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

C and 1 air compressor control Two compressors

,irc provided, each 100 ,7o-rated, one being selected to 'duty' and one to 'standby'. Their operation is normally

Jtoomatic, being governed by pressure switches on the storage vessel.

A manual control facility is provided at the 415 V sssitchboard.

5.4.2 Classification of plant areas

The classification of the various areas, as defined in BS5345 [5], is as follows and the electrical equipment is provided to suit (see Section 5.2.1 of this chapter):

•Methanol gas producer room — Zone 1.

•Hydrogen compressor room — Zone I.

Rectifier control The quantity of hydrogen produced the cells is controlled by varying the rectifier output i.urrent. This is normally automatic, but a manual ,orarol facility is provided. The rectifier output is , aried by switching the AC rectifier supply. Under livomatic control, pressure switches in the HP hydrogm network are used to vary hydrogen production and so to maintain pressure in the pipework and stor-

•Storage area:

(a)Within 1 m of methanol and hydrogen storage vessels or directly above them — Zone 1.

(b)Within 3 m of above storage vessels — Zone 2.

(c)Remainder — non-hazardous.

•Electrical equipment room — non-hazardous.

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TO DISTRIBUTION (25-30 bar)

5.4.3 Electrical, control and instrumentation

equip ment

production control panel The gas production

, [1!rol panel is located in the electrical equipment and is the focal point for control of the hydrogen 2:,mr. Accommodated in the control panel are the

•, I1 0‘king facilities for automatic and manual control:

•\ mimic diagram indicating the state of the plant.

•indicating and recording instruments for hydrogen

rrosure; methanol supply purity; hydrogen tempera- :tire, dryness and purity; and other data necessary or the operation and monitoring of the plant.

•I ! Orogen generation controls.

•il),Iroe.en compressor controls and instruments.

•(. and I air compressor controls and instruments.

•\ l am annunciator panel, incorporating 'alarm ac- p:', 'reset' and 'lamp test' pushbuttons, an audible

aLarrn and alarm contacts used to bring up alarms :0 he Central Control Room of the power station.

general control philosophy is 'failure-to-safety'.

it.drcwn generation control Automatic and manual

•, iirrol of the methanol generator is provided, the

'ruler being designed to maintain the selected hydroProduction rate, irrespective of variations in system

•"d dmbient conditions. The rate of gas production

..antrolled by switching or varying the voltage of :lc preheater AC supply to maintain gas pressure in 1-1 1' pipeline and storage vessel between 25 and bar. Production is stopped if the gas pressure rises

Gas producing and storage plant

to 31 bar or if the LP gas holder is greater than 95 07o full.

Manual control facilities comprise 'auto/manual' control selection, production 'on/off' and 'raise/lower' controls.

Preheater supply A contactor-controlled 415 V threephase four-wire supply is provided to the preheater, the individual elements of which are connected for 240 V single-phase operation. Overload protection is provided on the 415 V supply and arranged to trip the supply contactor and raise an alarm.

Protection devices trip the heater supply when the heater temperature exceeds approximately 570 ° C and restore it automatically when the temperature falls to a safe level. This prevents the gas generator from overpressurising.

5.5 Methane production plant

5.5.1 General description of plant

Methane (CH4) is produced by the catalysed reaction of carbon dioxide (CO2) and hydrogen (H2). Carbon dioxide is drawn from the CO2 storage tanks and hydrogen from the LP gasholder.

Hydrogen is supplied to the methanation plant via an LP compressor and mixer, the hydrogen and carbon dioxide mixing at sufficient pressure to drive it through the methanation process plant. Methanation takes place within a catalyst vessel, the methane produced being fed via compressors and separators to an HP storage vessel.

Methanation occurs in two stages within the tubes of the reaction vessel over a bed of catalyst packed inside the tubes. From the first stage reaction vessel, the gas produced passes via the tube side of a recuperative heat exchanger into the first stage condenser which removes water produced in the reaction.

The gas is then passed through the shell side of the recuperative heat exchanger to raise its temperature prior to entry into the second stage reaction vessel. Gas temperature is controlled by adjusting the volume of gas returned through the heat exchanger, using the inlet regulating and by-pass valves.

The heated gas then passes through the second stage reaction vessel and through the second-stage condenser, where the water produced by the reaction is again removed. From there, the methane gas is compressed and fed to the HP storage vessel at a pressure in the order of 44 bar.

The rate of methane production is controlled either automatically or manually by varying the supply of hydrogen and carbon dioxide from their respective storage vessels.

Once started, the process within the reaction vessels generates sufficient heat for it to be self-sustaining. However, to start the reaction, the first stage vessel

is heated to operating temperature by electrical fan heaters.

