10 Water Level Control
Mk8 MM Manual Page | 172
10.5 Capacitance Probes
10.5.1 Overview
The Autoflame capacitance probes can be used in conjunction with the Mk8 MM to detect the water level in
the boiler. 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.
The capacitance probes are designed to work with steam boilers where the chemical treatment is maintained
to the limits stated within these standards and guidelines. When the chemical treatment is maintained to
levels under the maximum limits as stated in the standard’s tables, the water level probes will work as
expected.
When using two capacitance probes, the water level is read to a resolution of less than 1mm in still water.
This resolution is maintained during normal operation by Autoflame’s patented “wave signature and
turbulence management” software. The patented movement detection of water feature ensures that no static
value can be accepted, i.e. the probes are in turbulent water. The swell management feature prevents
intermittent shutdowns from the 1
st
low being switching due to increases in steam requirements. The Mk8
MM knows the firing rate and boiler pressure and accommodates for this transient condition by increasing
the ‘control point’ level.
The capacitance probes are high integrity self-checked and continuously monitored by the Autoflame MM
control system.
Figure 3.5.1.i Capacitance Probe
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. 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 shutdowns 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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