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from the instrumentation department...
adjust axial bently nevada probes
Continuous process industries, DCS questions. topic
Posted by ggg on 16 December, 2010 - 6:19 pm
when i started to adjust an axial probes as i know (zero in middle -10 + or - .5 float). mechanical group pushed me to adjust zero (-10 ) at active side and told me that is man-turbo recommendation. is that true?


Posted by pascal on 17 December, 2010 - 4:56 am
I am agree with YOU the zero (or -10Vdc) is the middle position of the shaft NOT one side!

pascal


Posted by vahid on 17 December, 2010 - 1:48 pm
I suggest you to consider following points:

1- Note to the maximum travel that the shaft does have,

2- Position the shaft at the middle of the travel then fix the gap voltage to zero. Or Position the shaft at the end of the travel and adjust the gap voltage to its corresponding position.

3- Normally the shaft travels toward the direction that its pressure is less than the other side, as you are aware the pressure at the suction is less than the discharge. (It is Normal direction).

Hope this helps you.


Posted by galal on 19 December, 2010 - 7:26 am
thanks pascal and vahid for your replay, i know what you said but mechanical group told me its manufacture philosophy to adjust zero at active side (at edge of bearing). the question now is it a new philosophy in adjusting axial vibration probes ?


Posted by A. OZTAS on 20 December, 2010 - 3:05 pm
Did you check the linearity of the axial loop (probe + transducer and the extension cable if applicable)?
The range should be between -8V to -15V?

What is the current procedure used in your plant?
In general the manufacturer or the OEM doesn't give any clear procedures for axial calibration.

Is there any zener barrier in the loop?

Can you share with us the zero point setting in the rack (BN?)

Good Luck...
A. Oztas


Posted by pascal on 21 December, 2010 - 5:39 am
No there are no new philosophy as your mechanical team suggest at least that I am aware of.

in fact it would be very easy to you to check open the BN toolbox (I hope you have one) and check the range, alarm and trip (if any) set points for the thrust bearing displacement.

You should get Negative and Positive values for all of them (for range typical values are plus and minus one mm, meaning the zero, obviously, IS in the middle range.

NOW, if you get only positve or Negative values then THEY (mechanical team) are right and the zero should be on ONE side of the shaft, but frankly I doubt it!

So the BN 3500 (I guess) is able to tell you what he requires as input and therefore as zero position!

pascal


Posted by KGV on 22 December, 2010 - 1:51 am
In fact both adjustments: with rotor bumped onto active thrust pad and also setting of the voltage at middle of the float is in practice. While commissioning we had the rotor bumped on the active pad for our zero; later OEM suggested to do it on Middle position, and then we had done accordingly. I think the control trip settings for setting zero at middle. If it is to be bumped on active pad, the constants could change.

Another interesting part is that we had a problem of the axial probe mounting shifting in the casing expansion side giving more gap, while running. Thrust pad shims were replaced at that point.


Posted by galal on 24 December, 2010 - 3:11 am
ok friends,

the alerts and danger settings toward active side are less than non-active side example: alert for active is .2 mm and for non-active .3 mm , our compressor is axial type.

@ a. ostaz: we have a zener barrier at one machine and we haven't at other one

now what is more suitable: adjusting zero in middle or at active side?
note: if we adjusted it at middle, the machine will reach to alert and danger settings quickly as axial compressor will move toward active side and may machine stop because of lower active side settings

galal


Posted by Janbin on 8 February, 2012 - 1:44 pm
I agree with your mechanical, that setting can tell you that how worn out of your thrust bearing active side is? On the other hand, the gap voltage at zero is required for floating method if you want to know the same thing.

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