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MATLA POWER STATION
 Introduction

 MATLA a Sotho word meaning “strength” or “power”, has taken on a new meaning in the Eastern Transvaal now Mpumalanga.  The coal-fired power station bearing this name, is located near Kriel on the Mpumalanga Highveld about 200 km from Johannesburg. The word is now associated with electrical power and a massive 3 600 MW power station Matla when completed, was the biggest power station in South Africa (the first using 600 MW turbo-generators) and  one of the largest in the world. Matla was the first of the giant 3 600 MW coal-fired power stations in the world with a concrete boiler house superstructure.  This gives it a robust appearance, which is very different from that of other power stations in South Africa.  The unusual design evolved as a result of a world-wide steel shortage during the design stages.  The planning and design of Matla power station commenced in the early 1970s.  It was designed for an operating life of 30 years, but substantial coal reserves have extended its life span to 50 years. The station consists of six 600 MW units, giving an installed capacity of 3 600 MW.  Matla’s power is fed into both the 400 and 275 kV networks.  Construction commenced in 1976, and the station was fully commercial by July 1983.

 DSC_1477.JPG
Exterior view of Matla Power Station

 The planning and design of Matla began early in the seventies.  Construction started late in 1974 and the station was scheduled for completion before Eskom’s winter peak-demand period in mid-1983.  The first of the six 500 MW sets started feeding power into the national grid towards the end of 1979.  Matla had been designed for an operating life of 30 years, but substantial coal reserves may give it a lifespan closer to 50.  As a base load plant it will operate continuously except for regular scheduled stoppages on individual sets for inspection and maintenance.  A total of 3 800 tons of coal per hour can be transported by conveyor to the power station from the adjacent Matla colliery, which mainly makes use of long wall retreating mining methods.  Because the coals seams are at an average depth of between 48 and 105 metres, opencast mining methods will not be used.  Roughly 90 per cent of the reserves will be extracted.

 Matla is a six pack power station

 It has six 2 pole, 3 000 r/min synchronous generators, each with a rating of 688,9 MVA, 20 kV.  The generators stators are water-cooled and their rotors are hydrogen-cooled.  Power from each generator is transferred to the national electricity grid by means of an oil-cooled 700 MVA generator transformer.  Two generator transformers are designed to step up the voltage from 20 kV to 400 kV, and the remaining four are designed to step up the voltage from 2o kV to 275kV.  For each unit there is a 20 kV breaker between the generator and generator transformer.  The purpose of this breaker is to clear fault conditions between the generator and generator transformer.
 
DSC_1168.JPG
Matla Power Station

 The coal consumption of Matla power station is approximately 1,150 000 tons per month.

 A total of 3 800 tons of coal per hour can be transported to the power station by conveyor from the adjacent Eyesizwe Matla Colliery, which employs mainly long wall-retreating mining methods.  Because the coal seams are at an average depth of between 48 and 105 metres, open-cast mining methods are not used.  Roughly 90% of the reserves will be extracted.

 Water supply

 The raw water used at Matla was initially supplied by Kriel power station, but since the completion of the Vaal River system water has been supplied by the Grootdraai Dam, Grootfontein pump station and Rietfontein weir pump station. Matla power station raw-water consumption is about 3 500 Ml (megalitres) per month.

 At full load, the six units at Matla require 3 500 Ml of water per month.

 The bulk of this is used to make up for water lost by evaporation through the cooling towers.

 Other uses include cooling system blow downs, demineralised water makeup to the condense circuit, ad portable water for site drinking and domestic use.

WATER TREATMENT PLANT.jpg
Water treatment plant

 The raw water for Matla is obtained from the Vaal and Usutu river systems.  The Vaal supply is relatively plentiful, but the water normally contains a high content of suspended and dissolved solids.  This increases the costs of water treatment and de-mineralization.

Usutu water is substantially cleaner than Vaal water, but its supply to Matla depends on the consumption by Kriel and Kendal power stations.  Because it contains less suspended and dissolved solids.  Usutu water costs less than Vaal water to treat particularly to demineralise.  Although Usustu water has a lower unit cost for both supply and treatment than Vaal water.  Eskom’s allocation of this water is insufficient to meet Matla’s needs.

The shortfall is therefore made up by using Vaal water.

Fuel handling

 The coal from Matla mine is delivered, crushed to less than 25 mm in size, to the two coal staithes at 3 800 tons per hour.  The overland conveyor system is roughly 2 km long.  Each boiler has six coal bunkers.  The coal bunkers are concrete structures with a stainless steel lining.

Coal is stored in two coal staithes before being passed to the boilers for burning.  The first staithe, with a capacity of 50 000 tons, serves boilers 1 to 4, and the second with a capacity of 30 000 tons, serves boilers 5 and 6.

