Drum Level Control HP Drum Level Control

(Drum level will be corrected by the drum pressure).

Normal Level:

The drum level control will regulate the rate of feed water flow to maintain a proper drum level throughout the operation range of the steam generator. To get the maximum benefit from a three-element system, the feed water flow should be proportional to steam flow with reset action on drum level which means that the mass of feed water entering should always be equal to the mass of steam leaving, with continuous corrections made for deviations from level set points.

HRSG OPERATION AND MAINTENANCE

• HP FW By-pass Control Valve (V-108) will open & close (modulate) to maintain drum NWL set point, when HP FW Control Valve (V-100) is opened <20%.

• HP FW Control Valve (V-100) will open & close (modulate) to maintain drum NWL set point, when HP FW By-pass control valve (LV-D05B) reaches 80% opening.

• When HP FW Control Valve (V-100) is opened >20%, then HP FW By-pass Control Valve (V-108) shall be closed fully.

• HP FW Control Valve (V-100) shall be closed fully if valve opening < 15%.

Required actions:

High High level:

Sound the alarm.

Close the HP Feedwater Stop Valve (HV-102).

Ensure that HP Feedwater By-pass Control Valve is closed (V-108) GT protective load shedding (*).

HP Feedwater Stop Valve (HV-102) remains closed.

High level:

Sound the alarm.

Open the HP Evap Intermittent Blow-off Isolation Valve (HV-180).

Low level:

Sound the alarm.

Low Low Level:

Sound the alarm.

GT protective load shedding (*).

GT trip (after 2 min. of total elapsed time) if alarm not cleared.

(*) PLS will bring GT to zero load within 2 minutes (GT gas temperature is expected to be

~50% of its full scale). PLS will continue until correcting the cause of problem and process returns to normal.

IP Drum Level Control:

(Drum level will be corrected by the drum pressure).

Normal Level:

The drum level control will regulate the rate of feed water flow to maintain a proper drum level throughout the operation range of the steam generator. To get the maximum benefit from a three-element system, the feed water flow should be proportional to steam flow with reset action on drum level which means that the mass of feed water entering should always be equal to the mass of steam leaving, with continuous corrections made for deviations from level set points.

HRSG OPERATION AND MAINTENANCE

ALSTOM Power Revision: 0 4-3

Copyright 2005 04/02/05

• IP FW Control Valve (LV-430) will open & close (modulate) to maintain drum NWL set point.

Required actions:

High High level:

Sound the alarm.

Close the IP Feedwater Stop Valve (HV-400).

GT protective load shedding (*).

IP Feedwater Stop Valve (HV-400) remains closed.

High level:

Sound the alarm.

Open the IP Evap Intermittent Blow-off Isolation Valve (HV-480).

Low level:

Sound the alarm.

Low Low Level:

Sound the alarm.

GT protective load shedding (*).

GT trip after 2 minutes (maximum total elapsed time) if alarm not cleared.

(*) PLS will bring GT to zero load within 2 minutes (GT gas temperature is expected to be

~50% of its full scale). PLS will continue until correcting the cause of problem and process returns to normal.

LP Drum Level Control:

(Drum level will be corrected by the drum pressure).

Normal Level:

The drum level control will regulate the rate of feed water flow to maintain a proper drum level throughout the operation range of the steam generator. To get the maximum benefit from a three-element system, the feed water flow should be proportional to steam flow with reset action on drum level which means that the mass of feed water entering should always be equal to the mass of steam leaving, with continuous corrections made for deviations from level set points.

• LP FW By-pass Control Valve (V-819) will open & close (modulate) to maintain drum NWL set point, when LP FW Control Valve (LV-801) is opened <20%.

• LP FW Control Valve (LV-801) will open & close (modulate) to maintain drum NWL set point, when LP FW By-pass control valve (V-819) reaches 80% opening.

• When LP FW Control Valve (LV-801) is opened >20%, then LP FW By-pass Control Valve (V-819) shall be closed fully.

• LP FW Control Valve (LV-801) shall be closed fully if valve opening < 15%.

HRSG OPERATION AND MAINTENANCE Required actions:

High High level:

Sound the alarm.

Close the LP Feedwater Stop Valve (HV800) GT protective load shedding (*).

LP Feedwater Stop Valves (HV-800) remains closed.

High level:

Sound the alarm

Open LP Evap Intermittent Blow-off Isolation Valve (HV-825).

Low level:

Sound the alarm.

Low Low Level:

Sound the alarm.

GT protective load shedding (*).

GT trip (after 2 minutes maximum total elapsed time) if alarm not cleared.

(*) PLS will bring GT to zero load within 2 minutes (GT gas temperature is expected to be

~50% of its full scale). PLS will continue until correcting the cause of problem and process returns to normal.

Dearator Storage Tank and Condensate Preheater Condensate Preheater Bypass Operation

When operating the HRSG Unit with high sulfur fuel, the Condensate Preheater should be bypassed. To bypass the Condensate Preheater, the Condensate Preheater Feedwater Stop Valve (HV-002) is closed and the Condensate Preheater Bypass Stop Valve (HV-003) is opened, resulting in all of the flow bypassing the Condensate Preheater.

Online Switch Over to Engage Condensate Preheater Recirculation

In order to accomplish an on-line switch over while the HRSG is in operation and reengage the Condensate Preheater Recirculation, turn on the Condensate Preheater Recirculation Pump (PMP-040) and open the Recirculation Pump Discharge Control Valve to 100% open.

To prevent thermal shock of the Preheater manifold, keep the Condensate Preheater Bypass Stop Valve (HV-003) open for 15 minutes after opening the Condensate Preheater Feedwater Stop Valve (HV-002).

Set the Feedwater Heater Recirculation Control in AUTO mode.

Pegging Steam Control

Pegging steam flow from the IP and LP Systems will be controlled by the Pegging Steam Control Valves (PV-001D, PV-002D and PV-003D) to maintain the Deaerator Storage Tank

HRSG OPERATION AND MAINTENANCE

ALSTOM Power Revision: 0 4-5

Copyright 2005 04/02/05

4.2.2 Description

The feedwater control system is a control loop that, when in automatic maintains a balance of feedwater entering the boiler with the amount of steam leaving the boiler. The control system also keeps the volume of boiler water within the steam drum to an established set point level during operation.

The feedwater control system is referred to as a “three-element control system” because three system measurements, or variables, are used to determine the required feedwater rate to the boiler. The three elements monitored are:

• Drum level

• Feedwater flow

• Steam flow

The three monitored elements, steam flow, steam drum level and feedwater flow, are measured and converted into electrical signals. The signals are transmitted as feedback control signals, to the control room.

The use of three different elements provides a quick response when transients, or changes, occur during unit operation.

The Feedwater control system is an analog control system. Drum level is the controlled variable. Feedwater flow is the manipulated variable.