Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2003-01-29
2004-12-28
Wu, Daniel (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S635000, C340S626000, C073S30400R, C073S049200, C169S061000, C169S023000, C137S557000
Reexamination Certificate
active
06836217
ABSTRACT:
The present invention relates to a carbon dioxide fire extinguishing device.
PRIOR ART
For fire extinguishing devices with gaseous extinguishing media it is prescribed that the pressure vessel in which the extinguishing medium is stored under pressure is checked for gas losses. In the case of carbon dioxide pressure cylinders, it must be ensured that a gas loss of over 10% of the filling weight is reliably detected. In their periodic testing, transportable carbon dioxide fire extinguishers are weighed by means of a calibrated balance. As a result, a gas loss between two tests remains unnoticed. In the case of stationary carbon dioxide fire extinguishing systems, the carbon dioxide pressure cylinders hang individually in a weighing device, so that the weight of each individual carbon dioxide pressure cylinder is continuously monitored. If the weight falls below a fixed weight, an alarm is set off. Such weighing devices for suspending carbon dioxide pressure cylinders significantly increase the cost of stationary fire extinguishing devices. Moreover, they must be calibrated at regular intervals.
Until now, there has been no satisfactory alternative to the weighing of carbon dioxide pressure cylinders.
Pressure monitoring procedures are entirely unsuitable for detecting a gas loss from a carbon dioxide pressure cylinder, since, in the case of a customary filling ratio of 1:1.50 (i.e. a filling weight of 0.666 kg of carbon dioxide per liter of cylinder volume), below a temperature of 27° C. a gas loss of 10% no longer causes a significant drop in pressure in the cylinder (in the case of a filling ratio of 1:1.34, i.e. a filling weight of 0.746 kg of carbon dioxide per liter of cylinder volume, this lower temperature limit is even around 22° C.). Moreover, the pressure in the carbon dioxide pressure cylinder is highly temperature-dependent.
At least in the case of fire extinguishing devices, filling level gages with floats have also been unable to establish themselves as an alternative to the weighing of carbon dioxide pressure vessels. A valve with an integrated filling level gage with a float, as known for example for a carbon dioxide pressure cylinder from the U.S. Pat. No. 4,580,450, cannot be used in carbon dioxide fire extinguishing systems because the linkage of the filling level gage takes up considerable space in the valve base and this means that the inlet bore for the gas in the valve base has to be relatively small. It should be noted in this connection that carbon dioxide pressure cylinders for stationary carbon dioxide fire extinguishing devices have in the neck of the cylinder an internal thread of only W 28.8×{fraction (1/14)}″ according to DIN 477. It must be possible to screw into this internal thread a valve base which has a prescribed inlet bore for the extinguishing agent of at least 12 mm in diameter, in order that the carbon dioxide can flow into the valve with a low pressure loss after the fire extinguishing device is put into action.
The U.S. Pat. No. 5,701,932 discloses for gas cylinders with high-purity gases a gas cylinder valve with a built-in capacitive filling level measuring device as an alternative to a mechanical filling level measurement with a float. The capacitive filling level measurement described in U.S. Pat. No. 5,701,932 is based here on the principle that the liquid phase of a gas has a far higher dielectric constant than the gaseous phase, so that dropping of the liquid level in the pressure cylinder is reflected by a reduction in the capacitance of the probe. This measuring principle consequently presupposes that the measurement takes place at a given ambient temperature, at which it is ensured that there are two separate phases in the pressure cylinder, and that the level of the liquid in the pressure cylinder drops if gas is extracted from the pressure cylinder. However, by contrast with the application for high-purity gases described in U.S. Pat. No. 5,701,932, this is by no means always the case with a carbon dioxide pressure cylinder for fire extinguishing purposes. In fact, one application for fire extinguishing devices where carbon dioxide pressure cylinders are used is in machine rooms for protecting equipment, where it is quite possible for ambient temperatures of over 40° C. to be reached.
With a filling ratio of the carbon dioxide pressure cylinder of 1:1.50 (i.e. 0.666 kg of carbon dioxide per liter of cylinder volume), the liquid phase of the carbon dioxide then already takes up the entire volume of the cylinder when the temperature reaches 27.2° C., so that above this temperature a gas loss no longer necessarily brings about a change in the level of the liquid in the pressure cylinder. Moreover, the critical temperature of the carbon dioxide from which the carbon dioxide forms a supercritical fluid, because there is in any case no longer any difference between a gaseous phase and a liquid phase, is as low as 31° C.
Furthermore, it should be noted with respect to the valve with the filling level measuring device from U.S. Pat. No. 5,701,932 that it is also not suitable for flow-related reasons for carbon dioxide pressure cylinders in fire extinguishing devices. In fact, in a valve base with a screw-in thread of W 28.8×{fraction (1/14)}″, the fitting of the capacitive measuring probe takes up so much space that there is no space left for an inlet bore of at least 12 mm in diameter for the carbon dioxide extinguishing gas. To obtain enough space for such a 12 mm inlet bore in the valve base, the diameter of the capacitive measuring probe could of course be made even smaller. However, for this it would be necessary to accept stability problems with respect to the measuring probe, which cannot be tolerated in the case of an element with relevance to safety.
OBJECT OF THE INVENTION
The present invention is accordingly based on the object of reliably checking the carbon dioxide pressure vessel in a carbon dioxide fire extinguishing device for gas losses without weighing, at both low and high ambient temperatures. This object is achieved according to the invention by a device as claimed in claim
1
.
GENERAL DESCRIPTION OF THE INVENTION
In a carbon dioxide fire extinguishing device according to the invention, a capacitive measuring device which is calibrated for a temperature range above and below the critical temperature of the carbon dioxide is used for detecting a gas loss from the carbon dioxide pressure vessel. In other words, the present invention is based on the surprising realization that a capacitive measuring device can not only measure changes in the liquid level in the pressure vessel in a known way but a measurable change in capacitance can also be unequivocally assigned to a gas loss from the pressure vessel even above the critical temperature of the carbon dioxide, i.e. when there is no longer any physical difference between the gaseous phase and the liquid phase of the carbon dioxide. In this way, a simple solution is provided for detecting a gas loss from a carbon dioxide pressure vessel of a fire extinguishing device which can even be used at high ambient temperatures (i.e. temperatures above 30° C.) and makes laborious weighing of the pressure vessel superfluous.
Such a capacitive measuring device preferably comprises a capacitive measuring probe which extends over the entire height of the pressure vessel, a measuring module for measuring the capacitance of the capacitive measuring probe, a microprocessor for processing the measured capacitance values, which assigns to a measured change in capacitance a corresponding gas loss, and also means for generating an alarm message if the gas loss determined by the microprocessor exceeds a given value.
The calibration preferably takes place electronically, using for example a temperature sensor and a memory with calibration values for a temperature range above and below the critical temperature of the carbon dioxide. The microprocessor resorts temperature-dependently to the calibration values in the memory in order to assign to a measured change i
Huang Sihong
Kang Gregory B.
Luxembourg Patent Company S.A.
Nath Gary M.
Nath & Associates
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