4 replies to this topic

[ASH25320]

This doubt is regarding rotating machines like turbines & compressors. we generally monitor health of machine by watching vibration/displacement & casing velocity. However sometimes it happens that machine vibrations are high but there is no change in casing velocity(with respect to its previous reading), & sometimes the reverse case happens that means casing velocity increases but there is no change in vibrations of machine. I think both should corelate each other in the sense that if vibration increases it should also reflect in casing velocity & vice versa. Can somebody cleasr my doubts ? Thamks for your precious time.

[Art Montemayor]

Ash:

What you call "casing velocity" is assumed by me to mean machine volumetric flow rate. High speed rotating machines, such as turbo-expanders and centrifugal compressors don't necessarily rely on exhibiting vibrations due only to volumetric flow rates - certainly not to the point that they will reach an alarm point or shutdown level. Vibrations in centrifugal compressors (at least in the well-designed and proven models) are, in my experience, primarily due to mechanical design and mechanical limitations. These can be bearing design and loads as well as impeller type and designs. Varying machine speeds also have a marked effect. The type of capacity control(s) that you apply also can have an effect. If you are employing recycle gas as a capacity control versus speed control, there could be a difference. You just can't make a general statement without furnishing detailed background information as to type of machine, make, model, capacity ranges, types of controls and vibration ranges.

Bearing life, design, and condition can have a marked effect on any vibration characteristics.

[akslzf]

I presume your question is "when the vibration displacement on a turbine or compressor shaft -that is measured using a proximity probe & indicated in microns rms- increases, why does the casing vibration -that is measured using a seismic probe & indicated in mm/sec rms- does not increase?".

In general,
vibration velocity can be calculated as V = 2*pi*f*d mm/s peak-to-peak, where f=frequency in Hz & d=displacement in mm peak-topeak &
vibration acceleration can be calculated as A = 2*pi*f*V mm/s^2 peak-to-peak.
And your question is, when V changes, why do A or d not change.

Valid one.

The answer to the question is:

Casing vibrations are measured using accelerometers (siesmic probes) & the results are differentiated by FFT to velocity or displacement rms or peak-to-peak. Depending on the frequency range of the probes that feed data to the FFT & the capability of the FFT to accurately interpret the data, the result varies.

Further, the actual vibration originates at the rotor & measured at the bearings. This gets "conducted" to the casing & other parts of the compressor.

For shaft displacement measurement, two proximity probes are arranged orthogonally & the signals from the two are crunched by the Bently or other system to arrive at the position of the shaft or its diaplacement. This is far more accurate than the results provided by the accelerometers mounted on the casing, as the calculations are pretty straight forward.

You dont seem to be too familiar with the fundamentals of vibration theory. It is extremely important to use the right terms & units in vibration measurement. You may try these two extremely good books
Fundamentals of Rotating Machinery Diagnostics by Bently &
Machinery Malfunction Diagnosis by Eisenmann

Edited by akslzf, 19 February 2010 - 08:23 AM.

[kkala]

...the actual vibration originates at the rotor & measured at the bearings. This gets "conducted" to the casing & other parts of the compressor.
For shaft displacement measurement, two proximity probes are arranged orthogonally & the signals from the two are crunched by the Bently or other system to arrive at the position of the shaft or its diaplacement. This is far more accurate than the results provided by the accelerometers mounted on the casing, as the calculations are pretty straight forward.

Thanks akslzf for the interesting information. Mechanalysis (the term used in late 1970s) has been a useful diagnostic tool for preventive plant maintenance. Educative seminars took place as that time, but these were for mechanical engineers. I worked then in Fertlizers (Operations), and we notified maintenance immediately on observing vibrations of rotating equipment, though this was not planned maintenance. Only steam turbogenerators had systematic follow up of vibrations due to potential scales on turbine blades.
Once vibration data (schlumberger contact sensor used) of a grinding mill was sent to supplier, just after reception test. This could be due to resonance, since rotational speed of the mill was changed during the tests. In all other cases vibrations were signs of equipment malfunction, or loss of balance (more rarely). The latter could happen on induced air fans (e.g. of cooling towers), apparently when balancing rods on blades got shifted. But it was also due to scales or corrosion, e.g. on fans.
It is assumed that any diagnostic tool has some probability of failure /misinterpretation, though this seems rather low for mentioned method.
Increased flow rate within the specified limits for the equipment did not cause vibrations (it may be the case for increase beyond limits).
Hope above information is useful to those not being familiar with mechanalysis, considered (at least locally) as field of mechanical engineer.

Edited by kkala, 20 February 2010 - 03:03 PM.

[SteveG]

I don't know if this will be of help to you, but Plant Services ran an overview of vibration that explains many of the basics (the causes, effects, characteristics, etc.) I suspect you are already aware of most of the information in the article but I just thought I'd mention it in case it can be of use.