FR2522820A1 - Measurement of leaks in gas bottle, e.g. LPG bottle - using sensor for increase of gas in bell placed around leak zone - Google Patents

Abstract

In this process, the leak zone (2) is surrounded by a bell (3) which is connected to a sensor (7). This sensor measures the instantaneous concentration of the gas to be detected. The bell (3) is connected to the sensor (7) for a certain time so that an accumulation of gas is produced. From that moment, the increase in concentration of the leaking gas in the gaseous mix contained in the closed diffusing volume formed by the bell (3), the sensor (7) and the pipes between them is measured. By this process, the leaks are quickly detected with a high sensitivity. The diffusion can be forced by a circulator (9) which creates a continuous flow between the bell (3) and the sensor (7). The diffusion can also be produced naturally.

Description

 The present invention relates to a method for measuring leaks from containers containing a pressurized gas, as well as an installation for its implementation.

 The invention will be more particularly described with regard to the detection of leaks which may occur at the taps of bottles containing a liquefied petroleum gas, but this application is in no way limiting; as will be understood later, the method and the installation according to the invention can be used to detect and measure the leaks of a very wide variety of containers containing a gas under pressure.

 A known method for detecting leaks consists of enclosing the valve under a sealed bell in which a leak causes a rise in pressure. A device according to this process is described in particular in French patent 1,370,740 and its addition 2,092,965, both in the name of the Applicant: a first station installs a bell on each of the bottles parading on a conveyor (or on a carousel) and a second station, away from the first, connects a pressure sensor to the bell to measure the value reached by the latter inside the bell. The distance of the two posts gives the flight confined by the bell time to produce its effects, and the pressure to stabilize.

 The sensitivity of this process is however limited; one cannot in fact, for obvious practical reasons, excessively increase the time of confinement of the leak, nor use excessively sensitive pressure sensors, which would then be affected by all kinds of transient phenomena of pressure variation.

It was then proposed to replace the pressure measurement by a measurement of the concentration of the leakage gas; one then circulates in the bell a sweeping gas, neutral or of exactly known leakage gas content, then one takes a sample of the gaseous mixture leaving the bell; this sample is then analyzed, and the result makes it possible to know the leak rate ratio. This way of proceeding gives
This way of proceeding gives the flow of the sweeping gas in most cases satisfactory results, thanks to the use of highly sensitive analyzers, such as in particular infrared spectrometers or catalytic combustion detectors. However, it meets its limits in the necessary compromise between rapid detection of the leak (which requires a high flow of sweeping gas to rapidly stabilize the composition of the mixture to be analyzed, but requires significant dilution of the leak) and high sensitivity (possible with low dilution; but the low flushing gas flow lengthens the time required for stabilization).

 On the contrary, the method according to the invention makes it possible to reconcile these two imperatives, by ensuring a rapid measurement of the leaks which does not sacrifice the sensitivity of the detection. It is thus possible to ensure a more rigorous selection of "leaking" bottles, while increasing the production rate thanks to a reduction in the time the bottle is immobilized at the measuring station.

 To this end, the method according to the invention consists of: surrounding the leak area by means of a confinement bell placed in prolonged communication with a sensor for measuring the instantaneous concentration of the gas to be detected, so as to producing an accumulation of this gas; then measure the increase in the concentration of the leakage gas in the gaseous mixture contained in the closed, diffusion volume formed by the bell, the sensor in communication, and the pipes ensuring this communication.

 A sample taken from the stabilized mixture is thus replaced by a continuous measurement of the concentration, due to the diffusion of the leakage gas inside this closed volume.

 The diffusion can be natural, it can also be forced, by creating, during all the duration of the setting in communication, a direct current circulating between the bell and the sensor, in the gas mixture contained in the volume of diffusion. The stirring of this diffusion volume thus ensures a better homogenization of the mixture, reducing in considerable proportions the measurement time.

 Advantageously, before the bell is put in place, the diffusion volume is purged by introducing a sweeping gas. It is thus possible to fill the diffusion volume with a neutral gas or of a known composition with precision.

 It results from the fact that the diffusion volume is a closed volume that the gas which has escaped from the leak during the duration of the measurement accumulates in this volume. This accumulation results in an integration over this period of the quantity (mass) of escaped gas. The result of the measurement (of concentration) is therefore a variable value, increasing in proportion to time, the rate of increase of this value increasing in proportion to the leakage rate.

If this flow is constant, as we can suppose, the increase will be linear and therefore the concentration proportional to time.

 A first way of exploiting this result consists, after a determined period, in comparing the value reached by the concentration with a set value beyond which the container is considered to be leaking.

 It is also possible, from the increasing value of the concentration, to determine the time that it takes for this concentration to reach a predetermined value, and this time is compared to a set value below which the container is considered to be leaking.

 The bottle found to be leaking can finally, in known manner, be stored separately on an ejection conveyor, this ejection being carried out automatically from the result of the measurements.

