Leak detection and measurement of wind direction and velocity

Abstract: A leak detection system for gas, steam or the like in a plant yard (40) includes a plurality of sampling modules (10) and wind direction and velocity indicators (30) both arranged around apparatus or equipment in the plant yard. Samples of the atmosphere around the apparatus or equipment are drawn through the sampling modules, and the concentration of a substance leaked into the atmosphere is detected by a sensor module (20). At the same time, the direction and velocity of wind in the plant yard are determined. Leakage of gas, steam or the like in the plant yard can be detected based on the concentrations and wind direction and velocity. Methods for the determination of the direction and/or velocity of wind, wind direction and velocity indicators, and anemoscopes are also described.

Description: This invention relates to a leak detection system for gas, steam or the like, said system being suitable for the detection of gas, steam or the like leaking from equipment or apparatus of a plant where a flammable or noxious substance is handled in a gaseous, liquid or solid form, as well as to a method for the measurement of the direction and velocity of local wind, said measurement being needed for the estimation of a leak point and leak quantity. From every country there are reports of serious accidents or disasters caused by fire, explosion or leakage of noxious gas from individual machines, equipment or facilities, arising at factories with a plurality of machinery, equipment and/or facilities connected together via pipes, or in industrial complexes where a conglomeration of such factories is systematically linked one to another. Damage caused by such incidents is not limited to the loss of facilities and/or interruption of production. For example, accidents at nuclear power plants and the like where radioactive substances are handled involve such potential danger that they may even lead to global contamination. Causes of accidents include those pertaining to human error in operation and those attributed to a failure of machinery, equipment or facilities. In a large majority of reported incidents, leakage of gas or the like actually took place as an initial phenomenon; the gas so leaked then igniting due to the existence of a certain ignition source, resulting in fire or explosion. It is, therefore, possible to obviate many of such accidents and to minimize the loss of property of the society, if leakage of gas or the like can be discovered and stopped in an early stage, when the quantity of leakage is still small. With the foregoing objective in view, business establishments beyond a certain size are required to install gas leak detectors under laws such as the Fire Protection Law and High Pressure Gas Control Law in Japan. In particular, according to Section 8, No. 53 of the Safety Regulations for Industrial Complexes in the latter law, it is specifically ruled that gas leak detectors of a prescribed specification be installed at intervals of 20 m, with at least one gas leak detector at each point of installation along the boundary of each plant or the like. Besides these statutory regulations, business establishments which manage factories or various facilities have of their own volition also exercised tremendous efforts for the early discovery of leakage of gas. To date there exists neither a highly reliable leak detection method for the discovery of leakage of gas, steam or the like nor a system for estimating the point, quantity and the like of leakage when such leakage is discovered. Indeed, such detection is commonly dependent only on discovery by a maintenance crew member on regular patrol in a plant in an effort to discover any abnormality. Under these circumstances, the probability of detection of gas leakage in a plant by plural gas detectors discretely arranged in the plant is therefore very low. Moreover, even if a detector provides a warning, the retention of a warning signal may be brief and often becomes intermittent. It is, therefore, difficult to determine if the warning results from discovery of real leakage or is caused by some noise or is due to drifting of the zero point and, under the circumstances, the detection of gas leakage relies upon human judgment. US-A-3,416,357 discloses a system for detecting gas leaks employing a motor vehicle carrying a plurality of air inlet tubes. They are coupled to a blower which draws air through them and passes it to a mixer. The mixed air samples are passed to an analyser which analyses the mixture for leaked gas. SU-A-1,288,531 discloses a leak detection system employing an array of sampling points, concentration sensors, and a switch assembly for connecting selected sampling points to the sensors. The switch assembly operates in response to a controller which receives data from a wind direction meter. Data from the sensors and from a wind speed meter pass to a computer. Preferred embodiments of the invention may enable one to provide a highly reliable leak detection system for gas, steam and the like. Thus the present invention provides a leak detection system for gas, steam or the like in a plant yard where a flammable or noxious substance is handled in a gaseous, liquid or solid form, comprising: a plurality of sampling modules arranged in the plant yard, for collecting samples of the surrounding atmosphere, each said sampling module comprising a gas pipe having one open end, plural air inlet tubes attached to the gas pipe in communication with the interior of the gas pipe, and a like number of air samplers connected to the free ends of the respective air inlet tubes to collect samples of the surround atmosphere, at least one sensor module connected to said open ends of the gas pipes of the sampling modules, for receiving the samples of the atmosphere collected through the sampling module and having a built-in sensor to detect leaked substance in the atmosphere so received if the substance is leaking from equipment or apparatus in the plant yard; at least one vacuum pump for drawing the samples of the atmosphere which exist in the sampling modules, through the sensor module; at least one wind