Are there any hazards associated with leaving a Coke Drum headed up with both the drain and vent open for an extended period of time without any purge steam going into the Drum? We are looking for ways to conserve energy but don’t want to risk safety in doing so.
[From the Coking.com discussion forum archives. Submitted by Coker Squirrel 20 Jul 2004]
ATC/NC coker cat: How many coke drums do you have? Extended period of time could mean you are on T/A or Heater Decoke or whatever? If coke drum is not being placed on standby mode or ready for changeover as in the case of T/A situation or Charge oil Heater Decoke, then it will be safer not to have the coke drum in warm-up mode. Your savings will depend on a specific plan scenario & operation scheduled activity. Please be specific in your query.
Coker Squirrel: We have two Coker Units, eight Drums total. We are not considering putting a Drum in warm-up early. We are only considering saving steam energy by not steam purging a Drum that has been headed up for several hours. Currently we are on 14 hour coking cycles but at times we still have several hours to spare before putting a Drum in warm-up. We currently put a minimal amount of steam into the Drum after heading up but we thought that we could conserve even on that. I have heard that there is a remote chance that a relief valve could leak back into the Drum which would cause a hazardous atmosphere in a headed up Drum. That is why we have a minimal steam purge in a headed up Drum until time to pressure test and go to warm-up. If we mitigate that remote chance of a leaking relief valve back into the empty Drum, is there any other hazard concerning a headed up Drum without a steam purge on it? Thank you, Mr. Cat.
Coker Crazy: Can you give me the Coke Drum Cycle activities in times. I wonder how much you spare time for steam purging..in 14 hrs cycle? Do U have 3 drums in one block? Why Don’t you put spare drum in Vapor heating. What is the problem in puting drum in vapor heating…? It is better to heat drum before switch over to enhance coke drum life..
Coker Squirrel: We target a 1.5 hr warm-up. We also do a slow switch so Drum integrity does not suffer, 40 minute steam quench to Frac and Blowdown, and a 5 hour quench cycle. Usually, we don’t have a great deal of time to spare but at times we have a few hours before we need to go to warm-up. It is at these times that we are considering saving energy on steam purge.
Sudhakar Bonthala @ril I have a few Q’s for you – Mr. Squirrel – 1.What is the age of your Coke drums -there are two ages -one is actual years in service -other is actual thermal cycles it is exposed to -pl. tell us both 2. Is the warm-up time 1.5 hrs from the beginning ? We now have 3.5 hrs warm up for the 16 hour filling cycle. At 14 hrs. we plan to go to 3 hrs warm-up. 3. Are your drums inspected by CII(lowering video cameras etc.)? What are the findings? Coming back to your original Q -There is one more hazard scenario. If your drum is full of coke and for any reason , you keep it headed while keeping the vent valves open – cold air from bottom will gush thru the coke bed -eventually making so much of CO + CO2. Believe me -this can turn the coke bed redhot,provided sufficient time is available.
Coker Squirrel: Four Drums on one coker were changed out in ‘2001 so they are now 3 yrs old. Four Drums on the other coker are 7 yrs old. CIA periodical checks them. Warm-up time has less of an impact on the metalurgy than the quench cycle. Strain gauging is a useful tool that will tell you this. Thanks for the info on leaving a Drum full of coke with the vent and drain open. The coke will dry out and could result in what you say. For that reason we normally will leave the drum full of water if it will be a while before drilling it. This rarely happens unless we have a maintenance problem of some sort.
bruce kerr: Mr. Squirrel, I agree with Mr. Crazy you should head up the drum, air free, steam test and warm up as soon as you’re finished decoking.As far as the safety leaking back to the drum you should block the inlet to the safety and interlock it with the vent valve. The interlock should not allow to close the vent with the PSV inlet valve in the closed position.
Coker Squirrel: Thanks for the feedback. We may consider going straight to warmup after steamout rather than waste steam. We were also looking into an interlock between the PSV’s isolation and the vent as an option. Is this what most are considering doing about the PSV’s?
Jim Blevins, Chevron Texaco: Some good comments from the others on the question of leaving the drum headed up with the vent and drain open but no steam. Here are two other thoughts; if you leave it headed up you may find you build up some liquid in the bottom even with drain open so you may get some increased foam. Additionally you may find you leave oxygen in the drum which can cause polymers to build up in fractionator top trays and finfans. Be sure to purge the drum of all oxygen before starting the cycle.
Kaz Ganji: We have tried purging the drum with superhead steam instead of saturated steam in few refineries,which resulted in less amount of steam use with same period of purging time, surely purged all the oxygen out. Superheated steam does have more than 3 times the specific value than the saturated steam (lb/ft3). Also, it did help a little in heating the drum wall. A good news is that we are offering to use a new short warm-up line rather than the current warm-up line(i.e.less than 100 ft of warm-up line, instead of more than 300 ft). Warm up time will be 30 minutes, for purging with super heated steam followed by 60 minutes of hot vapor. The vapor temperature at the top of empty drum will be more than 800 F and at almost same pressure of the live drum). Future purgin and warm up time will be one and half hour instead of 4-6 hours. Temperature will be much hotter prior to switching the drum and in a shorter time.
Coker Squirrel: Thanks Kaz. I remember many discussions with you back when you worked at our Refinery. One such discussion concerned Drum warm-up and the possibility of making a shorter route. Good information on using super-heated steam for purge but it should have some saturation to help in heat transfer on the Drum wall. Thanks for the information.
bruce kerr: The interlock system we did at Exxon/Mobil Baton Rouge has this interlock and also BP in Toledo.