A typical system diagram is shown in Fig 10.13. Methane production plants include hydrogen production and storage facilities as outlined in Sections 5.3 and 5.4 of this chapter.

5.5.2 Classification of plant areas

The classification of the areas of the plant, as defined in BS5345 [5], is as follows and electrical equipment is provided accordingly (see Section 5.2.1 of this chapter):

•Electrolytic cell room — Zone I.

•Hydrogen compressor room — Zone 1.

•Methane reactor room — Zone 1.

•Methane compressor room — Zone I.

•Storage area:

(a)Within 1 m of hydrogen storage vessel and above it — Zone I

(b)Within 3 m of hydrogen storage vessel excluding

(a) — Zone 2.

(c)Remainder — non-hazardous.

•Electrical equipment room — non-hazardous.

5.5.3 Electrical, control and instrumentation equipment

The hazards associated with the production and handling of hydrogen and methane are similar and the general requirements in respect of electrical equipment are as detailed for hydrogen plant in Section 5.3 of this chapter.

Central control of the methane production process is provided at a gas production control panel located in the electrical equipment room, automatic and manual control facilities being provided.

5.6Nitrogen storage plant

5.6.1General description of plant

Liquid nitrogen is stored in a vacuum-insulated vessel. A bank of vaporisers, typically four, converts the liquid into nitrogen gas for distribution. The vaporisers may be any one of four types: electrical heat storage, ambient air fan-assisted, ambient air natural-draught, or steam (when auxiliary steam is available). The choice of vaporiser depends upon nitrogen production rate, that of power stations usually dictating the use of electrical or steam heated types because of their greater capacity and compact size. Gas pressure at the exit from the vaporisers is regulated by control valves within the range 7 to 17 bar for distribution purposes.

Distribution is isolated if gas pressure exceeds 17 bar or if its temperature falls below 10 ° C. Gas pressure

is further reduced outside the storage plant for final distribution. Make-up water supplies to electric heat storage vaporisers are trace heated to prevent freezing.

A typical storage-compound layout is shown i n Fig 10.14 and a bulk storage system in Fig 10.15.

5.6.2 Electrical requirements

Electrical requirements are limited to supplies for va_ poriser heater control cabinets, heaters, motorised valves, solenoid valves, trace heating and lighting. 415 V three-phase 50 Hz is provided for the purpose.

5.7 Carbon dioxide storage plant

5.7.1 General description of plant

Liquid carbon dioxide is delivered by road tanker and stored in a vacuum-insulated vessel. A fully automatic refrigeration system is provided which maintains the vessel pressure at 20 bar at – 17 ° C. Vaporisers convert the liquid into carbon dioxide gas for distribution. Vaporisers may be any one of four type,; electrical heat storage, ambient air fan-assisted, ambieN air natural-draught or steam (when auxiliary steam is available). The choice of vaporiser depends upon car. bon dioxide production rate, that of power stations usually dictating the use of electrical or steam heated types because of their greater capacity and compact size. Gas distribution pressure is nominally 19 bar. To prevent the passage of liquid carbon dioxide through the vaporisers and into the distribution system in the event of a fault, the gas temperature at the vaporiser outlet is monitored. If the gas temperature falls to

– 10° C, a control valve isolates the feeder to the distribution system.

A typical storage compound layout for generator purging is shown in Fig 10.16 and bulk storage system in Fig 10.17. Nuclear power stations need a larger storage capacity and a greater number of storage vessels and vaporisers, but the basic requirements are the same as for fossil-fired stations.

5.7.2 Electrical requirements

Electrical requirements are limited to supplies for vaporiser heater control cabinets, heaters, refrigeration units, motorised valves, solenoid valves and lighting. 415 V three-phase 50 Hz is provided for the purpose.

6 CW electrochlorination plant (sodium hypochlorite production and storage)

6.1 General description of plant

Sodium hypochlorite is produced for the treatment of cooling water (CW). Sea water is pumped through strainers to banks of electrochlorination cells. Thyristor

controlled rectifier units provide LV variable voltage DC supplies to the cells, thereby controlling the rate of production of sodium hypochlorite by electrolysis within the cells. A by-product of the process is hydrogen, which is of inadequate purity for use in a power station and has, therefore, to be exhausted in a controlled and safe manner. A solution of sodium hypochlorite, hydrogen and sea water leaves the cells and is fed to a hydrogen release tank, which exhausts the hydrogen to atmosphere. The hydrogen-free sodium hypochlorite solution then passes to a storage tank. Sodium hypochlorite is pumped from the storage tank to the dosing points in the water treatment plant. A system diagram for a power station site is shown in Fig 10.18.

