Applying IAR Technique

The above photo is that of an underground pipe carrying a fluid high in chloride concentration.

As can be seen in the photo — there is a big rupture of the pipeline leading to a unwarranted and unplanned plant outage.

What can engineers do about it? The usual way of thinking about is to quickly find a way to monitor the development of such failures in time so as to attend to the problem as quickly as possible with ruthless efficiency.

However, to monitor development of pitting / crevice corrosion – (the failure mode in this case) – is neither easy nor available. And even if there were a technique; its benefit, as in all cases of condition based maintenance of condition monitoring, is at best fundamentally limited in its effectiveness. Why? Simply because it doesn’t help improve the MTBF (Mean Time Between Failures). Without substantial improvement in MTBF, we can hardly expect to improve performance, productivity and profitability through maintenance of plant assets.

Such an approach to maintenance is more in keeping with the present requirements and needs of the industries. Maintenance is not about maintaining and restoring assets to its original condition. In present context, maintenance is directly linked to an organization’s profitability and survival under all economic conditions, at the minimum possible cost.

With this in mind, I invented the IAR (Initiator, Accelerator, Retarder) technique.

It means that for every failure or failure pattern or behavior pattern there would be at least one element in the system that would initiate or start the failure process. Similarly, there would be atleast one element in the system that would accelerate the process. Likewise, there would be atleast one element that would help retard the failure process.

Once these I, A, R elements are identified, the job of prolonging the life of a system at the least cost becomes relatively easy and more effective, which can be stated as follows:

1. Eliminate/Avoid the Initiator (s)

2. Prevent the Accelerator (s) from acting

3. Strengthen the Retarder (s)

The next step is to monitor the presence or development of the I, A , R s — if found contextually appropriate and effective.

Coming back to our case, the IAR s are the following:

Initiator — Material

Accelerator — Lack of cathodic protection

Retarder — Steady process that would prevent sudden increase in chloride concentration.

Therefore the set of solution would be:

Eliminate Initiator — Select material having high PREN (Pitting Resistance Evaluation Number)

Prevent Accelerator — install cathodic protection.

Strengthen Retarder — closely monitor the process to prevent sharp fluctuation of chloride concentration.

The above measures would not only help in preventing pitting/crevice corrosion but also prolong the life of the pipeline — helping the plant to become more productive and remain so for a longer time.

Moreover, it proves to be an effective maintenance planning tool than existing approaches.

Dibyendu De

Fretting Corrosion

In a plant it so happened that a machine with its shaft and pulley assembly was kept idle for little over three years.

Then one day the engineers decided to run the machine. After two months of running, the pulley came loose on the shaft and started rattling – making just enough noise for the operator to notice it and promptly stop the machine thus averting a nasty accident.
This is a case of fretting corrosion. This happens when things are kept in assembled condition for long without running or components are assembled loosely. The asperities at the contact surface that help to hold two components together are lost; thus loosing the vital grip forcing the components to come loose. This wear process is accelerated in presence of low frequency vibration that usually travel to such joints por assemblies from other running machines. The confirmation of fretting corrosion lies in observing reddish coloured powder in between the closely fitting joint interfaces and assemblies.

The pictures of fretting corrosion as seen in this case are the following:

Ways to manage this failure mode:

1. Take care to assemble correctly

2. Don’t leave a machine idle for a long time.

3. Prevent, as far as possible, low frequency vibrations to travel to a machine.

4. If an idle machine is to be commissioned then take care to inspect the joints and interfaces and replace assemblies as found necessary.

5. May be monitored by Wear Debris Analysis for lubricated joints and interfaces and by vibration monitoring for dry joints and interfaces or simply by visual monitoring.

Induced Force & Freedom for Movement

While tackling vibration problems (most machinery problems are oscillatory in nature) it is important to grasp the idea — “What causes vibration?”

The answer in its simplest form consists of two parts, which are: –

  1. Induced Force
  2. Freedom for Movement

We can say, that when we put these two phenomena into a relationship or when we discover a pattern involving the two phenomena, we have effectively understood the essence of a vibration problem in order to solve it or improve the situation. Without the “induced force” a piece of machinery would not continue to vibrate. And without “freedom for movement” machines would not vibrate either. Both must be present for a machine to continue to vibrate.

However, I find that students of vibration analysis often face difficulty in understanding these two related phenomenon and have a hard time linking them into a coherent pattern exhibited by a vibration problem.

So, I would first try to explain the phenomenon of “induced force.”

There are many ways of classifying vibrations. Vibrations patterns are also described depending on how they are induced. This is an important way of classifying vibration since the cause of vibration can be easily understood from such classification.

For instance, a shop floor may vibrate when a machine is switched on. Or an adjacent machine or structure may vibrate when another machine on the same floor is running. This would be called machinery induced vibration.

Similarly, a bridge or a tower may be subjected to strong winds causing those to vibrate. In that case, it would be called wind induced vibration.

