Feed System & Feed Tanks

Posted on Monday, June 04, 2012 by Unknown

Feed system

Shown above is a typical feed water system for a modern steam plant. Water is pumped from the main condenser by special centrifugal pumps having an inducer to allow suction from the vaery low pressures without vaporisation of the water. The water passes through the air ejector cooler to the Condensate cooled evaporator. A recirculation valve is available to return some condensate to the main condenser. The purpose of this is to increase the overall flow though the evaporator cooler thereby increaseing water production as well as to ensure a minium flow through the condensate pumps.
The water pass via the gland steam condenser and LP heater which in this case are shown as a combined unit on to the Main Condenser level controller. This prevent s the level in the condenser falling below a set level thereby causing the main condensate pumps to run dry.
Some times mounted after this is a deioniser and feed filter before the water is passed to the deaerator.
The deaerator is mounted as high as possible in the engineroom increaseing the suction head for the feed pump preventing vaporisation in the suction eye of the pump. Not shown is an automatic recirculation valve fitted to the main feed pump outlet to ensure a minimum flow through the pump. The boiler water then passes via the boiler water level controller to the economiser and then through to the boiler steam drum.
An second supply is available for use in emergencies to the drum either vai or by passing the economiser.
The drains tank condensate is pump via the drains tank level controller into the main feed system.

System level Control

Control of the amount of water in the system is by level control of the deaerator. One of the purposes of the deaerator is to act as a reserve capacity of high quality feed for the boiler. Water may be spilled to the feed tank or made up to the drains tank. An emergency filling valve is available for the main condenser the use of which is avoided as it introduces large quantites of gasses into the condenser reducing efficiency

Feed Tanks

Feed Tank Design

A well designed feed tank should be designed to minimise the oxygen within the feed system. This is especially important with open feed systems.

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    • Adequate ventilation with on or more vent pies determined by the volume of water
    • Condensate enters as low as possible via a slotted sparge pipe
    • Cold water make up enters at highest point
    • Sufficient tank volume to cope with transient flows from normal operations without necessitating spilling back to feed tanks or overflows.
    • Tank to have sufficient volume of water at normal working level to allow for 1 hours operation at maximum demand.
    • Take off to feed pumps to be at least 75mm from tank bottom
    • Tank to be located to provide head requirements at normal working level for feed pumps
    • The following are taken as parameters for a well designed tank

As a rule of thumb chemical reaction rates double for evey 10'C rise in temperature. For feed system this remains true upto about 80'C for open system. After this due to reducing solubility of oxygen the rate of corrosion reduces. Thus steam heating on the open feed tanks have thermostats set at 85'C or highe