Uptime

Finding Hidden Machinery Faults With Operating Deflection Shape Analysis

Case of the Whirling Pump
Finding Hidden Machinery Faults With Operating Deflection Shape Analysis
by Daniel T. Ambre

This article is a vibration case study using Operating Deflection Shape (ODS) and Experimental Modal Analysis (EMA) tools and computer animation techniques to solve difficult rotating machinery problems. Mr. Tim Schuyler and Mr. Steve Werness from Xcel Energy Fort St. Vrain Generating Station requested the services of Full Spectrum Diagnostics to perform a vibration investigation on a vertical river water supply pump whose vibration had significantly increased recently for unknown reasons.

The Fort St. Vrain facility is located in Platteville, Colorado, and was both that state’s only nuclear power plant, and the United States only commercial High Temperature Gas Cooled (HTGC) reactor design. The plant was decommissioned in 1989 and converted from a nuclear fueled facility to a natural gas fueled steam generation facility in 1996. The site added heat recovery steam generator (HRSG), and a second gas turbine in 1998. A third combustion turbine with HRSG was added in 2001, providing a current combined rated output of 720 megawatts. Two more combustion turbines are planned to supply up to 300 additional megawatts.

The generating station has a pair of vertical river water pumps used to control the cooling water in the facility. The P4119 and P4119S units are similar in design, both manufactured by Worthington and driven by Westinghouse 200 hp AC electric motors. Overall vibration levels on the P4119 unit were noted to have "jumped" from a stable trend of 0.150 ips to levels approaching 1.0 ips. The dominant frequency was found to be the 1x response at 1180 RPM. A resonance condition was suspected, however, a history of foundation issues suggested a deeper underlying problem or contributing factor.

Cantilever mounted vertical pump designs will typically have natural frequencies near operating speed points. These so-called "reed" modes are a function of the height of the cantilevered structure (above deck), the length of the pump shaft (below deck), the mass of the drive motor, and the "footprint" of the mount interface structures. Each of these parameters effect the mass and/or stiffness of the overall structure, thus the location of the natural frequencies. Also, as a note, symmetric systems (like this unit) will have two fundamental cantilever natural frequencies (N-S and E-W).

These modes are fairly close together, but may appear below operating speed, above operating speed, or even straddle the running speed point. This makes identification of the modes a top priority in establishing a proper structural correction, if required.

In addition to the planned Experimental Modal Analysis used to detect natural frequencies, an Operating Deflection Shape (ODS) analysis was also planned to investigate the operating characteristics. This analysis is helpful in resolving unbalance, looseness, misalignment, and bowed rotor faults. Operating analysis also provided an evaluation of historical repairs to the pump foundation on these units. Xcel Energy provided periods of both operating and non-operating test conditions for the ODS and Modal analyses, respectively.

Machine Design Information

Drive:

Westinghouse AC Induction Motor

200 HP @ 1180 RPM

Rolling Element Bearings:

Driven Machine:

Worthington Single Stage VTP River Pump

1180 RPM

Direct Drive Coupled

3-Piece Shaft Assembly

2 Spider Bearings

6-Blade Impeller

Capacity 6000 GPM, Total Head: 70 ft

Even though the operating vibration problems were found only on the P4119 pump, natural frequency testing was performed on each unit for comparison purposes.

Experimental Modal Analysis

An Experimental Modal Analysis is a diagnostic performed on structures to determine their natural frequencies, damping, and mode shapes. This analysis is performed with the machinery in a non-operating condition, which limits background vibration from other machinery sources. An impulse (usually a hammer blow) is applied to the structure to impart broad-band energy to the system. This energy functions to excite all of the system‘s natural frequencies, similar to the response of a bell or tuning fork to a sharp impact. This hammer has a load cell built into the head that measures the force parted to the structure and allows this signal to be compared to the accelerometer response, via a transfer function, resulting in a measurement of the frequency response of the structure.

Given enough response locations, each system’s natural frequency can be curve fitted and its mode shape animated with specialized modal analysis software. The Mode Shape is an effective visual representation of the deformed structure when amplified by a driving force.

The natural frequency analysis of each unit revealed some curious results. The problematic unit (P4119) appeared to have significant operating margin on its fundamental natural frequency modes. The operating speed was well above each mode. In addition, the secondary unit (P4119S) had no amplified response; however it was operating smoothly at a point between its EW and NS cantilever modes, and with significantly less speed margin?

