HP & LP Bypass Operation and Protection

HP & LP Bypass Operation and Protection

Purpose

• Boiler operation independent of the turbine
• Minimum steam flow through SH & RH tubes during start-ups, low load operation & sudden unloading of unit
• Utilized or building up of steam parameters
• Avoiding the boiler trip on Reheat protection
• No steam flow through Boiler steam tubes will lead to starvation & thereby overheating and lead to premature failure of tubes 
• The capacity of HPLP bypass is 60% of full load turbine steam flow 

Reheat Protection:

RH protection (delay=10 sec) will act if

1. TURBINE HP V/V & HPBP CLOSED [ either turbine HP stop v/v or HP CV in both HP line closed ] & [ both HP BP <2%]             OR
2. TURBINE IP V/V & LP BP closed [ Either IP stop v/v or IP CV v/v in both IP line closed ] & [ Either LP BP stop v/v or LP BP C/V in both LP line is closed] OR  
3. Generator ckt bkr open & [both HP BP closed OR both LP BP closed

HPBP Oil Supply Unit Interlocks:

• Pump Auto off Oil Press high >160ksc
• Pump Auto on Oil Press Low <120ksc
• Pump Auto off Oil Temp high >70 DegC
• Pump Auto off Oil Level low <18 Lit
• Alarm Oil Press too high >170ksc
• Alarm Oil Press too Low <90ksc

Pressure Mode:

Stg-2 Mode of Operation 

• Initial Pressure Mode  -
  Importance to the load 
• Fixed Pressure Mode -
  Importance to the Health of turbine.

HP BP Protection:

BP Valve Priority Close Conditions:

• Condenser Vacuum Low (<-0.6 ksc)
• Bypass Downstream Temp very high (>380 degC) 

                    Under these conditions 

• Steam Press Controller Switches to Manual from Auto 
• spray water valve will close
• This protection will override the Fast opening signal & open push button operation

HP Bypass Fast Opening Conditions 

• Generator Circuit Breaker Opening & Boiler on & no priority close
• Turbine Trip & Boiler on & no priority close
• Turbine Load Shedding Relay Operation
• HP BP Pressure Controller Deviation > 8 ksc
• Pressing the Fast Opening Button

BP Valve Auto Blocking Conditions  

• Bypass valve priority close command
• BP position transmitter failed 
• OSU-1 &  2 Oil Pressure <90 ksc
• Main steam Pressure transmitter failed

HP BP Interlocks:

BPE Valve Conditions  

• Steam Bypass Valves BP1 or BP2<2 % Auto Close Command to ensure positive shut off
• Steam Bypass Valves BP1 or BP2<2 % Spray Water Pressure Control Valve (BD Valve) close
• Steam Bypass Valves BP1 or BP2>2 % Spray Water System (BPE1, BPE2, BD Valve) will switch over to Auto

BPE Valve Auto Blocking Conditions  

• Bypass Downstream Temp very high (>380 degC)
• BP auto command blocking
• BPE valve position transmitter failed 
• BPE Temp transmitter deviation high 
• Therefore, it is necessary to make proper set values for Spray Water Header Pressure & HP BP downstream temperature before operating the Steam Bypass Valves

LP Bypass:

Functions of LP Bypass System are:

• Maintaining minimum Steam Flow through Re-heater
• Maintaining desired Steam Pressure in the Re-heater
• Providing an alternative path for dumping the steam (not accepted by the IP & LP Turbines) into the condenser

LP BP System comprises of : 

• Steam Pressure Control Loop
• Valve Position Control Loop
• Automatic Control Interface (ACI)
• Condenser Temperature Protection
• Tracking Unit

LPBP Protections:

Close LP Bypass under following conditions:

1. Condenser Vacuum Low (<-0.6 ksc)
2. Spray Water Pressure Low (<8 ksc)
3. Condenser Wall Temperature High ( > 90 DegC)
4. LPBP Down Steam Pressure High (>19 ksc)
5. Spray Water valve open command at Steam pressure before bypass valve >6.8 ksc