Corolisis Effect & Negative Damping – a Report

Report on Thaisen Fan (Scrubber)

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Brief description of the phenomenon:

After cleaning of the fan blades, vibration of the fan gradually increases during operation and in a span of 10 to 14 days vibration level reaches an unacceptable level, which necessitates the next cleaning cycle. However, for so long, this matched the scheduled production window provided by operation. However, after the recent changing of the rotor and the bearings, the fan now reaches unacceptable level of vibration within a short span of time that does not coincide with the scheduled “production window” of the operation, which causes “unplanned downtime.”

Goal of the investigation: To correct the imperfection in the system so that the fan cleaning cycle coincides with.the scheduled production window.

Result of the investigation:

 

1. The problem of rising vibration within a short period of time is an inherent problem (a birth defect) of the fan. The main reason is the Coriolis effect on the fan. Coriolis force is a force exerted by a moving fluid on the disc or impeller rotating in the fluid. If the rotation is CCW (counter clockwise) then the fluid moves to the right of the impeller and away from the centre. Similarly, when the impeller moves in the CW (clockwise direction) the fluid moves towards the left of the impeller and away from the centre.

In this case, with the fan moving in the CCW direction the Coriolis force moves toward the right of the impeller in the same direction as the damping force. This effect (the fan moves in the CCW direction) produces negative damping (since the two forces are in the same line of action).

Negative damping is a phenomenon, when damping force, which usually opposes the driving force, acts in the same direction as the driving force. In such a case the vibration of the system is amplified.

Combination of negative damping and Coriolis effect produces this phenomenon of gradually rising vibration of the fan in a short period of time, which goes away upon regular cleaning. In the present context nothing can be done to eliminate the phenomena of Coriolis Effect and Negative damping. However, if a similar system is to be installed in the future, we would be pleased to provide necessary suggestions and recommendations so that such phenomena are eliminated right from the start.

2. Present signatures indicate misalignment and dynamic imbalance

3. Weak foundations

Actions to be taken to increase the cleaning cycle to match scheduled “production window.”

Countermeasures

1. Take care to align the rotor properly. Care to be taken while putting shims.

2. Dynamically balance the fan in two planes to eliminate the imbalance

3. Cleaning cycle can be initiated when vibration of the fan on the bearings reaches 7 mm/sec (rms). It is safe to run the fan upto this point.

4. Monitor the condition of the foundation by taking vibration measurements in displacement and acceleration modes. Displacement should be taken in the horizontal direction on the topmost accessible point of the columns and at the base.  Acceleration should be taken in both vertical and horizontal directions. Displacement should not cross 50 microns in the horizontal direction or at the base of the columns. Similarly acceleration both in the vertical and horizontal directions must not cross 1.5 g. This would ensure safety of the equipment. In case it crosses corrective actions are to be taken to rectify the foundation.

Result:

After alignment and dynamic balancing in two planes vibrations came down to below 1 mm/sec and maintained its reliability till the next cleaning cycle (10 to 14 days) which matched the scheduled production window of operation — thus avoiding unplanned downtime.
Dibyendu De
dde@rgbwaves.com
9836466678
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Eccentric Stator

General Symptom: 2Lf (Lf = Line frequency)

Stator problems would create high vibration at 2Lf. Stator eccentricity produces uneven stationary air gap between the rotor and stator that produces a very directional source of vibration.

Soft foot is often the cause of eccentric stator.

Other key indicators:

  1. 2Lf peak would be comparably high
  2. For a 2 pole motor this peak would be close to 2N (N= running speed). Would need sufficient resolution to separate them
  3. A spectrum may reveal beating — 2Lf and 2N peaks may appear to rise and fall if we don’t have sufficient resolution to separate them.
  4. Time waveform  — a combination of 2N and 2Lf would reveal a beat type pattern if the time period covers more than a few seconds. If the time period isn’t long enough, then we would see a wobble or take on the classic M or W shapes due to combination of 1N, 2N and 2Lf.
  5. Thermal images would reveal heat bands in the direction perpendicular to the direction of high vibration
  6. Vibration would be highest at the point where the stator is closest to the rotor. Move the accelerometer around the motor housing to see if the peak is high in one or two locations.

Dibyendu De

Eccentricity in general

Symptoms are generally 1x radial (Vertical and Horizontal for a horizontally mounted machine).

Eccentricity occurs when the centre of rotation is offset (like offset misalignment) from the geometric centreline of a gear, motor rotor or a pulley.

It would generate strong 1x radial peak — in the direction parallel to the rotor/gear/pulley. This condition is common and mimics unbalance.

For gear eccentricity we would see 1x sidebands

For motor rotor eccentricity we would see pole pass sidebands.