The coal is drawn from the staithes and conveyed via a series of belts to the coal bunkers situated next to each of the boilers.  The bunkers have an approximate capacity of 1 000 tons, and there are six bunkers per boiler, giving an effective storage capacity at the boilers of 18 hours operation at full load.

 Each bunker feeds coal to a dedicated mill, via a coal feeder, through a gravity feed from the bunker to the feeder.

 When a boiler is started up, fuel oil is used as support for the coal, until the combustion of the pulverised coal is stable.  The fuel oil is stored in four storage tanks with a combined capacity of 5400 kl.

 Milling plant

 The milling plant consists of a volumetric coal feeder, a vertical spindle mill, a classifier and the piping to transport the milled coal (PF) as well as 8 burners.

The speed-controlled, coal feeder, delivers raw coal from the bunker to the mill to yield the desired quantity of coal to the boiler.  The coal falls by gravity from the feeder to the mill and is pulverised the grinding elements.

 An integral part of the mill is its classifier, which can be adjusted to obtain the required degree of fineness of the pulverised coal leaving the mill.

 The primary air fan delivers clean, hot, high-pressure air to the mill.  This air can be controlled in quantity and temperature.  It’s main purpose is to dry the coal and to transport pulverised fuel (PF) to the burners.

 The primary air delivers the PF to the burners through PF piping, the burners receive the PF/ primary air mixture, and the combustion of the coal is promoted by the addition of hot secondary air.

 There are six mills per boiler, five on load and one on standby under full load conditions, for which approximately 280 tons of coal per hour are required depending on the quality of the coal.

Turbines

Each of the six turbines unit has a high-pressure (HP) and an intermediate-pressure tandem configuration.

Each turbine operates at a rotational speed of 3 000 r/min.   The HP and LP cylinders have an internal and external casing to stablise temperature gradients, thus allowing for fast start-up and rapid load variation.

Live steam at a temperature of 535ºC and a pressure of 16.1MPa is admitted into the HP turbine via the HP steam stop an control valves.  It expands through the 10 HP stages, producing approximately 30% of the total power output.

Spent steam from the HP turbines is returned via the cold reheat piping system to the re heater part of the boiler, where its temperature is raised to 535ºC at a pressure of about 3.7MPa.  It then enters the IP turbine and expands via nine stages to reach 264ºC and 0.47MPa pressure.

During expansion in the IP modules, some steam is bled off to preheat feed water and thus to reduce the boiler heat input.

A thrust bearing is employed between the HP and IP cylinders to absorb the residual thrust from the rotors.

From the IP turbine exhaust, the team is carried into the LP cylinders via large crossover pipes one to each LP cylinder.

On entering the LP cylinder, the flow of steam splits nearly equally and expands in two directions through six stages on each side.  The steam becomes sub-atmospheric at about the penultimate stage, and the last stage handles wet steam with roughly 8-9% moisture.

The last-stage blade, which is approximately 1 080 mm long and one of the largest for a machine of this speed, has an average tip velocity of 510 m/s at full speed.  The steam is expelled at an axial velocity of 240 m/s through a bell-mouth exhaust leading to the condenser neck.

Condensation takes place at a back pressure of about 5 kPa (abs) in the dual-pressure surface-type condenser.

 Feed-heating plant

 At the design point the condensate temperature in the condensate temperature in the condenser hot well is 34,3ºC.

 Maximum system efficiency is obtained by preheating the condensate through three LP heaters and four HP heaters.  The FWT and LP pre-heaters are of the surface type, in which water flows through tube nests.  Steam extracted from different staged of the turbine heats the water by means of conduction through the tube metal.

 A spray-type de aerator between the low-pressure and high-pressure heaters removes entrained air in the condensate and heats the water by direct contact with steam extracted from the turbines.  The condensate is pumped from the condenser to the spray-type de aerator through the low-pressure heaters by the main condensate extraction pumps.  The feed water is then passed through high pressure heaters, reaching the boiler at a temperature of approximately 247ºC.

 Condensers

 The condensers are of the dual pressure surface type.

The vacuum inside the condensers is established by steam jet ejectors and maintained in normal operation by water jet air ejectors.  Steam exhausted from the LP cylinder condenses over 32 856 brass tubes with a surface area of 23 400 m².

 Each boiler turbine set has a dual condenser with a heat exchange capacity approximately 2 x 400 MW.