A summary is presented in Tables 1 and 2. Figures 2 & 3 show the measured natural frequencies in relation to the operating speed (cursor).

Vibration levels in the P4119 River Water Pump were found to be considerably above the vibration alarm levels typically recommended for a pump of this configuration. The historic overall vibration trend obtained from Mr. Schuyler indicated that the levels had recently increased. Operational vibration velocity levels were measured at 1.0 inch per second (ips) on the motor frame, and up to 1.5 ips on the outboard end (bell housing) of the motor. Typical alarm levels for vertical pumps would be set at roughly 0.290-0.340 ips, depending on vertical elevation. The harmonic content (multiples of 1x RPM) were considered low and acceptable.

From experience with vertical pumps of this type, an amplification of a natural frequency is typically the cause of unexplained vibration response(s). The symmetric cantilever design dictates that two natural frequencies should be present (inducing North-South, and East-West motions), i.e., the so-called "reed" modes of the pump. The biggest dilemma in correcting resonance problems is defining the locations (frequencies) of the modes. The two dominant corrective methods used to alter the natural frequency in resonant systems are the addition of a stiffness (bracing) and/or addition of mass to the physical structures. Unless the natural frequencies are first defined, the wrong correction method may be applied resulting in increased vibration levels instead of the desired reduction. With symmetric systems (like this one) the worst case is when the operating speed is between the two modes, which increases the potential of inducing additional problems no matter what corrective method is chosen.

In addition to the Experimental Modal Analysis, this analysis included Operating Deflection Shape (ODS) analysis. This test method is described below.

Operating Deflection Shape Analysis

An Operating Deflection Shape is measured with the machine at its’ normal operating condition. This analysis measures the machines’ response at a specific time or frequency. Both amplitude and phase information are collected at various locations on the structure and, via special software, the vibrating "shape" or response of the machine can be animated.

These animations show the analyst how the machine is moving during normal operation. Note that this is not a resonant response of the machine, but its operational response. The forces within the machine are responsible for the motion, or shape of motion measured with this analysis tool. For example, the unbalance response of any rotating system will produce a response or driving force at 1x RPM. Misalignment and looseness generally produce synchronous multiples of running speed (2x RPM, 3x RPM,etc.).

Examining the operating shapes gives the analyst additional insight into the root cause of many vibration related faults. Combining this analysis with an Experimental Modal Analysis is very effective in ruling-out suspected machinery problems associated with resonance.

Analysis & Conclusions

In this analysis, the P4119 pump indicated some unexpected characteristics. Typical resonance amplification will induce a linear response in the direction of, and similar to the mode shape defined in the EMA. The closer the driving force is to the natural frequency, the more mode shape resemblance in each animation.

This characteristic was not found in the ODS animations. In the P4119 pump operating shapes, a "whirling" or "orbital" ODS animation motion was noted. This visual response,along with the seemingly adequate resonant margin, was another indication that this system was unusual.

The whirling orbital motion in Figures 5 and 6 suggested that the excessive vibratory response was NOT due to resonant amplification.

ODS animations near a resonance will typically correspond to the linear mode shape. The animations suggest that the pump rotor (extended below deck) was driving the response in the motor. A possible reason for this is excessive clearances in the seals and "spider" bearings that support the rotor. Significant leakage from a problematic seal in this pump tends to support the theory that radial clearances are larger than desired.

Additional animations without the motor (Figure 6) more clearly show the whirling motion in the motor standoff support structures. The final animation in Figure 7 shows an isolated view of the support base plate structures. The foundation amplitudes suggest that vibration levels are higher than normal; however, the dense measurement mapping confirms that the plates are moving in unison (in-phase).

Out-of-phase motion between plates (grouted interfaces) or bolted joints would imply looseness response. These animations confirm that the foundation is not the source of the vibration problem. The relatively new bolted joints in the base plate are considered acceptable based on this analysis.

The final analysis performed was an "added mass trial" on P4119. Mass is added to resonant systems to alter the natural frequencies. Adding mass (with sand bags) should both reduce the natural frequency, as well as dampen its response via the shifting sand. Since the natural frequency for this unit was below operating speed, adding mass should increase the operating margin. If a stiffener support were applied instead, the likely result would be an increase in vibration since the operating margin would be reduced.

The Xcel Energy maintenance support acquired several sandbags totaling close to 120 lbs of weight. The unit was started up and the bags were stacked on the motor. No change in vibration levels were noted regardless of the amount of sand applied.

