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We are facing the problem of higher crude column top pressure in one of the our crude distillers. The problem gets worsened during hot ambient conditions and when bundles are in fouled condition and the same results in flaring/th'put reduction. The crude tower O/H is equipped with 06 banks of fin-fan condensers with each bank having 04 bundles (Total 24 bundles). The condensing duty is ~ 50 MMKcal/hr at design throughput and crude blend. Now, we want to expand the fin-fan capacity by adding one more bank of 04 bundles to reduce the column top pressure from ~ 1.2 to 0.8 kg/cm2g. However, we doubt that, this may aggravate the process side fouling as the velocity for each bundle will reduce. Also, the piping for new bank will not be symmetric and it may cause new bank to run cold & dirty. The present fouling pattern or performance of banks support this with extreme end bundles (last 8 bundles) taking less load and running colder than other 16 bundles. At the current load the inlet velocity is in the range of 24 m/sec. What should be the min recommended velocity in crude column O/H condnesers? What is the best strategy for expanding the capacity of crude column overhead condensers?
 
Answers
05/09/2009 A: Mike Watson, Tube Tech International Ltd, mike.watson@tubetech.com
Your comments are well founded, as low velocity has a definite effect on fouling rates. If average thru tube velocity is 0.5m to 1m per second fouling tends to occur. Anything over the magic 1.5m to 2m/s certainly helps minimise fouling. We are developing an on-line mechanical cleaning system that overcomes this very problem. What helps in the meantime, is attention to detail in cleaning i.e. a more regular cleaning campaign BUT using very quick technology providing a genuine back to bare metal condition. Traditional jetting of a O/H typically takes 1 to 2 days. TT technology cleans 2 O/H per day and combined with unique polishing technology minimises a "key way" for future fouling. A regular cleaning cycle at the right price, in return for the "right" technology will improve heat transfer and improve production. A little advice and check the headers "after" a contractor has jet cleaned the tubes. You may be surprised what is left behind. Many jetting contractors and clients alike forget "headers also need to be cleaned" otherwise sediment remains, is kicked up and compounds the fouling rate.
03/09/2009 A: Marcello Ferrara, ITW SrL, mferrara@itw.it
I think, the quickest way is to solve the fouling problem. We might provide an online/on-stream way for cleaning the coolers.
03/09/2009 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
The best way to keep the bundles clear and to maintain heat transfer is to limit overhead velocity to < 20 m/s and to have a good water wash at the inlet to each bundle. If the velocity is too high, you can get erosion corrosion and the wash water can cause an increase in corrosion rate. Ideally you would also have a water wash near the top of the of the main vertical run of the overhead line to scrub the bulk of the chlorides out of the vapor. The individual water washes at the inlet to each bundle would then scrub any residual chlorides out and keep any salts formed washed out of the exchanger. Minimum vapor velocity should be 10m/s to ensure that the velocity is high enough to entrain the water droplets from the water wash.
With such a large overhead air cooler bank and non-symmetrical piping there is no way that you will get even flow distribution. Water wash should be injected via spray nozzle into the center of the pipe and in the direction of flow. You want to get as small of droplets as possible to maximize the ability of the vapor to carry them into the exchanger tubes evenly.