In my last blog entry, I kind of left you hanging. Yes, there is a ‘cost’ benefit to starting out with an analog based vibration monitoring program but what do you lose when a digital device has the potential to do so much more. A digital vibration analyzer providing visual indication of both time domain (AC signal) and frequency domain (spectrum of contributing frequencies) is a valuable tool for any facility. And if you have a technician that can actually use the device to its full capability, then you have a winning combination. However, without an analyzer or more importantly, a competent technician, it can be very difficult to ‘troubleshoot’ vibration problems. Or is it? The sales guys will always insist you need the best, a super wamo spectrum analyzer, to monitor and troubleshoot, Or do you?
So let’s look at this problem from a common sense point of view. If I went into a facility cold, with no knowledge of previous work and collected simple analog Low Frequency vibration data (overalls, filter out RMS value), typically I would find 15 to 20% of similar equipment types on the high side of an established normal. And of that 15 to 20%, you may see up to 5% of those items that have any serious issues. And if I supplement the Low Frequency data collected with a round of High Frequency data, based on my experience, I would typically find less that 5% (closer to 3%) of similar equipment types with indications of potential bearing issues
And before I leave the discussion of common sense, I need to make one more point; most vibration issues that are of a serious nature, the ones that are destructive, the ones that are the reason for having a Shutdown List, are usually (better than a 100% probability) cause by design misapplication or installation error. WOW!!! These are the issues that are hard to troubleshoot and even harder to fix. However, once identified and corrected you should not have to deal with these again. These are generally issues that are a One Time Fix.
So let’s do the math on our initial vibration survey; if I had 60 small frame (ANSI frame) pumps, let’s say all between 60 and 100 HP, and 20% exhibit higher than normal values (Alarm status), than I have 12 pumps on my Watch List. 5% of those pumps, or one pump, would exhibit vibration readings that would warrant a Shutdown status. And, I would have an additional 2 pumps that indicate a potential bearing issue. In reality, of the 2 pumps identified with potential bearing issues, most likely one or both pumps would be in the group of 12 Watch List pumps. However, for this discussion, we’ll assume that there are 2 addition pumps that need attention. Final count; I have one pump out of 60 that requires immediate attention and I have 13 pumps that should be placed on a Watch List. Of the 13, 2 are bearing issue and 11 other issue.
So, do I need to spend the money on a spectrum analyzer to troubleshoot on my one Shutdown pump? I’m not sure that I do. Why? Between the cost of the analyzer and the training to use it, I could buy and install a new pump, maybe even two. And, keep in mind there is a good chance that the vibration issue on my one Shutdown pump is a One Time Fix. What if I just called an expert, a consultant (a vendor) and had him look at my pump(s); that would be way more cost effective than a digital analyzer and the technical training. Plus if the vendor did not provide an outcome to my satisfaction, I don’t have to pay him. I have to pay my technicians, no matter if they correctly identify the issue or not. Remember, with a program, I now have performance criteria (acceptance criteria) and I can include that criteria in a contract and hold that vendor’s feet to the fire; e.g. if he advertises he’s an expert, then his accounts receivable is based his performance and of course, my equipment improvement.
The important part of the above message; you now have a program using cost effective tools to categorize equipment into good, mediocre and poor performers. And based on performance criteria that you as the equipment owner took the responsibility to develop, you can now use that criterion to establish vendor contract requirements. WOW!!! There we go again, running our Maintenance Department like a business.
On the two pumps with potential bearing problems; just replace the bearings, its cheap insurance. Do you really give a rat’s butt weather it’s an inner races or outer race defect? That’s what a spectrum analyzer could tell you. I think not; you just care that you have an identified bearing issue that needs attention. And now that I have established Low and High Frequency vibration performance criteria, I can use the criteria to ensure my rebuilds are effective through post maintenance acceptance testing, e.g. we are holding ourselves accountable (like the vendor) to do error free work. WOW!!! There we go again, running our Maintenance Department like a business.
So now that I have a vendor contracted to sort out my one Shutdown status pump and I am replacing bearings on my two pumps with potential bearing issues, what about the 11 others that are in an Alarm condition and that I have placed on a Watch List? How do I troubleshoot those without a spectrum analyzer? Statistical Probability baby! It’s all documented on the internet. You say what?
Common Causes of Vibration…………..
If you have been in this business long enough, especially over the last 15 years, you have probably read just about everything there is to read on the internet associated with vibration troubleshooting. If you haven’t, you’re a fool. The internet is rich with comments, videos and technical articles that focus on vibration monitoring from around the globe. However, I do concede, there is also a lot of trash out there.
