Cuerpo teórico

Much work continues today on establishing standards for allowable overall vibration. Some of the better known standards now available include: a) ISO 2372 - “Mechanical Vibration of Machines with Operating Speeds

from 10 to 200 Revolutions per Second” - Basis for specifying evaluation standards (measurements made on structure).

b) ISO 3945 - “Mechanical Vibration of Large Rotating Machines with

Speeds Ranging from 10 to 200 Revolutions per Second” – Measurement and evaluation of vibration severity in situ (measurements made on

structure at various elevations).

c) ISO 7919 - “Mechanical Vibration of Non-Reciprocating Machines” - Measurements on rotating components and evaluation (measurements made on shafting).

d) ANSI S2.44, Part 1 - 1986, “Measurements and Evaluation of Mechanical Vibration of Non-Reciprocating Machines as measured on rotating shafts, “American National Standards Institute”, NY (1986).

e) AGMA Standard, “Specifications for Measurement of Lateral Vibration on High Speed Helical and Herringbone Gear Units”, Standard 426.01. f) API 610 “Centrifugal Pumps for General Refinery Services”, 1971,

American Petroleum Institute, Washington, DC.

g) API 617, “Centrifugal Compressors for Refinery Services”, 4th Edition 1979, American Petroleum Institute, Washington, DC.

h) API 670, “Non-Contacting Vibration and Axial PositionMonitoring System”, 1976, American Petroleum Institute, Washington, DC.

i) MIL STD-167-1 (Ships) 1974, “Mechanical Vibrations of Shipboard Equipment”, US Government Printing Office, Washington, DC.

j) MIL-M-17060B (SHIPS) 1959, “Military Specifications - Motors, Alternating Current, Integral Horsepower” (Shipboard Use), US Government Printing Office, Washington, DC.

Some attempts have been made to provide overall vibration criteria based on the type of machine and its drive configuration (centrifugal pump, direct coupled fan, belt driven fan, turbine/generator, etc.), and on its mounting (isolated versus non-isolated). It is recognized that there is often a significant difference in the amount of vibration for one machine type versus another. For example, a reciprocating air compressor obviously has significantly frequency, forcing frequencies of the machine itself, machine center of gravity relative to the placement of isolators, etc. Thus, it is important that users of today’s predictive maintenance hardware and software take into account the type of machine and its mounting when they begin to specify alarm levels of overall vibration for each machine that they will input into the computer database.

In addition, it is important for users to know how their particular predictive

maintenance data collector and software system measures overall vibration. Some systems have a fixed frequency range which is completely independent of any

frequency range chosen on any particular spectrum. In fact, this overall measurement is completely independent from the spectra measurement parameters specified for the data collector (F_max, # lines, # averages, etc.). In these cases, the analog time waveform is used as the basis for computing the overall vibration. In other data collector systems, the overall is computed directly from the spectra themselves using the following

formula:

(EQUATION 13)

The danger with the latter technique of calculating overall vibration is that significant vibration can possibly be occurring outside the maximum frequency (F_max) that was specified by the analyst. There would be no way one could be aware of this if the overall were not computed from the time waveform. For example, if a user specified a maximum frequency of 30,000 CPM and, unknown to him, there was a very large peak out at 60,000 CPM with an amplitude of .50 in/sec (12.7 mm/sec), the data collector may only display an overall of about .20 in/sec (5.1 mm/sec) whereas the true overall may be up on the order of .60 in/sec (15.2 mm/sec). Thus, it is most important for the analyst to know how the overall is computed and, if given the option, one should choose the analog time waveform technique.

If they still wish to know the total RMS energy within the spectrum as calculated by the above equation, they could possibly specify one of the spectral alarm bands that would go all the way from the low end up to the high end of the spectrum since the RMS

energy in each one of the bands is calculated using the same formula within the software of most condition monitoring vendors.

Table V is offered as a specification which takes into account the various machine types, how they are mounted and where measurements are taken when specifying peak velocity overall alarm levels. This table assumes that the data collector has used the analog time waveform technique to compute the overall peak velocity. In addition, this overall “peak” velocity assumes an actual measurement of RMS vibration multiplied by 2 since most all data collectors and analyzers today actually take RMS measurements and multiply them by this mathematical constant when displaying so-called “peak” velocity. Note that each of three “ratings” is provided in Table V including “GOOD”, “FAIR” and “ALARM. After reviewing of all spectra captured on a machine, if no

problems are found, the first two columns (“GOOD” and “FAIR”) are offered to give the analyst a general feel for the overall condition of each machine based on the highest overall level measured on their machine. However, even if the highest overall in the machine is still within the “GOOD” range, it is still possible for the machine to be in alarm, depending on what frequencies were generated and the amplitudes of those frequencies. That is where the spectral alarm bands come into play to ferret out the “apparently good condition” machines from those that truly have problems.