3 ELEMENT BOILER WATER LEVEL CONTROL


Three-Element Boiler Water Level Control


Reason for 3 element -

 SWELL 

✓ If however, there occurs a sudden increase in steam demand from the boiler, the pressure in the drum will fall.
✓ Some of the water present in the drum at the higher pressure will now ‘flash off’ and become steam.
✓ These bubbles of steam will cause the drum level to rise. 
✓The reduced mass of water in the drum will also result in more steam being produced, which will further raise the water level. This effect is known as ‘swell’.
✓A level control system which used only level as the measured value would close the feed water control valve, when it should be opening it. 

SHRINKAGE

✓ When the boiler load returns to normal the drum pressure will rise and 
steam bubble formation will reduce, causing a fall in water level. 
✓Incoming cold feed water will further reduce steam bubble formation and what is known as ‘shrinkage’ of the drum level will occur.


The problems associated with swell and shrinkage is removed by the use of second measuring element, ‘steam flow’. A third element, ‘feed water flow’, is added is added to avoid problem that would occur if the feed water pressure were to vary. 


 A THREE-ELEMENT CONTROL SYSTEM  


1. The measuring elements are ‘steam flow’, ‘drum level’ and ‘feed water flow’.

2. Since in a balanced situation, steam flow must equal feed flow, these two signals are compared in a differential relay.

3. The relay output is fed to a two-term controller and comparator into which the measured drum level signal is also fed. 

4. Any deviation between the desired and actual drum level and any 
deviation between feed and steam flow will result in controller action to adjust the feed water control valve. 

5. The drum level will then be returned to its correct position. 

6.  The sudden increase in steam demand would result in deviation signal from the differential relay and an output signal to open the feed water control valve.

7. The swell effect would therefore not influence the correct operation of the control system. 

8. For a reduction in steam demand, an output signal to close the feed water control valve would result, thus avoiding shrinkage effects. 

9. Any change in feed water pressure would result in feed water 
control valve movement to correct the change and maintain the correct drum level.

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