FIELD OF THE INVENTION
The invention relates to a method for determining liquid levels in vessels
employed steam generating plants and process industries.
BACKGROUND OF THE INVENTION
The pressure vessels and steam generating plant have the requirement of
monitoring and controlling liquid level at various parts of the system. In a
thermal power plant using steam generators the water level has to be
monitored and controlled at various components like boiler drum, condensers,
hot-well etc. Similarly in a process industry the liquid chemical levels have to
be monitored and controlled in components like process vessel, decanters,
reactors, etc.
Over the years, liquid level monitoring has been made by employing different
methods and techniques, starting from visual monitoring using thick glass
gauges, float switches for discrete level monitoring, and differential pressure
gauge. The method of employing sensors immersed in liquid has also been in
practice over the years. However detecting the abnormalities in the sensor
and at the same time monitoring the liquid levels poses a problem.
U.S. Pat. No. 4,482,891 describes a robust electronic liquid level gauge apparatus,
particularly suitable for use with boiler drum water column, which comprises simple
probe sensors in the water column;, differential amplifier sensors;, detectors
responsive only to the simultaneous presence of a minimum level signal and a
certain frequency;, and logic circuits continuously processing outputs from four
detectors: its associated probe detector, the one below, and the two above.
U.S. Pat. No. 4,646,569 describes a fluid level measurement system for the
measurement of a level of fluid in a vessel, in which, the electrical impedance
of a first conduction path between a central rod electrode and a surrounding
cylindrical electrode and the electrical impedance of the second conduction
path between the cylinder and the vessel are established. The first conduction
path is restricted to be below the minimum fluid level by an insulating coating
on the rod. The second conduction path varies with the fluid level. An output
signal is extracted from the electrodes potentiometrically which is indicative of
the fluid level. The signal may be linearized by application to a circuit having a
matched non-linear transfer function. With this arrangement, a continuous
output signal indicative of the fluid level is produced which is not subject to
errors due to changes in fluid resistivity.
U.S. Pat. No. 5,519,639 describes a system for monitoring water level of a
pressure drum having a pressure column, which comprises a plurality of
electrodes communicating with the column for contacting water and steam
within the column. Each electrode has its own location and produces a first
output which corresponds to the presence of water or a second output which
corresponds to the presence of steam. A discriminator is operatively
connected to the electrodes; and the discriminator has an analog-to-digital
converter for receiving the output of each electrode and converting the output
to a digital signal which represents the water conductivity for each electrode.
A central processor is operatively connected to the discriminator for powering
the discriminator as well as for receiving the digital signals in order to
determine a slope degradation between the conductivity and the location of
each electrode in the column. The central processor also determines an
inflection point between the water and the steam in the column. An LED
display is used to indicate which of the electrodes are located in water and
which of the electrodes are located in steam.
U.S. Pat. No. 6,650,128 describes a fault detection circuit in a boiler-water
level system, which includes a dual-frequency signal generator which
develops two AC components with no associated DC component. The two
frequencies are mixed and sent through an impedance matching circuit to
match the impedance of the signal generating portion of the system with the
impedance of the boiler water under measurement. The impedance-matched
signal is then directed to two legs, one leg directed through one of a plurality
of electrode probes and then to a first filter circuit, and the other leg is
directed a second set of filters. In combination, the filters pass either the
higher or the lower of the two frequencies to determine an open or short
condition in the level sensing circuitry/ as well as a steam vs. water condition.
U.S. Pat. No. 7,490,513 describes a liquid level sensor which comprises a
plurality of electrodes arranged in a defined relationship with respect to a
container filling trajectory, a signal generator coupled to one or more of the
plurality of electrodes, a signal receiver coupled to one or more of the
plurality of electrodes, and a sensing circuit coupled to at least one of the
signal generator and signal receiver. The sensing circuit is configured to
produce an output indicative of electrical resistance between at least one pair
of the plurality of electrodes. The sensing circuit may be associated with the
signal receiver and/or with the signal generator.
