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Curran919

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Hey all, another quite pedantic question from the world of vibration in pump contracts.

All Vibe Standards have limits based on overall values, and many also on narrowband values. Depending on the standard, they use different names with important implications. ISO X0816 standards use filtered values. To me, this infers a bandwidth of maybe +/-5% or a few Hz to capture all the energy for a single order of the pump's vibration.

On the other hand, API 610 uses discrete frequencies, described as "...the discrete frequency velocity, measured with a FFT spectrum using a Hanning window and a minimum frequency resolution of 400 lines." This infers that it applies to a literal discrete frequency in an FFT, not a narrowband.

This is of course an important distinction as all FFT algorithms of an amount of leakage into neighbouring FFT bins regardless of frequency resolution. If a frequency falls perfectly between 2 lines, you will have two equal magnitude of lines, whose sum (RSS) would equal the real amplitude at that order, which would then be as much as 1.41x higher than the spectral peak.

API 610 has a broadband limit (e.g. 3mm/s) always 50% higher than discrete limits (e.g. 2mm/s). This is strange though, as most pumps bearing houses will have two main peaks at shaft speed and BPF. If these both exceed the discrete limits at 2.1mm/s, the overall will still not exceed the overall limit, so it would make more sense for these values if we were referring to spectral peaks instead of filtered values.

The common GM vibe standard defines narrow bands to be used on around a pump's 'pumping frequency', but is very careful to state that the highest peak plus 2 lines on either side must be added (RSS) to give the actual value.

HI specifically avoids filtered values, which is unfortunate, as they are usually the voice of reason, though maybe that is the most reasonable thing to do.

So the standards somewhat favour the use of narrowband/filtered values over discrete frequencies, but we all know that this is much harder to accomplish than just reporting on discrete peaks, and as a result, that is exactly what we see programmed in all of our vibe analyzers. Everytime I get data from a contractor, it always reports the spectral peaks in the column (single lines), which are then compared to any filtered/discrete limit of interest. Its quite wrong, not conservative, and pretty deceitful sometimes, but somehow it evolved to being the norm.

What are your opinions on this? Do you use one or the other depending on if you are trying to make vibration seem high/low? I know I have...
John from PA

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I'm not sure about the specifics of the ISO specs but API 610 refers to vibration as measured during a shop test, not the operation in the field.  More comprehensive details about the instrumentation side of the field measurement can be found in API 670. 
OLi

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Reply with quote  #3 
Classic ISO is 10-1000Hz filter?
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RustyCas

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Reply with quote  #4 
I tend to question the usefulness/validity of a standard that is based on a "400 line" spectrum.... how can that provide meaningful results?
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John from PA

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Quote:
Originally Posted by RustyCas
I tend to question the usefulness/validity of a standard that is based on a "400 line" spectrum.... how can that provide meaningful results?


Keep in mind that API specs in general are purchase specifications and establish the minimum instrumentation that is to be used for a factory acceptance test.  Most factories have more sophisticated equipment as well which may be used should a problem exist on the test stand or for doing diagnostics later after field installation of the machine.

I would also add that the factory acceptance test specifications are quite detailed as far as plot formats, frequency ranges, etc. and for the most part are more than adequate for a new machine.  Additionally, the buyer has the right to request (and pay for) more sophisticated testing.
Curran919

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Quote:
Originally Posted by OLi
Classic ISO is 10-1000Hz filter?


10-1000 or 2-1000 for low speed. I've always been unsuccessful arguing for 2-200 as an alternative! They also have the filtered values in addition to the overall values.

Quote:
Originally Posted by RustyCas
I tend to question the usefulness/validity of a standard that is based on a "400 line" spectrum.... how can that provide meaningful results?


Just to let my age show again:

I've almost exclusively used NI for my DAQ, so having anything that has a hard limit on the number of lines of resolution all seems archaic to me. If lines of resolution are only a reflection of the available memory in the unit, most decent DAQs nowadays should be giving you way more than you would ever need. If you actually have a fixed display that gives you a 400 pixel domain, it still doesn't reflect true 'lines of resolution' if you can still zoom in.

The only people using analyzers with 400 lines anymore probably just have 15 year old equipment. Keep in mind that many of the ISO 10816 chapters are over 10 years old.
spciesla

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Reply with quote  #7 
The ASME OM Code used for Inservice Testing of safety-related pumps at nuclear power plants requires a frequency range of at least 1/3 of running speed to 1000 Hz.  This works well and is easy to implement for most common pump speeds (900, 1800, 3600 cpm).  It was challenging to accomplish with positive displacement charging pumps driven through a gearbox at approximately 196 rpm.

Getting back to the original post, doesn't your contractor(s) provide the overall amplitudes in addition to the list of spectral peaks?  The first line of your post indicated pump contracts.  Demand what you want/need in the contract and hold your vendor to it, whether it is an existing standard or an in house standard.

