Ultrasonic gas leak detector including a detector testing device

TECHNICAL FIELD

0001The present invention relates to fixed ultrasonic gas leak detection devices and systems used for instance for monitoring outdoor gas installations and more particularly to a self-test method and corresponding means for such devices and systems.

BACKGROUND ART

0002Various methods and corresponding devices are used for the detection of gas for instance leaking from high-pressure gas installation pipelines in industrial plants or offshore installations. Thus it is known to use catalytic point sensors or infrared point sensors where the sensor generates a signal after the sensor having been exposed to a certain minimum concentration of a gaseous agent for a given minimum of time. The concentration can be measured in LEL (Lower Explosive Level), which indicates when an explosive mixture is reached between the gas and the oxygen in normal air. Such sensors need thus to be in actual physical contact with the gaseous agent in order to generate a signal, which can for instance be used for triggering an alarm. According to a different method, so-called open path sensors are used, comprising an infrared (IR) transmitter and a corresponding receiver for receiving the IR signal transmitted by the transmitter and positioned at a certain distance from each other, i.e. with a certain signal propagation path therebetween. When a given concentration (LEL level) of a gaseous agent enters the propagation path between the transmitter and receiver, at least a portion of the IR signal is being absorbed by the gas and hence the portion actually received at the receiver decreases. Thus the presence of a gaseous agent in the propagation path is detected by the receiver, which can for instance elicit an alarm signal.

0003By either of the above methods it is possible not only to detect the presence of gas but also to determine its concentration. It may however be desirable, either in combination with the above detection means or in itself, to provide a gas leak detector, which does not require the building-up of a given minimum concentration of gas before eliciting a detection signal, but which is capable of generating an instant alarm signal, for instance at the escape of gas through a leakage in a pipeline etc.

0004A method and corresponding detector device can be based on the fact that the escape of gas at sufficiently high pressure through a leakage, for instance in a pipeline, generates airborne acoustic sound waves of a very large bandwidth. The amplitude of the sound signal is partly related to the mass flow rate (leak rate) from the leakage. The leak rate of the gas from the leakage is measured in the unity Kg/Sec. The leak rate is mainly determined by the gas pressure immediately before (upstream of) the leakage and the size of the leak. Other factors, for instance the molecular weight of the gas and the gas temperature have been found to have less influence on the sound signal generated by the leakage. Thus, the frequency spectrum of sound waves generated by this mechanism extends far into the ultrasonic region. The ultrasound portion of the signal generated by the leakage can thus be picked up by an ultrasound-receiving transducer, the positioning of which relative to the leakage will not be critical, which is of course important in installations comprising spatially widely distributed components that could potentially contain leakages. A very important advantage of this type of detector is that it does not require being in physical contact with the gas or that the gas be present in a predetermined propagation path. The correct functioning of a detector based on this principle is therefore less affected by wind carrying the gas away from the detector or by the gradual dilution of the gas as the gas spreads over a larger area.

0005Gas leak detectors of the above kind are known within the art and are known to function satisfactorily at distances between a leakage and a detector of up to approximately 15 meters.

0006When such gas leak detectors are installed at a site, such as an off-shore installation remote from a central alarm system, it is important continuously to ascertain that the detector is functioning reliably, i.e. that the absence of a signal from the detector indicating the presence of a gas leak is actually due to the fact that no such leak is present in the installation and is not caused by malfunctioning of the detector device itself. It is thus important that a detector of this type be provided with means for performing a self test procedure, the self test comprising all components of the detector, i.e. not merely the electronic circuits of the detector but also the detector transducer and optional wind screens or other protective devices encapsulating the transducer. Advantageously the test system and procedure should provide for a simple implementation hereof.

0007Gas leak detector systems based on the detection of an acoustic signal emitted from a leakage are described in the literature. Thus, GB 2 176 604 A describes a system comprising a detector containing an array of acoustic transducers thus forming a directional characteristic of the detector covering the area within which a potential leakage could occur. Typically the detector is located on a pipeline and the directivity pattern of the detector extends along the pipeline. At a distance from the detector an active acoustic array for emitting an acoustic signal towards the detector is located and this active array can be used to provide self-test means for test of the integrity of the detector. Specific self-test means are not described in this document. The placement of the detector and active array separate from each other furthermore make it possible to detect for instance gas bobbles escaping from the pipeline between the two arrays.

