reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

deionise the turbine condensate recovered from ''be condenser wells. Condensate is demineralised by ed-bed units and returned to the steam generator

ini

faclwater system. The levels of suspended solids and ,he pH value are controlled to minimise corrosion of :he boiler drums, tubing and auxiliary equipment.

Fi,,ures 10.21 and 10.22 show a simplified arrange- xrit o f t he pre-ireatment and ion-exchange plants at

pumps feed the water through mixed-bed units to the

roerve feedwater tanks.

Figure 10.23 shows a simplified regeneration system. Three regeneration water pumps supply water from the recerve feedwater tanks to acid and caustic dilution

tanks and thence to the respective anion, cation and mixed-bed units of the ion-exchange and condensate polishing plants.

Figure 10.24 shows the condensate polishing plant. The primary condensate feed is via the condenser extraction pumps into the mixed-bed units and thence to the de-aerator via the feedheating units as shown in Fig 10.25.

Figure 10.26 illustrates the feedwater chemical dosing system. Two 100% duty ammonia dosing pumps and two 100% duty hydrazine dosing pumps are provided to treat polished condensate and de-aerator inlet/outlet feeds.

Cooling water is treated to prevent scale deposition, organic growth and algae on heat exchange surfaces and to prevent marine growth.

Cooling water treatment consists of intermittent dosing with chlorine or of continuous dosing with sodium hypochlorite. Sodium hypochlorite dosing is pre-

STATIC MIXER

• • •

DISCHARGE

VALVES

RAW WATER

RUMPS

SUCTION

VALVES

•

II

RAW WATER

I NLET

LOW PRESSURE

AIR

FROM AIR BLOWERS

SAND

FILTER

AIR

BLEED

AIR BLEED

SAND

FILTER

'1/4...."4■-•■■■•••""

XDISCHARGE

VALVES

FILTERED

WATER

PUMPS

CAUSTIC 4- 4

TO MIXE BED UNITA

FROM

CAUSTIC

OILUTIONS TANK.

ferred for coastal stations because it has been shown to be more effective in the treatment of marine growth. It also eliminates the dangers associated with the storage of liquid chlorine.

The electrochlorination plant (sodium hypochlorite production and storage) at a nuclear power station is described in Section 6 of this chapter. Four dosing

pumps are provided to inject sodium hypochlorite into the cooling water system.

7.2 Electrical distribution system

3.3 kV and 415 V switchboards are provided to ensure maximum security of electrical supplies to the water

. MAIN SYSTEM FLOW

ACID

CAUSTIC

RESERVE

FEEOWA TER

I NLET

treatment plant. The distribution system is designed to minimise disruption to the plant operation in the event ot, .supply failure by feeding the plant from alternative w itchboards and providing interconnection facilities. For the water treatment plant described in the previous ection, the switchboards are divided into two sections, ach section being fed from independent sources and he two sections interconnected.

The largest individual loads are the electrochlorina- ! Ors — typically 750 kW and supplied at 3.3 kV. The

3.3 kV and 415 V distribution system for the electrochlorination plant is described in Section 6.3.7 of this chapter.

Other water treatment plant loads are fed from 415 V switchboards via latched contactors, this type of contactor being used in view of the essential nature of the plant and the need to safeguard against spurious tripping. The main 415 V three-phase loads are as follows — two 30 kW raw water pumps, three 40 kW filtered water pumps, two 30 kW booster pumps, two

3 kW recovered water pumps, three 50 kW effluent pumps and two 0.5 kW alum dosing pumps on the pretreatment and make-up plant; two 90 kW recirculation pumps on the condensate polishing plant; four 0.5 kW hydrazine dosing pumps, four I kW ammonia dosing pumps and, two 1 kW tank stirrers.

7.3 Electrical control

A suite of control panels located in the water treatment plant building houses all the equipment necessary for the fully automatic control, monitoring and alarm annunciation functions of the plant, and manual control facilities for maintenance and commissioning. Selected alarms are repeated in the Central Control Room.

Plant operation is controlled automatically by programmable logic controllers (PLC). Logic-initiated solenoid valves, which actuate the pneumatically-op- erated and motorised control valves, are fitted with override facilities which allow the plant to be controlled manually in the event of automatic control system failure.

7.4 Motor drives

The majority of electric drive motors are 415 V threephase, squirrel cage, totally-enclosed fan cooled types arranged for direct-on-line starting and having a degree of protection of 1P54 or CP55 weatherproof to suit the environment.

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CONDENSERS (THREE OFF)

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STEAM':

CONDENSER!

Ii.. 10.25 Condensate and feedhealing system

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R{G E A TIcN 2,1D

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VENT

AMMONIA

DOSING

DELIVERY

7.5 Frost protection

Trace heating and immersion heaters are provided, as necessary, to protect the plant against frost. A detailed description is given in Section 5.1.7 of this chapter.

7.6 Electrically- operated valve actuators

Motorised valves have actuators equipped with integral control gear. To facilitate maintenance and replacement, each actuator is equipped with flexible control and power cables fitted with connectors which couple with a disconnection box. Fixed cabling is glanded and terminated in the disconnection box in the normal manner.

