A S Sahney - Indian Oil Corporation Limited, Gujarat Refinery
N Venkatesh - Indian Oil Corporation Limited, Gujarat Refinery
The coke drum in a Delayed Coker Unit is subjected to continuous thermal stresses in a cyclic manner during the heating and cooling cycles. As a Coke drum is faced with severe temperature variation over a period of few hours and the same cycle is repeated over the years therefore, the Rate of Change of temperature of the Coke Drum walls is an important parameter and requires strict monitoring with the help of Skin thermocouples. Abnormally high ‘rate’ of temperature increase (during heating) or cooling (during quenching) implies a ‘severe’ operation and has a detrimental effect on the life of the equipment. Monitoring of the parameters around the Coke drum on a continuous basis is important as stresses developing due to the cyclic changes in these might lead to drum deformation at very early stage.
Close study of patterns of the coke drum skin temperature give a fair idea on the levels of stress the drum is subjected to. Analysis of the data collected from the field helps in identifying the problem and formulating a mitigation plan. From the operation perspective the actual gradients are to be smoothened to the extent possible without disturbing the Coke Drum cycle schedule.
Skin temperature indications are to be segregated and analyzed based on their location on the coke drum to zero down the exact time of the problem based on the coke drum cycle. On finding the exact time of the problem based on the drum cycle, required change in operation methods can be done.
Rate of vapor heating, time taken for Drum Switch, rate of water quenching and amount of water used as quench are adjusted based on observations made in the skin temperature monitoring reports.
Variables available for introducing changes in the coke drum skin temperature gradients leading to reduction in the rate of deformation of coke drums are steaming rate, quenching rate, introduction of water + steam cooling if not being used etc. The reduction in deformation rate can be monitored by the results of laser scanning of the drums. This has been done in one of our refineries and improvement was observed.
Present paper shares our field experience in an operating Delayed Coker Unit over a period of 2 years and effects of modified operating parameters in the cooling/ heating cycles of the coke Drum.