0008US 4,640,121 discloses a method and corresponding system where a plurality of probes are located spaced apart on the surface of a vessel for picking up high frequency solid-conducted sounds that could be generated by a leakage in the vessel. The provision of several probes distributed over the surface of the vessel makes a localisation of the site of leakage possible. Furthermore, the presence of several probes makes it possible to utilise these for self-test procedures, such methods being however not described in the document.

0009US 4,043,180 discloses a system for detecting a leakage in a pipeline, the system comprising a plurality of leak detection units distributed along the pipeline, each unit comprising a separate crystal detector for detecting structure-borne sound caused by a leakage in the pipeline and propagating through the pipeline and a separate crystal oscillator placed in the vicinity of the detector and coupled to the pipeline for generating a structure-borne signal for propagation through the pipeline, whereby the integrity of the crystal detector can be tested. The detector and the oscillator are coupled via electrical leads to a local electronic circuit provided in each of the leak detection units.

0010EP 0 315 199 describes a system comprising ultrasonic detectors located in an installation at positions where for instance leakages could be expected to occur. Thus, for instance a detector could be located in the vicinity of a valve or a joint in a system of pipelines. No self-test means are described in this document.

0011Finally, JP 58100729 describes a system somewhat resembling the above and comprising a detector for detecting structure-borne sound propagated through an installation and where a piezoelectric exciter generates a structure-borne sound of a frequency different from the frequency of sounds generated by leakages in the installation, whereby an ongoing test of the integrity of the detector can be performed without interruption of the normal function of the detector.

SUMMARY OF THE INVENTION

0012On the above background it is an object of the present invention to provide an ultrasonic gas leak detector device comprising self-test means for monitoring the correct functioning of the complete detector including detector transducer(s) and optional protective devices, such as wind screens, surrounding said transducer(s).

0013It is a further object of the present invention to provide a detector device of the above kind, which can be mounted as a fixed installation in an explosion-hazardous environment.

0014It is a further object of the present invention to provide a complete detector system comprising a detector device of the above kind in communication with a central alarm system, which can be located remotely relative to the detector device, the detector device being for instance mounted in an off-shore natural gas installation and the alarm system being mounted in non-explosion endangered environment where it can raise an alarm.

0015These and other objects are attained with an ultrasonic gas leak detector device according to the characterising clause of claim 1, the system according to claim 9 and the self-test method according to claim 11 Various advantageous embodiments of the detector device according to the invention are defined by the dependent claims.

0016According to the present invention there is thus provided an ultrasonic gas leak detector device comprising a housing provided with a first fixed transducer for receiving an ultrasonic signal S1 generated by a gas leakage and propagating through air, said housing being furthermore provided with at least one second transducer for emitting at least one ultrasonic signal S2, which can be received by said first transducer, and self-test means by means of which said device can perform a self-test, enabling the correct functioning of the device to be monitored whereby said housing is designed for use in explosion endangered environments and said at least one test transducer is provided on an external surface of said housing. By basing the self-test function on the emission of an acoustic signal in the ultrasonic frequency range used for the detection of a gas leak in an installation, the correct functioning of the complete device, inclusive of leak detector transducer, electronics and a possible windscreen or other protective device surrounding this transducer, can be ascertained in a simple and reliable fashion.

0017The gas detector according to the invention outputs at least three signals: (1) one signal indicating a gas leakage for eliciting a corresponding alarm, (2) a real time DC signal that relates to the ultrasonic sound level measured by the microphone, also called: "the 4-20 mA" interface, (3) one signal indicating that the detector does not function properly, (4) digital communication interface, and (5) AC analog microphone signal output.

