptime” Magazine. Read it for yourself here:

“Using Process Reliability Analysis for Strategic Business Decisions”

SUBMITTED BY: John H. Williams

TITLE: Vibration Analyst

EQUIPMENT: 700 HP Frick ammonia compressors.

TECHNOLOGIES APPLIED:
Vibration analysis and Ultrasound.

PROBLEM DESCRIPTION:

Vibration readings came into alarm on #6 Frick ammonia high stage compressor. Vibration readings taken on the outboard side of the male and female compressor screw bearings continued to rise. Readings went into alarm in April. Work order was written to repair compressor. Work was submitted for approval. Unit was shut down until work was completed.

INVESTIGATION:

Base line reading on male screw conveyor 12/14/2005

Readings into alarm on 4/14/2006

Readings after repairs 11/6/06

CONCLUSION: Compressor refurbished and bearings replaced. New readings were taken above. The unit was able to be repaired in place with significant costs savings versus sending compressor out for repairs. Problem was located before any damage happened to screws or compressor head. Bearings were found to have significant wear after 10 years of running time and poly metallic bearing cages had broken down. A spacer for the thrust bearings was found to be in backwards also.

Thrust Bearing                                  Bearing Cage Gone

IN SUMMARY:

When suspect readings show up, you can check bearings for cage failure by shutting down the compressor and turning machine coupling over by hand. Listen with mechanic’s stethoscope and you should hear the rollers hitting as the shaft turns over. The bearing cage is usually the first thing to fail on these compressors.

It’s much better to locate the problem now before catastrophic failure.

SUBMITTED BY: Jamie Duart

TITLE: Level III Vibration Analyst

EQUIPMENT: Carbon Furnace Draft Fan

TECHNOOGIES APPLIED:  Vibration

PROBLEM DESCRIPTION:
The Carbon Furnace Induced Draft Fan is an overhung, radial blade fan mounted on top of the carbon furnace. It is a direct-drive, variable speed application. Vibration amplitudes vary greatly from month to month and overall vibration is over 1 in/sec on some surveys. A long history of work orders exist in our CMMS system ranging from physical removal of fan for cleaning to bearing replacement to adding mass and stiffness.  

INVESTIGATION:

Analysis: With many variable speed applications vibration data goes in and out of alarm from survey to survey. In many cases this is a result of resonance.  Resonance is when a forcing frequency such as imbalance, misalignment, looseness, bearing defects, gear defects, etc., coincides with a natural frequency. The natural frequency may be that of the rotor itself or may be that of the support frame or foundation or even drive belts. It was suspected that the Carbon Furnace Induced Draft Fan at our facility was experiencing resonance and different testing methods were employed and much research was done to prove our theory. 

Comparative Analysis:  On the following page is a horizontal, vertical and axial view of the coupling side fan bearing on the Carbon Furnace Induced Draft Fan. The spectrums show that the readings in the horizontal direction are significantly higher than in the other two directions. One of the characteristics of resonance is that it causes much higher vibration in one direction as compared to the other tri-axial directions. When resonant, it is common for the resonant direction to be 5 to 15 times higher than in the two other directions.

Natural Frequency and Resonance Detection: Component natural frequencies can be detected utilizing several different methods including Impact/Impulse Natural Frequency Testing (also referred to as a “Bump Test”) and Run-up and Coast-down Testing.  Impact Naturally Frequency Testing should be performed with the piece of equipment shutdown and could not be performed due to production schedules. A Run-up Coast-down Test was the best option in this case and we employed a technique of this test called a Bode’ Plot. This Plot consists of two different plots – one of amplitude versus RPM and the other of phase versus RPM. Past vibration history showed that the Coupling side fan bearing, horizontal position showed the most response and was the point chosen for our testing. The results of this test can be found on the next page. Data shows that the characteristic 90-degree shift and rise in vibration amplitude seen at resonance occurs at approximately 64% of these units operating speed or 1360 RPM. 

Questions arose as to whether this resonance may have been caused by the fan being out-of-balance or misaligned. These conditions could have been the forcing frequencies that coincided with the structures natural frequency and resulted in resonance excitation. A laser alignment had been performed immediately before conducting the Run-up/Coast-down test and the assumption was made that this was not the forcing frequency responsible for the resonant condition. The phase shift that had occurred at 1360 RPM and the steady decrease in vibration as RPM was increased to 2081 RPM was uncharacteristic of an out-of-balance condition. Attempts to balance this fan would likely have been unsuccessful without changing the natural frequency of the structure. This natural frequency may be lowered by adding mass or raised by adding stiffness.

Physical and Analytical Research: Closer examination of the structure confirms that support and stiffness is inadequate. Conversations with Robinson Fans (the manufacturer of this fan) stated that they recommend the foundation supporting the assembly weigh a minimum of five (5) times the weight of the assembly (motor, fan, bearings, shaft and base). In this case the assembly weighs approximately 3700 lbs. total.  Five times this amount would be an approximate value of 18,500 lbs. Since this fan sits on two I-beams mounted to the top of a furnace it is doubtful that the base approaches this value. The tank-like construction of the furnace may not be able to support a massive foundation like what is recommended. 

The fan company representative also addressed our concerns about the A-frame base that the motor and fan bearings are mounted to not being adequate to dampen frequencies excited by the fan. He explained that the weight of the base assembly (including the motor and fan bearings) provided by Robinson was roughly ten (10) times that of the fan and should provide adequate damping (fan and shaft weight is approximately 320 lbs. and the motor, base and fan bearing weight totals approximately 3700 lbs.).

Another concern we had was that this fan may not have been designed for a variable speed application and that we were exceeding the fans operating speed or operating at a critical frequency of the fan. Robinson explained that they typically design their fans with a critical speed at least 40% above the maximum speed rating. Maximum speed listed on the fan curve is 2183 RPM and currently the fan does not exceed 2081 RPM.

RECOMMENDATION:
After careful review of the data including vibration analysis, fan company recommendations and work order history in our CMMS system it has been determined that the best course of action would be to relocate this fan to the level above the furnace. This would allow installation of a more substantial base to change the natural frequency of the unit and eliminate the resonant condition that is occurring.

CONCLUSION:

In April of 2006 the Carbon Furnace ID Fan was moved from the top of the Carbon Furnace to a newly constructed platform on the level above. Structural I-beams were added and a concrete pad was poured. Fan vibration was checked upon startup and levels were found to have risen at 2081 RPM from approximately .25 in/sec to nearly .5 in/sec. Although vibration amplitudes had risen there was little change in phase. Phase only fluctuated about 30 degrees from 40 to 100%. Vibration amplitude did see a slight rise at about 80% but did not appear to be a resonance (characteristic phase shift was not present). Time constraints prevented completion of a precision field balance but running speed vibration was lowered to approximately .25 in/sec. Balancing was accomplished the following month with final readings of .04 in/sec at 2081 CPM.

Below are pictures of the new structure as well as pictures of the old location on top of the furnace.