Drum Level Control Conv Master

In the boiler the steam flow changes according to the load demand from the turbine or from the process consuming the steam energy. To match the steam take off from the boiler, the feed water flow has to be increased or decreased as required.


Single Element Control
During low load operation, three-element control is not required. Drum level measurement with it's set point is adequate. Hence the output of the Drum Level Controller is directly given to the Feed water by-pass Control Valve.


Valve capacities are generally measured in terms of Kvr. The formula for Kv is Flow rate and Kvr is equal to Kv*(DP)^0.5. More specifically, Kvs relates to the pass area of the valve when fully open, whilst Kvr relates to the pass area of the valve as required by the application.

The Kvs can be thought of as being the actual valve capacity required by the installation and, if plotted against the required flow rate, the resulting graph can be referred to as the 'installation curve. The graph looks like linear due to a few data


Boiler Feed Pump discharge pressure usually fixed 125 bar. After Feedwater by-pass control valve almost maximum, this is the time to change master control from single element to three element control called Boiler Feed Water Pump Conversion Master.





If Steam drum pressure is 85 Bar, from pump curve with pressure 95 bar and 3400 RPM, the flow is around 380T/h. With the flow 380T/h, the by-pass control valve is opening around 85%. Feed water main valve will open gradually from 10% 20% 30% 40%. After 40% the valve directly to 100% open. When the valve is open 10%, Boiler feed pump discharge pressure will reduce its pressure to 10 bar above steam drum pressure by fluid coupling with scoop tube adjusted, and then by pass control valve will shut slowly. To avoid steam drum level rapid increase, try to make same the flow rate from chart when use control valve by-pass and use main valve as describe above.

Three Element Feed water Control
For automatic control of the feed water flow to the boiler, following three(elements) primary inputs are normally being considered, 1. Drum level, 2. The different between Main Steam flow and Feed water flow and 3. The different pressure between Pump discharge and drum.


The valve DP is the difference between the pump discharge pressure and a constant boiler pressure of 10 barg. Note that the pump discharge pressure will fall as the feed water flow increases. This means that the water pressure before the feed water valve also falls with increased flow rate, which will affect the relationship between the pressure drop and the flow rate through the valve.

Fluid coupling with a scoop tube adjust is maintain pump discharge is 10 bar above steam drum pressure.

related link:
Boiler feed pump curve.