May 22, 2017 at 2:13 pm #25519
I am currently working on the coke drum overhead line modification project to reduce the pressure drop through the overhead line to the fractionator tower. What are the design criteria for sizing the coke drum overhead line to the fractionator?
May 25, 2017 at 3:22 pm #25529
A common hydrocarbon vapor line velocity guideline which sets the line size in a new design is based on pressure drop during normal conditions. The guidance says that the pressure drop should be less than 0.1 psi per 100 equivalent feet of line. A typical maximum velocity in ft/sec is 100 divided by the square root of the vapor density in lb/ft3.
The overhead vapor line should be insulated and sufficient HCGO added as quench to lower the vapor temperature down to less than 800 F. The HCGO quench should wet the walls all the way to the fractionator. If the walls of the pipe dry out, coke will begin to form and restrict the overhead line.
Some refiners remove the insulation on the coke drum overhead line as the heat loss keeps the line wet and prevents coking of the overhead line. The heat loss changes day to night and summer to winter or during a rain storm. This excessive heat loss causes excess amount of recycle and will increase coke yield. If you are operating the Coker to maximize liquid products, the recycle should be closely controlled which will require the overhead vapor line to be insulated and enough quench added to keep the line clear.
May 26, 2017 at 2:01 pm #25539
Thank you for your response. Well, reducing the DP and velocity through the overhead line will affect the other drum warming process? Take longer to warm the other drum?
October 26, 2017 at 10:31 pm #27553
If the warm up vapors are routed back to the fractionator, the pressure drop through the coke drum overhead vapor line is a part of the driving force bringing vapors back into the coke drum to warm the drum. The reduced temperature of the coke drum as compared to the overhead vapor temperature results in condensation that reduces the pressure in the coke drum and draws vapor back into it.
Most systems have a “forcing valve” in the combined overhead vapor line. This is sometimes referred to as the SP-3 valve that is sometimes a ring valve and sometimes a gate valve with a minimum stop. I have also heard of a butterfly style valve being used in this service. The purpose is to impose more pressure drop in the combined overhead vapor line to force more vapors back into the drum being warmed and increase the rate of heating and the maximum temperature of the drum prior to switch.
The warm-up line has to be adequately sized out of the utility header to prevent a two-phase choked flow limiting the mass flow rate and the warm-up of the drum.
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