Anyway, one item you will see pop up every now and then on the information super highway is a chart that shows the most common causes of equipment vibration. I have developed my own variation based on my experience in power production, DOE nuclear remediation projects and manufacturing segments. And unlike the other talking heads and their internet variations, my top cause is alignment, not balance. The following is what I present in my training courses.
Maintenance Concepts
Table of Common Vibration Sources
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Source
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Probability of Occurrence
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Misalignment (soft foot, coupling, belts and sheaves)
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50%
|
Balance
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25%
|
Resonance (At Run Speed. If VFD involved, will replace Balance as #2 Cause)
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15%
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Loose Parts or Components (Loose Nuts and Bolts)
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5%
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Electrical (voltage/current unbalance, air gap, overload, eccentricity. If VFD involved, will replace Loose Parts as #4 Cause)
|
3%
|
Bent Shaft, Shaft Dimensional Defects
|
1%
|
Blade Pass, Vane Pass, Gear Mesh or Bearing Defect AMPLIFIED by Resonance
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1%
|
Rotor Rub
|
Less Than 1%
|
Leprechauns and Gremlins
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Less Than 1%
|
So on the remaining 11 Watch List pumps; I will schedule things I have immediate control over; things I can generally self-perform, things that are in the top 10 as far as causes of vibration. Initially I want to check the easy stuff, the low hanging fruit. I will check mounting bolt tightness, electrical (balanced current/voltage between phases), inspect the structure (resilient mounts, cracks and breaks) and I will schedule precision alignment checks. Precision alignment checks? What the heck is precision alignment? The answer is nothing more than we what have been discussing from the beginning of this blog;
Precision alignment is establishing and documenting a measurable tolerance (acceptance criteria) for flexibly coupled equipment based on equipment speed. And most important, meeting that criteria when completing the alignment.
WOW!!! Not only do I now have Performance Criteria for my dynamic equipment (vibration monitoring), I have an Acceptance Criterion that allows me to meet that performance criteria. Again, the following table is what I present in my training courses for general coupling alignment.
Maintenance Concepts
Table of Alignment Tolerances
Small Frame Pumps
Electrical Motor Driven
| ||
Shaft Speed
(RPM) |
Offset
(mils) |
Angular
(mils/in.) |
600
|
5
|
2.5
|
900
|
4
|
2
|
1200
|
3
|
1.5
|
1800
|
2
|
1
|
3600
|
1
|
0.5
|
> 4000
|
0.5
|
0.25
|
These tolerances are based on the Reverse Dial Indicator method of alignment. All commercial laser alignment sets on the market today are based on this method of alignment. If you don’t know or understand this alignment method, you need to research it; use the internet. Below are just a few links that look at different alignment methods, shows when they are applicable and provide instructions on how to complete each alignment method.
So what if your facility does not have the skills or equipment to perform this type of alignment? Based on your established and documented precision alignment acceptance criteria, that you as the equipment owner took the responsibility to develop, you can now use that criterion to establish vendor contract requirements. WOW!!! There we go again, running our Maintenance Department like a business.
And to double check the vendor’s work, besides just meeting alignment tolerance criteria, remember that I now have an established Low and High Frequency vibration performance criteria and I can use that criteria to ensure the vendor’s alignments are effective based on post maintenance acceptance testing. WOW!!! There we go again, running our Maintenance Department like a business.
Based on my 30 plus years in the industry, checking nuts and bolts, correcting any electrical issues (balanced current/voltage), verifying structural integrity and verifying alignment, better than 75% of your Watch List pumps will be brought into an acceptable dynamic (vibration) performance range.
And now we have just 2 remaining Watch List pumps. Here, the cause of vibration may just be a bit harder determined and correct. Based on our referenced probability, there is a good chance of a mechanical balancing issue(s). We will review mechanical balancing in an analog world in future blogs; hint you don’t need a spectrum analyzer here either. Anyway, now it’s really time to call an expert, a consultant (a vendor) so that he can provide support to your maintenance staff. Just remember, in his contract, establish criteria for him to work against. Make it clear he only gets paid if there is an improvement. WOW!!! There we go again, running our Maintenance Department like a business.
Using Reason, Logic, Common Sense and World Class Business Practices, leveraging access to global media as a tool to determine the most probable causes of dynamic equipment degraded performance, establishing specific performance criteria and establishing specific acceptance criteria, I can improved the dynamic condition of my 60 small frame pumps without investing in a spectrum analyzer and the associated training. And, I am left with the advantage of a ‘validated’ and ongoing Condition Based Maintenance (CBM) program at the best price.
