Drum Level Control in Larger Capacity Boiler

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Drum Level Control in Larger Capacity Boiler

In a large capacity boilers feed pump motor is not directly connected to the boiler feed pump.
Between motor and feed pump a hydraulic coupling is used in which an arrangement is given for speed increasing and then regulating through the scoop tube. This provide a variable speed operation while motor is running at constant speed.
With the help of scoop tube motor is started with no load and pump is running with it on a re-circulation mode.


No direct mechanical connection is there between the motor and the pump shafts. 
The coupling consists of a radial pump impeller mounted on the motor shaft and a radial flow reaction turbine runner mounted on the pump shaft. Both the impeller and runner are identical in shape and they together form a casing which is completely enclosed and filled with oil. When motor shaft is rotated, the oil starts moving from the inner radius to the outer radius of the pump impeller. This oil enters the runner of the pump shaft. The oil while flowing through the runner, transfer its energy to the blade of the runner and the pump shaft rotates. The oil from the runner then flows back into the pump
impeller and continuous circulation takes place. The position of a scoop tube regulates oil flow in to the circuits. By adjusting the scoop tube position, oil low in the working circuit and hence, the torque transmission capacity of the coupling varies. The position of a sliding scoop tube is adjusted automatically through suitable actuator and can also be adjusted manually.

Following two methods are adopted for the operation of scoop to control the drum level:

  • Differential pressure (DP) mode
  • Three-clement mode
Opening and closing of feed control valve is closed as per three-clement drum level controller to maintain the drum level. DP is the difference between the pump discharge pressure and the boiler pressure and s measured across the feed control valve. At constant pump speed. Discharge pressure falls as the feed water flow increases. To take care of this problem, scoop is operated in DP mode (Figure 9.18). The scoop is adjusted automatically to adjust the pump speed to regulate the pump discharge pressure around 7 kg/cm2-10 kg/cm2 above the boiler drum pressure

Due to throttling operation of feed control valve, energy lass is more in this mode of  operation This mode of operation is normally adopted in case of emergency are fluctuating conditions

In case of three-element mode, feed control valve is kept wide open and scoop is adjusted automatically by three-element controller to adjust the pump speed and maintain the drum level.


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