FCC Power recovery train axial vibrations 2

[amar122amar]

In our FCC power recovery train flue gas expander, air blower and steam turbine are on one shaft @4185 rpm and motor/generator is on other shaft@1500 rpm. both these shafts are connected via a gear box.All the machines are protected by giving axial displacement high high trip.
For Expander it is +0.57mm & -0.45mm, for blower +0.4mm to -0.65mm, for turbine +0.4mm to -0.65mm and for gear box 0.5mm. my questions are:
1. How these values are decided (+ and - values are different).
2. For blower and turbine - value is greater than + but during normal running it shows around +0.2mm reading.
3. gear box axial vibration trip is on + value only but during normal operation it -0.2 mm.

Thanks and regards.

 

[JJPellin]

There are two different philosophies when it comes to setting the zero point for thrust probes. Some plants (including my own) set the zero point in the center of the thrust float. So if the total clearance within the thrust bearing were 0.012 inch. We would set the zero in the center and with the shaft thrust hard in the active direction it would read +0.006 inch. The other method of setting the zero is to set it with the shaft in the hard active position. In this system the reading would be 0.000 with the shaft thrust active. In both cases, the high thrust trip would be set at a point where it is believed that the thrust bearing has failed and there is a risk of internal metal to metal contact. If the zero is set at the center of thrust float, it would tend to make sense to set the trip points equally in the positive and negative directions. The assumption would be that if the shaft thrusts perhaps 0.025" beyond the normal thrust float then the thrust bearing has failed and you are wearing through the babbett into the base metal. If the zero is set hard active, then it would seem logical to set the thrust trip in the negative direction as a higher value since the rotor must first thrust by the amount of the bearing clearance before it even contacts the inactive thrust bearing. The exact value used depends on the design of the bearing, the configuration of the machine and the cost of a false trip. Some machines tend to be naturally thrust balanced and so there may be a perception of a lower value in a thrust trip. A gearbox with double helical gears, for example does not produce a normal thrust in either direction. For a compressor or turbine, a thrust failure can be one of the most destructive failures and there is a great need to make sure the machine trips before the rotor contacts the casing.

Johnny Pellin

 

[spciesla]

A star for Johnny - Excellent reply.