 The installation according to the invention comprises a bell for confining the leakage zone, as well as a sensor placed in extended communication with the bell, allowing the instantaneous concentration of the leakage gas in the gaseous mixture contained in the volume to be measured. previously defined.

 Advantageously, this installation also comprises means for creating in the gas mixture contained in the diffusion volume a direct current flowing between the bell and the sensor.

 Preferably, it also comprises means for introducing a sweeping gas into the diffusion volume.

Other characteristics and advantages of the invention will appear on reading the detailed description below, made with reference to the appended drawings in which
FIG. 1 represents a first embodiment of the invention, where diffusion is forced by a circulator,
FIG. 2 is a graphic representation of the results of the measurements, for different values of the leak rate,
FIG. 3 is a variant of the first embodiment,
FIG. 4 represents a second embodiment, with natural diffusion,
Figure 5 is a variant of this second embodiment.

 Figure 1 refers to the embodiment with forced diffusion of the leakage gas. The valve 2 of the bottle 1 is capped by a containment bell 3; the bell can be applied tightly to the bottle, but this sealing is not necessary. A first pipe 4 connects the bell to one end 6 of the measurement cell of a leakage gas concentration sensor 7, by means of a two-way valve 5. The sensor 7 can advantageously be an infrared absorption sensor, using in particular the photoacoustic effect. The other end 8 of this same measuring cell is connected, by means of a line 10, to the bell by means of a circulator 9 such as a fan-pump.

 Line 4 can for example take the headlight atmosphere confined by the bell by means of orifices provided at the base thereof, and line 10 return to an orifice opening at the top of the bell.

 The valve 5 can take two functional positions A or B. In position A, the pipe 4 is directly connected to the sensor 7, thus establishing a closed loop or circuit in which a current will be created by the circulator 9. In position B, a sweeping gas, for example clean air or air containing an exactly known proportion of the gas to be detected, is introduced, from a supply line 11, to the sensor 7, the circulator 9 and the bell 3. It is thus possible to purge these circuits.

 The operation is as follows: by known means, the bottle is brought to the measuring station, below the containment bell 3. In this state, the valve is placed in position B, and the volume of the sensor-circulator- bell, as well as the connecting pipes, are purged by sweeping the gas supplied by the pipe 11. Simultaneously, the sensor 7, crossed by this gas, can provide a reference measurement.

 Then, the bell is brought into contact with the bottle, and the valve 5 placed in position A, thus establishing a closed volume of diffusion and a closed circulation circuit. Any leak will then produce a progressive increase in the concentration of the gas contained (and circulating) in this closed volume, this increase in concentration being monitored by the sensor 7.

 Once the measurement is complete, the bell is raised, and a next bottle admitted to the measuring station. In the meantime, the valve 5 is returned to position B to proceed with the sweeping and purging of the circuit. The signal from the sensor, processed, can be memorized and then restored when the bottle passes in front of an ejection conveyor: the restored signal corresponding to a leaking bottle can actuate a cylinder pushing it back on the ejection conveyor and therefore excluding it from further handling for the purpose of verification or repair.

 As shown in Figure 2, the presence of a leak results in a constant increase in concentration over time, the hatched area being representative of the quantity (mass) of gas that has leaked. If the leak rate is constant, the increase in concentration will be linear and therefore represented by a straight line whose slope will depend on the one hand on the value of the leak rate, and on the other hand on the size of the diffusion volume. (a reduced volume of diffusion allows a faster increase in the concentration, so we see that there is interest in minimizing this volume).

The three lines D1, D2, D3 correspond to three different, increasingly high, values of the leak rate.

 In Figure 2, the lines have been shown from the origin, which means that the initial concentration is zero, and therefore that the sweeping gas is a clean gas free of all traces of the detected gas. If this were not the case, the lines would have the same appearance, but would shift up the ordinate the origin corresponding to the concentration of the sweeping gas: it is easy, as is easily understood, to proceed has a differential measurement to eliminate this initial value, thanks to a prior measurement, of reference, carried out during scanning.

 It will be noted that this measurement, where only the concentration and the time intervene, is independent of the pressure prevailing in the diffusion volume, and therefore is not affected by the variations of the latter, in particular its transient variations.

 The measurement can be used in different ways. First of all, it is possible to integrate the leak for a fixed, predetermined duration TO. The final concentration reached at the end of this time Tg is then compared to a set point; if the measured concentration is above this threshold, the bottle is considered to be leaking. This predetermined duration can advantageously be of value substantially equal to the time of presence of the bell on the container; the time during which the bottle is at the measuring station is thus optimally used.

 Conversely, it is also possible to set a given CO concentration threshold, fixed, and to measure the time required to reach this value.

This duration is then compared to a set duration value; if the measured time is less than this set value, the bottle will be considered as leaking. To avoid overly prolonged measurements, it is possible to set a CO concentration threshold such that, in all circumstances, the setpoint duration remains less than the total duration of the bell's presence on the container.

