AUXILIARY PRESSURE REDUCING & DESUPERHEATING STATION (APRDS)

OBJECTIVE

• Auxiliary steam is the steam that is required for various auxiliary requirements in a power plant.
• A part of main steam is tapped off for auxiliary steam station from MS/CRH line, after pressure reducing and de-superheating up to 16 kg/cm² & 320 °C. At the HT header (16 Kg/cm² & 320⁰C) and then at the LT header (16 Kg/cm² & 210 ⁰C) through desuperheater b/w the HT & LT headers.
• Spray water quantity required for the temperature reduction of the steam is controlled by separate spray water valve (taken from CEP discharge).

TYPES OF STATIONS

• High capacity PRDS (LTPRDS) with tapping off steam from main steam line or CRH line .
• System consists of two auxiliary steam pressure reducing and desuperheating stations (APRDS).
• Low Capacity PRDS (LTPRDS) from HT header.

APRDS SERVICES

Auxiliary steam system is designed to provide steam for following systems.

• Turbine & Turbine auxiliaries.
• Boiler auxiliaries.
• Fuel oil heating system.

USE FOR PLANT

• During startup.
• Low loads.
• Normal running.

However auxiliary steam systems of all existing units are interconnected through HT and LT auxiliary steam headers, through HT station header & LT station header so that unit start-up requirement of one unit can be meet from other unit or vice-versa.

• An auxiliary boiler has been envisaged for initial start-up of the unit and fuel oil heating.
• For starting the units, boiler has to run on light oil and steam thus generated will be used for unit start-up.)

SYSTEM DESCRIPTION

• APRDS is used for Steam Conditioning Services for reduction of pressure and temperature of steam.
• It is a combination of Control Valve for the pressure reduction purpose and atomizing nozzles through which water is sprayed into steam for reducing the temperature.
• Water spray control valve for each HT & LT header.
• APRDS safety valves are installed for prevent of excess pressure of header.
• Nos. of safety valves 3 nos.
• Pressure setting 18 Kg/cm².
• Combined PRDS: Both PRV and DeSuperheater in a compact single unit.
• Split PRDS: Separate PRV (Pressure Reducing valve) and DeSuperheater.
• Combined PRDS will help to save the space and also cost of piping, as the unit is single and compact. For PRDS control system.
• There will be one Pressure loop and one temperature loop. Pressure control system includes pressure sensor, pressure transmitter, controller, Pressure gauges, etc.

USE OF AUXILIARY STEAM

• Deaerator pegging steam.
• Deaerator feed storage tank heating.
• Main ejector & starting ejectors
• Turbine seals.
• TDBFP gland sealing.
• Turbine wet steam washing.
• Fuel oil atomization, Steam tracing lines heating station.
• APH soot blowing during startup.
• SCAPH.
• Boiler acid cleaning.
• Mill inerting (mill fire fighting).
• AC equipment for main building.
• HT & LT station header.

STEAM TAKEN FROM FOLLOWING POINTS

• MS line tapping.
• CRH line tapping.
• Interconnection from station header (station header is fed by other unit auxiliary boiler).

PARAMETERS OF APRDS FOR 500 MW UNITS

HT PRDS USE

• TG gland sealing
• Ejector
• TDBFP
• AH soot blowing

LT PRDS USE

• Deaerator paging
• FST initial heating
• HFO heating
• SCAPH
• Wet steam washing

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