116.Why to maintain 6 – 7 Kg/cm2 dp across the feed water control valve

Normally in a boiler the spray water will be tapped of at the inlet of Feed Water Control valve. At full load the pressure drop between the tapping point and the attemperator nozzle will be 10 bar without any pressure drop across the FWCV. On minimum auxiliary power consumption point, this is ideal operating condition. The boiler feed pump speed will be modulated to provide the quantity of feed water flow to drum.

The 10 bar dp across the spray control valve is enough to provide the required spray water flow.

At 50% the dp across the spray water control valve, with same 0 dp across FWCV, will be 2.5 bar. This dp will not be enough to provide the spray flow. More over the quantity of spray at 50% will be maximum and that at full load will be minimum.

The required dp across TCV can be provided by maintaining a minimum dp across the FWCV. Hence in Drum Level control loop, the feed water flow is controlled by LC and the dp across FWCV is controlled by the BFP’s speed control.

The steam temperature control is a case cade loop. The secondary will control after attemperaor temperature. Assume that spray water required to reduce 1 Deg.C at full load is 1 T/H. The spray water demand at 50%  load to 1 Deg.C. will be 0.5 T/H. If the dp across the TCV is maintained constant, a Equal % valve will provide satisfactory results in this application. 

If the dp across the FWCV is controlled constant at 7 bar from 50% to 100%, the dp across TCV will be  9.5 bar at 50% load and at 100% load it will be 17 bar. In this case an adaptive gain shall be applied .

The dp across TCV can be maintained constant at 17 bar by setting the FWCV dp at 14.5 bar at 50% and 7 bar at 100%.