A large carry-over of coke or partially coked resid from the coke drum is called a foamover. Preventing such foamovers is vital to continued opera­tion of a delayed coker. Depending on the volume of material carried over, the effect on unit operability will range from bad to disastrous. Several problems are initiated by foamovers.

Coke lay-down in the coke drum overhead vapor lines. This will cause an increase in the coke drum operating pressure. Reduced unit capacity may result.

Partial plugging of the combination tower bottoms screen. This screen keeps large pieces of coke out of the suction of the coking healer charge pump. Figure below shows the effect of a carry-over on the screen. A minor foamover will plug its bottom section. A symptom of this is the need to raise the combination tower bottom’s level to secure a steady flow of liquid to the heater’s charge pump.

coke drum carry over

Complete plugging of the combination tower bottoms screen. The unit must now be shut down to clean the screen. In some units, this screen is 25 ft high, but a serious foamover can plug it in one day.

Coking heater tubes are coked. A sudden loss in heater feed will lead to coking of the tubes. If the firing rate is rapidly reduced, severe tube damage may be avoided. One can count on the need to steam-air decoke the tubes after the coker is forced down because of a foamover.

A massive foamover will carry coke well up into the combination tower. If the coke gets up into the trayed section, many days will be required to clean the tower before it can be returned to service.

Plugging the blowdown system. After the drums are switched, the full drum is vented to a blowdown system. If the drum then foams over, the entire blowdown system can be plugged. As several pairs of coke drums will often share a common blowdown system, this type of foamover can shut down an entire coking complex. This unfortunate scenario actually happened to one six-drum unit.

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