The unit was shutdown for a natural frequency (impact) test. The test indicated that the natural frequency at 1027 CPM was shifted lower by the added 120 lbs of mass to 990 CPM, or approximately 6%. If resonance were the root cause of the elevated vibration levels, a significant drop in the 1x RPM amplitude would have been noted during pump operation. This proved that resonance was not the dominant contributor to the excessive vibration levels.

Based on this analysis and discussions with Xcel Energy management and maintenance personnel, it was recommended that the unit be removed and inspected. It was suspected that some internal unseen fault was responsible for the elevated vibration levels.

Disassembly and inspections of the hardware showed that a locked coupling was likely the root cause of the excessive vibration. Continued operation in this "locked" state resulted in significant wear in the spider bearings, the shaft, and rotor bushings. This extra clearance allowed the rotor to "whirl" as a unit, inducing the orbital motion noted above deck.

The operating animations (ODS analysis)proved to be invaluable in isolating the root cause of the rotor problems.

Dan Ambre, P.E. is a Mechanical Engineer and founder of Full Spectrum Diagnostics, PLLC, a Full Service Predictive Maintenance Consulting company. Dan specializes in Resonance detection, Experimental Modal Analysis, and Operating Deflection Shape machinery diagnostics.

Full Spectrum Diagnostics provides Vibration Analysis level I, II, and III training and certification, as well as training in advanced diagnostic techniques. Dan is a certified software representative for Vibrant Technology, Inc., the creators of ME’scope VES software tools. He also provides ME’scope VES Software Training targeting the In-Plant Vibration Analyst.

Animations referenced in this article are available via email at modalguy@aol.com. Please visit www.fullspec.net

There is also an online Modal discussion at MaintenanceForums.com

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RCM & the Sub-Prime Mortgage Meltdown

Turning A Blind Eye
RCM & the Sub-Prime Mortgage Meltdown

by Neil Bloom

You might be curious and wonder what in the world RCM has to do with the current financial crises, so let’s explore what it is that they have in common. As we will see, RCM has a distinct similarity to the current global sub-prime financial fiasco. Many of you have heard about Reliability Centered Maintenance, or RCM as it is called. But how much do you really know about it?

RCM is a logical way of identifying what equipment in your facility is required to be maintained on a preventive maintenance basis rather than a let-it-fail-then-fix-it basis. Many have heard the phrases "don’t fix it until it breaks" or "don’t break it by trying to fix it." There is a grain of truth to these axioms, but they depict a very shallow approach if you are striving to achieve reliability and safety levels for your facility that are the best they can be.

Many plants have tried the "hit-and-miss" approach, or the old "how-we-used-to-do-it" approach, or the "runon-luck" approach to maintenance. These methods will get you only so far until your luck runs out, and the potential for a disaster looms right around the corner.

In the absence of a structured RCM approach, reliability will rest solely on the basis of seat-of-the-pants experience, with a strategy consisting of a best-guess decision process. That approach falls far short of modern-day expectations.

However, in real life everything is not perfect. For example, there is no universally perfect climate or environment. Likewise there is no universally perfect reliability environment. For example, there are those who are responsible for plant reliability that may still be in a relatively early stage of getting their program going. They are really doing the best they can with the resources they are allocated. In meeting and talking with many of the people in this stage of their reliability program development, I am deeply encouraged by their devotion and passion. They are continually striving to do better. They implement predictive maintenance (PdM) in all the ways they can and use the best judgment available to them.

Then there are those reliability professionals that find themselves in a more experienced and progressive reliability environment. These folks have the benefit of being in a more highly developed situation with a greater flexibility to implement more advanced preventive maintenance methods and tools such as RCM.

Even with those employing a methodology such as RCM, there are those just beginning in their journey, there are those who have been through the challenges of attempting to begin an RCM program and then there are those who have successfully completed the RCM process.

RCM originated in commercial aviation and, unfortunately, since it left that industry, it has become a very difficult and costly process. It was never intended to be that way. In fact, RCM today hardly resembles what it was intended to be by its founders Stanley Nowlan and Howard Heap.

It has been conservatively estimated that less than 10% of all RCM programs attempted will succeed. Looking at it another way, over 90% of all RCM programs attempted are destined for failure! For those of you with first-hand experience, who have attempted to implement an RCM program, you understand why RCM has been so difficult.

Remember, there is a major difference between a program such as RCM, which is designed to truly enhance safety and reliability and its cost avoidance benefits, such as avoiding potential disasters….. and a more generic PdM or betterment program which is strictly an economic exercise implemented solely to reduce known costs. There is nothing wrong with PM betterment efforts or using standardized PM task templates.

