4.2 Low pressure system configuration - part 3
All of the system components upstream of the isolating valve are primarily concerned with controlling the flow through the condensate system to maintain a constant water level in the condenser.
The prescribed level is required to give surge and cavitation-free extraction pump performance and is maintained by throttling the extraction pump flow. A level control device on the condenser, typically as manufactured by Fisher and sold under the trade name 'Level-Troll', is used to give an air signal which controls the position of a pneumatic control valve and maintains the level in the condenser. The level control valve fails to the 'open' position on loss of air or control signal, thus preventing a unit trip due to loss of forward flow to the de-aerator and hence eventual loss of feed to the boiler.
The position of the control valve is immediately upstream of sectionalising valve. Its function in trimming the pump discharge head to suit the system resistance is shown in Fig 3.40. To protect the extraction pump from damage due to low flows, the pump manufacturer specifies a flow below which the extraction pumps should not be operated. To meet this need a recirculation connection and a control valve is provided and is commanded to open as the main control valve goes below the 25% flow position. The way in which the recirculation valve operates in antiphase to maintain a minimum flow through the extraction pump is also illustrated in Fig 3.40.
Between the condenser level control valve and the sectionalising valve, a dump connection is provided, which is used to discharge contaminated condensate from the system during system clean up.
The non-return valve upstream of the control valve prevents accidental discharge of water from the de-aerator tank into the condenser in the event of an extraction pump trip with a faulty discharge non-return valve.
The gland steam condenser is placed upstream of the recirculation connection to ensure a cooling condensate flow at all times. The extraction pumps are provided with isolating valves on both suction and lischarge to allow isolation for maintenance. Non-eturn valves are provided on the discharge of the pumps to prevent backflow of water through an idle pump from the discharge of the duty pump.