IM-P402-93 AB Issue 7
31
8.15 Two and three element control for boilers
These notes are included at this stage because the subsequent display sequence will vary
according to the selections made:-
Two element control
When a sudden increase in steam demand occurs, boiler pressure will drop, and the steam
bubbles in the water will expand. This causes the water level to rise, and the feedwater valve to
close. Although the measured level of water has increased, the mass of water will be decreasing,
requiring the feedwater valve to open. A two element control system (probe / controller and steam
flowmeter) uses the output from the steam flowmeter to alter the control action and prevent the
valve closing.
For effective control, it is important to make an accurate assessment of the rise in water level under
full steam flow (maximum demand). This rise in water level can vary according to operating
conditions, for example steady maximum steam demand, or intermittent, sudden demand, as well
as factors such as boiler pressure and water TDS level. There will also be a difference in level
between the boiler and the gauge glass under different firing conditions and steam demand.
Three element control
Under certain conditions where the boiler feedwater pressure varies considerably, perhaps due
to other boilers drawing water, three element control is used. A water flowmeter is added to
compensate for variations in flow due to pressure variations. The diagram shows the selections
available, and how they relate.
8.16 Maximum water
The water meter needs to be scaled so that it reads 100% at the maximum boiler output.
For example, if the maximum boiler capacity is 60% of the maximum on the meter scale,
'Enter' 60%.
8.17 Minimum valve
The valve has a feedback potentiometer which tells the controller the valve position. Fully close
the valve (physically check that it is fully closed) then press 'Enter' to calibrate the minimum
potentiometer setting.
8.18 Maximum valve
The valve has a feedback potentiometer which tells the controller the valve position. Fully open
the valve (physically check that it is fully open) then press 'Enter' to calibrate the maximum
potentiometer setting.
8.19 Maximum steam
The steam meter needs to be scaled so that it reads 100% at the maximum boiler output.
For example, if the maximum boiler capacity is 60% of the maximum on the meter scale,
'Enter' 60%.
8.20 Steam offset
This is the estimated rise in 'SET POINT' (as a percentage of the gauge glass) which would
occur if the boiler output changed from 0 - 100%. A figure of 20% may be a good starting point for
many boilers.
For the display sequence see pages 32 to 35.
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