4 replies to this topic

[Bodhisatya]

Dear All

We are having an oil Brake Expansion Turbine in our Air Separation Unit.After a sudden trip the machine is showing unexpectedly high Vibration for the Same speed (38400 rpm).The reason for this sudden abnormality needs to find out.

On Confronting the Vendor he said please measure the Gap Voltage.Gap Voltage as per my knowledge is the magnetic EMF generated in between the probe tip and the shaft.The measured range was 6.8 which vendor says is fine.But what I fail to fathom is what physically does this Gap Voltage imply.If the generated gap voltage is high,then does it mean the Vibration is High or vice Versa and How.

Looking for some sort of insights from the esteemed Delegates.

Regards
Bodhisatya.

[Bodhisatya]

Dear All

Waiting for some sort of Insights.

Bodhisatya.

[Pilesar]

As I understand it, gap voltage is an indication of the
distance between sensor and shaft. When the machine was
aligned, the sensor was calibrated so that the voltage
reading would be in the middle of the range. Too high a
reading or too low a reading now would indicate that the
machine has shifted in relation to the original alignment and
would suggest problems with the bearings.

[Bodhisatya]

Exactly that is what I want to know....how is an internal wear and tear of the machine (assumed) resulting in change of GAP Voltage.Physically what's happening with machine if the value is too low or if it's too high.

[Pilesar]

An expansion turbine spins super fast -- probably faster
than a car engine at its highest revs. The fast rotation
means that unless the shaft is aligned perfectly straight,
the rotating weight is perfectly balanced, and the vanes on
the wheel all are pushed by the expanding fluid evenly, then
the rotating momentum will be converted to net directional
forces that will try to move the rotating equipment out of
its case. The expander is designed so that the forces will
balance one another as much as possible and the opposing
forces will counteract one another. The expander case is
made so that the balance is maintained.

I think of an expander as analogous to a toy gyroscope or a
spinning basketball that can be balanced on one finger. It
does not take much effort to keep a well-balanced spinning
item steady if it is rightly aligned. But once the axis
begins to wobble, the hand has to try to compensate. If the
wobble is too great, the effort becomes unmanageable and the
spinning item crashes.

The vibration in the expander is the 'wobble'. The bearings
try to compensate for the wobble to keep the axis aligned.
If the bearings are doing their job properly, the correct
distance between the sensor and the shaft is maintained. A
voltage gap change in the proximity sensor would indicate
the bearings are wearing away. The magnitude of the voltage
change gives a clue to the extent of the wear.