3 Water Level Control
04.09.2016 Mk8 MM Expansion Features Installation and Set-Up Guide Page 35
3.4 Capacitance Probes
3.4.1 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 Autoflame Micro-Modulation (M.M.) module knows the firing rate and boiler
pressure, and accommodates for this transient condition by increasing the ‘control point’ level.
Figure 3.4.1.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 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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