 Boilers

 Matla is one of the few power stations in the world with a concrete boiler house superstructure, giving it a robust outward appearance very different from other stations in South Africa.  The rather unusual design evolved as a result of a worldwide steel shortage during the initial design stages.  The use of concrete resulted in a reduction in the construction lead time and in savings in capital outlay.  The six boilers are of the coal fired radiant furnace, natural circulation type with rehearing.  Each 65 m high boiler is suspended from the top of the boiler house to overcome expansion problems.  Highly purified demineralised water circulates upwards within tubing making up the boiler furnace walls.  Heat is transferred to this water within the furnace wall tubes, and boiling results.  The steam and water mixture produced is collected in a steam drum (on the 65m level in the boiler house) where the steam separates from the water.  Saturated steam is then led to the super-heaters where it absorbs more heat the temperature of the steam being increased to 540°C at a pressure of almost 17 MPa (abs).  Superheated steam is used to drive the turbines which in turn spin the generator rotor coupled to the turbine shaft.  The main steam pipework incorporates a high-pressure bypass system by means of which steam can be made to bypass the high-pressure turbine and flow directly into the cold rehear pipework.  After passing through the re heaters into the hot reheat-pipework, the steam passes through the low pressure bypass system and discharges into the main condenser after passing through spray de-super-heaters.  The bypass systems are designed to handle 35 per cent of the boilers’ maximum rated flow and are installed for the following reasons:

  •  to obtain the correct steam pressure and temperatures before starting-up the turbo generators, and
  • to allow the boiler to continue operating on a low load after the turbine has been tripped.
matla1.jpg
Boiler as seen from the inside

 Each boiler is equipped with 48 burners arranged in three rows of eight on both front and rear furnace walls.  Mounted in the centre of each burner is an oil burner which is used for starting up and for stabilizing the pulverized fuel flame at low loads.  The constant speed forced draught fans supply secondary air to the burner wind boxes whilst primary air fans supply primary air to the coal mills to carry fuel to the burners.  Combustion gases are drawn from the furnace by two induced draught fans, over the surfaces of the steam super- heater, the economizer, air pre-heaters and via the electrostatic precipitators to the chimney.  Approximately 99 per cent of the dust, or fly ash, is collected by the precipitators.

matla2.jpg
Electrostatic precipitator

 Plant

 Control:  There are three control rooms, each serving a pair of sets.  Each boiler-turbine set will be run as a separate entity, with controls and instrumentation accordingly incorporated into individual unit control desks and panels.  From here all major operations associated with start-up, normal operation, shutdown and emergency operation can be carried out.  These control rooms are in constant touch with other major nerve centres making up the integrated Eskom transmission network.  The control room of set 1 also accommodates the high voltage yard monitoring and operating functions.  A data logging computer constantly monitors the main operating and alarm systems, providing operating personnel with a constant flow of information on video screens and printouts.

Mills

Each pulverized fuel mill is fed by a coal feeder which controls the flow of coal into the mill, the amount being determined by the boiler steam output requirements.  AS the coal enters the mill it is ground to a fine powder by steel balls running in a circular track, then discharged into the boiler by means of an airflow supplied by the primary air fan.  When working at full capacity each mill crushes about 75 tons of coal per hour.  There are 36 mills in all providing a spare capacity of 2 mill per boiler when design quality coal is being burned.  At full load, each boiler burn about 250 tons of coal per hour.

Ash collection and disposal

 When on full load, each boiler produces 250 tons of coarse ash and 1 400 boiler tons of dust per day.  The total quantity of ash and dust produced by the six sets will amount to some 10,000 tons daily.

 Dust collected in the precipitator hopers is sluiced away to an ash sump (on per boiler).   During de ashing, coarse ash and mill rejects are sluiced to ash crushers, and then fed into set ash sumps.  The dust/ash slurry is pumped from the sumps to ash dams.  All water used to convey the ash is decanted from the ash dams and returned for reuse in the ash system in a closed cycle.

Generators

 Each generator coupled to the end of the LP turbine shaft generates 600 MW at full load at the terminal voltage of 20 kV.  Generator cooling is done in two ways using hydrogen and demineralizer water.  The stator and rotor are pressurised to a maximum pressure of 500 kPa by means of hydrogen which is circulated through coolers by 2 fans solidly locked to the main shaft. 

The coolers are of the surface type and cooled by circulating water. 

 A circulating demineralized water cooling system assists the hydrogen cooling generators of units 1 and 2 feed the 400 kV grid, while the four remaining generators will feed the 275 kV network.

 High-voltage yard

 The 400 kV yard consists of a three bus-bar system with one transfer bay, two bus coupler bays and one bus section bay.  The 275 kV yard consists of a two-bus bar system with two bus coupler bays and two bus section bays.  The outdoor yard layout at Matla allows for the interconnection of the 400 kV system with the 275 kV grid by means of coupling transformers.

