APRDS in THERMAL POWER PLANT

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 16 Kg/cm²  & 320⁰C) and then at the LT header (16 Kg/cm2 & 210⁰C) 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

System consists of two auxiliary steam pressure reducing and desuperheating stations (APRDS).

High capacity PRDS (LTPRDS) with tapping off steam from main steam line or CRH line .

Low Capacity PRDS(LTPRDS) from HT header.

APRDS SERVICES

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

1:- Turbine & Turbine auxiliaries.

2:- Boiler auxiliaries.

3:- Fuel oil heating system.

USE FOR PLANT

1:- During start-up.

2:- Low loads.

3:- Normal running.

Auxiliary steam system is normally operating on unit basis.

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

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

1. Deaerator pegging steam.
2. Deaerator feed storage tank heating.
3. Main ejector & starting ejectors
4. Turbine seals (Gland sealing steam)
5. TDBFP gland sealing
6. Turbine wet steam washing
7. Fuel oil atomization, Steam tracing lines heating station
8. APH soot blows during startup.
9. SCAPH.
10. Boiler acid cleaning
11.  Mill inerting (mill fire fighting)
12. AC equipment for main building.
13. HT & LT station header
14. TDBFPs (initially) standby steam source

STEAM TAKEN FROM FOLLOWING POINTS

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

PARAMETERS OF APRDS

• 500 MW         
• HT header                                
• 16 Kg/cm²                                
• 320°C                                                 
• 115 tph                                              
• LT header
• 16 Kg/cm² 
• 210°C
• 75 tph

HT PRDS                                                                                 

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

LT PRDS

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