Or for example, a pipe carrying fluid in a power plant or a pump may be subjected to flow induced vibration. Common problems of pumps like cavitation, re-circulation, erosion and water hammer are all examples of flow induced vibration.

Likewise, unusual vibration of an anti-friction bearing may be induced by electromagnetic forces emanating from electrical cables. We would say that the bearing is subjected to electromagnetic induced vibration.

Similarly, vibration of machines, buildings, towers, bridges can be blast induced owing to sudden application of explosive forces, like the way it happens in mining industry.

In the case of earthquakes, bridges and towers are subjected to ground induced vibrations.

We may think of “induced force” as the necessary stimulus imposed on a structure that forces it to vibrate. Structure, from the vibration point of view, may be a piece of machine, building, tower, pipe, bearings, foundation — or simply anything that has stiffness and mass.

However, a structure would only vibrate or continue to vibrate if it has freedom to move. A machine can move in many directions provided it is allowed to do so. More the number of directions a machine is allowed to move more difficult it becomes to understand a problem. However, the question is “How do we know a machine’s Freedom to move?”

One easy way to find it out is by finding the number of natural frequencies exhibited by the machine. This may be effectively found out by conducting a “bump test” on the machine where the number of natural frequencies show up on the frequency spectrum. The number of natural frequencies is just equal to the number of directions a machine is free to move. For example, if a machine has five natural frequencies within the operating range that consists of the operating speed and its harmonics then the machine is free to move in five different directions.

So, when we know the nature of the induced force and the number of directions a machine is likely to move, we may then try to find the proper relationship between the two phenomena to complete our understanding of the essence of a vibration problem. Once such relationship is understood the solution(s) to a problem is self evident.

 

 

 

Learning Vibration Analysis

Every year we gather at NTPC, Noida, for our animated dialog on real life Vibration problems. This year there were 39 of us happily engaged for four fun filled days. It is a type of annual conference where engineers and practicing vibration specialists across the country come together to interact, exchange and learn from each other.

This year, the workshop was designed differently. We gently moved away from the traditional methods of vibration analysis and instead emphasized the application of complexity science in analyzing system problems through vibration patterns. I think this approach is the first of its kind in the world.

So, what was new?

First, only cases from the real world of engineering were discussed and explored. Twenty cases were discussed. Each case was unique. They were something like Zen koans waiting to be cracked for enlightenment.

Why?

There are two sides to reality. One is the phenomenal one — what we can sense. The other is the essential one — what we can’t “see” through our senses. The phenomenal side manifest as events that we experience while the essential side provides the cause that precipitates such events. Problems of vibration offer us the opportunity to explore both sides of reality. Through measurements, we can easily see the phenomenal one (the degrees of freedom, amounts of vibration and their frequencies) — that is all about sensing oscillatory movement and its nature. But to understand the cause of vibration we must be able to “see” the essential part of reality – what induces vibration?

The cases forced the participants (practicing specialists) to take multiple takes and interpretations of the cause of vibration before the reason finally clicked. Initially, each case left the participants perplexed.They sort of provided the proverbial “whack” on the head for realization to dawn.

Why is this so? Cracking one problem does not ensure that the next problem can be solved by following the same method. If one tries to use the same method that helped one to solve a problem one has to use thoughts and concepts culled through previous experiences. By trying to apply a standard method and tactic one can’t see the essential part of the reality, which often proves to be a frustrating experience. Any effort to solve a vibration problem with a standard approach ties up a practitioner in knots. Not surprisingly, even vibration specialists find vibration problems paradoxical. They are paradoxical in the sense that seemingly logical, rational and conceptual thinking held in the minds of a practitioner are challenged when dealing with vibration problems.

Therefore, for each case, the essential part — the induced cause(s) — had to be built separately — bit by bit — connecting one bit to the other till the essential nature of the problem was self evident.

At the end of the four days the participants were left smiling, relieved to know that they need not remember any standard method or approach or a formula to tackle vibration problems — more so, for the most complex ones. They only need to see through a problem with patience or perseverance to develop deep intuitive capability, which would then help them see through the essential nature of any real life vibration problem quickly and accurately.

On the whole it was great fun and we all basked in the enjoyment.

 

Note: In conducting this course, I was helped by Mr. Anil Sahu, my co-facilitator. He had a bunch of paradoxical cases to share.

Doing Nothing yet Everything is Done

From 21st June to 23rd June I conducted a live workshop on Streamlined Reliability Centered Maintenance (SRCM) at the Power Management Institute (PMI) of National Thermal Power Corporation (NTPC).

But what the heck is SRCM?

It is a structured process of risk based decision making against black swans.

In brief, it is about:

  1. How to detect an incipient black swan in time?
  2. How to improve the stability of a system?
  3. How to improve the longevity of a system?
  4. How to mitigate consequences of failures?

When we are able to do all that to a system we may call it “smart maintenance.” After all as human beings we create, maintain and destroy systems. Given a system, smart maintenance is about doing all the three – create, maintain and destroy. Surely, it is one of the most complex project management we can engage with.

However, the smart maintenance can really happen when one simply does nothing yet everything is done.

