ROTATING THROAT ASSEMBLIES

The throat slot (throat plate ring as seen in figure 6) openings in a mill act as a venturi and produce very high PA velocities (up to 80 m/s), which creates primary pulverised fuel (PF) classification. This action causes extreme erosion on the throat slots, which in turn leads to intense maintenance. The maintenance is intensive and very time-consuming, as the throat slots need to be welded up, cut out and/or replaced. Where throat plates are welded up, the maintenance (welding inside the mill) also poses a possible health risk to the welding personnel performing the work.

Figure 4 - Typical stationary mill throat (throat plate ring) in a Babcock & Wilcox E-Type Mill

A Rotating Throat Assembly (RTA) in turn is where the rotating throat is fixed to the mill table by a clamped or bolted arrangement. This throat rotates with the mill’s table when in operation and has an air seal on the ledge cover. Refer to figure 7 for a general arrangement drawing of a RTA.

Figure 5 - Schematic drawing of a Rotating Throat Assembly (RTA)

When rotating throats were first introduced into South Africa, the initial objective was: “The use of Rotating Throats to enhance Milling Plant Performance”. This implied that with the

introduction of the RTA’s there would be a positive performance improvement associated with the upgrading to a rotating throat assembly. Rotating Throats were tested on the following power stations for possible performance improvements:

• Hendrina. • Arnot. • Duvha. • Kriel. • Matla.

The initial key questions of the research however addressed the final conclusion and focus of the “modified research objectives” very well. This is where the research project moved from a performance (PF fineness) and throughput point of view to a maintenance point of view. The reason for this was that the performance improvement and throughput of the RTA were not conclusive on all installations tested.

Figure 6 - Rotating Throat Assembly in a Babcock and Wilcox E-type mill

The current maintenance philosophy being applied on the B&W mills at Kriel Power Station is time based (used based maintenance with scheduled interventions), with a mill service interval of 5 000 hrs. This interval is derived from the level of wear on the throat slots as well as for the replacement requirement of the mill wear plates. This is however not the only maintenance intervention needed on the mills, as the mill ball wear rate also impacts on the

Mill Ball Bottom ring Ledge cover (stationary) Throat vanes (rotating)

At the start of cycle 0 (zero), the mill is equipped with new grinding rings and 11 off steel balls. The mill balls are worn down from φ 768 mm to an intermediate size of 690 mm. This set of balls is then removed, stored and replaced with a new set of φ 768 mm balls which will then be referenced as cycle 1. This is repeated two more times till the end of cycle 3 is reached at a ball size of φ 690 mm. Now the φ 690 mm balls are not removed, but an additional 690 mm ball added. There are now 12 (twelve) 690 mm balls for cycle 4 (four) that are worn down to 640 mm which is the ball scrap size. The 640 mm scrap balls are removed and then replaced with the 10 off balls from cycle 0 plus 2 balls from cycle 1. This process continues till cycle 6 is reached on the mill. At the end of cycle 6 the ring is usually also at the end of its life and the mill grinding media (rings and balls) are then replaced with new rings and balls where the mill cycle starts at cycle zero again. A typical ring life for a Kriel 10.8E B&W mill is 60 000 operating hours. This equates to a wear rate of 980 hrs/mm.

A typical wear rate on a steel cast mill ball on a Kriel 10.8E B&W mill is 105 hrs/mm. This equates to operating hours for each cycle from 0 to 3 to [(768-690) x 105] 8 190 hrs. A factor that will override a mill service before the typical 5 000 hrs services will be the ball size. A mill in cycle 2 for example was serviced at 5 000 hrs, now only (8 190 – 5 000) 3 190 hrs later the mill has to be opened for the mill balls to be changed as part of the ball cycle regime. Usually the maintenance department then does a complete mill service as is prescribed for every 5 000 hrs. Logically this does not make any sense and the aim should be to only service the mill the next service interval of 5 000 hrs is reached. Due to the cost of manpower and the time it takes for an activity like the opening of a mill for a ball change, the maintenance activity is “drawn forward” to be performed as part of the ball change/add.

Figure 7 - Schematic representation of the mill cycles

768-mm (11 off balls) 690-mm (12 off balls) 640-mm 0 1 2 ₃ Ball add 4 _{5 6} Mill cycles

The utopia will be if mill maintenance intervals and ball change/add activities could be coordinated. This implies a condition-based approach as the wear rate on the mill balls vary because of changing coal qualities, different mill cycles, various mill conditions, etc.

4.2.1 PROCEDURE FOR MILL THROUGHPUT / PERFORMANCE TESTING Before the test procedure is discussed, it must be reiterated that there is a definite split between mill performance and throughput of a mill. A basic definition of the two terms as follows:

• Mill performance is the fineness achieved in the pulverising process.

• Mill throughput is the tons of coal pulverised per hour by the mill in question.

By carrying out performance tests a mill would be tested before a rotating throat installation and the same tests repeated after the RTA installation. This methodology to indicate performance parameters/figures before and after the modifications. With both sets of results a proper evaluation can be performed on the modification’s impact.

Typical tests that were performed as part of the test program are: • Clean Air Curve.

• Load Line. • PF Fineness. • Coal Analysis - Abrasiveness Index. - Ash Content. - Total Moisture. - Surface Moisture. - Bulk density.

- Calorific Value – Coal.

- Calorific Value – Rejects.

- Hardgrove Index.

• Mill reject rate.

• Mill Power Consumption.

• PA Fan Power Consumption (only where dedicated PA Fans per mill).

For the analysis of performance and throughput of a mill, two Kriel Power Station mills were identified for testing after the testing on all the vertical spindle mill power stations were complete. The reason for this was twofold:

(i) No coal samples were taken during previous tests (if they were taken, they were not analysed or were not conclusive).

(ii) To be able to compare “apples with apples”, it is important to look at the complete mill system. This comment reflects to the system resistance of PF pipe work from the mill outlet to the PF burner inlet. Two identical mills with the same PF outlet system needs to be considered for analysis, otherwise false results will be obtained. The ideal situation would be to test all the performance parameters on a mill with the stationary throats, shut it down, install a RTA and then re-test. Due to installation pressures the Plant Manager did not have the luxury of having a mill readily available for testing before and after a RTA installation.

4.2.2 MAINTENANCE INTERVAL TESTING ON RTA’S

One of the most important aspects of an RTA installation is the life of the new technology compared to the original stationary throats. As was shown in section 4.2.1 the stationary throats needed a preventative maintenance intervention every 5 000 hrs, otherwise the mill’s performance becomes unacceptable. With a RTA installed the trend on the performance of

the mill as well as the wear on the hardware needs to be tracked. Consistent performance results over time will indicate a superior product compared to the stationary throats.

The key questions in section 5.4 introduced 4 (four) other technologies to be able to extend the service life of a mill to be able to go beyond the indicated 5 000 operating hours. These technologies mentioned are:

• Mill spider the wear plates

• Mill loading cylinders

• Classifier cone

• High chrome mill grinding media

These technologies do not impact on the performance or throughput of the mill directly, but will support longer maintenance times for maintenance activities or interventions. Each of the new components will be tested and compared to the old/original installed component’s lifetime. This to test the technical success of the modification.