1 INTRODUCTION
1.1 Overview of Water Level Control
1.1.1 Water Level Control Philosophy
The Autoflame water level control focuses on safety and accuracy in controlling the water level in a
steam boiler. The system has a typical level control accuracy of ± 2mm in still water. This accuracy is
maintained during normal operation by Autoflame’s patented “wave signature and turbulence
management” software.
The system safety is guaranteed as the level measurement is managed by two identical capacitance
probes both of which measure and control to the level switching points entered at the time of
commissioning.
Both probes control typically “high level”, “required level”, “first low” and “second low”. The actual
water level readings taken from both the probes are constantly compared and checked against each
other, as well the commissioned water levels. When controlling the required level this data stream is
combined with a PI algorithm which controls either the two port feed water control valve or the
variable speed drive to the feed water pump. Each probe is self-checked for electrical and mechanical
integrity by hardware references and self-checking software routines. Each probe and its control
electronics are compensated for ambient temperature variations and component drift, guaranteeing
absolute safety of operation.
By our method the probes control the required level by learning the wave signature and managing the
turbulence within the boiler shell. This “wave signature management” takes into account the changes in
burner firing rate and any variance in pressure in the boiler shell. Incorporated within the system
hardware are all necessary electronic switching functions to control audible alarms, mute/reset and
indication lights required to meet standard North American and European codes. Safety, accuracy and
integrity are guaranteed.
Movement Detection of Water
When the burner is running it is expected that a wave turbulence signature of in excess of 20Hz / 1mm
will always be present (due to vibration of thermal energy). Both probes are checked for this value.
The default setting is 20Hz, a range of 0-100Hz is possible, if set to ‘0’ this feature is turned off. This
feature ensures that either probe cannot read a still water condition when the burner is running. This
safety check ensures no static or stuck value can be accepted, thereby checking that the probes are in
water.
Swell Management
When there is a sudden drop in boiler pressure an increase in water level will be observed. This is due
to the expansion of the steam bubbles in the water causing the water level to increase. It follows that
the water feed would then turn off or go to a low flow condition. The Autoflame system identifies this
ambiguous condition by monitoring the sudden increase in burner firing rate to meet the load demand
and increases the “required water level” by up to 50% of the distance between normal “required
level” and “high water level”. When normal conditions are reinstated and the boiler firing rate
stabilises, the “required level” returns to the normal setting. This stops spurious shut downs due to 1st
low being switched during these transient conditions. The Autoflame system knows what the firing rate
and boiler pressure is at any one time and uses this information to identify the above condition. This
feature is one of the main elements in the patent claim.
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