Q & A > Question Details
Following is a brief overview of the problem we are currently facing at our Diesel Max Unit (Mild Hydrocracking Unit),
Incident:
Our Diesel Max unit reactor having four beds, is equipped with three Quenches. MV for the 3rd Quench Gas Flow Valve started to increase and reached to a maximum value of 100% within 08 hrs. With increase in the MV opening of this Quench valve, flow across the valve remained consistent initially at around 4700 - 5000 Nm³/hr and then gradually started to decrease to a much lower value of 3,100 Nm³/hr at 100% opening at DCS at 70% Unit load.
Observations:
Field observation was taken for the Quench valve and maximum opening found was 85% from field.
Field observation for any abnormal sound across the NRV was checked and found normal.
Similarly, Pressure drop across the reactor in the field on local PIs and across DPT,Delta T and Radial Spread across the reactor beds is observed and found no abnormality.
Actions Taken:
FT installed at the Quench valve was also drained and purged and found no error.
Unit load reduced to turn down ratio.
2nd Quench Gas flow at bed#3 increased to compensate for the reduction at Bed#4 Quench (3rd Quench).
Your expert opinion and guidance is requested on the Issue.
 
Answers
24/05/2011 A: Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
Have you attended the control valve to match the opening with DCS and field opening? All your quench flows are from the common header which comes from RGC? If so, the problem could be partial blockage in the 3rd quench system.
Generally quench control valves' plug are moving up and down in a cage like fitting and the cage is provided with mesh to allow only gas not any particle. If your quench control valve has such an inner arrangement, then possiblility of foreign particles entering your quench distributor is remote.
You can check for the following to identify the problem:
1 Establish flow through quench bypass, close the particular quench control valve gradually and try to identify with your old data for the flow through it for a particular opening.
2 If flow is ok then the problem could be with the control valve. In that case, isolater the control valve block valves at u/s and d/s and drop the control valve for cleaning. Before dropping confirm that your block valves are not passing.
3 If there is no blockage in the control valve, then the problem could be in your quench line itself.
4 Ensure that flow problem is not there in other quench valves and the valve openings are maintaining in the same level as you had when there was no problem. If so, then try to confirm the delta pressure across the valve and line of problematic onel. It is not easy but if you have any opening like drain or vent across the control valve or better if it is available little far away from the control valve then pressure gauges can be fitted temporarily. Calculate the delta pressure. Compare it with old records. Come to a conclusion as whether the block is in the control valve or in the line itself. If it is in the line, then it is only a shut down job.
21/05/2011 A: Rajesh Gajera, Essar Oil Limited, rajesh.gajera@essar.com
Are your plant just commissioned ? If yes, then there is chances of plugging the quench nozzle, stuck up of material in NRV and\or control valve due to improper flushing of line. Have you checked your H2 supply pressure to reactor quench ? Is there any press. variation in your RGC dis.?
21/05/2011 A: keith bowers, B and B Consulting, kebowers47@gmail.com
If the 'field determined values' are believed accurate, then it seems the transmitter to the control system computer (DCS?) might be faulty--sunlight heating it up etc.? or the DCS input point hardware is faulty.
I suggest replacing the all the instrument hardware (electronics) and renew all the connections associated with this very important Quench Control circuit. It would be proper to establish a written procedure for Operators to cross check any such irrational or suspect indication against bed outlet temperatures (inputs to the Quench Controller) and the downstream -lower bed top temperatures to see if they are indicating increased temperatures and demand for more quench.
This is a situation in which extreme caution and precautionary reduction in reactor inlet temperature is appropriate until multiple 'back-up' and alternative data points positively confirm all is really OK and the Quench valve position reading is faulty and flow indicator is also suspect. A few hours of reduced hydro-cracking is a very small 'insurance policy premium' to pay when contrasted to the potential for cataclysmic damage /destruction of the unit and possible loss of life if the reaction 'runs away.' Having watched in horror as bed temperatures in a Unicracker climbed past the indicator/recorder upper limit of 1200F with a very steep slope on a recycle gas failure and then depressuring system failure, I strongly urge immediately reducing temperatures in the beds above and making absolutely certain the reaction is fully stable and under control before resuming normal conditions.
In the actual case above, the unit was out of service over 6 months while the sintered catalyst was mined out with jack-hammers, warped and sagged bed supports, distributors, quench distributors etc were re fabricated, replaced, new catalyst obtained, reactor re-certified (acoustic emission testing using high pressure nitrogen as test fluid). The root causes--a technician dropped a wrench on the unshielded vibration detector on the recycle gas compressor causing an immediate trip AND the safety blind at the exit of the depressuring valve header had not been removed following the just completed catalyst change-out.


One would not normally desire to operate with the Quench valves open more than 50% of max flow rate to ensure adequate control range is available to mitigate an incipient 'runaway'.
21/05/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Unfortunately, data observed after the feedrate was reduced does not help much due to the reduced space velocity. Question: When the quench flowrate was decreasing while its control valve was opening, what was the temperature that drives the quench flow doing? If it was increasing, a blockage, temporary or otherwise, may have been occurring in that part of the quench system. Is there a sparger with holes, wrapped with screen, maybe, that could have experienced a restriction?