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Level Detectors: NBS vs O/V Continuous

Home Forums Coking Design and Reliability Cokedrums, Structure, Inspection Drums Level Detectors: NBS vs O/V Continuous

This topic contains 2 replies, has 2 voices, and was last updated by  antonio saura Cebrian 12 years, 10 months ago.

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  • #3693

    Lucibar Davalillo
    Participant

    Does anybody have both, Continuous and NBS level detectors installed? If so, could you tell which one is better for coke/foam detection and/or antifoam usage? do you see any advantage of having both? or would continuous + density point detectors be enough?
     
    My understanding is that continuous do not differentiate between light/heavy foam, so may cause higher than required antifoam injection.
     
    Can anybody help or comment on this?

  • #6930

    We have installed Ohmart Vega system, and  the advantage is that the operator can monitor the level change throughout the entire measurement range of the continuous detector system.  There will be no blind spots along the continuous level measurement range.  It is also important to remember that we are looking at the top of the process as it rises in the drum.  If the top of the process is foam, we will see that.  If the top of the process is liquid with no foam, we will see that.  After charging and as the drum is filled with water, we will not see the water rising in the drum until the water level is above the coke bed and becomes the top of the process.  the  system measures the level and does not provide a distinction between foam, liquid hydrocarbon, coke or water.  You will always know where the top of the process is at all times.
     
    Also, another advantage that I´ve heard are:
     

    Once the operators understand the continuous level system and gain confidence in its ability to track the leading edge of the process repeatedly (whether it is foam or liquid hydrocarbons) they will understand it is safe to push the level in the drum (decrease the outage) resulting in significant increase in the thru-put of the drums over a year’s time.  With the top gauge mounted as above there will be no need to alter the physical location of the continuous level system.  The operators can simply increase the level with confidence.  With a point detector system, if the end user wants to increase the level in the drum, he must physically change the location of the point detectors on the drum and reinstall the point detectors at the new desired level.  That becomes expensive.
    With longer lengths down along the side of the vessel, the operators will have a larger picture into the process.  This can permit your organization to optimize the charge rates with the benefit of decreasing the cycle time of the drum which can increase the thru-put.
    By knowing the level sooner, you can look at the effectiveness of anti-foam injection sooner.  Different crudes will have different foaming characteristics.  If you wait until the level gets to the top 1/3 of the drum with a very fast foaming blend of crude, you may have to inject a large amount of anti-foam quickly to break down the foam front in the top of the drum to avoid a foam-over.  If you could see the effectiveness of the antifoam earlier in the cycle, you might adopt an intermittent philosophy of anti-foam over the top half of the drum thereby reducing overall anti-foam usage.
    Decreased catalyst usage can be significant.  using this system some people said that they have saved a lot of money in reducing the frequency of downstream catalyst change-outs.  By effectively reducing the amount of antifoam being used on each cycle, the amount of silica being carried over is reduced, so the effect on the downstream catalyst is reduced, creating longer life of the catalyst
    Some refinery have utilized the Vapor Density Compensation gauge this monitor process carryover into downstream equipment.  By running and monitoring the 4 to 20 madc from this gauge, the end user will see that as the drum gets full near the crossover point into another drum, the vapor density will increase dramatically.  As that gauge output goes from 4 to 20 madc, it indicates that more particulate matter is potentially being carried over.  This carryover of matter, whether coke particles, or anti-foam material will cause fouling of the downstream equipment

     
     
    Best Regards

  • #6928

    Lucibar Davalillo
    Participant

    Thanks for the good information.
     
    Any criteria or best practice for their location? I mean, something like: lower at 25% of normal coke level or coking cycle, mid at 50%, etc..?.
     
    Thanks again,
    Lucibar

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