Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-02-19
2003-11-18
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S693000, C340S620000, C073S30400R
Reexamination Certificate
active
06650128
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates generally to the field of boiler water level detectors and, more particularly to systems for the detection of electrical faults, as well as water versus steam detection, in boiler water level detectors.
(2) Description of Related Art
The control of water level in a steam boiler is extremely critical, in that if a fire tube is exposed, a boiler explosion could result. The rate of thermal conductance from a fire tube by water is many times greater than that of steam, thus if steam blankets the tube, there is insufficient heat conduction from the tube and the tube rapidly overheats. In order to avoid this potentially catastrophic occurrence, steam systems are required to provide redundant water level indicators for each boiler.
One common solution to the requirement for redundant water level indicators has for some time included two independent visual level indicators. Such visual level indicators require penetrations for top and bottom of the indicators and a sight glass in which the level of water is visually apparent. However, boilers typically operate at high temperatures and pressures, often at a normal operating pressure of 3,000 psi. This factor presents an engineering challenge to make the sight glasses robust in such a hostile environment. As a consequence, sight glasses often leak, and a small leak at such a high pressure can quickly develop into a serious problem.
In recognition of this problem, standards setting organizations, such as for example the ASME, have provided alternative standards for providing redundant level indications. For example, one such redundant visual indicator may be isolated and an electronic level indication used instead. Such electronic level indications must themselves be robust and ideally are redundant, typically parallel circuitry, and not scanned, polled or multiplexed systems, but fully independent systems. However, electronic level indicators present challenges in design all their own due to the high temperatures involved. A typical electrical-type level indicator uses a water column system with spaced-apart conductive electrodes (also referred to as probes), with the water column mounted directly to the side of the boiler. An electrical potential is impressed across each of the spaced-apart electrodes, and the current is measured. If water is present between the conductors, the low impedance of the water results in a relatively high measured current. Conversely, steam is much less conductive, and thus if steam is present between the conductors, the high impedance results in a relatively low measured current. Commonly the higher current is used to illuminate one color or type of lamp, and the lower current illuminates another color or type of lamp, thus presenting to an operator an easily visible level indication. Additional logic circuitry may provide steam over water error indication, high and low level trip points, alarms, pre-alarms, displays, relay closures, SSR conduction, and other indications.
Some such electrical level indicators use a low DC potential with an impressed square wave periodic signal impressed upon it. The DC potential may cause hydrolysis of the water, and can result in explosive concentrations of hydrogen and oxygen within the steam system or electroplating. Thus, there remains a need for an electronic level indication system for boilers that avoids the hydrolysis problem.
Further, standards setting organizations have mandated various self-diagnostic features, such as for example power supply monitoring, clock function monitoring, electrode short detection, open wiring detections, and water over steam detection. Many systems have been offered to serve these various functions, but there remains a need in the art to provide an integrated electronic system which can effectively detect a circuit short, a circuit open, and/or a water vs. steam condition, wherein the electronic system sensing circuit has no direct current component which may cause hydrolysis.
Martin el al., in U.S. Pat. No. 4,020,488, described an apparatus for indicating when the value of a parameter lies above or below a predetermined limit. The apparatus includes two or more sensors each arranged to provide a warning signal when the parameter lies beyond a respective specified value, the value for at least one of the sensors being equal to or beyond the predetermined limit; and logic means including a validation circuit connected to the two sensors and arranged to provide a “parameter-beyond-limit” signal. This signal may be used to operate an alarm and/or an emergency trip, only when both sensors provide a warning signal. Various forms of apparatus are described in which the parameter sensed is the level of water in various items of a steam plant as used in power stations to provide an indication of high and/or low water level.
Bartles, in U.S. Pat. No. 4,224,606, describes a fluid level control circuit with a safety test function for a boiler water level probe of a fail safe type. The control circuit is connected to a field effect transistor controlled amplifier having a well defined threshold of operation. A safety test switch is provided to pull the amplifier's input down below the threshold point, but not to a shorted condition. By observing the output of the system, it is possible to determine whether the fluid level detector is properly installed, and whether the amplifier has had any type of unsafe operating failure.
Spencer, in U.S. Pat. No. 4,482,891, describes a robust electronic liquid level gauge apparatus, particularly suitable for use with boiler drum water column. The apparatus 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.
Cosser, in U.S. Pat. No. 4,646,569, describes a fluid level measurement system for the measurement of a level of fluid in a vessel. 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.
Colley et al., in U.S. Pat. No. 4,692,591, describe a controller for controlling an electrode boiler steam humidifier, of the type operable from a source of polyphase alternating current. A plurality of current loop transformers, coupled to the boiler electrodes, sense the individual currents drawn in each phase of the polyphase alternating current. A plurality of full-wave bridge rectifiers individually couple the secondaries of the current loop transformers to the input of a voltage amplifier such that the output of the amplifier is indicative of the highest of the currents sensed in each phase of the boiler electrode current. When the output of the voltage amplifier exceeds a predetermined threshold, a control effect is produced such that operation of the humidifier is controlled in accordance with the highest of the individual phase currents drawn through each of the boiler electrodes.
Similarly, Colley et al., in U.S. Pat. No. 4,792,660, describe a controller for controlling an electrode boiler steam humidifier. The controller includes a plurality of current loop transformers, coupled to the boiler electrodes, for sensing the current drawn by the elect
Browning & Bushman P.C.
Bushman C. James
Dole Timothy J.
Le N.
Tyco Flow Control
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