Thus, the prior art discloses multiple liquid level monitoring systems in a
steam generating plant to detect various abnormalities while detecting the
presence of liquid / vapor viz. power supply monitoring, excitation signal clock
function monitoring, sensor short detection, excitation and feedback signal
wire open/short detection, water over steam detection, etc. However, there
remains a need in the art to provide a method and an apparatus which can
effectively detect a sensor short, wire open / short conditions during the
detection of liquid or vapor presence by the sensors in the vessel.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a method for monitoring
liquid level, in a vessel, for providing outputs for control of liquid level in the
form of raising an alarm for low/high levels of liquid and provisions to trip the
process.
A further object of the invention is to propose a method, for processing the
response signal from each sensor to provide information on the presence of
liquid or vapor including the healthiness of the sensor and its interconnecting
wires for validating the liquid level measured in an apparatus.
SUMMARY OF THE INVENTION
The liquids employed in steam generating plants and process industries have
different electrical characteristics in liquid phase and vapor phase. The
method and apparatus described herein employs the difference in electrical
characteristics to detect whether a sensor is immersed in liquid or in vapor
phase of the liquid. The differences in the electrical characteristics between
the liquid and vapor phase of the medium are used in such a manner that
four types of information are extracted from one response signal from the
sensor. The magnitude of the response provides two information based on low
and high from a reference level and the phase of the signal (time lag)
provides another two information. The liquid level and healthiness of the
sensors are derived from a combination of the four information by appropriate
logic.
In this method the difference in electrical parameters of the medium (liquid /
vapor) constitutes the basis for detecting whether the sensor is in the
proximity of the liquid or is in proximity of its vapor phase by arranging
multiple sensors along the height of the vessel. Detecting the sensor outputs
enables monitoring of liquid level in stepwise (discrete) levels and suitable
control / alarm outputs can be derived to annunciate high / low level alarms
and to effect control action for feeding /draining the liquid to maintain the
desired level. The sensors are designed and mounted in such manner that they
are electrically insulated from the body of the liquid containers which are
made-up of metals.
An electrical excitation signal, in this invention, a square wave is injected
between the sensor and body of the vessel. The capacitance which is formed
between the sensor and the vessel is dependent on the medium in the vessel.
Considering the vessel geometry is same, the different sensors will have
different capacitance effect to the vessel based on whether the sensor is
immersed in liquid or vapor of the medium. Capacitance is the function of
electrical permittivity of the medium. In general the permittivity of the liquids
will be higher as compared to the permittivity of the vapor. Hence a sensor
immersed in liquid will have higher capacitance than a capacitance in vapor.
The response to the excitation signal will then be different for sensor in
proximity with liquid and sensor in proximity with vapor. The difference will be
in terms of the magnitude and the phase difference between the excitation
signal and the response signal. In case of liquids having good electrical
conductivity (salt solutions), the sensors response will be predominantly
resistive when it is in liquid and the response signal will be capacitive in
nature when it is in vapor. In such cases for identifying the sensors located in
the liquid medium, the magnitude of the response signal in conjunction with
zero or minimum phase will be used. Similarly when the sensor is located in
the vapor medium the capacitive response of the signal will be used with
magnitude and considerable phase lag.
In other cases where the liquid is not conductive, example de-mineralized
water used in steam generators the response signal will be dependent on the
capacitance of the medium. In such cases the magnitude of the response
signal as well as the phase lag information have to be used together to detect
whether a sensor is in liquid or in vapor medium.
The method and apparatus described herein can be employed for sensing
liquid levels in vessels employed in steam generating plants and process
industries. The monitoring and control of the liquid level is important from the
process control point of view.
The method and apparatus described here is an electrical sensing method
based on certain electrical characteristics of the liquids and its vapor that are
used in boilers and pressure vessels, the liquids being either water or
chemical solution.
The apparatus described herein also has provision to detect abnormalities of
the sensors in addition to the liquid level measurement.
In brief, the method and apparatus employs the principle of using multiple
sensors mounted along the height of the vessel, exciting each sensor by a
square wave, analyzing the magnitude and phase lag of the response signal
from each sensor and providing output indicating liquid level and also
providing outputs for control of the liquid level. The method also incorporates
an algorithm for identifying the fault in the sensor and its associated wire
system and thereby providing validated level output.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig.1 - Illustration of a Typical Sensor System Model
Fig.2 - Illustration of the Sensor System in a Liquid Medium and Signal
Waveforms according to the invention.