PS matla 84349.jpg
Matla Power Station

TECHNICAL DATA

Generating capacity

3 600 MW

Mining company                                         


Eyesizwe

Fuel

 

Calorific value                                 

Ash content                         

Sulphur content                                            

Coal consumed at full load                        

Total annual consumption                           

Coal staithe capacity                                    

Staithe 1                                                         

Staithe 2                                                       

Stockpile capacity                                         

Boiler bunker capacity                                 

Fuel oil – Bunker                                        

Storage capacity                                           

Annual consumption (total)                       




24 MJ /kg (dry basis)

19.1%

±1%

±1800 tons/h

±13-14 million tons

80 000 tons

50 000 tons

30 000 tons

28 million tons

7 000 tons per boiler

150

2000 tons

±3 000 KI

Boilers


Number                                                         

Manufacturer                                               

Type                                                              

Height (roof to hopper)                               

Maximum continuous rating                     

Final steam pressure                                 

Final steam temperature                            

Re heater steam pressure                         

Re heater steam temperature                   

Width at burner level                                  

Depth at burner level                                 

Boiler expansion (downwards)                

Boiler water content cold                           

Furnace volume                              

Total heating surface area                        

 

 



6

Babcock

Natural circulation

62m

508 kg/s

17.2 MPa

540ºC

3.84 MPa

540ºC

24m

13.7 m

280 mm

122400 kg

12193 m²

6008 m²

 


Boiler circulation pump



None


Cooling Towers


Number                                             

Type  

                                 

Overall dimensions

Height                                                           

Diameter at base ring beam          

Pond diameter                                 

Depth of pond                     

Capacity of tower pond                  

Throat diameter                   

Diameter at top                                

Air inlet area

                                   

Evaporation at 600 MW                 




6

Concrete construction natural draught


 

149 m

98.25 m

South 107.3 m - North 94.6 m

South 1.58 m   - North 1.75 m

14300 m³

54.62 m

61.03 m

2 317 m²

 

1 250 m³/h


Chimneys

 

Number                                             

Height                                                                                                 - 273 M north, multi-flue

 

Diameter (base)                                                                                              

 

Diameter: (top )ID                                                                                       




2

213 M south

273 M north, multi-flu

25 M base South

28 M base North

 

14 M top South

28 M top North

Turbines


Manufacturer                                               

Type                                                  

Speed:                                                          

Steam flow to HP turbine                          

HP Exhaust steam flow to RH:     

Work rate of HP cylinder:                           

HP cylinder blading efficiency:    

Steam flow to IP Turbine                           

Work rate of IP cylinder                              

IP cylinder blading efficiency                   

Steam flow to LP 1                                      

Steam flow to LP 2                                      

Work rate of LP 1 & 2                     

Total work rate of turbine train                  

Total efficiency                                




MAN

Multi-cylinder impulse reaction

3 000 r/min

492 kg/s at MCR

462 kg/s

30% or 180 MW

89.2%

462 kg/s at MCR

50% or 300 MW

93%

188.4 kg/s

188.4 kg/s

20% or 120 MW

600 MW

43%

Mills

 

Manufacturer                                               

Type                                      

Number                                 

Rotational speed                                         

Rated output                                    

Motor                                                             




Babcock & Wilcox (SA)

E 12.9 vertical spindle

6 per boiler

26.7r/min

70 tons/hr

683 kW


Feeders

 

Manufacturer                                               

Number                                                        

Minimum coal throughput                         

Maximum coal throughput                        

Throughput control                                     




 

 

Stock equipment

6 per boiler

15 tons/hr

75 tons/hr

Variable speed DC 380 V motors

Forced-draught fans


Manufacturer                                                

Type                                      

Number                                 

Volume flow per boiler (design)               

Mass flow per boiler (design)        

Impreller shaft coupling                             

Motor output                                     

Motor speed                                                 

Gas-control method                                    




James Howden & Co Ltd

Z design 6

2 per boiler

576m³/s

720kg/s

Wellman Bibby

2000 kW

745 RPM

Vane inlet control system

Condenser

 

Type                                      

Cooling principle                            

Type and number of tubes                        

                                                                       

Total tube surface area                              

Condenser pressure          

Condenser steam pressure                      

Condenser steam flow                               

 

 




Dual pressure surface

Conduction

30556 Admiralty brass

2300 Titanium

23400 m²

5.6 and 8,3kPa (abs)

42ºC

165.3 x 2 kg/s

Extraction pumps

 

Motor manufacturer                        

Power output                                                

Speed                                                             

Pump manufacturer                                    

Type                                                              

Delivery pressure                                        

Delivery pressure                                        




GEC

1600 kW

1450 r/min

Mather and Platt

400-600

98kPa

1668kPa

Load pressure feed heaters

 

Bled steam temperature                             

                                                                       

                                                                       

Water inlet temperature                             

Water outlet temperature                           

                                                                       

                                                                       

Steam safety valve                                     

Water safety valve                                      

 




No 1 - 77.3ºC

No 2 -159.4ºC

No 3 - 264.5ºC

No 1 – 39.4ºC

No 1 -74.1ºC

No 2 – 110.3ºC

No 3 – 146.4ºC

No 3 – 3.70kPa

3000kPa

De-aerator

 