However, with almost the same amount of effort, one can successfully achieve a true comprehensive RCM program instead of settling for something with far less value and effectiveness.

RCM and Reality

From many decades of first hand experience with RCM and associated preventive maintenance program initiatives, approximately 80% of those people who are contemplating implementing an RCM process to improve their plant reliability, do not have a grasp of what it really is. The majority of these folks have good intentions, however, what they are settling for is a means to convert time-directed overhauls into condition monitoring PdM tasks. Or they want to use cookie-cutter PM templates, or find better ways to schedule their PM’s, or review the 20% of their known problem components that they believe cause 80% of all of their plant problems. I wholeheartedly endorse all of these measures but let me be very clear; these betterment techniques are not RCM.

It’s The Unexpected

It has been proven over and over and over that the vast majority of major disasters that occur… that were not due to either acts of nature, human error, or sheer negligence…were caused either by equipment failures whose consequence of failure was unexpected and never analyzed, or from components which were incorrectly analyzed to be run to-failure. The disasters caused by these two reasons are "surprises" because they were totally unknown, unexpected, and unanalyzed.

In this light, think about the following statement……."Plant reliability and safety is directly related to the existing vulnerabilities that have NOT yet been identified because the failure consequences surrounding those vulnerabilities have not yet occurred. Shortcuts in the RCM process will most likely result in those plant vulnerabilities remaining unidentified until an unwanted event does occur. Real-world RCM is all about finding those vulnerabilities before they can occur and result in an unwanted consequence of failure."1

RCM And The Sub-Prime Mortgage Fiasco

Now that we have a better understanding of what RCM is and what it is not, what does all of this have to do with the sub-prime mortgage fiasco? Can you see how the above quotation can be directly compared to the vulnerabilities that existed in the sub-prime mortgage market? Can you see how shortcuts in the mortgage application process, such as verifiable income statements and job validation not being required, contributed to the debacle? How about the wishful thinking that, through some uninterrupted string of luck, home prices would continue to escalate indefinitely? Can you see how the absence of ascertaining the real vulnerabilities, such as overstating the inherent value assessment and understating the inherent risk of the mortgages that lenders wrote, resulted in a catastrophic failure of the investment banking industry?

The vulnerabilities that existed on Wall Street, prior to the mortgage meltdown, are eerily, yet specifically similar to the vulnerabilities that existed prior to the countless number of plant catastrophes that have occurred previously, and, in fact, vulnerabilities which currently exist in many plants and facilities today!

What can we in the reliability community learn from the mistakes that were made in the financial community?

It is very easy to fall into the mind-set of cruising right along on a wave of "Reliability Nirvana".

What do I mean by this? What I mean is that the plant or facility is humming along just fine without any significant problems. There have been no calamities recently; there have been no explosions, no fires, no fatalities, and no negative front page headlines or litigation concerns. So how does this Nirvana continue?

It usually continues on a prolonged streak of luck. But, at some point in time that luck will run out.

Quite often, plant managers know that all is not right. However, the inertia of the status quo becomes too great to overcome and they continue to depend on that streak of luck. Unconsciously, they may even turn a blind eye to "reliability reality", thinking that since everything is running well today, everything is just fine with their plant or facility. There may even be a reverse incentive to continue to do nothing, since the option of doing nothing saves money in the short term.

Can you see where this faulty logic is taking us?

A Faulty Logic

That same "cruising right along" logic prevailed at our most prestigious investment banks on Wall Street. Let’s look at the makings of that global financial misfortune in detail. Not unlike the plant management with visions of "Reliability Nirvana", the Investment Bank CEO’s and Managers had that same kind of misappropriated confidence without being realistic about the risks involved. They were in what I call a "Financial Nirvana." Many of those in risk management at the leading Wall Street institutions inadvertently set up "blinders" so they could not really peek at the true risks involved in the undertakings they were dealing with.

How about the profits? They were enormous. Sounds a little like some of those folks in a state of "Reliability Nirvana", making more money by doing nothing and cutting costs capriciously.

Did any of those financial leaders really want to challenge the status quo that was bringing in all the money they could possibly hope for? Could they not see, even for a moment, that they were running on luck? Did it take that much insight to figure out that when they loaned money to people who had very little chance of repaying that debt, they were playing all their cards on the hope that home prices would continue going up by the day. And if that upward trend suddenly stopped, they would pay dearly. Surely it didn’t take a genius to see that their formula for success also put them on the brink of disaster?

