Plant Engineering: Mines

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Plant Engineering: Mines

GCC: Electrical & Mechanical Engineers

Past Examination Papers

June 1998

Question 1:

The Mine Health and Safety Act Regulations require that, where the rope of a winding engine is securely attached to the winding drum, the brake, without the assistance of any counterbalancing effect of any load supported by the other drum, shall be capable of holding, without slipping, a load on the rope equivalent to the combined mass of the conveyance and its attachments, double the maximum permitted mass of material and the mass of the rope between the sheave and the conveyance, when the conveyance is at a point which produces the maximum torque.

(a) Describe how you would conduct a test on a double drum winder operating in a vertical shaft to ensure compliance with this requirement.

(b) Calculate the total braking torque necessary to comply with this requirement if the fully suspended rope length is 1 450 m, the mass of the rope 8,6 kg/m, the authorised material load 6 000 kg, the drum diameter 2,3 m, and the mass of the cage and attachments is 4 800 kg.

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Question 2:

(a) Briefly describe an explosion protected enclosure for electrical equipment used in a hazardous area.

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(b) How should bolts, screws and nuts of an explosion protected enclosure be, to minimise unauthorized interference?

[4]

(c) Describe the checks to be carried out before using a phasing stick on a high-voltage circuit. Describe the procedure to be used when using a phasing stick as well.

[4]

(d) Use Kirchoff’s law to calculate the insulation of a single-core cable. The insulation resistance is connected in series with a voltmeter across a 480 V DC supply and reads 5 V. When the voltmeter is connected in series with a 50 kΩ resistor across a 240 V DC supply, without the insulation resistance, the voltmeter reading is 90 V.

[8]

Question 3:

(a) Your mine purchased a load haul dumper for use in a coal mine. You are tasked to ensure that the potential hazards, while using such machine, are properly identified and also, to recommend how the risks, associated with the hazards, are minimised or controlled. Show clearly how each hazard is rated in terms of probability of it happening, the consequences and how it should be managed.

[10]

(b) With regard to oxy-acetylene flame cutting equipment:

i. Describe the cause(s) of a flash-back

ii. Discuss the circumstances under which flash-back arrestors are effective

iii. Where flash-back arrestors should be placed in the system

[10]

Question 4:

(a) A district of a mine consists of an intake airway 2 km long, a face and a 2 km return airway. The face has a resistance of 2 Ns2/m8 and the intake and return airways have a resistance of 0,1 Ns2/m8 and 0,15 Ns2/m8 per kilometre respectively. A Booster fan is to be placed in the return airway between the face and the return crosscut junction to ensure that the airflow through the pump chamber and the face is to be 5 m3/s and 15 m3/s respectively.

Ignore other leakage effects and

Calculate:

i. The pressure of the booster fan

ii. The differential pressure required between the inlet and the outlet of the district

Resistance coefficient = (Ns2/m8)

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(b) What diameter hydraulic cylinder is required for lifting a load of 1 ton with the crane sketched below if the hydraulic pressure is 7 MPa? The mass of the crane beam may be neglected in your calculation.

Question 5:

(a) Sketch and describe a variable displacement axial piston pump. State where they are used, their expected efficiency and their disadvantages.

[5]

(b) A slimes pump has an efficiency of 48% and must pump slime against a static head of 25 m. The density of the dry solids is 2,4 t/m3 and the solid to water ratio is 1,1 to 1 by mass. The pump column is 750 m long and has a diameter of 150 mm. Calculate the tons of solids that can be pumped per day if the pump is driven by a 30 kW motor.

[15]

Question 6:

A lift id supported by three 16 mm diameter ropes. It was found that one of the ropes slips with a jerk over a driving sheave. It was found that it slips 3,5 mm on the sheave when the fully loaded car is descending and the length of rope, from the car to the sheave, is 6 m.

(a) Describe the cause of this behaviour

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(b) Calculate the pull in this rope immediately before it slips if Young’s modulus for the steel is 200 GPa and the effective cross-sectional area for the rope is 150 mm2.

[10]

(c) Calculate the percentage of the car weight carried by the rope under such circumstances. The mass of the loaded car is 1 750 kg.

[5]

Question 7:

(a) Explain a practical method to determine the power factor of an induction motor with the aid of a clip-on ammeter and a three-phase capacitor with the motor running on light load.

[5]

(b) A 5,6 kVA three-phase capacitor was connected to a 10 kW, 380 V squirrel cage motor and the following readings were obtained:

Current to motor …………………………………………. 13 A

Current to capacitor ……………………………………… 9,2 A

Current from supply ……………………………………… 5 A

Determine the power factor with this arrangement and the capacitance to be installed (if necessary) to ensure a power factor of 0,95 on a light load.

[15]

Question 8:

(a) Describe the potential danger of an explosion occurring in a rotary type compressor.

[5]

(b) Two tenders for a rotary type compressor were received to provide 40 m3/s free air at a pressure of 450 kPa. The first tender is for a compressor which is uncooled, that is the compression process is irreversible adiabatic (pV1,49 = constant). The second tender is considerably more expensive and is for a water cooled compressor, that is the compression process is reversible polytropic (pV1,3 = constant). The atmosphere pressure is 103,5 kPa, the ambient temperature is 32°C and Cp = 1,004 kJ/kg.K. Compare the two tenders on the basis of power consumption. Briefly discuss other elements which may influence your recommendations.

=.-

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Plant Engineering: Mines – Pas exams: June 1998

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