As mentioned in my last blog entry, acceptance criteria is one of the most, if not the most important aspect of successful Preventive Maintenance (PM) and Condition Based Maintenance (CBM) programs. And I had also stated that “all criteria will be accepted and refined by the asset owner’s engineer and documented”. Specifically when referencing National and International Consensus Standards as a basis for acceptance criteria, that criteria needs to be evaluated to ensure it is representative of the normal operating characteristics of the asset.
In all CBM (read this Predictive Maintenance; PdM) courses I have developed for various clients over the years, I have included the following;
It is the Facility Owners Responsibility to establish Performance Standards (Objective Criteria). Any Defined Performance Standards should provide Alarm Values and Shutdown Values.
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Then I go on to explain that Consensus Standards and Original Equipment Manufacturer (OEM) recommendations should be used as a basis for developing specific asset criteria. However, those recommendations need to be evaluated and in most cases adjusted to fit a specific set of asset operating conditions. Generally, the facility owner’s engineer is responsible for that final decision plus, that decision needs to be published in an operations document and then training provided to ensure that coherent and consistent data is produced.
Low Frequency Vibration Monitoring………..
The following discussion is focused on broadband, filter out, low frequency vibration monitoring (less than 2 KHz).
In the days before ISO 10816, Mechanical Vibration – Evaluation of machine vibration by measurement of non-rotating parts, Parts 1 through 6, there was ISO 2372, Mechanical vibration of machines with operating speeds 10 to 200 rev/s – Basis for specifying evaluation standards. ISO 2372 was replaced by ISO 10816 in 1995.
One of my favorite examples of acceptance criterion that needs to be evaluated and adjusted to fit a specific set of asset operating conditions is the old, like 1974 old, ISO 2372 Vibration Severity Chart. I am sure you have seen this chart or one of its many variations. This chart is packaged with just about every vibration meter, vibration analyzer and vibration data collector on the market today. Yes, it is still referenced 20 years after ISO 2372 was withdrawn. WOW!!! And one would think that after 40 years of presentation, someone would recognize that this chart is not an absolute but provides ‘guidance’ only, a place to start, when developing a CBM process. I guess the green, yellow, red interpretive colors hide the intent.
It should be noted that this chart was also referenced in the original ISO 10816 (ISO 10816-3 to be specific) as an appendix. However, in the more resent revisions of ISO 10816, the severity chart has been removed (or at least rewritten). And within ISO 10816 (the original and the latest iteration) there is the following statement;
The ISO does NOT provide Pass or Fail criteria. It provides REASONABLE guidance ensuring that gross deficiencies (unsafe) or unrealistic requirements (too limiting) are avoided.
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Below is a variation of the severity chart that I use in my CBM training courses. I use it as an example to make several points; 1) that the severity chart is no longer referenced in a consensus standard, 2) if the chart is referenced, it is not used as pass or fail criteria but as general guidance, and 3) that it is the Facility Owners Responsibility to establish Performance Standards (Objective Criteria), not a third party comity.
Overall RMS Velocity IPS
(MM/S)
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Class I
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Class II
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Class III
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Class IV
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.01 (.25)
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A
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A
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A
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A
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.017 (.4)
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.028 (.7)
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.044 (1)
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B
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.07 (1.7)
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B
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.11 (2.8)
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C
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B
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.18 (4.6)
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C
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B
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.28 (7)
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D
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C
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.44 (10)
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D
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C
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.7 (17)
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D
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1.1 (28)
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D
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1.8 (46)
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As can be seen, the ISO 2372 vibration severity chart is an absolute presentation; with numbers and colors it is very authoritative.
One of my latest clients, a global manufacturer with a fleet of facilities in the U.S., Taiwan, China, Japan and Korea and one that has been in business for well over a hundred years, drank the ISO 2372 Kool-Aid. During my initial visits to each facility, I had asked the technicians to walk me through their vibration monitoring program(s) for small frame pumps. They would show me their latest manual data collection and on-line monitoring schemes. Not surprisingly, none of the individual programs were consistent or even similar to the other facilities as far as data collection and analysis (that’s yet another blog discussion). However, I could count on one thing being the same, a devout following of the ISO 2372 severity chart. The alarm points in all of the monitoring schemes reviewed were always in line with the severity chart. A red line at 7 mm/s RMS was drawn across all of the trending data; a solid ‘D’ in accordance with the ISO severity chart.
That’s a wonderful effort except; the mean value of all vibration data collected on the clients small frame pumps was less than 2 mm/s RMS. The alarm value was established at 300% of any actual monitored condition. WOW!!! In effect, the equipment could fail (and actually had a substantial history of failure) prior to identifying a condition that would cause a failure. Based on the alarm value alone, the program could not provide a Return On Investment.
When I pulled the string on how this alarm value was derived, the common response;
· the vendor that sold us the equipment set the alarm values,
· and these are ISO alarm values.
And when I asked them to show me there copy of the ISO, they pulled out the old ISO 2372 severity chart that came in the box with the monitoring equipment or device; no standard, just the severity chart. And when I approached management and asked why they allowed a vendor (a third party) to define what was, or more importantly, what was not acceptable criteria for their equipment, there was only a look of astonishment followed by embarrassment. They invested heavily in a program (training, equipment and software) and yet had not seen any real return; a high expectation but unfortunately, no condition improvement. And it was their error.
Did they change course? Yes they did. If you actually read ISO 10816 you will see a caveat (I think it in Part 1) that states, changes in vibration amplitude greater than 25% warrant investigation. So with a mean value of 2 mm/s RMS for all readings, the Alarm Value was adjusted down to 2.5 mm/s RMS. And for a Shutdown Value, the facility went with 3 mm/s RMS. Once the client established these limits, assets that performed in a range greater than the Alarm Value but had not yet reached the Shutdown Value were put on a watch list. And assets with readings in the Shutdown range were…..….you guessed it; shutdown.
Shutdown assets became a priority to correct deficiencies and at least get vibration performance below the Shutdown Values and placed back in service. Watch list equipment was worked as a secondary priority to bring performance in line with similar assets with better performance. Just to finish this chapter; the majority of performance issues associated with the client’s small frame pumps at all of their facilities were coupling and alignment issues. WOW!!! There goes another myth; balance being the most common cause of vibration issues (yet another future blog discussion).
This is what CBM is all about folks; finding indications of potential failure, prioritizing, planning and correcting the cause before we see actual failure(s). This is where you see your Return On Investment; asset performance improvement.
That 25%..................
Above, you probably noted that I used a value in listed in a Consensus Standard (in this case ISO 10816, Part 1) to establish an alarm value. However, past experience and how that experience rolls into any training I give on vibration acceptance is a bit different. Generally, for common equipment types, I have the client collect data on all bearings in all plains, e.g. a baseline. Then on a spread sheet, I have the client list the equipment on the ‘rows’ and the data collected at each bearing position in the ‘columns’. Then we look at the mean value of data from each column. Then we look at the mean value of all of the means. The first standard deviation from the mean of the means is the Alarm and the second standard deviation is the shutdown. Then I roll the values up to the next whole number (of half value). It’s a pretty simple process. What’s really interesting; the Alarm Value generally comes in at ………25%.
Basically, what we are doing here is establishing performance criteria based on the equipment’s operating characteristics in a specific set of circumstances. We are using the consensus standard as guidance and reference to ensure our decision is reasonable; not too high and not too low. We, as a facility owner, are taking responsibility for establishing performance criteria to evaluate the condition of our equipment. WOW!!
Maintenance, what a Concept!!!!
MMJennings
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