 FIG. 3 relates to a variant of the embodiment, where an additional pipe 12 makes it possible to short-circuit the sensor 7 by diverting the gas stream. This bypass line is associated with a second two-way valve 13. The valve 5 retains the functions it had previously. When the second valve 13 is in position C, the sensor 7 and the circulator 9 are directly connected, and the situation is therefore identical to that of FIG. 1.

 On the other hand, when the valve is in position D, the sensor is short-circuited. It is possible on the one hand (position B of valve 5) to carry out a sweep avoiding the sensor, therefore making it possible to keep a sample of the mixture while carrying out the operations prior to the next measurement. It is also possible (position A of valve 5) to continue the diffusion of the gas mixture while "storing" an instantaneous value without stopping the integration process.

 In a second embodiment, illustrated in FIG. 4, natural diffusion of the leakage gas is used, without using closed-loop circulation.

 The bell 3 is directly connected to the sensor 7. A pumping member 14 is constituted r L example of a piston 15 moving in a cylinder 16, the volume of which (when the piston is in the suction position arrow F) is substantially equal to the volume of the bell 3 and the pipe which connects the latter to the sensor. The purging gas can be introduced, by controlling the valve 17, from the supply line 11 towards the end 6 of the sensor located opposite the bell.

 In a variant shown in FIG. 5, the cylinder 16 and the measuring cell 7 of the sensor are combined, the piston 15 then penetrating directly into the cell of the sensor.

 The operation is as follows: first of all, as before, the bottle is brought to the measuring station under the bell, the piston then being in the delivery position (arrow E). The valve 17 is then opened to allow the intake of the sweeping gas and the purging of the sensor-bell-pipe assembly.

 Once this purge has been carried out, the bell 3 is brought into contact with the bottle, and the valve 17 closed.

From this moment, any gas leak will result in an increase in the concentration inside the bell and therefore, by diffusion, in the sensor. The change in concentration will, as before, have the pace shown in Figure 2; these measures can be used identically.

 At the end of the measurement, the pump is activated by operating the cylinder in the suction position (arrow F).

The atmosphere existing in the bell-sensor assembly is then sucked into the sensor-cylinder assembly; it is thus preserved by conservation of the sample despite the lifting of the bell.

 Before taking the next measurement, the piston 15 is pushed back towards the discharge direction (arrow E) and the valve 17 reopened, thus allowing the assembly to be purged beforehand.

 Of course, the present description is given without limitation and many variants of the process or of the installation can be envisaged without thereby departing from the scope of the invention.

Claims (11)

 1. Method for measuring leaks from containers containing a pressurized gas, in particular a liquefied petroleum gas, of the type consisting in: surrounding the leakage zone (2) by means of a containment bell (3), communicating extended with a sensor (7) for measuring the instantaneous concentration of the gas to be detected, so as to produce an accumulation of this gas, characterized in that the increase in the concentration of the leakage gas is then continuously measured in the gaseous mixture contained in the closed, diffusion volume formed by the bell, the sensor in communication, and the pipes ensuring this communication.  2. Method according to claim 1, characterized in that, throughout the duration of the communication, creating in the gas mixture contained in the diffusion volume a direct current flowing between the bell and the sensor.  3. Method according to one of claims 1 and 2, characterized in that, prior to the installation of the bell, the diffusion volume is purged by introducing a sweeping gas.  4. Method according to one of claims 1 to 3, characterized in that, after a determined period (To) the value reached by the concentration is compared to a set value beyond which the container is considered to be leaking.  5. Method according to one of claims 1 to 3, characterized in that, from the increasing value of the concentration, the duration that this concentration takes to reach a predetermined value (CO) is determined, and this duration is compared at a setpoint below which the container is considered to be leaking.  6. Method according to claim 4, characterized in that the determined duration (Tg) is substantially equal to the time of presence of the bell on the container.  7. Installation for implementing the method according to one of the preceding claims, of the type comprising a bell (3) for confining the leakage zone (2), characterized in that it comprises a sensor (7), placed in prolonged communication with the bell, allowing the measurement of the instantaneous concentration of the leakage gas in the gaseous mixture contained in a closed diffusion volume formed by the bell, the sensor in communication and the pipes ensuring this communication.  8. Installation according to claim 7, characterized in that it further comprises means (9) for creating in the gas mixture contained in the diffusion volume a direct current flowing between the bell and the sensor.  9. Installation according to one of claims 7 and 8, characterized in that it further comprises means (5, 11) for introducing a sweeping gas into the diffusion volume.  10. Installation according to one of claims 8 and 9, characterized in that it further comprises means (12, 13) for short-circuiting the volume of the sensor so as to temporarily divert the circulation of the gas stream out of that -this.  11. Installation according to claim 7, characterized in that it further comprises means (14; 15, 16) for sucking out of the bell, and through the sensor, the gaseous mixture resulting from the diffusion, so as to keep a sample of this mixture in the sensor enclosure after removing the bell.