Fig.3 - Illustration of the Sensor System, in a Vapor Medium and Signal
Waveforms.
Fig.4 - Illustration of the Sensor System, in a Sensor / Wire Short condition
and Signal Waveforms.
Fig.5 - Illustration of the Sensor System, in a Wire Open condition and Signal
Waveforms.
Fig.6 - Functional Block Schematic of Liquid Level and Fault Detection
Apparatus according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The preferred embodiment of the method and apparatus to detect and
monitor the presence of liquid or its vapor in a vessel have been illustrated in
Figs. 1, 2, 3, 4, 5 & 6. Fig.l shows the typical representation of an equivalent
circuit pertaining to a sensor and its connectivity with a low voltage low
frequency square wave generator. The impedance offered by the medium
shall be represented by the elements viz. Rs & Cs, which are respectively the
resistance and capacitance of the medium with respect to the body of the
vessel. The excitation signal generated from the square wave oscillator is
injected to the Rs + Cs combination through an impedance network Ri and
the response signal is sampled from the Rs + Cs combination. 2 illustrates the
wave shapes of the excitation and the time synchronized response signal
sampled when the sensor is in the proximity of liquid medium which has
relatively more capacitance. Due to this, the time taken to attain the set
threshold by the response signal is relatively more. Similarly, 3 illustrates the
wave shapes of the excitation and the time synchronized response signal
sampled when the sensor is in the proximity of vapor medium which has
relatively less capacitance. Due to this, the time taken to attain the set
threshold by the response signal is relatively less. The time lag or the phase
shift between the excitation and the response signal is obtained by using a
signal conditioner and phase detector logic. Thus, the same excitation signal
being responded differently based on the presence of liquid or its vapor in the
proximity of the sensor illustrated in the subsequent wave shapes in Fig. 2 &
3. In case of abnormality in the healthiness of the sensor and its associated
wires, the abnormality can be categorized as Sensor / Wire short or Wire open
which is illustrated by Fig. 4 & 5 for Short fault and Open fault detection
respectively. In both Short and Open fault conditions, the Cs offered by the
sensor system is very minimum or zero, thereby the deciding factor is only Rs
which makes the response signal output similar to excitation signal, that is
without any time or phase shift. The factor to distinguish the short and open
fault is the magnitude of the response signal with zero or very minimum time
lag / phase shift. Thus, by using this methodology, the excitation signal and
its corresponding response signals can be processed to generate four outputs
namely presence of liquid or its vapor mediums and the healthiness of the
sensor system for short and open fault.
Further, Fig.6 illustrates the overall schematic block diagram of the proposed
apparatus. The typical sampling vessel (100) is used to position the multiple
discrete sensors in the vertical alignment, a few of the sensors in liquid
medium and a few in the vapor medium. These sensors (101) are excited by
a low voltage square wave generated from a square wave oscillator (103)
through an impedance network (102). The impedance network element and
the impedance offered by the medium around the sensor (101) forms a
potential divider. Thus based on the medium (liquid or vapor), the sensor
gives back a response signal to the electronics. The multiple response signal
from all the sensors (101) are routed through an analog multiplexer (104)
which gives a sequential output of one signal at any instant for further
conditioning though a signal conditioner (105). The conditioned response
signal is further routed through a Phase Comparator (106) and a Magnitude
Comparator (107). The Phase Comparator (106) gets the excitation signal as
a reference input for comparison process. The Phase Comparator (106)
generates two levels of outputs as High and Low based on the phase
difference between the Excitation signal and the Response Signal from the
concerned sensor which is under measurement at any instant. The second
comparator used for Magnitude comparison (107) receives the corresponding
Response signal from the concerned sensor and a Liquid / Vapor reference
signal. Based on this reference magnitude and the response signal magnitude
two levels of output are generated by the Magnitude comparator (107). The
outputs from the Phase Comparator (106) and the Magnitude Comparator
(107) are fed to a Logic block (108) which processes and generates status
regarding the healthiness of the concerned sensor, and its associated
interconnecting cable, (viz. Sensor / Wire short and Wire Open) as well as that
of the medium ( Liquid or Vapor) in proximity or sensed. The Truth table for
the Logic Block (108) is shown as under. Also the Excitation and Response
Signal Wave shapes applicable to the medium under detection namely Liquid
or Vapor and healthiness of the sensor and interconnecting wire are
illustrated in Figs. 2, 3, 4 & 5.
