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We are facing problem of shorter run length of our Delayed Coker Unit(DCU). The furnace coil inlet pressure increases to design limit within a very short interval i.e. 30-35 days after the SAD, however the tube skin temp remains well below the design limit.
The feed to the DCU unit is VR from VDU, having asphaltene about 5% & CCR about 17%. The COT, BFW flow and other operating parameters are maintained as per the design operating guidelines.
Can anyone help to identify reasons for the drop in the run length of our DCU?
 
Answers
02/07/2019 A: John Bettinger, Qualex Engineering, john.bettinger@qualex.com
I would look downstream of the furnace to see if some restriction in the transfer line to the drum is the problem.
28/06/2019 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
The asphaltene and concarbon numbers are fairly typical for a DCU. I suspect most likely cause of rapidly increasing furnace dP is accelerated coking due to localised high film temperatures (caused by flowmeter error, pass flow imbalance, uneven firing, flame impingement, or inadequate/incomplete steam-air decoking).
27/06/2019 A: RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com
Please recheck your feed side properties especially salt content. Also, please check if there is a change in the feed side conditions? Like increase in CIT? What is the crossover temperature? Are you using any additives?
27/06/2019 A: Dipankar Phukan, Indian Oil Corporation, dipankarphukan79@yahoo.com
Just an experience I wanted to share, one of the visbtreakers I have worked in had high Start of run delta P which quickly reached EOR condition with abnormally short run length. During shutdown, the heater tube bends were first radiographed and it was observed that that there had been an abnormal bulging in the tube, what seemed like oxidation of tube metal thus restricting the flow path. This was peculiar and although we ended up never running running that Visbreaker after that turnaround as a new DCU had come up to take its feed, you may compare the start of run delta P over a number of shutdowns to analyse if its increasing gradually. Repeated and prolonged steam air decoking may end up oxidizing the tubes which we suspect in our case.

Having said that, you need to ensure the cold tube velocities are within range, 6 m/sec is normally the desiqn, thermocouple map of the heater is adequate to monitor each zone properly. Increasing the turbulising water flow upto 2 to3% is recommended and found to be favorable as per available literature as well as personal experience.
27/06/2019 A: keith bowers, B and B Consulting, kebowers47@gmail.com
There are two possible reasons for the indicated high coil pressure ---faulty pressure indicator, or flow restriction in the flow path of coil or transfer piping.
The first thing to verify is the coil inlet pressure indicator. The second is to verify the drum inlet pressure.

Partial plugging of the transfer line between the coil and drum are very common. ALL the piping between the pre-heat exchangers and coke drum proper is subject to obdurate coke deposits, including pressure relief piping and relief valves. Some piping designs are more prone to plugging and require mechanical cleaning every 'run'. Severe plugging in pressure relief piping and relief valves is all too common and can allow dangerous over-pressure accidents.

Best Practices call for 'rupture disc' protection of pressure relief piping and relief valves of Delayed Coking units.
27/06/2019 A: Bruce Carr, Suncor Inc, bcarr@suncor.com
My first guess would be poor heater operation... isolated hot spots. I'm assuming the skin temperature is measured by thermocouples welded to certain points not representative of all the skin. How effective is your SAD? Is there a chance the tubes are not free of coke or ash.... Have you tried pigging... then you would be sure the tubes are clear.....