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FLY ASH LEVEL DETECTION IN ESP
HOPPERS
# INTRODUCTION :-
In Thermal Power Stations, some amount of ash particles
are prone to get carried away with the exhaust air of
the Boiler.
In order to prevent these ash particles from being
exhausted out of the stack, they are collected in the
Economizer of the Boiler by gravity method. However, a
large portion of the Fly-ash particles which can not be
trapped in the Economizer are collected in the ESP
Hoppers below the Electrostatic Precipitators (ESP)
having high peak voltage of 71 KV on the collecting
plates to polarize the Fly-Ash Particles.
Fly-Ash is thus collected in a number of hoppers below
the ESPs. To prevent over-spilling of Fly-ash from these
hoppers, de-ashing is performed after a fixed time
interval. But this process results in high power
consumption as many a times the de-ashing operation
takes place after the pre-fixed interval even though the
hopper is not filled completely with Fly-ash. Further
there is a lot of wear & tear due to moving parts of the
de-ashing system.
This calls for an Automatic Level Detection System to
control the High and Low Level of Fly-ash in the ESP
Hoppers and start the emptying process only when the
pre-set max. Level is reached. This prevents build-up of
the ash particles in the portion of the High Voltage
Plates where build – up could cause a short-circuit
between the collecting plate and the electrode
(especially under Humid conditions ) thus destroying the
plate arrangement and the electrical equipment.
That is why the level detection of fly ash in ESP
Hoppers remains a relatively difficult application in
industry even today.
The challenge is :
* Fly Ash density varies widely and its composition
changes.
* Fly Ash has very low density, hard to sense.
* It tends to stick to everything particularly when
moist.
* Fly Ash has a very low dielectric constant.
* It carries static charge.
* Fly Ash is at high temperature ( Approx. 200 deg. C.).
* It is abrasive in nature.
* There is Suspended (floating) dust in vicinity of the
level sensing probe.
The solution lies in using an RF Admittance level
controller with an extra strong immuno coat Rod Probe.
The Evaluation Unit measures the change of admittance
accurately by using the Radio Frequency ( R.F.)
techniques coupled with the coat-immunizing circuitry.
The R.F. signal transmission between the Probe and the
evaluation unit is via coaxial cable with driven shield
arrangement. This eliminates cable capacitance & drift
due to change of cable capacitance with temperature. The
processed signal is used to energize or de-energize a
Relay whose output contacts are available for
annunciation or control. A change of Level of Fly-ash in
the hopper causes a change in di-electric which in turn
causes a change in admittance of this imperfect
capacitor. An accurate measurement of this change
affords an indirect measure of the level of fly-ash in
the hopper.
# DESIGN CONCEPT ( Immuno-coat Probe – Active Shield )
• The Probe comprises of Sense and Shield electrodes.
These are electrically isolated from each other and from
the Vessel Wall (Metallic Tank) by means of suitable
insulators ( say, PTFE or ceramic).
• The Sense electrode and Vessel Wall serve as two
electrodes of an imperfect capacitor with service
material ( say, Fly Ash ) as dielectric.
• A change in material Level causes a change in
dielectric which in turn causes a change in the
Admittance of this imperfect Capacitor which is
proportional to the Level of the material in the Tank.
• The evaluation unit measures the change of Admittance
accurately by using RF techniques coupled with Coat
immunizing Circuitry.
• RF signal transmission between Probe and Evaluation
unit is via tri-axial cable with driven shield
arrangement. This eliminates cable capacitance and drift
(due to change of cable capacitance with temperature.)
• The processed signal is used to energize or
de-energize a relay whose o/p contacts are available for
annunciation / control.
Now, how the coat immunizing circuitry works :Refer to
Fig. 1 below.
The Drive current to the Active Shield (Metal Sleeve) is
at the same Frequency and Polarity as the probe. ( i.e.