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.

Failure Mode of SS 304 & its prevention

Chromium in concentrations above 12% renders steel stainless. 304 stainless steel (SS), contains 19% Cr and 9% Ni. The function of nickel is to strengthen the alloy and to provide greater corrosion resistance. Unfortunately, if the steel contains nearly 0.1% residual carbon from the iron and steel-making processes then the chromium in the SS has a strong affinity for carbon and slow cooling through the red heat range allows chromium carbides to nucleate heterogeneously on the grain boundaries. The adjacent regions in the grains are depleted of chromium to far below the 12% threshold and are no longer as corrosion resistant. Thus, the steel is said to be “sensitized” and is susceptible to intergranular corrosive attack. The slow cool after the welding of the 304 SS allows such precipitation and triggers sensitization. This is technically known as the sensitization temperature of SS which is around 650 degrees C (ball park figure). It can get activated during welding or due the application like using 304 SS in very hot working conditions.

 

Even when sensitized, the steel is adequate for many applications, such as household products (SS utensils that we use on the gas ovens) and even containers for less concentrated nitric acid. However, while sensitized steel is adequate for 75% nitric acid, it could not be used for the 90% solution.

Whenever corrosive attack on 304 grade of SS is the predominant failure mode it might be prevented in several ways. For example, low carbon stainless steel, designated as 304L could be used. Or, addition of niobium during steel making would tie up the carbon as fine, harmless intragranular niobium carbides. Alternatively, we can anneal, especially welded joints on 304 (if possible — size of the furnace often becomes a constraint) at a bright-red heat to dissolve the carbides and then water quench to prevent their re-nucleation. Any of these techniques can be effective, but the additional cost has to taken into account.

When Engineers fail to detect bearing failures?

It is not unusual to see condition based maintenance engineers engaged in vibration monitoring and analysis, sometimes miss detection of bearing damage. This usually happens with pumps and agitators.

Anti-friction bearings fail by fatigue. And they fail very quickly when alternating stresses approach static stress imposed on a bearing.

A German engineer named Wohler, who tested materials under conditions of rotating bending, made the first systematic study of the effect of alternating stress on fatigue.

He found that if alternating stresses were only slightly less than the static stresses which would cause breakage, only a few cycles of loading were required to cause failure.

He also found that as alternating stress was reduced in amplitude the number of cycles needed to cause a failure also increased. This tendency was maintained until the alternating stress level had been reduced to about a quarter or a third of the maximum sustainable static stress, at which level the life of the bearings or a specimen appeared to be infinitely long. This limiting stress has become known as “endurance limit” of the material.

Therefore, in many engineering applications, say anti-bearings, material is not called upon to resist alternating tension and compression (as in case of shafts and axles) but has instead to resist a fluctuating stress superimposed upon a steady stress.

Often the steady stress in a particular component is determined by the load to which it is subjected in service, while the alternating component of vibration arises from unwanted vibration in the system.

In case of anti-friction bearings, when lightness and smallness are important criteria in the design, the mean stress level in the part must approach as closely as possible to the static strength. It is therefore of great importance that the alternating component due to vibration be kept as small as possible. Hence, if the alternating component is sufficiently large the failure would take place within a few cycles, which would clearly escape the notice of a vibration analysts who chooses to monitor the bearings at regular preset intervals. In such cases, the possibility of a condition monitoring person missing out on the potential damage signal is high enough. He/she would then fail to detect a bearing failure in time for any corrective action.

Note:

Fatigue is not really a feature of vibration as there is no necessity for the stress cycles to be regularly repeated; neither is the number of stress fluctuations in a given time important — at least under normal conditions. The point is that the number of stress cycles to cause failure of a component is usually large and execution of vibration is a common way of achieving the necessary large number in a relatively short time. The other important thing is that alternating stress has to be more or less near to the static stress imposed on a material to cause rapid and sudden fatigue failure of a specimen.

Oscillations at Severn Crossing

The important frequencies of whole structures and of machines, in general, are mostly less than 50 cycles per sec and rarely more than 500 cycles per sec.

The lowest frequencies of a system can, in fact, be quite small. A clothes line slung between two posts and having plenty of sag, for instance, may oscillate freely at only one or two cycles per sec

An oscillation of this sort was observed during the autumn of 1959 in the grid system of the Central Electricity Generating Board at the Severn Crossing.

The frequency concerned was unusually low, being of the order of 1/8th cycle per sec. The crossing has two large pylons just over a mile apart, supporting transmission cables of 1.69 in diameter. It was found that, provided it blew from the right direction, a moderate wind would make the cables sway with low frequency and large amplitude in such a violent fashion that cables normally spaced 27 feet apart actually touched, leaving broken strands and burn marks, as well as short-circuiting the electricity supply.

A probable explanation of this behavior was eventually found and a cure effected by wrapping the conductors with thin plastic tape thus altering  the geometry of the surface presented to the wind.

(Excerpt from the book – Vibration by R.E.D. Bishop, Cambridge University Press, 1965, page 29)