Occasional VI(Radial Vibration) spot peak in HYC R/G Comp. turbine

[1906]

Re the Hydrocracker recycle gas comp. driven by HP steam turbine, recent four VI(Radial Vibration) trends of turbine shaft show occasional spot peak around 3 times per day from normal 12~ 18 mm to peak 40 mm.(We have four VI in Turbine shaft both in exhaust and governor side). However, comp. side four VI shows no sign of spot peak, it's very stable. Re this operational phenomena, anybody who experience same problem in recycle gas comp. turbine as our VI increase, please comment me any information to solve this turbine VI spot peak.

Just for reference, there is no interlock in VI, it only generates alarm at 52 mm.

 

[JJPellin]

What is the fequency predominant during the events? What does the phase angle do? Are there any irregularities in the seal steam or gland condensor systems during the events? Does your steam system have a tendency to lay down minerals? Does the turbine have an electronic governor or mechanical/hydraulic? The items that I would consider most likely would be a governor drive coupling that is locking up. I have seen this with a gear-type coupling driving the worm gear drive for a mechanical/hydraulic governor. I have heard, but not seen that a gear coupling locking up between the turbine and compressor can show up more strongly on the governor end than the coupling end. If you have an oil or grease lubricated gear coupling, that could be an option. The drive turbine on our FCC main air blower has a tendency to build up minerals on the rotor. This has not tended to cause a vibration problem. Instead it shows up as a performance (horsepower) problem. We do an occasional on line water wash of this turbine. We put in atomizing nozzles so we can de-superheat the steam and wash the turbine on the run. We use conductivity in the condensate from the surface condensor to judge when the wash is complete. If you can provide some more information, I would be able to comment more.

 

[1906]

Re the comments of JJPELLIN, Please find information about your questions as below.
Our Hydrocracking unit recycle gas compressor use flexible coupling and electronic governor. Also,main steam has no minerals.(checked LAB data such as SiO2)

However, I need your additional explanation to understand your phrase more precisely.

q1> Does your writing "Are there any irregularities in the seal steam or gland condensor system during events?" means seal steam or gland condenser could be the causes of occasional VI(radial Vibration) peak? If it is true, would you explain why that could be ?

q2> In our manual doesn't recommend seal steam(use Low pressure steam) injection to the gland seal during normal operation. However, I found some amount of seal steam is serviced to gland seal during normal operation. So, i'm thinking of cutting seal steam just for the case of seal steam quality is not good enough. Does this action has a chance of improving VI problem ?

q3> Would you explain the meaning of "phase angle" ?

q4> Any comments would be appreciated.

 

[JJPellin]

I have seen problems with multi-stage condensing steam turbines if the seal steam and gland condensors were adjusted incorrectly. In one particular turbine this is what happened. The operators mistakenly opened a vent on the leak-off line that diverts steam from the high pressure end of the turbine to the low pressure end of the turbine during normal operation. This particular turbine is run from full speed down to slow roll several times per day. When the turbine would come down to slow roll, the case pressure would drop and cold air would be sucked in that open vent. When the cold air hit the hot shaft, the rotor would bow and the vibration would spike up very, very high. If there is any chance that vacuum at the gland packing could be drawing air in along the shaft, that can definately cause a vibration spike. For most condensing tubines, seal steam is introduced from an outside system for startup. Once you are up and running, the leakoff of steam from the packing case on the high pressure end supplies the sealing steam to the vacuum end. In some cases, if there is excess, it may be necessary to vent off some of this steam. A slight amount of steam leakage from each end is important to be sure you do not draw in cold atmospheric air along the shaft. By phase angle, I am refering to the referance angle of the vibration. If a vibration is occurring at one times run speed and the phase angle is changing, it could indicate that the balance of the rotor is changing, perhaps from mineral buildup. If the vibration is occurring at a non-synchronous frequency, and the phase angle is shifting, it could indicate some part of the machine is moving in and out of resonance. A rotor resonance could be excited by a light rub. Even a very faint rub in a shaft laby seal can excite a rotor resonance in some machines. I would have thought that this could not exist long term since it seems like the steel shaft would wear away the softer aluminum and the rub would stop. But I have seen long-term vibration phenomenon that were traced back to a light rub in an aluminum laby seal. A rub could be diagnosed by looking at the vibration spectrum and the orbit. A rub might have a frequency at about 1/2 times run speed and the orbit might have an internal loop indicating a rub with a bounce on every second revolution. It is more likely that your tubine has steel laby seals (J-strips or L-strips) or perhaps carbon rings. In the final analyis I would suggest the following:

Make sure that the seal steam and gland condensor systems are adjusted as the manufacturer recomends. Make sure you cannot be drawing in cold air along the shaft.

Examine the vibration spectra to determine the frequency at which the high vibration is occuring. Examine the shart orbit during the event to look for a possible internal rub.

When you say "Flexible Coupling" I assume you mean disk pack, diaphragm, etc. If the main drive coupling is a gear coupling, it could be intermittently locking up and causing the vibration. But I think it is more likely a rotor bow or internal rub. I almost forgot about bearing instability as a possibility. An oil whirl in a radial bearing could be an intermittent problem. This would also tend to show up in the spectra or shaft orbit.