The Sense Probe and the Active Shield are maintained at
equi-potential.) This prevents flow of RF current from
Probe to the Vessel wall (Ground) in case of build up on
the probe. But when the actual bulk material in the
vessel fills & touches the probe, the RF current from
the Probe flows towards the Vessel Wall ( outside the
saturated region for the build-up due to active shield
).
This technology will ignore build-up upto a thickness of
approx. 15 – 20 mm on probe ( due to Moisture present in
the material forming Lumps or due to sticky material eg.
Molassis ). In any Power Plant, Fly Ash particles are
air prone in the exhaust of the Boiler. In order to
prevent the Fly Ash being exhausted out of the stack, a
large amount is collected in the Economizer
Mechanical considerations:
The construction of the probe is such that the 19 mm
OD x 15 mm ID sensing element along with its 2 mm thick
PTFE insulation is supported by means of a tapered nut
mechanism which makes it rugged for continuous heavy
duty application.
This also provides large sense area so as to achieve
adequate change in reactance for reliable performance.
2 mm thick PTFE ensures firm insulation between sense
and shield and between shield and ground. It can
withstand more than 2.5 KV for 1 minute. Moreover this
minimizes the chances of deformation when working at
high temperature.
The SS shield pipe support has 31 mm OD and 24 mm ID
which serves as a rigid support to the 19 mm sense
element for all practical purposes since the force
required to bent the shield pipe is much greater than
the force required to bent the sense element. Our design
is almost two times stronger as compared to any other
make.
Our design not only utilizes the shield pipe as driven
element for immunity to build up and coating but also
provides a rigid support to the sense element to
increase the mechanical strength.
Long Standoff ( L-435mm, OD-42.5mm, ID-35mm ) makes
wiring easy & cools the probe thus protecting terminals
and cable from high temp.
# ELECTRICAL CONSIDERATION :-
The shield and sense electrode are insulated from each
other and the ground. Therefore Capacitance is formed
between the ground and the two electrodes with the fly
ash as dielectric.
Variations in the temperature and the amount of charge
carried by the ash can affect its dielectric constant as
well as its conductivity. There could be variations in
electrical properties of the coating on the electrodes
as well as in the floating fly ash in vicinity of the
electrode. This can affect the reliability of the
performance if these factors are not given due
considerations in the probe design.
The length of the shield and sense insulation is an
important criteria for reliable operation of the probe
in difficult environment as encountered in ESP
application. A shorter length of insulation decreases
the resistive path increasing the possibility of
spurious switching whereas longer insulation increases
the cost. our design utilizes adequate length of
insulation that is necessary for trouble free operation.
Our RF Admittance Level Limit Switch LEVTESTER SLA..
Series is specially developed for stringent applications
such as Fly Ash Level Detection where the conventional
Capacitance type level controllers, Rotating Paddles
etc. are prone to failure due to above criticality.
Our RF Admittance Level Limit Switch with immuno coat
Probe uses 3 electrode technique with driven shield
arrangement to achieve high degree of immunity.
No moving parts, no mechanical wear, no electronic
insert in probe head.
Remote Calibration is possible. A special 10 LED dot
display simplifies calibration and indicates material
trend in vicinity of the point Level.
Hi/Lo fail safe selection reduces inventory.
A Gas filled Surge arrester in probe head discharges
the static charge present on the fly ash as it falls in
the hopper below the electrostatic precipitator
preventing any possible damage to the electronics.
A change of Level of material in the hopper causes a
change in dielectric which in turn causes a change in
admittance of this imperfect capacitor. An accurate
measurement of this change affords an indirect measure
of the level of material in the tank.
The Evaluation unit measures the change of Admittance
accurately by using the Radio Frequency (R.F.)
techniques coupled with coat immunizing circuitry. The
R.F. signal transmission between probe and evaluation
unit via special co-axial cable with drain wire
arrangement eliminates cable capacitance and drift due
to change of cable capacitance with temperature. The
processed signal is used to energize or de-energize a
relay whose output contacts are available for
annunciation or control.
This is how the critical application of fly-ash Level
detection is addressed in industry.
Hopper Level Detection
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