The disposition of equipment within a typical CW electrochlorination plant is illustrated in Fig 10.19. Electrical equipment rooms housing 415 V switchgear, transformer/rectifier units, control/mimic panel and control air pumps are situated adjacent to the electrochlorination cell room. Outside the building is a storage area in which the sodium hypochlorite storage tanks, sodium hypochlorite distribution pumps and hydrogen release tanks are situated, the latter being above the

storage tanks and at a high level to ensure the safe discharge of hydrogen.

6.2 Classification of plant areas

The classification of areas, as defined in BS5345 [5], is as follows, electrical equipment within those defined areas being provided accordingly (see Section 5.2.1 of this chapter).

•Electrochlorination-cell room — Zone 1.

•The area directly above the hydrogen release tank — Zone 1.

•Within 2 m of the hydrogen release tank — Zone 2.

•Electrical equipment room(s) — non-hazardous.

6.3 Electrical, control and instrumentation equipment

6.3.1 General

-

Many similarities exist between the electrical require ments of this plant and of the hydrogen production

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111CIIIS which are similar for both are described :c following sections: DC connections and busbars, harrier unit cubicle, warning notices and gas

back - up protection, electropneumatic con- equipment, and C and I air compressor control.

6 12 Production control panel

• production control panel is located in the electrical

.. pinent room and constitutes the control centre for production and storage of sodium hypochlorite or its distribution for water treatment purposes.

\„. , iii modated in I he control panel are the following for automatic and manual control:

•■ mimic diagram indicating the state of the plant.

•fransformer/rectifier controls and indicators.

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CO2 GAS TO

GENERATORS

PLAN

FIG. 10.16 Carbon dioxide storage plant layout

•Alarm annunciator panel, incorporating 'alarm accept', 'reset' and 'test' buttons, an audible alarm and repeat alarm contacts to initiate alarms in the Central Control Room.

The control system is designed for automatic control of the plant under normal operational condition, with the facility for manual control, if necessary. Overall control of the system is via a programmable logic controller (PLC) housed in the control panel.

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6.3.3 Sea water feed pumps and strainers control

The scheme shown in Fig 10.18 has three 415 V threephase 50 Hz sea water feed pumps each capable of delivering half of the total teedwater flow required. Two of the three pumps are selected to 'duty' and one to 'standby'. Loss of a duty pump automatically causes the standby pump to start. Two motorised sea water strainers are provided, each having sufficient capacity to handle the full plant output.

Automatic operation of the dosing system is con. trolled by the production control panel logic system, the number of pumps started being determined by the number of hypochlorite generators needed to satisfy chlorination requirements.

Hypochlorite flow from the dosing pumps is auto. matically regulated by two motorised valves installed in the pump outlets.

6.3.7 Electrical distribution

6.3.4 Transformer/rectifier controls

The scheme shown in Fig 10.18 has eight transformer/ rectifier equipment, each supplying DC current to its associated electrochlorinator. The transformer/rectifier units can be selected for automatic or manual control either individually or as a group, thereby ensuring maximum control flexibility.

6.3.5 Sodium hypochlorite storage

Production of sodium hypochlorite is governed by the level in the storage tanks, leading to shut down if the maximum level is reached. Similarly, a low level will shut down the dosing pumps.

6.3.6 Dosing pump controls

The scheme shown in Fig 10.18 has four 415 V threephase 50 Hz dosing pumps, each capable of delivering one-third of the maximum dosing rate. A 'three out of four' duty selection scheme is provided; the fourth pump is on standby and starts automatically upon failure of one of the duty pumps when automatic control is selected. Manual control of each pump can be selected, if required, thereby ensuring maximum flexibility of operation.

The majority of supplies to the plant drive-motors are 415 V three-phase 50 Hz and are derived from two contactor boards, each board being supplied separately. An interconnector is provided between the two boards. This ensures security of supply with the maximum operating flexibility. Multiple drives, such as transformer/rectifier equipment and dosing pumps, are spread over the two boards to minimise the effect on plant operation of the loss of one board due to supply failure.

The electrochlorinators present the largest individual loads, each being in the order of 750 kW, and are supplied at 3.3 kV three-phase 50 Hz by circuit-breakers on two switchboards. Hypochlorite dosing pumps, typically 55 kW, sea water feed pumps and ventilation fans are supplied by electrically-held contactors on the 415 V boards.

7 Water treatment plant

7.1 Description of plant

The design of water treatment plant varies according to the quality of raw water which may be sea, river , lake or towns water; the first two sources are the more