Steam pressure                               

Safety valve                                                 

Water temperature                                      





961kPa

1100kPa

176ºC


Boiler feed pumps (electric)

 

Manufacturer                                   

Number                                             

Type                                      

Discharge capacity                                     

Discharge pressure                                    

 

Motor power output                         

Motor speed                                                 

Main pump speed                                       

 




Sulzer

2 per unit

Multistage

300kg/s

19.5MPa (abs)         

 

9800 kW

1492 r/min

5000r/min

PA Fans


Manufacturer                                                

Number                                             

Type                                      

Coupling                                                      

Motor                                                             

Motor output                                                 

Motor speed                                     

Fan delivery temperature              

Fan delivery pressure                                



 

Hwden SA Fan Co

6 per unit

Single inlet type

Wellman Bibby

3,3 kV squirrel-cage induction

2050 kW

1488 r/min

50-250ºC

15-12kPa


Pulverised fuel burners

 

Number                                                         




48 per boiler

Fuel oil burners

 

Number                                 

Delivery capacity (each)                            

 




48 per boiler

650 kg/h

 Re heaters

 

Type                                      

Heating surfaces                                        

                                                                       

Protection                                         

Total discharge capacity                            



 

Convective

13602 m² Primary

7900 m² Secondary

8 safety valves

540 kg/s

Super heaters

 

Number                                                         

Type                                      

Heating surfaces                                        

                                                                       

                                                                       

 

Protection                                         

Total discharge capacity                            

 




3 per boiler

One radiant and two convective

3655m²  Primary

1790m²  Platen

3824 m² Secondary


4 safety valves

168kg/s

Economiser


Type                                      

Heating surface                              




Plain tube

15690 m²

Steam drum

 

Length                                              

Inside diameter                                            

Thickness                                         

Weight                                                          

Protection                                         

Total discharge capacity                            




24 metres

2.2 metres

152 mm

280 tonnes

4 safety valves

745 kg/s


Boiler feed pumps (turbine driven)


Manufacturer                                   

Number                                                         

Type                                      

Discharge capacity             

Discharge capacity             

Turbine speed                                 

Main pump speed                           

Flow rate control                                         




Sulzer

1 per unit

Multistage

kg/s

25.8MPa (abs)

4850 4/min at full load

Turbine speed

Turbine speed


High pressure feed heaters

 

6A & 6B steam safety valve          

Water inlet temperature                             

Water outlet temperature                           

Bled steam temperature                 

Bled steam pressure                                  

7A & 7B steam safety valve                      

Water outlet temperature                           

Steam inlet temperature                            

Steam inlet pressure                                  

Water safety pressure for all HP heaters 




2.5MPa

173ºC

207ºC

±435ºC

±1.8MPa

4.5MPa

247ºC

332ºC

3.9MPa

25MPa

Cooling Water system


Motor manufacturer                                    

Type                                                  

Rated power                                                

Motor poles                                      

Pump                                                            

                                                                       

Discharge capacity                                     

Speed                                                           

Pump power consumption                        

Suction branch bore at impeller               

Discharge branch bore at              

Impeller

Impeller

 

Quantity of air at specified barometric pressure passing through tower when cooling 9.32 m³ water/s under following conditions

 

Circulating flow rate                                    

Cooling range                                             

Re-cooled water temperature                   

Atmospheric wet-bulb temp                      

Atmospheric dry-bulb temp           

Relative humidity                                        

Mean atmospheric pressure         

 

Circulating water

 

pH                                                      

Total alkalinity CaCO                                 

Hardness CaCO                                         



Mitsubishi

3.3 kV squirrel-cage screen protected

1 700 kW

18

Vertical, mixed flow, centrifugal, concrete volute

6.5 m³/s

328r/min

1 332 kW

1 150 mm

1 500 mm

 

Stainless steel BS 3 100

 






12.3 kg/s

13/3 m³/s, 15.8ºC

22.5ºC

11.05ºC

15.45ºC

61%

84.9kPa

 

 

 

8.4

120 mg/l (max)

400 mg/l


Generators

 

Manufacturer                                                

Rated capacity                                             

Terminal voltage                                         

Power factor                         

Cooling medium rotor        

Cooling medium stator                   

Total efficiency of generators                   

 

 



Alsthom

689 MVA

20 kV (50 Hz)

0.9 (lagging)

Hydrogen at 400kPa

Demineralised water

99.12% @ 620 MW @ unity

Main Contractors

 

Boilers                                   

Precipitators                                                 

Turbines                                                       

Generator transformers                              

Cooling towers                                            

Cooling water pumps  boiler feed pumps

 

Chimneys                             

Cabling and switchgear    

Instrumentation and control                      

Process computers                                     

Low pressure services       

Fire control system                         

Coal conveyors                                           

Water treatment plant                     

Civil works                                        

Steel structure                                             

 