Those subprime mortgages were further packaged into Collateralized Debt Obligations (CDO’s), then even further re-packaged by the CDO’s into Structured Investment Vehicles (SIV’s). Each financial instrument in the process brought in more money and more bonuses for the Wall Street crowd. At each step of the way the financial leaders turned a blind eye to risks and to reality.

The Similarities Of A Disaster

In summary let’s look at the similarities of the mortgage disaster and the impending equipment disaster just waiting to happen at a plant or facility.

A FALSE STATE OF "NIRVANA":

In the Financial World: Senior management believed their current sub-prime mortgage process could continue indefinitely and their "house of cards" could not fail.

In the Reliability World: Senior management believes their facilities will continue cruising right along and their facility could not incur a disaster.

TOTALLY OBLIVIOUS TO THE REAL RISKS:

In the Financial World: Senior Investment Banking management, without an in-depth understanding of their mortgage underwriting vulnerabilities, were totally oblivious to the real risks involved.

In the Reliability World: Senior Plant and Facilities management, without an in-depth understanding of their real plant vulnerabilities, are also oblivious to the real risks they are taking.

THE PROFIT MOTIVE TRUMPED EVERYTHING ELSE:

In the Financial World: The investment banks on Wall Street were too busy making money to perform a "self-assessment" of their true vulnerabilities.

In the Reliability World: Plant management is often so focused on reducing costs and saving money that they become unmindful of the true vulnerabilities in their plant.

THE ADDITION OF COMPLEXITY AND CONFUSION CAMOUFLAGED THE RISKS:

In the Financial World: The mortgage documents were so complicated that the people in China, Japan, Europe and even here in the USA had no idea of the "exposure to danger" that they were investing in.

In the Reliability World: The PM program becomes so convoluted that it camouflages the real "exposure to catastrophe" in ones plant.

COVERED THEIR EYES TO REALITY:

In the Financial World: They inadvertently put on a set of blinders to prevent seeing the deficiencies that existed in their mortgage lending program.

In the Reliability World: They may inadvertently put on a set of blinders to prevent seeing the deficiencies existing in their preventive maintenance program.

If your plant management team’s primary goal is to "keep things humming along" and only pay lip-service to preventive maintenance, then disaster is right around the corner. While it may not happen on their watch, especially if they are close to retirement, it could quite possibly happen on your watch.

Was there ever a doubt that a "day of reckoning" was going to happen in the financial world with so many billions of dollars being lent to people, many of which had no job and offered bogus applications to substantiate their income? Was there any doubt that home prices could not continue to skyrocket 15% to 20%, year over year over year?

Likewise, was there ever a doubt that a "day of reckoning" was going to happen and cause the hundreds of plant shutdowns, fires, explosions, and fatalities that have occurred due to equipment problems whereby a less than robust preventive maintenance program was in place? Is there any doubt that unanalyzed or incorrectly analyzed failure modes or hidden failures in standby, backup, and redundant systems are just waiting to wreak havoc on our plants or facilities?

Is there any doubt that this situation exists, and just might happen at your plant or facility?

If so, it doesn’t have to be that way.

Different reliability professionals and different corporate organizations will no doubt be at different levels of understanding and different levels of implementation in their quest for achieving reliability. A large number of you are just beginning to realize and understand that the unknown and unanalyzed consequences of failure pose the most serious threat to the safety and reliability of your plant, and to the well-being of your plant personnel.

There is a new wave that is bringing classical RCM back to its simple original roots like it was always intended to be. Many astute Maintenance and Reliability professionals have already begun to realize the benefits of riding this new wave, and let’s hope that many more will ride the wave to reliability success.

Neil Bloom is the author of "Reliability Centered Maintenance – Implementation Made Simple" published by McGraw-Hill. He is a mechanical engineer with over 35 years of both hands-on and senior level engineering and maintenance management experience in RCM and Preventive Maintenance Programs in the commercial aviation and commercial nuclear power industries. He is an international guest speaker on RCM and an Instructor of RCM in the Continuing Education Division at the University of California-Irvine (UCI). Neil provides 3-day RCM Training Seminars/Workshops and can be reached at neilbloom@RCMAuthor.com or (949) 218-1286. His website is

www.rcmtrainingseminars.com

.

References

1. RCM

–

Implementation Made Simple, Bloom, Neil B., McGraw-Hill Professional, (2006)

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