Time waveform would be sinusoidal when viewed in velocity. Vibration from gear will also have gear mesh vibration and modulation of the turning shaft of the offending gear.

Phase: If belt driven, phase readings taken parallel and perpendicular to belts will either be in phase or 180 degrees out of phase. For a direct driven component, vertical and horizontal readings will be 90 degrees out of phase.

Rotor Bow

General symptom: 1x radial (Vertical and Horizontal direction of horizontal machines)

Usually a rotor bow in a motor looks like a static imbalance. Broken bars and loose connections (at motor terminals and at MCC) cause motors to heat up (localized) owing to uneven current flow through the phases causing rotor bow — uneven weight distribution around the rotor’s centreline. Hence we see high amplitude peak at 1x running speed in the radial and horizontal directions.

Localized overheating can be seen on the motor body through infrared thermal imaging.

The effect of can also be seen on the rotating air gap — a high peak at 2xLf with pole pass sidebands around 1x and 2x peaks. The 2x peak often comes up when the effect is more severe.

The time waveform would be sinusoidal when viewed in velocity.

Phase: expect 90 degree shift between vertical and horizontal axes. The inner race will move in and out once per revolution with a bent shaft

External Noise in Vibration Analysis

Quite often, vibrations external to a machine (emanating from other machines or structures) can be transmitted through the foundation (from other machines) and structural supports (e.g. grinding frequencies generated from grinding of materials). It can also be transmitted through liquids (e.g. water hammer, turbulence) and air (acoustic pressures, electromagnetic radiation).

In most cases, low frequency vibrations are transmitted in this manner. This is because low frequency vibrations travel great distances.

In case such transmitted vibrations match the resonant frequency of the machine or any of its components, vibrations are greatly amplified (resonance).

Such vibrations can damage components like anti-friction bearings through a phenomenon called false brinelling if the affected machine is in the stand-by mode.

It is wise to suspect presence of such external noise if a frequency peak is found in a vibration spectrum (FFT) which can’t be identified or appears strange.

In that case we can check whether any machine near to the machine of interest exhibits that particular frequency. Or we can stop the machine to check whether the unusual or odd frequency still appears on a stationary machine. Alternatively, we can stop other local machines (usually not possible) to see whether the odd frequency disappears from the signature.

In case, the frequency happens to coincide with 2x, 3x, 8x harmonics of 1x (fundamental frequency) then we may use time synchronous averaging to see whether the amplitude contributed by the external noise averages away.

 

Rethinking Maintenance Strategy

As of now, maintenance strategy looks similar to strategy taken by the medical fraternity in themes, concepts and procedures.

If things go suddenly wrong we just fix the problem as quickly as possible. A person is healthy to the point when the person becomes unhealthy.

That might work fine for simple diseases like harmless flu, infections, wounds and fractures. And it is rather necessary to do so during such infrequent periods of crisis.

But that does not work for more serious diseases or chronic ones.

For such serious and chronic ones either we go for preventive measures like general cleanliness, hygiene, food and restoring normal living conditions or predictive measures through regular check ups that detects problems like high or low blood pressures, diabetes and cancer.

Once detected, we treat the symptoms post haste resorting to either prolonged doses of medication or surgery or both, like in the case of cancer. But unfortunately, the chance of survival or prolonging life of a patient is rather low.

However, it is time we rethink our strategy of maintaining health of a human being or any machine or system.

We may do so by orienting our strategy to understand the dynamics of a disease. By doing so, our approach changes radically. For example. let us take Type 2 diabetes, which is becoming a global epidemic. Acute or chronic stress initiates or triggers the disease (Initiator). Poor or inadequate nutrition or wrong choice of food accelerates the process  (Accelerator) whereas taking regular physical exercise retards or slows down the process (Retarder). Worthwhile to mention that the Initiator(s), Accelerator (s) and Retarder (s) get together to produce changes that trigger of unhealthy or undesirable behavior or failure patterns. Such interactions, which I call ‘imperfections‘ between initiator (s), accelerator (s) and retarder (s) change the gene expression which gives rise to a disease, which often has to be treated over the entire lifecycle of a patient or system with a low probability of success.

The present strategy to fight diabetes is to modulate insulin levels through oral medication or injections to keep blood sugar to an acceptable level. It often proves to be a frustrating process for patients to maintain their blood sugar levels in this manner. But more importantly, the present strategy is not geared to reverse Type 2 diabetes or eliminate the disease.

The difference between the two approaches lies in the fact — “respond to the symptom” (high blood sugar) vs “respond to the “imperfection” — the interaction between Initiators, Accelerators and Retarders”. The response to symptom is done through constant monitoring and action based on the condition of the system, without attempting to take care of the inherent imperfections. On the other hand, the response to imperfections involve appropriate and adequate actions around the I, A, R s and monitoring their presence and levels of severity.

