4.1.9 Feed Water System
The objective of the system is to de-aerate the condensate and supply all pressure levels with feed water. The deaerator/feed water tank is part of the feed water system. From the outlet of the preheater the condensate is supplied to the deaerator dome. The amount of condensate is controlled by the preheater control valve, located downstream the preheater. Pegging steam is added to the feed water tank and/or deaerator dome to de-aerate the water and maintain a minimum pressure (and temperature).
During normal operation pegging steam is coming from the LP SH steam line (pegging into the feed water tank). If LP steam is not sufficient or the GT is running on solar oil, cold reheat steam is added (pegging into the dome). From the feed water tank LP pumps and combined HP/IP pumps supply the feed water to the HRSG. The pumps are equipped with minimum flow lines.
4.1.9.1 Deaerator/Feed Water Tank
The deaerator is integrated with the feed water tank. The working principle of the deaerator is based on the thermal de-aeration. This means that at increased temperature and/or pressure less gas can be dissolved in water. Thus increasing the heat and the pressure of the water will force the dissolved gases to evaporate from the water as shown in Figure (4.8). The water and the steam flow counter current through the deaerator. The water to be de-aerated enters the deaerator/feedwater tank at the top in the so-called deaerator dome. The steam required for heating up the water phase enters the deaerator/ feed water tank, through the steam distributor tank.
In the deaerator dome the water is sprayed into small droplets, thus creating a large contact area between the water and the gas phase. This is beneficial for the de-aeration process as both the heat transfer from steam to water and the diffusion time of the non-condensable gases (oxygen, carbon dioxide) to the surface area is shorter.
The non-condensable gases leave the deaerator with some steam through the deaerator vent. The de-aerated water is collected in the feed water tank and is fed to IP and HP economizer systems and the LP drum by means of the respective feed water pumps.
4.1.9.2 Feed water Pumps
A total of three combined IP/HP feed water pumps and three LP feed water pumps are installed. The pumps extract water from the feed water tank through three separate lines. The water is pressurized and supplied to a common HP, IP or LP feed water line. During normal operation there is a pump in operation for each HRSG in operation. When both HRSGs are in operation the third pump is on standby. The pumps are equipped with a minimum flow line which returns the water to the deaerator in case no water is required by the HRSG or if the flow is below the minimum flow. The minimum flow line supplies the water to the deaerator dome as shown in Fig. (4.9)
The objective of the system is to de-aerate the condensate and supply all pressure levels with feed water. The deaerator/feed water tank is part of the feed water system. From the outlet of the preheater the condensate is supplied to the deaerator dome. The amount of condensate is controlled by the preheater control valve, located downstream the preheater. Pegging steam is added to the feed water tank and/or deaerator dome to de-aerate the water and maintain a minimum pressure (and temperature).
During normal operation pegging steam is coming from the LP SH steam line (pegging into the feed water tank). If LP steam is not sufficient or the GT is running on solar oil, cold reheat steam is added (pegging into the dome). From the feed water tank LP pumps and combined HP/IP pumps supply the feed water to the HRSG. The pumps are equipped with minimum flow lines.
4.1.9.1 Deaerator/Feed Water Tank
The deaerator is integrated with the feed water tank. The working principle of the deaerator is based on the thermal de-aeration. This means that at increased temperature and/or pressure less gas can be dissolved in water. Thus increasing the heat and the pressure of the water will force the dissolved gases to evaporate from the water as shown in Figure (4.8). The water and the steam flow counter current through the deaerator. The water to be de-aerated enters the deaerator/feedwater tank at the top in the so-called deaerator dome. The steam required for heating up the water phase enters the deaerator/ feed water tank, through the steam distributor tank.
In the deaerator dome the water is sprayed into small droplets, thus creating a large contact area between the water and the gas phase. This is beneficial for the de-aeration process as both the heat transfer from steam to water and the diffusion time of the non-condensable gases (oxygen, carbon dioxide) to the surface area is shorter.
The non-condensable gases leave the deaerator with some steam through the deaerator vent. The de-aerated water is collected in the feed water tank and is fed to IP and HP economizer systems and the LP drum by means of the respective feed water pumps.
4.1.9.2 Feed water Pumps
A total of three combined IP/HP feed water pumps and three LP feed water pumps are installed. The pumps extract water from the feed water tank through three separate lines. The water is pressurized and supplied to a common HP, IP or LP feed water line. During normal operation there is a pump in operation for each HRSG in operation. When both HRSGs are in operation the third pump is on standby. The pumps are equipped with a minimum flow line which returns the water to the deaerator in case no water is required by the HRSG or if the flow is below the minimum flow. The minimum flow line supplies the water to the deaerator dome as shown in Fig. (4.9)
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