Babcock

Lurgi

MAN

ASEA Electric (SA) Ltd

Knight, Piesold/Hammond

Sulzer Bros (SA) Ltd


Kareena Africa

Hubert Davies Construction (Pty) Ltd

Siemens AG c/o Siemens Ltd

Citect Scada

Stewarts & Lloyds Ltd

Mather & Platt (SA) (Pty) Ltd

Spencer (Melksham) SA (Pty) Ltd

Foster Wheeler

Gillis Mason

Dorbyl



DSC_1163.JPG

MATLA11.jpg


MATLA LIST.jpg

Matla Chimney


The multi flue chimney at Matla power station was the highest slip form structure in the Southern Hemisphere was completed by Futurus of Kempton Park.

 The 275m high chimney’s superstructure was completed in only 16 months, thanks largely to Futurus’ own system of locally developed slip forming.  This was the first time that a structure of this size and configuration had been slip formed in South Africa.

 An added challenge was that the chimney windshield tapers from a diameter of 24.4m at a level of 40m to 20.9 at the 100m level.  This required Futurus to develop a sliding shutter which enabled the necessary adjustments to be made to decrease the windshield’s diameter and circumference as the taper progressed.

 Working 24 hours a day, Futurus’ slip forming of the chimney progressed at a rate of 4m a day at times, resulting in a labour problem as new workers had difficulty adjusting to the rapidly increasing working heights.

 On completion of the project, 14 500m³ of concrete, 1 900 tons of reinforcing steel, 78 000 m² of slip form work and 1,2 million semi acid resistant refractory bricks for lining the flues, were used.

Picking up pieces at Matla

On August 27, 1980, the collapse of one flue in the triple flue chimney No 2 killed one worker and injured 15.  The accident was indeed a freak by world standards and certainly unparalleled in South African construction history.  The first phase of the investigation into the accident progressed and it was felt that the results should be made public on these findings. When the investigation began, removal of the rubble at the collapsed site was delayed because it was thought that its removal could prove dangerous for those workers involved in the operation. However, after extensive investigations and calculations, it was established that the rubble could be removed without serious risk to life and property.  The only risk that did exist was that of falling bricks within the windshield.  The steelwork had been designed for temporary erection at the top of the chimney.  It was supported by the windshield and enabled moving platforms to be erected so that further examination of flues Nos. 4 and 5 could take place.  A recheck of design calculations had indicated that the windshield i.e. the outer concrete surrounding the flue’s, was sound and that there was no danger of collapse of the remaining flues.

Some damage to the two other flues in the triple flue chimney had been reported, however but the damage was not seen as being too serious to allow repair.  A design to further increase the stability of these two flues was own eating completion and repair work commenced.

The chimney was designed by Ove Arup and Partners

Stanton was originally called in after the collapse of the flue.  At that stage no decision had been made regarding possible demolition of the chimney and the Manchester-based company was asked simply to remove the remains of Flue 6.  Their wok was in fact, half complete when the decision was made to demolish the entire structure.  Santon, in conjunction with American explosives expert Jim Redyke, then submitted a detailed proposal, which was accepted for demolishing the chimney using explosives

The blast due on July 18, has involved the drilling 1 000 holes – some 700 in the windshield itself and the remainder in Fule’s 4 and 5.  The drilling, which was completed in eight weeks, took place at the 40m level where a slab has created a virtual “ground level” to distribute the mass and other forces through the base.  All drilling was carried out internally.  The remaining 40m will be demolished conventionally after the blast.

The success of the demolition will revolve around the hinge arrangement at the 40m level.  The hinge was made up of vertical steel and concrete columns of differing lengths which forced the chimney, which was hoped to “walk”.

The demolition was fraught with problems, not least of which is the fact the Flues 4 & 5 were still standing within the windshield. 

The two remaining flues naturally had a great deal of bearing on the direction of the fall.  “If they start moving about inside the windshield as the chimney cmes down there could be major deviations in direction”.

To overcome this problem, horizontal wires had been fitted to fasten the flues back onto the wind shield.

Situated on top of the chimney were many tons of steelwork which originally formed part of Futurus’s internal hoists and once again this was tied by wires to the wind shield ensuring the minimum movement.

Basically, they were trying to keep everything as together as possible.

Blasting mats were to be fitted round half the circumference to prevent possible damage caused by flying concrete chunks.

The so called “target area”, lies between the coal staiths on one side and a cooling tower and the generator hall on the other.

Before the ballast Futurus had moved 50 000m³ of soil onto the site to create an impact cushion for the stack.

 Sunday 19 July 1981

 07:50 - A siren wailed in the bitter cold of thee mid-winter morning warning us that the blast would take place in ten minutes.

 07:55 - A voice, on the public address system, told us that the explosion was only five minutes away.