0018According to a presently preferred embodiment of the invention there is thus provided a detector for detection of gas leakages by means of the ultrasonic signal generated by a gas leakage, where the detector is able to sense an acoustic signal in the frequency region above approximately 20 kHz. The detector is provided with self-test means where the appropriate function of an acoustic receiver (detector transducer) is subjected to an ongoing test procedure by means of an integrated, external noise source (a so-called "fail safe" procedure). The detector according to this presently preferred embodiment of the invention is furthermore designed for installation in explosion endangered environments (EX approved) and furthermore designed for fixed installation. Furthermore, the detector according to this presently preferred embodiment of the invention is provided with communication means for communication with a central alarm system, implemented as a so-called 4-20 mA Interface, relay output for both alarm trigger level setting and error alarm for indication of internal instrument failure. Furthermore, there is provided a digital communications interface for digital communication.

0019Although the self-test procedure utilising a test signal generated by a test transducer and picked up by the detector transducer is advantageous from the point of view of ascertaining the correct performance of the complete detector device, including detector microphone, protective means such as a wind screen provided around the detector transducer and the various electronic circuits and connections of the detector, other self-test procedures can also be incorporated in such devices, either as an alternative to the above procedure or in combination herewith as an option. Thus, it is possible to provide the detector with self-test means for performing an electronic test of the detector transducer (for instance a so-called charge injection calibration of the transducer) and/or the electronic circuits in the detector device.

BRIEF DESCRIPTION OF THE DRAWINGS

0020The invention will now be described in more detail with reference to the accompanying drawings, in which <ul id="ul0001" list-style="none"><li>figure 1 is a perspective view of an embodiment of a device according to the present invention;</li><li>figure 2 is a cross sectional side elevational view of the embodiment shown in figure 1, and</li><li>figure 3 is a graph illustrating the functioning of the self-test method according to the invention.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

0021In the following, a detailed description of an embodiment of the ultrasonic gas leak detector according to the invention is given.

0022With reference to figs. 1 and 2 there is shown an embodiment of the detector device according to the invention generally referred to by reference numeral 1. The detector comprises a housing consisting of an upper section 7 and a lower section 6 with internal cavities 13 and 14, respectively. Between these cavities extends a partitioning wall 15 provided with a communicating passage 17 providing access for electrical connectors from between said two cavities. The lower section 6 is provided with outlets 9 and 10 for communication between the detector and external equipment such as a central alarm system. The lower section 6 is furthermore provided with display means 16.

0023In the lower section of the detector device there is located the detector transducer 2, which is able to detect an ultrasonic sound signal emitted by a gas leakage. The detector transducers can in principle consist of any transducer capable of generating an electrical output signal in response to an ultrasonic signal, although the transducer in practice must meet certain requirements relating to high frequency sensitivity, stability, robustness and the choice of material, etc. to make it compatible with the environmental conditions in which it is to operate. The detector transducer 2 is situated within a suitable housing 3 that also provides the detector with certain acoustic characteristics and can optionally be provided with protective means 4, for instance a windscreen to avoid noise from wind at high air speeds. In case of a gas leakage, the detector transducer 2 receives an ultrasonic signal S1 emitted from the leakage.

0024On the lower section 6 there is furthermore provided a test transducer 11 accommodated within a suitable housing 5 and communicating with the surroundings through an opening 12 in said housing 5. The opening 12 preferably faces the detector transducer 2 although other configurations could also be used. During operation, the test transducer 11 emits an ultrasonic test signal S2, which is picked up by the detector transducer 2. An example of a test procedure will be described below.

0025The housing of the detector device is provided with holes 18 with thread by means of which it can be fixedly attached at a proper location in the installation to be monitored.

0026The housing of the detector is divided into two "chambers" where the lower chamber 6 contains all electronics. The upper chamber 7 contains connection terminals for detector communication and power supply as well as gland access holes for external cable entrance.

0027The detector shall be available for both the European market and the North American market, where the requirements to the design rules for designing instrumentation in explosive areas are different.

0028The detector has been designed so that the same mechanical/electrical design will be valid in both Europe and North America, but the marking on the detector will be different for the two markets.

0029In the European market, the lower chamber will be classified as a solaced Ex d chamber that can withstand an internal explosion, whereas the upper chamber 7 will be classified as an Ex e chamber also called "increased safety area".