So a successful strategy to reverse diabetes would be to eliminate or avoid the initiator (or keep it as low as possible); weaken or eliminate the Accelerator and strengthen or improve the Retarder. A custom made successful strategy might be formulated by careful observation and analysis of the dynamics of the patient.

As a passing note, by following this simple strategy of addressing the “system imperfections“, I could successfully reverse my Type 2 Diabetes, which even doctors considered impossible. Moreover, the consequences of diabetes were also reversed.

Fixing diseases as and when they surface or appear is similar to Breakdown Maintenance strategy, which most industries adopt. Clearly, other than cases where the consequences of a failure is really low, adoption of this strategy is not beneficial in terms of maintenance effort, safety, availability and costs.

As a parallel in engineering, tackling a diseases through preventive measures is like Preventive Maintenance and Total Productive Maintenance — a highly evolved form of Preventive Maintenance. Though such a strategy can prove to be very useful to maintain basic operating conditions, the limitation, as in the case of human beings, is that it does not usually ensure successful ‘mission reliability’  (high chance of survival or prolonging healthy life to the maximum) as demonstrated by Waddington Effect. (You may refer to my posts on Waddington Effect here 1 and here 2)

Similarly, predictive strategy along with its follow up actions in medical science, is similar to Predictive Maintenance, Condition Based Maintenance and Reliability Centered Maintenance in engineering discipline. Though we can successfully avoid or eliminate the consequences of failures; improvement in reliability (extending MTBF — Mean Time Between Failures) or performance is limited to the degree of existing “imperfections” in the system (gene expression of the system), which the above strategies hardly address.

For the purpose of illustration of IAR method, you may like to visit my post on — Application of IAR technique

To summarize, a successful maintenance strategy that aims at zero breakdown and zero safety and performance failures and useful extension of MTBF of any system may be as follows:

  1. Observe the dynamics of the machine or system. This might be done by observing  energy flows or materials movement and its dynamics or vibration patterns or analysis of failure patterns or conducting design audits, etc. Such methods can be employed individually or in combination, which depends on the context.
  2. Understand the failures or abnormal behavior  or performance patterns from equipment history or Review of existing equipment maintenance plan
  3. Identify the Initiators, Accelerators and Retarders (IARs)
  4. Formulate a customized comprehensive strategy  and detailed maintenance and improvement plan around the identified IARs keeping in mind the action principles of elimination, weakening and strengthening the IARs appropriately. This ensures Reliability of Equipment Usage over the lifecycle of an equipment at the lowest possible costs and efforts. The advantage lies in the fact that once done, REU gives ongoing benefits to a manufacturing plant over years.
  5. Keep upgrading the maintenance plan, sensors and analysis algorithms based on new evidences and information. This leads to custom built Artificial Intelligence for any system that proves invaluable in the long run.
  6. Improve the system in small steps that give measureable benefits.By Dibyendu De

 

 

The Case of Burning BagHouse Filters

Recently I was invited to investigate a case of frequent burning of baghouse filter bags.

There were five such baghouses connected to five furnaces of a steel plant.

The client reasoned that the material of the bags was not suitable for the temperature of the gas it handled. However, with change of material the frequency of bag burning did not change. So it needed a different approach to home onto the reasons for the failures.

Hence, this is how I went about solving the case:

First I did a Weibull analysis of the failures. Engineers use Weibull distribution to quickly find out the failure pattern of a system. Once such a pattern is obtained an engineer can then go deeper in studying the probability distribution function (pdf). Such a pdf provides an engineer with many important clues. The most important clue it provides is the reason for such repeated failures, which are broadly classified as follows:

  1. Design related causes
  2. Operation and Maintenance related causes
  3. Age related causes.

In this case it turned out to be a combination of Design and Age related causes.

It was a vital clue that then guided me to look deeper to isolate the design and age related factors affecting the system.

I then did a modified FMEA (Failure Mode and Effect Analysis) for the two causes.

The FMEA revealed many inherent imperfections that were related to either design or aging.

Broadly, the causes were:

  1. Inability of the FD cooler (Forced Draft cooler) to take out excess heat up to the design limit before allowing the hot gas to enter the bag house.
  2. Inappropriate sequence of cleaning of the bag filters. It was out of sync with the operational sequence thus allowing relatively hot dust to build up on the surface of the bags.

Next, the maintenance plan was reviewed. The method used was Review of Equipment Maintenance (REM). The goal of such a review is to find maintenance tasks that are either missing or redundant for which new tasks are either added/deleted or modified. With such modification of the maintenance plan the aim is to achieve a balance between tasks that help find out incipient signals of deterioration and tasks that would help maintain longevity and stability of the system for a desired period of time.