Truth Table for the LOGIC BLOCK (107):

Thus the output from the Logic Block (108) provides the Status of the
individual Sensor / Wire Healthiness and the medium (viz. Liquid or Vapor)
deltected by the said Sensor. The explanation given is for one sensor channel
and the same is applicable for any number of sensor used in the system to
detect liquid or its vapor proximity. The process signal and its status data are
further routed through a control unit (113) which gives out the validated
Level status (114) of the all the sensors mounted in the sampling vessel in
proximity with liquid or its vapor. Also based on the liquid level low / high
threshold programmed in the controller unit (113), the validated level High or
Low relay outputs (115) are generated for the user front.
It is to be understood that the description of the preferred embodiment(s)
herein is (are) to be considered only illustrative, rather than exhaustive, of the
present invention. Those of ordinary skill will be able to make certain
additions, deletions, and/ or modifications to the embodiment (s) of the
disclosed subject matter without departing from the spirit of the invention or
its scope, as defined by the appended claims.
WE CLAIM :
1. A method for determining liquid levels in vessels employed in steam
generating plants and process industries, the liquids being water or chemical
solutions and vapor produced from the liquid, the method comprising :
- exciting a plurality of sensors mounted at different levels of the vessel with
an excitation source (103) providing square wave signal;
- conditioning the response signals from the sensors with a signal
conditioner (105);
- comparing the output from the signal conditioner with the excitation
signal using a Phase Comparator (106) and a Magnitude Comparator(107)
to detect the time lag and the magnitude of the response signal;
- logically interpreting the magnitude and the phase or time lag using a
Logic Block (108) to identify at least four different statuses of the sensors.
2. The method as claimed in claim 1, wherein said four different statuses
comprises Sensor in Liquid medium (109), Sensor in Vapor medium(110),
Sensor system Short=(11) and Sensor system Open=(112).
3. The method and apparatus as claimed in claim 1, where in the logic block
and the functional blocks use response signal from the sensor to interpret four
statuses by ingeniously comparing the excitation signal in terms of magnitude
and phase lag.
4.The method as claimed in claim 1, wherein the method is enabled to
determine the healthiness of the individual sensor and its connecting wires to
generate reliable liquid level indication.
5. The method as claimed in claim 1, wherein depending on the type of liquid,
the frequency and magnitude of the excitation signal can be modified without
changing the basic circuitry.
6. The method as claimed in any of the proceeding claims wherein when an
ionic solution having relatively more conductivity is employed, the capacitance
effect can be significantly improved by increasing the excitation signal
frequency.
7. The method as claimed in claims 1 to 5, wherein when a low conductivity
liquid is employed the excitation signal frequency is lowered while its
magnitude is increased.
Dated this 8™ day JUNE of, 2011

The invention relates to a method and apparatus for determining liquid
levels in vessels employed in steam generating plants and process
industries, the liquids being water or chemical solutions and vapor
produced from the liquid, the method comprising exciting a plurality of
sensors mounted at different levels of the vessel, with an excitation source
(103) providing square wave signal, conditioning the response signals
from the sensors with a signal conditioner (105), comparing the output
from the signal conditioner with the excitation signal using a Phase
Comparator (106) and a Magnitude Comparator(107) to detect the time
lag and the magnitude of the response signal, and logically interpreting
the magnitude and the phase or time lag using a Logic Block (108) to
identify at least four different statuses of the sensors, viz. Sensor in Liquid
medium (109), Sensor in Vapor medium(110), Sensor system Short (11)
and Sensor system Open (112).