CAUSES OF SHORTER DELAYED COKER HEATER RUNS REPORTED

Feb. 12, 1990
Norman P. Lieberman Process Improvement Engineering Metairie, La. Feed interruptions and localized high heat fluxes are common causes of shortened delayed coker heater run lengths. These causes were discussed by operators of delayed coking units who attended a delayed coking seminar held by Process Improvement Engineering in New Orleans in November 1989.
Norman P. Lieberman
Process Improvement Engineering
Metairie, La.

Feed interruptions and localized high heat fluxes are common causes of shortened delayed coker heater run lengths. These causes were discussed by operators of delayed coking units who attended a delayed coking seminar held by Process Improvement Engineering in New Orleans in November 1989.

Feed interruptions can cause shortened heater run lengths because heater tube-skin temperatures can rise whenever feed flow is reduced or stopped. The higher tube-skin temperatures accelerate coke deposition on the heater tubes, shortening the run length of the heater.

Examples were presented that illustrate how feed can be interrupted by inadequate draining of a coke drum, level control problems in the fractionator bottom, a plugged flow-meter orifice, and by reduced steam supply to a steam-turbine-driven feed pump.

Localized high heat fluxes can cause localized heavy coking on heater tubes. High heat fluxes can be generated by attempts to reduce energy consumption by reducing the excess air in fired heaters and by plugged burner tips.

One cause of burner tip plugging is vacuum offgas in the coking heater fuel.

Thorough decoking of heater tubes is necessary to achieve long heater run lengths. The procedure for steam spalling of heater tubes to decoke them was also reviewed at the seminar.

FEED INTERRUPTIONS

Operating conditions or procedures that cause an interruption in feed flow, rapid reduction in feed rate, or erratic feed flow, will increase the tube-skin temperature of the tubes, causing rapid coke deposition inside the tubes, thus shortening heater run length.

For instance, not fully draining a decoked drum before switching can cause a sharp reduction in feed flow or stop it completely.

In a normal coking cycle, during coke drum warm-up, hot vapors from the on-line drum are backed into the top of the empty drum. The bottom of the empty drum is drained to the blowdown system.

One operator gave an example of how an incompletely drained coke drum caused a feed interruption to the coking heater. For that particular coking unit, the flow of heating vapors is controlled by the drain valve shown in Fig. 1. On occasion, the drain valve would plug with residual coke remaining in the empty coke drum.

During a drum switch, operators failed to notice that the line to the blowdown system was cool, which would have indicated that the coke drum was not draining properly. Thinking the drum had drained fully, they switched 900 F. resid feed into a coke drum containing water and light, liquid hydrocarbons.

The pressure in both the operating and empty coke drums rose to 60 psig from 20 psig because of the rapid evolution of vapor from the empty drum. The increased drum pressure in the operating drum reduced the coker heater charge significantly for about 5 min.

After the pressures in both drums had peaked, the pressure fell rapidly. The highfoam alarm on the operating drum indicated a carryover of coke into the fractionator, as a result of the rapid pressure drop in the system.

The small foam-over caused the heater charge pumps to briefly lose suction. After the coker had again stabilized, tube-skin temperatures were noted to have increased by 25 F.

It is important to determine that the empty coke drum is fully drained before switching feed to the drum. One way to check for proper draining is to ensure that the drain line is warm during draining.

Level control problems in the fractionator bottom can lead to a feed interruption. In one situation, the tower bottom's level rose above the vapor inlet, causing the coke drum pressure to rise slowly.

Although it took an hour, the pressure in one drum rose to 45 psig from 30 psig. When the higher coke drum pressure was finally noticed, the tower bottom's level was lowered to below the vapor inlet.

This caused the drum pressure to drop 15 psig in less than 5 min, causing a foam-over. The foam-over caused coke particles to lodge in the active heater charge pump's suction screen, causing the pump to lose suction.

By the time the unit operators had switched to the spare charge pump, tube-skin temperatures had risen 30 F.

Therefore, close monitoring and good control of fractionator bottom level can help avoid feed interruptions.

Feed flow can be reduced substantially by improper indication of feed flow. In one case, the orifice plate of the feed-low metering system on one heater pass was partially plugged by a lump of coke. This resulted in an erroneous high flow indication.

The erroneous flow measurement caused the pass flow control valve to close. With the valve closed, the pass outlet temperature increased, causing high tube-skin temperatures in the tubes in that pass.

All metering orifices should be checked periodically to make sure they are unobstructed,

For those operators running steam-turbine-driven heater charge pumps, steam pressure variations in the header supplying the turbine can cause charge interruptions. In one case, an operator noted a drop in steam header pressure when a full coke drum was steam-quenched.

Unfortunately on that coking unit, the charge pump turbine received its steam from the same header. The drop in steam pressure during quenching slowed the turbine and reduced the heater charge rate.

The coking unit experienced a daily increase in tube-skin temperature caused by the drop in charge rate. The problem was corrected by changing the charge pump driver to an electric motor.

LOCAL HIGH HEAT FLUX

Reducing excess air in coker heaters to lower energy requirements can shorten heater run lengths. Although reducing excess air is a good energy saving goal, it can also cause high localized heat fluxes in heater tubes, leading to coke deposition.

Any time air flow is reduced to a heater, the tubeskin temperatures of the tubes along the radiant wall of the heater will increase. This is because the flow of flue gas through the convective section of the heater drops, and the radiant heat duty must increase to offset the loss in convective heat transfer.

Also, lower excess air raises the flame temperature at the burner. This promotes higher peak heat fluxes and poorer heat distribution in the fire box.

Reduction in combustion air not only causes an immediate increase in tube-skin temperature, but also leads to a longer-term acceleration of tube coking rates.

Preheating combustion air also raises tube-skin temperatures, leading to shortened run length. For example, if air is preheated to 420 F. from 60 F., the flame temperature might increase to 2,800 F. from 2,500 F.

The hotter flame causes higher peak heat fluxes, and encourages the growth of hot spots on the tube exteriors.

Although reducing excess air to lower energy requirements can improve coker economics, that savings can be diminished if heater run lengths are shortened as a result.

PLUGGED BURNER TIPS

Flame impingement is a common problem for all refinery process heaters. Flames impinging on heater tubes can cause high localized tube-skin temperatures that lead to excessive coking in the tubes.

Flame impingement is caused by improper air/fuel mixing. Plugged burner tips are a common cause of improper mixing.

One common cause of burner tip plugging is the introduction of vacuum tower offgas into the delayed coker fuel supply. The vacuum offgas contains oxygen and hydrogen sulfide which react to form water and elemental sulfur. The sulfur deposits on, and partially plugs, the burner tips.

Therefore, when vacuum tower offgas is part of the coker heater fuel supply, burner flame pattern should be monitored closely to determine the frequency of burner tip cleaning.

ON-LINE STEAM SPALLING

Operators reported continued success with on-line decoking of heater tubes by steam spalling into the coke drum. The typical procedure is presented in the accompanying box.

Two problems with steam spalling were noted, however. One problem is thinning of the last few heater tube return bends (and especially at the outlet elbows). This difficulty was readily improved by replacing the Utube return bends with muleear type plug headers.

Several refiners had coke drum foam-over problems during steam spalling. The foam-over was caused by permitting the coke drum to run below its normal operating temperature, and then switching coke drums while steam spalling continued.

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