As far as HI being the voice of reason, I don't agree with you.  In my limited experience with HI standards, they are pretty lax on acceptable vibration amplitudes.
Curran919

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Reply with quote  #8 
Quote:
Originally Posted by spciesla
Getting back to the original post, doesn't your contractor(s) provide the overall amplitudes in addition to the list of spectral peaks?  The first line of your post indicated pump contracts.  Demand what you want/need in the contract and hold your vendor to it, whether it is an existing standard or an in house standard.


We're the producer. I am in the R&D, basically doing global troubleshooting to different production sites. We are currently trying to roll out harmonized data acquisition for all our test beds, but it went so poorly when they tried this 12 years ago that everyone is resistant to changing their tools. Anyway, this has nothing to do with their ability to give quality data, we are working on that. It is simply about how to define the energy of a single harmonic/peak and when it should be done with a spectral peak value or a narrowband/filtered value.

Quote:
Originally Posted by spciesla
As far as HI being the voice of reason, I don't agree with you.  In my limited experience with HI standards, they are pretty lax on acceptable vibration amplitudes.


Why should more conservative vibration limits equate to a more reasonable approach? Generally, the less conservative standards have more analysis put into them so they can push that number up without compromising the machinery. This can be reflected in the specificity of the standard and the extent of categorization.

Compare HI to API then, which is usually considered to be a very conservative technical standard. Looking at the OH and BB pump limits, this holds true, but the HI limits on VS pumps are 30% lower than API's.

When I say HI are the voice of reason, I generally mean that ISO X0816 leaves a lot of loose ends for interpretation, and they know it. Chapter 7 applies to pumps, including VS and says to use Chapter 3 for driver vibration. Those allowables are less than for the pump. So the limit at the top of the motor is much less than at the bottom of the motor. Its ridiculous. HI is the only one that has an appendix explaining how to treat driver vibration on vertical pumps specifically, which is always a source of tendering issues with customers... ALWAYS.

To take my bias out of it, our standards rep is on the HI and ISO TC108 Committees and he has said that he generally agrees with the ISO better, but I'd argue that he just can't criticize his baby. [tongue]
RustyCas

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Reply with quote  #9 
Since we are discussing standards now, the RustyCas "standard" works like this:  reasonable 1X given the system size, speed, design; no identifiable FTF vibration;  low 2X is OK, but no discernable 3X - 5X (looseness);  no discrete bearing defect frequencies that rise above the noise floor; maximum peak acceleration value < 2 g's (motors) and < 4 g's for other equipment.

You'll notice no mention of an "overall vibration level" because I am always more concerned about the existence of an identifiable fault on a new or rebuilt machine.  For example, on a large baghouse fan recently with a new wheel, the balance levels were 0.8 mils IB, and 1.4 mils OB.  The vendor said that was "in spec" -- it may be, but I don't care because there is no good reason for a wheel that is shop balanced to have IB/OB variance of that magnitude.  They had sent a balance tech (which my customer paid for) for the startup, and he didn't see any need to balance it (I wasn't involved at that point).  When I pointed out the discrepancy after my next survey, they (vendor) said the OB vibration was due to a fan housing seal rub (I called BS, and it didn't change when they adjusted the seal).  I routinely field balance these fans to 0.25 mils or less, and they often come from the shop at 0.5 mils.  So in this case this wheel didn't meet my balance standard, which is "as low as can be reasonably be achieved with reasonable effort."

But I don't expect we'll ever seen that in any standard.

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Curran919

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Quote:
Originally Posted by RustyCas
"as low as can be reasonably be achieved with reasonable effort."

But I don't expect we'll ever seen that in any standard.


I think you underestimate how important of a word 'reasonable' is in the engineering world. Of course, what is the point of a normative standard if you are just telling people to use reason?
Shoveldr

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Reply with quote  #11 
Quote:
Originally Posted by RustyCas
Since we are discussing standards now, the RustyCas "standard" works like this:  reasonable 1X given the system size, speed, design; no identifiable FTF vibration;  low 2X is OK, but no discernable 3X - 5X (looseness);  no discrete bearing defect frequencies that rise above the noise floor; maximum peak acceleration value < 2 g's (motors) and < 4 g's for other equipment.


I had to create an "acceptance critera" for new gearboxes at a former employer, these were gearboxes we manufactured.  The former criteria was an overall level and when the $500,000 gearbox failed that they freaked out a little; detailed testing was worse until we realized the vertical gantry mill they used for a driver for the testing had the same number of teeth as the input pinion of the gearbox, oops.

We created an experience based criteria which our quality department immediately rejected, things like no discernible pattern weren't quantifiable enough for them.  We ended up doing a lot of statistical analysis on all the gearboxes we had tested and the results of physical measurements on the gears and came up with an almost identical standard, only with a better justification and they accepted it.
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