Anti foam is a unique problem to many cokers and we had many discussions at the coker I worked at about it. We had more problems with foaming toward year end, when RP&S (receiving, Pumping and Shipping) would cook up “cocktail” crudes for us to run and sometimes we’d have light materials mixed into the crude slates as well as crudes with some chemicals in it from normally downstream units. That having been said, under “normal” if there is such a thing on a coker with the varying crude slates, we determined that the coke drum builds a head of foam right from the start of charging it and the foam level (as mentioned above) varies anywhere from 2 to 10 feet and sometimes more. A major determining factor as to coke foaming is the temperature at which you start charging the coke drum. The hotter the better, thence a longer warmup time is the best. Now with shortening cycles, sometimes coke foaming is a trade off for shorter cycle time. Also, anti foam consideration needs to be addressed so that you don’t contaminate your hydrotreater catalysts or your reforming unit catalysts. After all that anti foam has to go somewhere once it’s used in the coke drum and goes in the overhead back to the fractionator on most if not all cokers. It is going to go overhead with the naphtha streams and where is it going to end up? Best case scenario is that you don’t use but a minimal amount of antifoam after the drum you’re switching into in already into warmup and getting ready to switch into it. Worst case is you put antifoam into the drum halfway through it’s charging cycle due to terrible back foaming from raunchy crude slate cocktails and insufficient drum warm up before switching into it. Good luck with your antifoam endeavors. Hope this helps.