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Plant Engineering: Mines
GCC: Electrical & Mechanical Engineers
Past Examination Papers
June 1995
Question 1:
The following data applies to a double drum automatic rock winder:
Mass of empty skip ………………………………………………………………………….. 8,225 t
Rate of acceleration (uniform) ……………………………………………………………… 0,9 m/s2
Rate of acceleration (fast braking uniform) ……………………………………………….. 2,5 m/s2
Length of rope (from drum, over sheave to top of skip with skip in tipping position) … 70 m
Rope breaking load …………………………………………………………………………. 1,715 MN
Rope nominal diameter …………………………………………………………………….. 46 mm
Rope mass …………………………………………………………………………………… 9,289 kg/m
Rope construction …………………………… 6 x 31 compound triangular strand with E = 103 GPa
After a skip is tipped, the winder begins the acceleration cycle but the empty skip remains stuck in the tipping path. The winder only trips out 6 seconds after the commencement of acceleration. Shortly after the winder is brought to rest, the skip breaks loose and falls.
Calculate the maximum force exerted on the rope and explain what the result of the slack rope will be.
[20]
Question 2:
The following data applies to a double drum, twin motor, geared AC winder:
Motor voltage …………………………………………….. 6,6 kV
Motor power ……………………………………………… 1,95 MW each
Pinions ……………………………………………………. 32 teeth
Pitch of gear ……………………………………………… 44,45 mm
Bibby coupling ……………………………………………. 80 slots
Outside diameter of slotted portion …………………….. 560 mm
Both motors are controlled through a common single liquid controller. Due to an earth fault on one of the motors, the spare stator has been placed in position.
(a) Explain in detail how you would proceed to connect up and re-commission the winder using the spare stator.
[10]
(b) When attempting to phase in the two rotors, it is found that the slots on the Bibby coupling are 3,8 mm out of line. Explain how you would proceed to phase in the two motors.
[10]
Question 3:
(a) Name the elements of Risk Management and discuss how a risk management programme may be used to enhance safety and health in the mining industry.
[6]
(b) Describe how you, a member of senior management team on the mine, would start to introduce a new safety culture onto the mine.
[6]
(c) Name the dangers of diesel engine exhaust emissions and describe how these can be managed to within safe and tolerable limits.
[8]
Question 4:
A three-deck man cage, with an empty mass of 6 t carries 50 persons per deck to an underground station 2 000 m below the bank. The normal rope stretch at this depth is approximately 1,8 m. In order to improve safety, guide gripping devices are to be installed on the cage gripping on all 3 guides, consisting of steel-on-steel clamps operated by 100 mm diameter hydraulic pistons.
(a) If the coefficient of friction for steel-on-steel is 0,24 and an allowance of 100% is made for possible shock loading, calculate the:
i. Hydraulic pressure required for each clamp to hold the cage steady while persons are being loaded or unloaded.
[6]
ii. Minimum air pressure required to generate the hydraulic pressure for the hydraulic system if the power pack pressure booster consists of a 150 mm diameter pneumatic cylinder and a 20 mm diameter hydraulic piston.
[6]
(b) Draw up a code of practice for the operation of a guide snatch device, noting any safety devices to be included in the design.
[8]
Question 5:
A detachable grip chairlift is installed to transport persons underground from a main shaft to a subvertical shaft.
The following data applies:
Length of installation ……………………………………………. 3 km
Inclination towards subvertical shaft ………………………….. 1 in 250
Distance between rollers ……………………………………….. 15 m
Rope breaking strength …………………………………………. 154 kN
Mass of rope ……………………………………………………… 0,93 kg/m
Mass of chair ……………………………………………………… 20 kg
(a) Calculate the:
i. Maximum number of persons per hour that can be transported
[3]
ii. Minimum time for a trip from the main shaft to the subvertical shaft
[2]
iii. The drive unit motor capacity, assuming:
[8]
· Maximum amount of persons traveling towards sub shaft and no persons traveling out.
· Total efficiency of 75%
· Friction allowance 19,6 N/roller for unloaded and 39,2N for loaded rollers
(b) Describe, with the aid of sketches, how the chairs are automatically attached and detached from the rope and describe a device to ensure that the minimum spacing of chairs is maintained.
[7]
Question 6:
(a) Sketch and describe a water treatment plan for water for domestic use. The water is pumped from a slow flowing river. The raw water has a high pH value and may contain disease bacteria.
[5]
(b) A pump is to raise 40 litres against a static head of 30 m and through a pipe, 400 m long. Determine the most economical pipe size to use (that which gives the least annual expense)
[15]
Power cost is R2,00 per kWh.
The pump operates 12 hours per day.
Interest on capital investment is 13% annually.
Annual depreciation is 2,5%.
The cost of pipe laid is R250 for 150 mm, R398 for 250 mm, R470 for 300 mm and R550 for 475 mm.
The pump and motor efficiencies are 70% and 85% respectively.
Question 7:
(a) Briefly discuss the advantages of earthing the neutral point of a three-phase power distribution system.
[8]
(b) Distinguish between neutral earthing and equipment earthing.
[4]
(c) A 33 kV, three-phase, 50 Hz overhead line to a remote pump station is 50 km long and each line has a capacitance to earth equal to 0,01 μF/km.
Determine the inductance and kVA rating of the arc suppression coil, also called the Peterson coil. What is the purpose of this coil?
[8]
Question 8:
(a) The cylinder of an oil engine is to be charged with air at 96 kPa and 60°C. Compression takes place to 1/14th of the original volume, according to pV1,35 = C. Oil fuel with a mass of of that of air is injected into the cylinder and combustion is assumed to occur at constant pressure.
If the calorific value of the oil is 44 MJ/kg, determine the:
i. Isothermal pressure and temperature after compression
[6]
ii. Theoretical temperature after combustion if the specific heat capacity at constant pressure for the mixture = 990 J/kg.K
[8]
(b) Name SIX constituent parts of a diesel engine that require special provisions before the engine may be used in a hazardous area. Give a brief description of the requirements for each part mentioned.
[6]
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Plant Engineering: Mines – Past exams: June 1995
Printed on 8/16/2005 12:37 PM