Electricity + Control February 2016

ENERGY + ENVIROFICIENCY: FOCUS ON VALVES + ACTUATORS

o Atmospheric pressure

- P

[kPa]

For the remainder of the day a pressure of 520 kPa will be sufficient to sustain the system air pressure requirements of the end users in this shaft. Most of the mine’s compressors are also installed at these shafts. This allowed the compressed air supply pressure set point to be regulated at the pressure required for the high pressure side of the compressed air ring. For the implementation of the ring split strategy, two control valves were required. The location of the valves in the compressed air ring is indicated on Figure 6 . Figure 7 shows a typical pressure usage profile for a normal production day of these shafts. This was also the pressure profile of the entire compressed air ring that was used before the ring split strategy was implemented.

i

o Isentropic sonic volumetric flow constant - C 1

[7,3587m/s K 0,5 ]

o Conversion constant [3600s/h] o Isentropic coefficient of discharge for square edged orifice - C d [0,8] o Leak diameter - D [mm] o Conversion constant - C 3 [10 6 mm 2 /m 2 ] It is clear from this formula that the air flow through the hole in the pipe will decrease when the system line pressure, P 1 is decreased. Pressure control methods Automatic pressure control valves are used to control the air flowing through a pipe. The valve opening can be adjusted so that pressure loss over the valve will result in the required reduced system pressure downstream of the valve. Pressure transmitters installed in the air ring are used, not only to provide pressure metering at the specific location where it is installed, but also as a feedback process variable. This feedback process vari- able is used in the PID control loop to adjust the valve opening so that the pressure loss across the valve will provide the correct, reduced downstream system pressure. Case study The surface air ring of the mine that was used in this case study con- sisted of a pipe network that was approximately 40 km long. Figure 6 is a simplified schematic layout of the mine’s surface compressed air network. - C 2

High pressure side

650.00

600.00

550.00

500.00

450.00

400.00

350.00 Pressure in kPa

300.00

250.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Time in hours

Shaft pressure

Figure 7: Shaft pressure.

The two valves are controlled from the central control room and isolate the high pressure side of the compressed air ring from the low pressure side. A low pressure set point of 440 kPa is maintained in the low pressure side. It can be seen in Figure 6 , that compressors are located on either side of the compressed air ring. The compressors located on the low pressure side are presently used as a backup in case additional air is required on the low pressure side of the

ring. Because of the location of these compressors and the fact that they were used before the ring was split, no accurate flow data was avail- able to compare the difference in flow rates before and after implementation. The effect of the reduced

pressure and air flow was ob- tained by comparing the total amount of compressed air used over a period of one month with the corresponding values when the power baseline was originally measured. After imple- mentation, the total amount of compressed air used was an average of 191 728 tons per month.

Figure 6: Surface layout of case study.

The bottom three shafts shown in Figure 6 are themine’smain produc- tion shafts. These shafts require a supply pressure of 590 kPa during peak production hours which start at 0700 and continue to 14:00.

Electricity+Control February ‘16

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