8 Coal, ash and dust plant

8.1 Coal handling plant

8.1.1 General description of plant

Coal is delivered to a power station either by sea, canal, road or rail: rail is the most common in view of the large and regular deliveries of coal required by large modern power stations, especially since most large coal-fired power stations are located inland.

On those stations supplied by water, three methods are used to unload coal from ships or barges:

•Various designs of continuous-unloaders which remove the coal from the barges or ships and feed it

directly onto a twin conveyor system.

• Harge-unloaders which hoist and tip the barges one a ti me to discharge their loads of up to 200 t \! coal into hoppers, from where it is fed by paddle

. :20,.lers onto a twin conveyor system.

• rra%elling/grabbing cranes which load the coal into 'oPPers, from where it is fed by paddle feeder onto ,i[o.in conveyor system.

commonly, coal is delivered by trains working power station on a merry-go-round system in the trains remain coupled and move contin-

•. 1 `IY .at a speed in the order of 0.8 km/h during the

oading sequence. The wagons, which are of the :Ter-bottom type, pass through the unloading area ,re they discharge their loads into underground hop-

pers and are automatically weighed 'full' and 'empty' to establish the amount of coal delivered. A minimum of two conveyor systems, each of which can be operated independently, are fed by twin-paddle feeders from the associated wagon unloading hopper. A typical rail unloading plant is illustrated in Fig 10.27.

From the wagon unloading hoppers, the coal is transported by a system of electrically-powered conveyors and flap (diverter) valves, either directly to the boiler bunkers or to the coal store. Metal detectors and magnetic separators divert tramp iron and metallic objects from the coal stream. Trash screens remove non-metallic objects and one or two stages of vibrating screens and crushers ensure that the coal is reduced to a size and consistency which is suitable for the mills to

handle. Belt weighers are provided to determine the weight of coal being delivered to store, reclaimed from store, and delivered to the bunkers'. Coal fed onto the bunker feeder system is automatically sampled. The conveyor system incorporates junction houses in which plant is located. A typical junction house is shown in Fig 10.28.

At most stations coal is delivered to or reclaimed from store by travelling stacker/reclaimer bucket-wheel machines. These machines are boom conveyor systems which stack or reclaim coal whilst travelling the length of the store on rails.

Dust suppression or extraction systems are provided at strategic locations, such as wagon unloading hoppers, bunker tops, crusher buildings and junction houses, where the concentration of airborne coal dust might otherwise constitute a hazard.

Figure 10.29 shows the coal plant conveyor system at a large power station.

8.1.2 Electrical supplies

\lost loads for flap valves, conveyors, vibrating feeders, crushers, paddle feeders, magnetic separators, fans and trash screens generally are supplied at 415 V three-phase 50 Hz from the 415 V Coal Plant Switchboard, although 3.3 kV supplies may be used when the duty demands. Stacker/reclaimer machines are supplied at 3.3 kV. Electrical distribution is designed to safeguard the

independent operational requirements of the duplicated coal plant facilities and to ensure that an electrical fault will not result in the total loss of coal supplies to the boilers.

A typical distribution diagram is shown in Fig 10,30.

8.1.3 Electrical control

A control desk and mimic panel in the Coal Plant Control Room forms the central control facility for the coal plant. Manual/automatic control facilities and plant status indicators are provided. Control of the stacker/reclaimer machines is from the machines, only status indications being provided in the control room. Wagon unloading and lineside signalling control is from either the control room or the wagon unloading house, supervision of this and other important aspects, such as the coal stores, being assisted by television monitors and cameras (Fig 10.29).

Sequential control of conveyors and paddle feeders is provided for start-up and shutdown. This is essential to avoid coal spillage which might otherwise occur- due to the differing rates of acceleration and decelera tion of the conveyors attributable to the wide variation in their lengths and lifts.

8.1.4 Conveyors

Conveyors are driven by 3.3 kV or 415 V three-phase 50 Hz squirrel-cage induction motors through ALIA

couplings to protect the conveyors during start-up. The motors are totally-enclosed, fan cooled, with a degree of protection of I1 3 55. Each conveyor is equipped "ub an emergency trip-wire system running the full

length of the conveyor alongside the access way. Switches are located at intervals along the trip wire. The trip wire may be a wire rope arranged to operate trip switches directly, or a cable designed to trip relays

rOWER 3

TRANSFER Tav.ER 2

SHUTTLE cONVE fOR

BUCKET WHEEL

MACHINE NORTH

STORE NORTH

26

JUNCTION HOUSE 1

JUNCTION HOUSE 2

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'AERGENCY sTocKCuT

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TRASH SCREENS

BUCKET WHEEL

MACHINE SOUTH

31 30

CAMERA

UNLOAERNG HOPPERS

:UNCTION HOUSES

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SEPARATOR

EMERGENCY

RECLAIM HOPPER

N3 V-GT HE RS

RECLAIM FEEDER 2

RECLAIM FEEDER ,

JUNCTION HOUSE 5

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