 08:00 – An American Twang can be heard over the loudspeaker. “Ten, nine, eight, seven, six, five, four, three, two one … fire

 A red flare drifted lazily across the area and suddenly the steel plating across the tower erupted as the explosion smote the ear drums.  Then it happened.  Instead of falling like a tree the tower just crumpled seeming to crush itself as it came down.  \immediate reactions were those of a total disaster, but nobody could say for sure until the massive 80m high dust cloud had settled.

 As soon as was physically possible teams were sent in to assess the damage.  They went on site about an hour after the blast.  As they climbed the berm wall which had been constructed as part of the original plan they saw what looked like a comparatively small heap of rubble, rather less than one would have expected from such a massive structure.

 Miraculously, all that had been damaged was the right hand section of the precipitator at Unit Five.  This had been completely flattened and an ash line had been slightly fractured. They could not believe that it was physically possible to do that to that stack, said master blaster, Jim Redyke, afterwards.  “In all my years and all my experience I have not seen anything like that – I just wouldn’t have believed it possible”.

 ESCOM’s Chief Civil Engineer, added, “Initially when we decided to demolish the chimney we would have liked it to have fallen like it did but we took advice and everybody said that it couldn’t be done.  The blast in fact took place as we would like it to have happened but certainly not as we planned it to happen”.

                                                                                                                                                                                                                               

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Jim Redyke, master blaster inspects the dynamite charges just before the blast

 The controversial chimney stack had been beset with difficulties since the number six flue collapsed.  The flue collapsed within the windshield leaving two portions intact.

 ·         The spire, 54m high, being the back part of the bottom of the flue

·         25m of the top of the flue which had come down through the 275m of the chimney with a clearance of 50mm on its circumference where it had passed through the middle         slab of the stack.

 The fall of the flue did not bring down the whole chimney, is itself a miracle.  ESCOM was faced with the difficult decision whether to attempt repair to the collapsed flue or whether to demolish.

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A comparatively small heap of rubble wa left when the dust ​had settled

The Contractors

 Just another job?  The fact the first job for Hi-tower Contractors was the demolition of the R3.3 million chimney which had been built by Futurus.  HI-tower a newly formed company specialising in chimney and cooling tower construction.  The initial brief was to attempt the repair of the fallen flue.  Hi-tower engaged the services of Santon Steeplejacks, a UK firm specialising in jobs of this nature.  In attempting to do the repair, the safety of personnel in the tower could not be guaranteed an the accessibility of the work places as almost impossible, thus Santon was called in.  As work advanced it could be seen how badly the chimney was damaged and it was decided to think along the lines of demolition.

 The decision to demolish was only taken after a lengthy feasibility study had been carried out to determine what the effect of such an explosion would be on the surrounding structures at Matla.  The study showed that the kinetic energy released by the impact would disperse into the earth sufficiently rapidly to prevent damage.

 Once the go ahead had been given for demolition.  Hi-tower again called in Santon who in turn engaged Jim Redyke of Dykon Explosives in Tulsa, Oklahoma, to do the actual blasting.  Redyke consulted Netherton and Associates also of Tulsa.

 The preparations, for the blast was exhaustive.  The scope and the magnitude of the problem were beyond formal education and there was no precedent.  No similar structure as high as this stack had ever been demolished.  Yet Redyke commented after the failure of the stack to go the way he wanted it to go that he would do it exactly the same way again.

 Contractor discussed preparations for the blast with Hi-tower on site a week before the event.

 Hinges

 In the original plan for the fall, the tower would have collapsed along a line which would miss the cooling tower and the coal staith.  The direction of the fall was determined by two factors.  First there was the axis about which the flues had to fall and secondly the ground area available. Had the stack fallen along the axis determined by the two standing flues it would have come too close to the coal staith for comfort. Thus it was necessary to shift the axis of fall 10º west of that direction in which the flues would tend to fall.

One of the measures undertaken to help ensure this, was the introduction of hinges to the inside of the chimney.  

These hinges were placed at each side of the “smile” of bird’s mouth” blasting pattern at the tower’s 40m level.  Four rolled steel joists were placed behind each end of the pattern and four were placed in stepped formation actually in the “bird’s mouth”.  Weighing one ton and measuring six and a half metres each these were bolted on to the windshield and then encased in concrete.  In addition, a concrete slab, 250mm thick, was cast at the 30m level.  This was placed parallel to the hinge points and 90º to the direction of fall; it covered two thirds of the area of the stack.  This was done to create a diaphragm and a measure of stiffness and it was intended that the hinges and the slab would absorb most of the downward fore of the stack during the blast.

“We introduced hinges into the structure, maybe the hinges failed we don’t know. Sand ESCOM’s Chief Civil Engineer.”

“But, said Redyke, every chimney we ever shot never had a hinge.  We just allowed it to bear on its own mass.  We added the hinges to increase the stability at the hinge point, to provide greater cross sectional hingery.”