Finally the investigation was wrapped up by formulating the Task Implementation Plan (TIP). It comprised of 13 broad tasks that were then broken up into more than 100 sub-tasks with scheduled dates for completion and accountability.

 

Observing Complexity

To me, observing real life systems is something like this:

A real life System comprises of a meaningful set of objects, diverse in form, state and function but inter-related through multiple network of interdependencies through mutual feedbacks enclosed by variable space, operating far from its equilibrium conditions not only exchanging energy and matter with its environment but also generating internal entropy to undergo discrete transformation triggered by the Arrow of Time forcing it to behave in a dissipative but self organizing manner to either self destruct itself in a wide variety of ways or create new possibilities in performance and/or behaviour owing to presence of ‘attractors’ and ‘bifurcations’; thereby making it impossible to predict the future behaviour of the system in the long term or trace the previous states of the system with any high degree of accuracy other than express it in terms of probabilities since only the present state of the system might be observable to a certain extent and only a probabilistic understanding may be formulated as to how it has arrived at its present state and what would keep it going, thus triggering creative human responses to manage, maintain and enhance the system conditions, function and purpose and create superior systems of the future for the benefit of the society at large.

Such a representation of an observation looks quite involved. Perhaps it might be stated in a much simpler way. Most real life systems behave in a complex manner creating multitude of problems of performance and failures. But how do we get rid of complexity and uncertainty as exhibited by systems? We may do so by deeply observing the complex behaviour of the system to improve our perception to gain insights about the essence of the system; find out the underlying ‘imperfection’ that causes the apparent complexity and uncertainty and then find ways to improve the existing system or create new system and maintain them in the simplest possible manner. We do this by applying the principles of chaos, reliability and design. Surprisingly, the same process might be used to troubleshoot and solve problems we face on a daily basis. If done, we are no longer dominated or dictated by the ‘special whims’ of the system.

The crux of the matter is how we observe reality and understand it so as to make meaningful choices as responses to life and living.

A baffling problem

Last week I was invited to engage with a baffling problem. It left engineers and managers of the power plant baffled for the last four months.

Instead of writing the technical details of the problem, which might not be of much interest to non-technical people I would instead write about my approach to solving baffling problems along with the principles involved. This is because we often wrongly interpret problems as technical, non technical, financial, social, etc. Essentially all problems are problems of the human mind. Invariably they always originate and live in the human mind. However, problems, as a rule, are always solved or resolved through enlightened (understanding, wisdom) consciousness.

The process, which I intend to highlight below, may be used to successfully solve both personal and professional problems or quickly adapt when a baffling problem (or a black swan) strikes.

The seven steps of the integrated process are as follows:

1. Look at the event as perceived in the present and the events that went before it.

Principles:
a) See all, sit still, breathe deep, care for the ill.
b) When drinking water think of its source.

2. Examine the responses of human beings to a problem. Spot the responses that emanated from stored memories (usually collective).

Principles:
a) Buddha is a toilet stick.
b) The gateway is found only after a long search and the long search is a spiral going inward.

3. Check implementation of the responses for assumptions, acceptance of imperfections, ignorance of facts and avoidance of details.

Principles:
a) See the whole universe in a glass of wine.
b) Can you see the cloud in the book?

4. Scan the various perspectives people have about a problem, i.e. how individuals feel about the problem. Identify the blind spots.

Principles:
a) No question; no answer
b) Cut the grass; pull the roots. 
c) Pine cones look at different directions but do they?

5. Listen intently to what they say and what they don’t say about the issue.

Principles:
a) Noise of the unsayable, unspeakable and unsaid is music.
b) Go up the mountain to meet the sage, know that it is you, come down to mix with the lakes and rivers.
c) A ruler and a worthy person who don’t connect, a musician and a listener who don’t find each other, or two potential friends who never meet; all lose.

6. Gain deep insight about the problem.

Principles:
a) Don’t catch the snake by its tail.                          
b) Birds are not afraid of directions.
c) Question, test and use every sense you have and believe that dawn would come.
d) Imaginary lines between stars guide us to new harbors.

7. Form understanding of the problem. Formulate viable solutions for minimal intervention. Exchange the understanding in the simplest possible terms as logically as possible to change collective consciousness. Leave it there.                                         

Principles:
a) No question; no answer.
b) Spirit is wrapped in reason and reason is wrapped in spirit.
c) One raindrop creates thousand ripples.
d) Don’t wish for spring; spring gives life to wish.

Chances are that baffling problems would no longer appear baffling and would soon be solved/resolved by people.