Where is my Value Added…………
The last step is to document the improvement and show the value added by the improved performance of my 14 small frame pumps. Please don’t expect to see a complete payback for your monitoring equipment, training and program investment. However, showing small gains form this one specific equipment type can improve your credibility with CBM program investment. The easiest way to show value added for common equipment types, like pumps and fans, is to show deferred overhaul (rebuild) schedule; from a 3 year overhaul cycle to a 4 year overhaul cycle, showing the cost deferment over a 10 year period; e.g. instead of 3 overhauls in 10 years, you now have 2. And, if luck is on your side, one of your 14 deficient pumps will be a business critical pump. Here you could model the cost saving through an avoided unscheduled outage; e.g. my $5000 investment just saved you $1,000,000 in production losses. Again, I have several spreadsheet based models that I will share in future blogs.
Nuts and Bolts and Washers………..
Before we leave this article, I want to talk about fasteners that secure dynamic equipment to a foundation. I can include a myriad of horror stories here but I will stick to my three favorites.
· Story 1 – 60 HP, Gould 3196 Pumps, Tainan, Taiwan
Part of our job scope in Taiwan was to assist the customer in establishing a basic vibration monitoring program. We set up a basic program similar to the one shared above; all analog. On our first round of data collection, I noticed that better than half of their small frame pumps were running a fairly steep vibration level; at the lower end of the scale we were seeing < 1 to 1.5 mm/s RMS at all monitoring points and at the higher end, we were seeing 2.5 to 3.5 mm/s. And there were several readings in excess of 4 mm/s. The facility set their Alarm value ant 2.5 and their Shutdown at 3 mm/s. So after we collected and then reviewed the data as part of the training, I asked the question; was there any work recently completed on the pumps that had higher readings? The answer was yes. During a recent outage, a vendor was hired to check and correct alignment on a group of pumps. Initially all of the pumps ran quite smoothly and then as time passed, they ran rougher and rougher. So I asked if they checked alignment. The answer; “no, the vendor had realigned the pumps during the outage, the pumps are in alignment”.
After I finally convinced then to check pump alignment, we started out with a simple soft foot check. Dial indication on the motor foot, loosen the mounting bolt and see if there is any lift. In this case no motor foot lift, however what really surprised me was how a 135 lb man (Taiwanese men are fairly small individuals) with very delicate hands and arms could loosen the motor mounting bolts with nothing more than a 10” crescent wrench. WOW!!! And when I ask if the bolts had ever been tightened with a torque wrench, I got a blank stare.
Folks I’m gonna’ tell you right here, if you don’t tighten the bolts at or near their yield, they’re going to loosen up over time. And that’s just what happened. The bolts were not tight and the motors shifted over time causing alignment issues and a corresponding higher vibration.
Moral of the story; always torque your mounting fasteners or at lease use a cheater to tighten the heck out of them. A 10“ crescent on a 5/8“ bolt just isn’t going to cut it.
· Story 2 - 300 HP, Richland Washington
At the Hanford nuclear reservation, a cross site transfer system was installed to transfer highly radioactive contaminated waste over several miles from one storage area to another. The project dollar value was in the billions and completion was being held up by high vibration from a pair of 300 HP multi stage horizontal booster pumps manufactured by Sulzer Bingham. Just to relieve the suspense; the high vibration was not caused by an alignment issue, it was a resonance issue and of course a VFD was involved (my substitute #2 cause). However, when we went in to double check the common causes of vibration, the ones we have immediate control over (alignment, loose nuts and bolts, electrical and structural), we found that the washers that were part of the motor mounting were not rated for graded fasteners, e.g. they installed a SAE Grade 2 washers under SAE Grade 5 bolts. And as you torqued the bolts up, the washers would collapse under the bolt heads and would actually ‘extrude’ into the motor feet bolt holes.
We discovered the issue when checking bolt tightness; we torqued the bolts to specific criteria (a torque value for that size fastener) and as the washer pulled through the motor mounting holes, it would affect the alignment. Basically, we could ‘snug up’ the bolts and maintain alignment but when we went to torque the bolts after the final alignment adjustment, we would be outside of our alignment tolerance. The fix; hardened washers - ASTM F436. The hardened washers fixed this problem and allowed us to focus on the real issue, the resonance issue.
