March 10, 2017 at 2:02 pm #25028
Our drum quench pipe is suferring a huge corrosion. The corrosion seems to be by aeration corrosion. The pipe is carbon steel. We need to replace part of the pipe each 3 years.Is it common in other units ? What is the solution ? Replace by another material ? Is there any spec for the water quality including the decoking system ? Chloride ? NH3 ? Coke fines ?
March 12, 2017 at 10:30 pm #25043
Corrosion in the quench and cutting water system is fairly common. Oxygen dissolved in the water is one of the reasons and that leads to deep, sharp pits in the top of the pipe or any connection that comes off the top of the pipe. In addition to oxygen there is H2S dissolved in the water. At ambient conditions, H2S, oxygen and liquid water will form elemental sulfur which can be corrosive.
I am aware of the use of duplex stainless steel, 2205, for branch connections and areas that have had repeated corrosion failures with good success. Austenitic stainless steel, either 304 or 316 (I think both have been used), has been used in the decoking equipment to reduce corrosion. I believe that constructing the entire decoking water system from those materials would be too expensive and difficult to justify.
Reliance in India published an article in Hydrocarbon Processing in May 2008 on this subject. They tried an oxygen scavenger in the decoking water system and that lowered the corrosion rate significantly. Even so, the corrosion rate was higher than desired so they added a second chemical and the corrosion rate dropped to acceptable levels.
The primary cutting water quality concern is the amount of coke fines as they will cause erosion in the cutting equipment and will deposit in areas of low flow and inhibit performance of the decoking valve and cutting tool. A suspended solids content of less than 500 ppm is considered to be good andn lower is better. Routine analysis of the cutting water should be done and analyzed for pH, Fe, H2S, NH3, Cl, suspended solids. Weekly analysis is adequate.
March 13, 2017 at 10:42 am #25046
How can I measure the suspended solids in the water decoking ? Any specific method ? The coke fines is decanted in the water. And the other contaminants ? Is there any spec ? We analyse chloride, NH3 and H2S in the water.
March 14, 2017 at 5:12 am #25048
There are several methods to measure total suspended solids in water. They all give similar results for small to medium particles. The methods are; 1) USEPA method 160.2, 2) APHA 2540 D, 3) ASTM D-3977. The ASTM method is the most accurate, regardless of particle size.
All three methods filter water through filter paper that is weighed before and after with the solids are dried.
Typical water quality to minimize corrosion is to have the pH between 5.5 and 8.0, chlorides less than 50 ppm, oxygen content less than 20 ppb. I have seen H2S content in the cutting water tank at 5 ppm. Lower is better. Iron content should be less than 1 ppm, but the trends are the important data. If the iron content is 5 ppm for several weeks and then increases to 10 ppm, that is an indication of a change in corrosion in the water system. There should be a correlation between NH3 and H2S. If the ammonia is high, then the H2S will be high as well. Again, the trends are the important part.
March 14, 2017 at 3:52 pm #25049
Our tipicaly data for Chloride is 50 – 70 ppm, pH = 9, NH3 = 2, RSH = 2 and now we have a huge problem with coke fines. This coke fines are plugging the decoking valve in our pump decoking. Also, the Rotary joint are leaking due this fines.
March 14, 2017 at 7:01 pm #25051
Please paste the entire link, all the way through to .pdf in your browser. That should work.
Your water problem seems to be coke fines. The maze should be dredged out daily until the cutting water quality improves. Additionally, the cutting water tank should be purged to remove the coke fines that made it to the tank. Agitating the tank and then draining from the bottom back to the pit will move fines out of the cutting water tank. If the maze is sized properly the coke fines will settle in the maze once it is clean and not make it to the cutting water tank.
March 14, 2017 at 7:16 pm #25052
Just correcting the link. Great paper
March 17, 2017 at 6:58 pm #25078
In order to improve the decoking water quality we are doing purges in the cutting water tank more often and clean more the pit. What we can do to remove more coke fines in the cutting water tank ? Anything in the DCU process ? Is it common to use hydrociclone in the water to the tank ? Centrifuge ?
March 21, 2017 at 7:23 pm #25136
Hydrocyclones have been used to remove solids from the cutting water before storage in the cutting water tank. I would not say that they are common. The flow rate through and pressure drop across the hydrocyclones in important to their efficiency, so the control scheme has to take that into account, if you plan to install them.
A common filter press can be used to remove solids from the system. Other types of filters have been used to remove solids from the cutting water with varying success.
If the drum temperature is not hot enough during the coking phase, sandy coke can be produced that makes lots of fines. A target drum outlet temperature of 440 C should be high enough to fully convert the hydrocarbons in the drum. This target temperature may need to be adjusted depending on feed type and recycle ratio. A high recycle ratio will require a higher drum outlet temperature. A high concarbon feed and low recycle does not need as high of a drum outlet temperature, but 440 C is a good target to start. Measuring the coke HGI (Hardgrove Grindability Index)and coke VM (volatile matter or VCM meaning volatile combustible matter) will provide data to tell if the drum is hot enough during the coking phase.
Coke cutting technique can increase the amount of fines generated removing the coke from the coke drum. Removing coke in as large of chunks as possible should minimize the coke fines and minimize the time to remove coke. Even so, there will be approximately 30% fines in the coke pit so managing the maze and purging the fines out of the cutting water tank are important to cutting water quality.
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