Added to all the other imponderables it was felt that the construction of a hinge strong enough to carry the total load was impractical.  The design time material requirements and construction time thereof would not be reasonable.

One of the biggest problems in preparing the chimney for the last was the stabilisation of the collapsed flue.  Santon actually had to tie it to the wind shield with steel cables.  The top of the 25m section of flue was resting against the wind shield while the bottom was resting on the mound of rubble.  Any activity or work mound of rubble.  Any activity or work carried out on it caused it to move and until it had been tied own it was extremely unstable.

It was further necessary to chip away what was left of the flue’s spire from the 54m level to the 40m level and to cut away some of the fallen flue.  This was done so that access could be obtained to that part of the wind shield against which it was leaning.  This access was required to admit personnel to drill the blast hoes to accept the dynamite charges.

Protection

In all, 7650 holes were drilled in a “smile”, or ”bird’s mouth” pattern into the wind shield.  This was done at the 40m level around half the circumference of the tower.  At widest the pattern was two and a half metres tapering to two thirds of a metre at the hinges.

In order to protect the cooling tower and coal staith a nine metre high berm was constructed in trough formation around what should have been the tower’s line of fall.  Some 40 000m³ of earth were moved to build this Hi-tower called in Johannesburg-based earthmoving plant specialists.

Shock trenches, seven metres deep had to be dug to help absorb the impact of the fall.  So successful were these that one of the seismographic instruments placed by ESCOM did not even record a reading.

A skirt of steel plating was placed around the wind shield to cover the blast holes at the 40m level.  Santon also placed steel plating at the opening at the bottom of the tower.  The intention of these was to prevent any material shooting out of the tower during the blast and damaging other structures.  These were successful as we could not spot so much as a broken window after the blast.

 Aftermath

 We had all been very lucky.  Granted ;the blast did not go as planned.  “The best laid plans of mice and men gang aft agly…” and this maxim seems to have plagued the Matla stack for a long while  The construction of such a tower is a testament to man’s engineering ability, its destruction must cause sadness in the hearts of those who devoted their lives to building.

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THE COAL FIRED THERMAL POWER STATION – BASIC CYCLE

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THE BOILER

Coal is fed from coal staithes (1) to the boiler bunkers (2) by a conveyor belt, from where it is fed into pulverising mills (3) which grind the coal to powder.  The pulverised coal is carried by a stream of air from the mills to the boiler burners (4), where it is blown into the furnace (5) to burn like a gas.  The products of this combustion are dust and ash. The ash falls to the bottom of the boiler where it is sluiced away for treatment and the dust is carried in the flue gases to the precipitators (6) where most it is collected electrostatically.  The cleaned flue gases pass through the chimney (7) to the atmosphere.

SUPERHEATED STEAM

 Heat released by the burning coal is absorbed by boiler feed-water inside many kilometres of tubing which form the boiler walls.  The boiler feed-water is converted to steam at a high temperature and pressure.  The steam is super-heated in further tubes (8).  Superheated steam passes to the high-pressure turbine (9) where it is discharged onto the turbine blades.  The energy of the steam striking the turbine blades causes the turbine to rotate at 3 00 rpm.  After exhausting some of its energy in the high-pressure turbine, the steam is reheated in the boiler re-heater (10) then passes through the intermediate-pressure turbine (11) and from there to the low pressure turbine (12).  Coupled to the turbine shaft is the rotor of the generator (13).  The generator rotor is a cylindrical electric-magnet which is enclosed within a gas-tight housing.  The stator (14) consists of large coils of copper bar in which electricity is produced by the rotation of the magnetic field in the rotor.  The electricity produced passes from the stator windings to a transformer (15) where the voltage is raised from about 20Kv to the national transmission voltages (275 kV and 400 kV/2 at Matla).  Electricity passes through the high-voltage yards from where it is distributed to consumers via the national transmission network.

Cooling and recirculation

 After exhausting its energy in the turbines the steam is condensed in a condenser (16) and pumped back as water through the de-aerator to the boiler economise (17) by means of the boiler feed pump (18) for reheating.  Water from the economiser is feed to the steam drum (19), which contains both a water and steam drum and conveyed to the super heater (8) for further hearing before passing to the turbine.  The water is fed to the furnace tube walls via headers at the bottom of the combustion chamber to recommence the cycle.

The condensers contain many kilometres of tubing (about 328 km at Matla) through which cold water from the cooling towers (20) is constantly pumped.  Heat which the cooling water extracts from the steam circuit is removed by spraying the water out in the lower levels of the cooling towers; the cooled water is then collected in ponds beneath the towers cools the water.  The cooled water is then re circulated to the condensers.  Some of the sprayed water inevitably rises with the draught of air, forming the familiar clouds of water vapour at the top of cooling towers.