Reference no: EM13898921
1) The first form of electrical power employed in mines was DC, why was this the case and what particular advantages did DC offer at the time? Why did AC replace DC power?
2) What is the difference between the distribution voltage and the utilisation voltage? Why use a distribution voltage and utilisation voltage? Define the following with respect to a mine electrical power system:
a) Demand
b) Peak load
c) Maximum demand
d) Demand factor
e) Diversity factor
f) Load factor
g) Coincident demand
3) List the design criteria as specified by the IEEE that apply to industrial systems particularly with respect to mining. Two types of flameproof electrical equipment exist for mines, Class 1 and Class 2. What does the term flameproof mean with respect to mining and how is equipment flame proofed? In addition, what is the difference between class 1 and class 2 flameproof or FLP equipment?
4) Define the following terms:
a) Resistance
b) Impedance
c) Step up transformer
d) Step down transformer
e) Useful power
f) Reactive power
5) Define the term intrinsically safe with respect to mine electrical systems. What is the power factor? Why is the cost of delivering electricity at lower power factors high? How is it possible to increase power factors?
6) Describe the components of a trailing cable system. Cable handling is an important aspect in mining operations, describe in detail one method of cable handling employed in either surface or underground mining.
7) For the transformer system shown below calculate the following:
a) The voltage V2
b) The current I2
c) The current I1
8) For the system indicated below determine the following:
a) The voltage V2
b) The impedance (The resultant resistance) of the circuit on the V2 side of the circuit
c) The current I2
d) The Average Power on the low winding side of the transformer kW
e) The reactive power kVAR
f) The effective power kVA
g) The power factor
h) The Current I1
9) Describe in detail using diagrams where appropriate the following power distribution systems that can be applied in open pit mining:
a. Radial
b. Secondary selective
c. Ring main
10)A 350kVA transformer is at full load with a 0.85 lagging power factor. To correct the power factor to 0.95 a capacitor pack is placed in parallel to the secondary of the transformer. What is the required kVA of the capacitor pack? Also how many kVA's is the transformer providing at the new power factor?
11) A motor that draws 160kW at a 0.75 lagging power factor is placed in parallel with another that draws 90 kW at 0.5 leading power factor. Determine their combined power factor.
12) List the main advantages and disadvantages of compressed air driven machinery. What types of compressors produce compressed air? Describe the operation of one of these compressors, in particular detail the parameters that influence the pressure and flow developed and techniques that are used to vary these parameters.
13) In compressed air systems why is it normally required to have a receiver located somewhere in the system? What purposes does a receiver serve and detail some possible designs of receivers?
14) In a compressed air system in a deep mine considerable savings are achieved by placing the compressor on surface rather than underground, explain why? How can this effect be calculated?
15)A 200mmdiameter main carrying 3.6 kg/s of compressed air at peak periods services a section of a mine. The main is 2,600m in length, what would be the maximum pressure loss across it? The air in the column has a mean temperature of 35°C and the barometric pressure is 100 kPa. Average gauge pressure is 600 kPa. The Universal gas law can be applied and that the gas constant is 287 J/kgK.
16)Compare and contrast compressed air and electrical drives as power sources in mining. A stoped out area with a volume of 5,000 m3 acts as a receiver. It is pressurised to an absolute pressure of 2900 kPa during off shifts. The intention is to draw air from the receiver during the 4hour drilling shift until the absolute pressure drops to 550 kPa. What is the rate at which the receiver boosts the main air supply during this period? Air temperature is 30°C, gas constant for air R=287 J/kgK.
17)A 356 mm diameter main services a section of a mine and carries 2.4 kg/s of compressed air at peak periods. If the main were 1,000 m in length what would be the maximum pressure drop across it? The air has a mean temperature of 30°C and the barometric pressure is 100 kPa. Average gauge pressure is 500 kPa.
18)A 25 mm diameter hose services a rock drill and carries 0.07 kg/s of compressed air at peak periods. If the hose is 30 m in length and the hose has, five meanders along its length what would be the maximum pressure drop across the hose? The air has a mean temperature of 30°C and the barometric pressure is 100 kPa. Average gauge pressure is 500 kPa.
19)Briefly describe the methodology of employing hydropower in mines. What limitations exist on the application of this power source and what are the advantages of this power source?
20) List eight problems that occur in open pit mining by excess water buildup in the mine. Answer the following:
a) In what ways does water enter an open pit?
b) In what ways can water be prevented from entering a pit?
21) Describe inpit dewatering systems that are in common usage.
22) One detrimental aspect of groundwater is the reduced stability it develops in both rock and soil slopes. What is usually the major difference between dewatering a rock slope as opposed to a soil slope? Describe three possible methods of slope dewatering.
23) For each of the following dewatering jobs, select a suitable pump type:
a) An expit dewatering well
b) A sump at the bottom of the pit
c) An inpitdewatering hole for a sinking cut
d) Dewatering installation for a lake
e) Temporary buildup of water in the pit
24) Describe the basic steps that engineers use to deal with floodwater in an underground mine, wherever possible referring to particular case studies to illustrate the effectiveness of these measures.
25) The following data presented is from a nest of piezometers installed side by side at a single site.
Piezometer

A

B

C

Elevation at surface (m AHD)

850

790

830

Depth of piezometer (m)

150

100

50

Depth to water (m)

