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A surge tank is always part of either a two-tank or two-chamber deaeration system. In a two-tank system, it functions as a freestanding unit. In a two-chamber system, it functions as one of the two chambers contained in the receiver that encompasses both the deaerator and surge tank functions. This animation illustrates the operations of a freestanding surge tank in a two-tank system. (See the two-chamber deaerator animations for a two-chamber system).
A surge tank functions like a boiler feed except that it feeds a deaerator instead of a boiler. It is typically needed in steam systems with 20% or more condensate returns.
A surge tank serves as the holding tank of the makeup water for the deaerator. Normally, all returning condensate (except small amount of returns above 212°F) are blended with the colder makeup water. This blending helps avoid sudden changes in temperature or pressure, depending on the type of deaerator being used. Keeping the temperature of water at the saturation point (i.e., boiling point) is critical for effective removal of oxygen and non-condensable gases.
Therefore, preventing wide temperature fluctuations within the deaerator is critical to achieving and maintaining its rated performance level (e.g., .005 cubic centimeters of oxygen remaining per liter of water).
All deaerator units are rated in pounds per hour (lbs/hr) of steam. The transfer pumps on a surge tank are sized (i.e., the pump rate) based on the deaerator rating. The pump rate is expressed in gallons per minute (GPM). On all two-tank systems, the transfer pump rate equals the evaporation rate of the deaerator. The evaporation rate is calculated by dividing the deaerator's rating by 500.
The transfer pumps must run continuously pumping water directly into the modulating transfer valve on the deaerator. The modulating water-level controller, on the deaerator, sends a signal to the modulating transfer valve, on the deaerator, letting the transfer water into the deaerator as needed.
The standby pump on a surge tank is activated by a low-level switch on the deaerator. The standby pump runs automatically if the lead pump fails or can't keep up.
Condensate returns flow into the receiver. The transfer pumps of the surge tank run continuously so that the modulating transfer valve on the deaerator is always filled with water that is under pressure.
The solenoid valve on the surge tank is closed; the pipe into the solenoid is always filled with makeup water.
The water level in the surge tank falls as it is pumped to the deaerator.
At the low water level setting, the float switch sends a signal to open the solenoid valve letting city makeup water flow into the receiver.
As the city makeup water flows into the receiver, the water level begins to rise.
At the high water level setting, the float switch sends a signal to close the solenoid valve stopping the flow of city makeup water into the receiver.