11 Top Blowdown
Mk8 MM Manual Page | 204
11 TOP BLOWDOWN
11.1 Overview
11.1.1 Importance of Maintaining TDS
To manage a steam boiler for optimum efficiency and reliability, an important requirement is to ensure that
the Total Dissolved Solids (TDS) in the water are measured and controlled to the right level for that boiler. It
is generally accepted that for water tube boilers the level of TDS measured should not exceed 1,500 PPM by
volume and for fire tube boilers the TDS should not be higher than 2,500 PPM by volume. The figures stated
are not definitive and in all applications the recommendations of the boiler manufacturer or water treatment
chemist should be implemented.
It has been established that the conductivity of water is proportional to the measured TDS as long as the
temperature remains constant. Any variations in temperature will affect the measured conductivity by
nominally 2% per 1°C. It follows that the temperature of the water must be measured, and the conductivity
reading must be adjusted before a TDS reading can be extrapolated from this line of data. The Autoflame
system incorporates a temperature measurement sensor in the steam drum to establish the steam
temperature, this stream data is used to constantly correct the conductivity value.
A second variable that effects the conductivity measurement is polarization of the water sample, this occurs
when electrical energy from the probe builds up a relatively tiny offset above or below the earth (0 Volt
value). This polarization value is typically noticeable when a continuous frequency is being emitted from the
probe as part of the conductivity measurement method.
The Autoflame system deals with the potential problem of polarization in the following manner: The probe
measures any build-up of voltage potential above or below earth or 0V in the water sample. The measured
polarization voltage data is used to modify the conductivity calculation. The Autoflame system emits electrical
energy at a rate of 10 x 300 microsecond pulses every second. This translates into a method where we are
emitting electrical energy for 0.6% of the sample time. All other manufacturers who use the frequency
method are emitting electrical energy for 100% of the sample time. It follows that the polarization problem in
these cases would be 167 times greater.
A third problem that affects the accuracy of the TDS measurement is the build-up of scale on the probe
electrode. By design the water sampling container has been arranged so that the turbulence created during
the blow down sequence will ensure that the probe remains effectively free of scale or deposited solids that
could be held in suspension. The probe is self-cleaning.
The sampling container has a known orifice size. From this it is possible to calculate the percentage losses
due to surface blowdown. This is possible because the following parameters are known which include hole
size, temperature, pressure, pressure drop across the solenoid and the time that the solenoid is open for.
It can be seen from the above that the Autoflame TDS system deals succinctly with three of the main
problem areas that are encountered when designing an accurate TDS control solution.
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