27

47

36

If A, B and C refers to the points of measurement of piezometers a, b and c, determine:
a) The hydraulic head, pressure head and elevation head at A, B and C
b) The vertical hydraulic gradients between A and B, and between B and C if they are spaced 100m apart
26) Define the following terms
a) Hydraulic conductivity
b) Porosity
c) Effective porosity
d) Transmissivity
e) Aquifer
f) Perched Aquifer
g) Water table
h) Artesian well
i) Confined Aquifer
j) Hydraulic gradient
k) Specific discharge
l) Seepage velocity
27) Calculate the discharge and seepage velocities for water flowing in a pipe filled with sand, with a hydraulic conductivity of 0.05 m/day and an effective porosity of 0.25. The hydraulic gradient is 0.4. How far will the water move in one year?
28)A cylindrical column, 0.3 m in diameter and 5 m long is filled with relatively homogeneous sand. If the porosity of the sand is 35% and distilled water is continuously introduced into the column, saturating the sand and establishing steady state flow conditions for which the discharge is measured to be 18.6 x 102 m3/day, determine:
a) The specific discharge
b) The average velocity of flow through the column
29) If atmospheric pressure pushes mine water up a suction line due to the vacuum created by a pump, is there a limitation as to the maximum length of suction line? If so what is the value?
30) Determine the required power to pump 25 l/s against a total dynamic head of 120 m if the pump operates at 78% efficiency?
31)A surface mine requires to pump a quantity of 2,500 litres per minute from a main sump located at the bottom of the pit to a settling pond. The vertical lift of the pipeline is 700m and the total length of the pipeline is 4000m. Assuming the pipeline to be straight with gate valves located at 300m intervals along the pipeline, that the pipe friction factor is 0.017 determine:
a) The flow rate in litres/second
b) If a flow velocity of 0.5 m /s is the design velocity in the pipeline, determine the pipe diameter.
c) Using the determined pipe diameter calculate the head loss due to friction in the pipe.
d) Determine the head loss due to a single valve and total valve head loss in the system.
e) Assuming no entry or exit losses determine the total head loss in the pipe.
f) Determine the head required by the pump
g) Determine the power added by the pump
h) If the pump has an efficiency of 75% determine the power required by the pump.
i) Select a suitable pump to undertake this task
32) Pumping occurs from a shaft bottom sump to surface via a 300m vertical pipe. Ignoring entry and exit losses, assuming a pipe friction factor of 0.02, a pipe diameter of 250mm and ignoring other losses in the pipe, if there is a flow rate of 3000 litres per minute; determine the pump head required?
33) What is the required power to pump 4000 litres/min against a total dynamic head of 250 m if the pump operates at 82% efficiency?
34) Define what is meant by the term acid mine drainage. Describe some methods that can be employed to reduce the effects of acid mine drainage.
35)Why in a mine drainage system both water and mud pumping occurs?
What techniques exist to clarify water in a mine drainage system?
36) Why in a typical mine pumping system is it usual to pump out of an underground mine in stages instead of the full head in one go?
37)A Continuous miner has two 74.6 kW, 550V, and 3,600 rpm three phase squirrel cage induction motors. Calculate the current drawn from the line and total power factors when:
a) The first motor is being started
b) The second motor is being started and the first motor is running at full speed
c) Both motors are started simultaneously
d) Both motors are running at full speed
The motors have a power factor of 70% lagging at normal full speed and 20% lagging at the time of starting. Assume the motors are started directly on line with full load.
38) A threephase 660V alternating current substation supplies power to the following loads:
(a) 70 kVA at 75% lagging power factor, and
(b) 22kW at 100% power factor. Determine the operating power factor of the substation.
In this case a 40kW synchronous motor is to be installed at the substation to improve the station power factor to 95% lagging. Determine the power factor at which the synchronous motor must operate to achieve this.
39) A 100 kW, 450V AC shuttle car uses a No. 4 threeconductor flat cable. Two layers of the cable will remain on the shuttle car's reel at all times. Does this cable have adequate ampacity if efficiency is 80%, a power factor of unity also applies. The ampacity for the cable is 104 Amp. Details for the No 4 three conductor flat cable are:
Ampacity = 104 A per conductor at a conductor temperature of 90°C
40) A long wall section has the following equipment.
Equipment motors

kW

V

Shearer

298

950

Pump pack 1

37.5

550

Pump pack 2

37.5

550

Face conveyor 1

112

550

Face conveyor 2

112

550

Stage loader

37.5

550

Load factors for all the motors can be assumed to be 0.8 except the pump packs, these have a load factor of 0.9. Determine the current supplied to the high side of the load centres transformer. The distribution voltage is 6,600 V. (Hint: When motor power factors are unknown, assume that they are equal to the load factors (up to about 0.85)).
41) A confined sand aquifer has an effective porosity of 25% and a hydraulic conductivity of 30 m/day. The hydraulic gradient in the aquifer is 0.02 The overlying confining bed is composed of clay with a porosity of 45% and a hydraulic conductivity of 0.0002 m/day. Hydraulic gradient in the clay is 0.15. Determine the specific discharge and seepage (average linear) velocity for flow of ground water in both the aquifer and the overlying aquitard.
42) Field investigations indicate that the hydraulic gradient in a 20m thick sand aquifer is 0.01. The transmissivity of the aquifer has been determined from pumping tests to be 500 m2/day:. The porosity of the sand is 0.25. Calculate the discharge and linear velocity.
43) The hydraulic gradient measured in two observation bores positioned along the direction of groundwater flow in a 30m thick confined aquifer is 0.001; the aquifer's transmissivity is 750 m2/day. Determine the groundwater flow through a 6 km wide section at right angles to the direction of groundwater flow.
44) A unconfined aquifer is 33 m thick and 7 km wide. Two observation bores are located 1.2 km apart in the direction of flow. The head in bore 1 is 97.5 m and in bore 2, it is 89m. Hydraulic conductivity is 1.2 m/day.
a) What is the total daily flow of water through the aquifer
b) What is the elevation of the potentiometric surface at a point located 0.2 km from bore 1 and 1 km from bore 2