· Story 3 – 125 HP Belt Driven Overhung Fan, Richland Washington
At the Hanford nuclear reservation, the Plutonium Finishing Plant main exhaust ventilation system consists of 8 electric driven 125 HP belt driven overhung fans and 2 back-up steam driven (approximately 75 HP) fans. These fans all take suction on a common plenum and exhaust through a common stack. These fans have been (and still are) in operation since 1943. Many still have the vintage 1943 open drip proof motors that were originally installed. However, this is not to say that these fans operate with no vibration issues. Because of the parallel operation and slightly different fan curves, these fans have a tendency to produce flow oscillations, a pressure disturbance, as if the fans are operating way to the left side of the curve. The result is a high axial vibration. This has taken its toll on these older fans, historically plagued by fan wheel cracks and bearing issues. In 2011, one of these fans failed catastrophically, the fan wheel hub shattered allowing the fan wheel to unwind itself in the housing, and actually shearing the fan shaft bearing bolts from the mounting pedestal allowing the fan shaft to crash into the motor.
Shearing the Fan Shaft Beating Bolts; these bolts are ¾” and the wheel failure sheared them flush with the bearing pedestal. WOW!!!! No matter what the fan wheel or shaft does, the bearings should never be ripped from the mount. The problem, over time with many, many bearing replacements, a spacer plate was added and welded to the bearing deck on the pedestal. Instead of drilling through the spacer and the bearing deck to relocate the new bearing bolt holes, the plates were drilled and tapped and the bolts were installed in blind holes, e.g. the spacer plate (a piece of A36 cold rolled, very soft material) was acting as the nut. And for whatever reason, over the years as new bolts were procured, they were ordered way too long. Instead of reordering the correct length or at least cutting the bolts down, washers were stacked under the bolts for a spacer so they could be tightened. And the bolts installed were 304 stainless steel, a very, very soft material. So between the soft bolting material, extended bolt length and limited thread engagement, when the bolts where stressed they failed and turned a wheel failure into a complete fan failure. This fan has been permanently removed from service.
It is hard to believe this type of ‘cob job’ came from a nuclear facility, but it did. They let their design safeguards (configuration management) degrade and it bit them in the buttock. If you are securing a dynamic component, use SAE Grade 5 fasteners or better. Use hardened flat washers and use lock washers.
The objective of these stories is to highlight the importance of fasteners and dynamic components. If any of the asset managers (responsible owner(s) of the equipment) had thought through failure prevention, they would have their component fastening systems spec’ed out and acceptance criteria documented and defined for torqueing the fasteners.
My recommendation for mounting dynamic equipment;
Maintenance Concepts
Motor and Bearing Mounting Fastener Sets
| |
Components
|
Specification
|
Bolts
|
(length, diameter), NC, SAE J449 Gr. 5, Zinc Electroplated IAW ASTM B633
|
Nuts
|
(diameter), NC, SAR J995, Gr. 5, Zinc Electroplated IAW ASTM B633
|
Flat Washers
|
(diameter), ASTM F436, ANSI B.18.22.1, Zinc Electroplated IAW ASTM B633
|
Lock Washer
|
(diameter), CS Lock Washer Wire, ANSI B.18.21.1, Zinc Electroplated IAW ASTM B633
|
Maintenance Concepts
Basic Torqueing Criteria
| |
Fastener Diameter
|
Torque Value SAE Gr. 5
|
1/2”
|
48 ft-lbs
|
9/16”
|
70 ft-lbs
|
5/8”
|
96 ft-lbs
|
3/4”
|
160 ft-lbs
|
7/8”
|
240 ft-lbs
|
1”
|
370 ft-lbs
|
Maintenance Concepts
Fastener Installation Criteria
|
For securing motors, motor mounting plates, mounted bearings or any item supporting a shaft where shaft velocities exceed 1500 FPM, as a minimum, SAE Gr. 5 fasteners sets shall be used. A fastener set includes; bolt, nut, 2 flat washers and 1 lock washer. When installing fastener sets, a minimum of 5 threads and a maximum of 10 threads will extend from the nut. Bind mountings will not be used. All fasteners sets shall be torqued after final adjustments.
|
For adjustable motor mounting plates generally associated with belt driven equipment, where mounting studs are provided as a component of the mounting plate, SAE Gr. 5 nuts, hardened flat washer (F436) and lock washers will be used to secure the motor to the plate. In lieu of a final torqueing, tightening the nuts to fully crush the lock washers plus ¼ turn is acceptable.
|
Maintenance, what a Concept!!!!
MMJennings
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