0030In North America, both the lower 6 and the upper 7 chamber will be classified as Ex d chambers.

0031The microphone 2 and the piezo electric transducer 11 will be classified as intrinsically safe devices outside the lower 6 Ex d chamber.

0032The complete detector housing is made of stainless steel 316 according to proper design requirements for both European ATEX requirements and North American design regulations for instrumentation to be installed in explosive atmospheres.

0033Referring now to figure 3 there is shown a graph illustrating an example of the operation of a detector device according to the present invention. The upper portion of the graph illustrates the ultrasonic sound pressure level (SPL) at the detector microphone as a function of time. A certain level of background noise will be present in any installation as indicated on the figure. At a predetermined and adjustable level above this background level, an alarm trigger level is located, this level being as an example set to 74 dB SPL in the example shown in figure 3. During normal operation the installation occasionally emits ultrasonic signals indicated as spontaneous ultrasonic noise events and maintenance noise in the figure. The emissions are thus not caused by a leakage in the installation and should hence not lead to an alarm being elicited. In order to prevent such false alarms from occurring, the detector device is provided with an adjustable timer function by means of which a delay for triggering the alarm can be set. Thus, ultrasonic noise events in the installation with a time duration shorter than the set delay do not give rise to an alarm. Only if the ultrasonic sound pressure level exceeds the alarm trigger level and the duration of this ultrasonic sound pressure level exceeds the specifically chosen delay, an alarm signal will be generated by the detector device for eliciting an alarm, for instance in a central alarm system at a remote site.

0034As furthermore indicated in figure 3, a test signal T emitted by the test transducer 11 is also received at the microphone 2. This test signal T is repeated at a certain predetermined interval and the test signal has a given duration ΔT. According to the embodiment of the self-test method shown in Figure 3, the level of the test signal is above the alarm trigger level and also above the (highest expected) level of ultrasonic maintenance noise. Specifically the duration of the self-test signal is set to 2 seconds in the present example and the signal could be repeated every 5 minutes, but these specific values are of course only examples of values, which could be chosen in an actual set-up.

0035Every time the microphone registers the reception of the test signal, the sound pressure level of the received test signal must fall within a given predetermined tolerance range, and if the level of the test signal falls outside this range, an alert signal indicating an operational error of the detector is generated and communicated for instance to said central alarm system.

0036Specifically the test signal could consist of a sinusoidal signal of a frequency of for instance 40 kHz, but many other choices of test signal would of course also be possible - and might even prove advantageous in relation to the overall nature (for instance spectral content) of the particular installation. Thus for instance single frequency sinusoidal signals at other frequencies within the ultrasonic region could be used as well as narrow band ultrasonic noise signals.

0037Although a specific embodiment of the present invention has been shown and described in the preceding parts of the detailed description, it is understood that a person skilled in the art may conceive other embodiments of the invention without departing from the scope of the invention as defined by the following claims.

LIST OF REFERENCE NUMERALS

0038<dl id="dl0001"><dt>1.</dt><dd>Ultrasonic gas leak detector</dd><dt>2.</dt><dd>Detector transducer</dd><dt>3.</dt><dd>Housing of detector transducer</dd><dt>4.</dt><dd>Protector of detector transducer</dd><dt>5.</dt><dd>Housing of test transducer</dd><dt>6.</dt><dd>Upper section of device</dd><dt>7.</dt><dd>Lower section of device</dd><dt>8.</dt><dd>Connecting flange</dd><dt>9.</dt><dd>Output/Input</dd><dt>10.</dt><dd>Output/Input</dd><dt>11.</dt><dd>Test transducer</dd><dt>12.</dt><dd>Emission opening</dd><dt>13.</dt><dd>Cavity of upper section</dd><dt>14.</dt><dd>Cavity of lower section</dd><dt>15.</dt><dd>Partitioning wall</dd><dt>16.</dt><dd>Display</dd><dt>17.</dt><dd>Communication passage</dd><dt>18.</dt><dd>Threaded holes</dd></dl>