Some weeks ago we saw some cracks in the FCC expander blades in one of our FCC units. The cracks appeared suddenly, from one month to another.
The fresh catalyst addition rate are very low, so catalyst turnover is slow. It has provoked the ageing of our e-cat inventory. We have measured the attrition of the e-cat, with Jet Cup method (Davison Index), and there is a decrease from 2-3 to 1-2. My question is could this decrease in DI of the e-cat (harder catalyst) be responsible for the mechanical problem in the expander?
NITIN KUMAR KANJELE, LIL, email@example.com
I believe it is the process conditions but neither may be the hardness nor may b the flow rates which has effected cracks on the expander blades. As expanders work on the principle of isentropic expansion and thus the blades are designed for these conditions,most likely the pressure,temperature and flow would have been the main reason for causing the cracks.
Please refer the following:-
a) Considering Filter or Separator upstream the expander:-The hardness of the catalyst carried over would have definitely effected the filter material or efficiency with that flow rate.Please check it.
b) If some how the harder catalyst would have entered the expander casing,definitely it would have caused excessive vibration because of the speed of impact on the blades in the expander and the expander would have tripped.It is very much like a dirty steam enters a steam turbine and the vibration monitoring system trips because of excessive vibration.
c) The excessive windage would have created the wind-wall effect and prevented the turbine from delivering the desired flow.The back pressure and the excessive load on those high speed blades would have caused cracks on the impellers.
d) Remember,the amount of expansion(i.e. the amount of isentropic reaction which an expander creates) is a critical point to decide the metallurgy of the blades.If the blade material or geometry is not meant for that duty,it will fail very frequently which I doubt the most in your case.
e) In worst case scenario,it may be the the accessories coupled with the expander, remember expander are very critical to what they are coupled with.For example where the turbo expander which is coupled with a generator will act as a power inducer if operated below its destined rpm.As i guess in FCC it may be coupled with an air compressor (steam turbine driven, very critical) which is feeding the regenerator with air?
More or less, I clearly doubt the balance of the reaction rather than the geometry and metallurgy provided that the actual case with complete datasheets and process conditions shall be shared with me with your concern and with not liability to anyone.
keith bowers, B and B Consulting, firstname.lastname@example.org
Catalyst hardness and particle size distribution (toward a double hump size distribution) could cause increased erosion of the expander blades if catalyst carry-through to the expander increased.
However, that would not cause blade cracking --it would cause erosion. A severe temperature excursion such as an 'afterburn' could cause severe expander blade damage in just a few minutes.