Discontinuous Consumption Processes

Discontinuous electrode consumption has been studied extensively by Lefrank et al. with the objective to improve electrode performance specifically for DC operations.¹⁰The discontinuous

140 120 100 80 60 40 20 0

16 20 24 28 32

Electrode current (kA)

Electrode diameter (inch)

DC

AC

Fig. 10.42 Current carrying capacity vs. electrode diameter for AC and DC furnaces. (Courtesy of SGL Carbon Corp.)

consumption processes are results of mechanically or thermally induced stresses which exceed the electrode strength limits. Stress levels of this magnitude occur as electrode tip stresses at the arc foot point or as hoop stresses around the electrode joints. Tip stress leads to tip spalling. Hoop stresses lead to column breakage and butt losses.

10.3.5.1 Butt Losses

Current, temperature, and stress distributions for a 28 inch electrode, operated at 120 kA under AC and DC conditions, are shown schematically in Fig. 10.43.

Under AC conditions, most of the current is flowing through the peripheral region of the electrode.

Under DC conditions, more current will flow through the electrode center resulting in an almost evenly distributed, and lower maximum current density than under AC conditions. As a result the DC electrode generates more energy inside the electrode and therefore the temperature gradient is much higher in DC electrodes. Consequently, the hoop stress gradients are much greater for DC conditions. As the electrode diameter and current loads are increased this situation will be ampli-fied further. This phenomenon is a result of the finite graphite thermal conductivity and the reduced capacity of the larger electrode for radiant heat transfer resulting from a reduced surface to volume ratio. Electrode joints are affected even more by the central flow of current in DC oper-ations and potentially, the nipple can overheat resulting in joint opening and socket splitting.

Table 10.6 R_DCand R_ACof 110 in. Electrodes

Electrode Diameter, (in.)

20 24 28 32

R_DC, microOhm 70 49 36 27

R_AC, microOhm, at 60 Hz 83 65 53 44

I_DC/I_AC 1.09 1.15 1.21 1.28

AC DC

Temperature distribution ( C)

Hoop stress (%) Current density

(A/in²)

AC DC AC DC

(DC Avg.)

0 1650

100%

0 30

2000

160%

Compression Tension

370

94

Fig. 10.43 Current, temperature and electrode stress distribution. (Courtesy of SGL Carbon Corp.)

10.3.5.2 Tip Spalling

Very high current densities develop at the arc spot on an electrode due to the self-magnetic pinch effect. For DC operations with only one electrode, the tip stresses are higher than for AC opera-tions. The arc spot will move around the tip of the electrode in a random manner but sometimes in DC operation, the arc spot will tend to fix preferentially on one location. This will result in the gen-eration of longitudinal splits and severe butt losses. Improvements in arc deflection control have improved this situation reducing butt losses significantly.

10.3.5.3 Breakage

State of the art AC operations experience few electrode breaks, typically less than two to three per month. However, frequent top joint opening and breakage some time after an electrode addition have been reported by some DC shops. It was hypothesized that a rotational arc movement in some DC operations might generate a clockwise tangential force to the electrode axis leading to the unwinding of the top column joint. For this reason, Lefrank et al. conducted a comprehensive high speed video motion analysis of the arc behavior at major DC furnace operations of different fur-nace design in the U.S.

The videos show that the DC arc spot is moving with high speed randomly over the electrode tip.

The arc spot movement seems to be influenced by the combination of totally random events with controlled, directional forces.

The directional forces are the result of the powerful magnetic fields generated by the DC current loop of the furnace. They can restrict the arc spot to a limited area of the electrode tip and may direct the arc jet preferentially to one furnace side which is shown in Fig. 10.44. Depending on the furnace design, they can also force the arc to rotate either clockwise or counter clockwise for a few hundred milliseconds, but then the arc spot will again move in a totally random and unpredictable way over the electrode tip for prolonged time periods.

The results of this study clearly indicate that a stable DC arc direction or continuous arc rotation did not exist in any of the investigated DC furnaces. Therefore, the phenomenon of top joint open-ing of DC electrode columns must be generated mainly by the absence of the self tightenopen-ing,

I

F B Reactor DC

Rectifier Transformer

Fig. 10.44 Influence of external magnetic field on DC arc furnace without electro-magnetic compensation. (Courtesy of SGL Carbon Corp.)

counter clockwise, electromagnetic force which helps AC operation to maintain tight joints as shown in Fig. 10.45.

Lower powered DC operations suffer specifically from the heat sink effect which is caused by every electrode addition as demonstrated in Fig. 10.46. At low power levels, the necessary thermal nipple expansion supporting the tightness of the top joint does not happen fast enough. The absence of the self-tightening AC effect will be aggravated under these conditions through furnace vibrations, excessive water spray cooling, and, if pitch plugs are used, through joint lubrication by the liquid pitch between 150–250°C.

For higher powered DC operations, the failure mechanism seems to be different. In this case, the contact resistance between the electrode end faces may become too high for the extreme current levels. This phenomenon can be the combined result of the missing self-tightening AC effect, ques-tionable joint design, inadequate tightening torque, or poor electrode addition practice. As a con-sequence, the current will flow preferentially through the nipple. This can lead to overheating of the nipple to its creep temperature which will result in joint breakage.

A number of counter measures can be taken to prevent these failures. The design of electrodes and nipples should be adjusted by the electrode producer to the demanding DC joint conditions (con-tact resistance as a function of surface finish, expansion coefficients, and dimensions). DC elec-trode joints should be locked by nailing after each elecelec-trode addition. DC operations should not use the electrode water spray system for at least twenty minutes after each electrode addition. DC shops, with an abnormal number of top joint breaks should apply automated, off- furnace electrode additions or use the automatic on-furnace addition devices available in the marketplace. DC shops should apply the recommended, high tightening torque moments. Table 10.7 presents published torque levels.