direction and velocity indicator for the real-time determination of the direction and velocity of local wind; and a data processor for the real-time analysis of wind direction and velocity data obtained from the said wind direction and velocity indicator and concentrations of the leaked substance in the atmosphere, which have been collected by the respective sampling modules, as detected by the sensor module, said data processor including a graphic screen for providing a display showing the area containing all of said modules, said data processor being adapted to draw on said display the area or areas extending upstream of the or each module for which leaked substance is being detected, said area or areas each probably encompassing the point of leakage; said areas changing over time as the wind direction changes, and said data processor being arranged to draw successive areas to narrow down successively the overlapped area likely to contain the point of leakage. Plural sampling modules are arranged around apparatus or equipment in a plant yard. Through the sampling modules samples of the atmosphere surrounding the equipment are drawn by a vacuum pump, whereby the concentration of a substance leaking into the atmosphere and being drawn throughthe individual sampling modules is detected by corresponding sensor modules. When there is no leakage of gas, steam or the like in the plant yard, all the sensor modules generally indicate zero concentration. If the sensor module connected to any one of the sampling modules detects a concentration higher than 0, this indicates some leakage near that particular sampling module. In general, there is wind over the plant yard and outside the boundary of the plant yard. Accordingly there is also wind near each point of leakage. The direction and velocity of local wind near the point of leakage are substantially different in many instances from those above the plant yard or those outside the boundary of the plant yard because of influence by obstacles such as many apparatus or equipment in the plant yard. Further, they always vary at certain time intervals in accordance with meteorological fluctuations in wind direction above the plant yard. When plural sampling modules have been installed around apparatus or equipment in the plant yard, for example, in such a manner that the apparatus or equipment are surrounded by the sampling modules, any leakage in the plant yard, if any, can be detected generally by a sensor module connected to at least one of the sampling modules because of fluctuations in wind direction. Due to such fluctuations in the direction of wind around a point of leakage, the sampling module by which the leakage is detected may change from one to another with time and the detected concentration may correspondingly vary along the passage of time. In some instances, leakage may be detected by more than one sampling module. With the foregoing in view, a wind direction and velocity indicator is preferably arranged in the vicinity of each sampling module, and data of a concentration from the corresponding sensor module and the direction and velocity of wind from the wind direction and velocity indicator are transmitted to a central data processor. On a graphic screen with all the module sets displayed thereon, an area probably encompassing the point of leakage is indicated at the windward of the wind direction measured by the wind direction and velocity indicator against a sampling module which has detected the leakage (more than one sampling module may detect the leakage). Variations in the area, which is likely to encompass the point of leakage, depending on time-dependent variations of the wind direction are overlapped as time-dependent variations and are traced. A leakage point usually remains at a fixed point and is generally considered not to move suddenly. By conducting a trace in accordance with such processing of information as described above, the area probably encompassing the point of leakage is successively narrowed down in accordance with time-dependent variations in wind direction and velocity, whereby the point of leakage can be specified in 1-2 hours if the leakage is 1 m 3 or greater an hour. When the area probably encompassing the point of leakage has been specified practically, time-dependent variations of the concentration measured by the sensor module detecting the leakage and those of the wind direction and the wind velocity data obtained from the wind direction and velocity indicator are taken into parallel analysis so that the quantity of leakage can be calculated because a certain relational expression can be established between these variations as will be described hereinafter. Therefore, time-dependent variations of the quantity of leakage can also be recorded and displayed. As has been described above, the present invention has the following advantages. Plural sampling modules and at least one and preferably a correspondong plurality of wind direction and velocity indicators are arranged around apparatus or equipment in a plant yard. Samples of the atmosphere around the apparatus or equipment are drawn through the sampling modules, and the concentration of a substance leaked into the atmosphere drawn through each sampling module is detected by a sensor module. At the same time, the direction and velocity of wind in the plant yard are determined. Leakage of gas, steam or the like in the plant yard can, therefore, be detected in a most economical manner, reliably and with a high probability. Inclusion of a data processor can provide a highly reliable system for the detection of the leakage of gas, steam or the like by analyzing and processing concentrations which have been detected by individual sampling modules and data from wind direction and velocity indicators and then estimating the point and quantity of leakage. The above and other objects, features and advantages of the present invention will become-apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which: FIGURE 1 is a plan view of a plant yard to which a leakage detection system according to one embodiment of this' invention has been applied; FIGURE 2 is a side view of a sampling module 10; FIGURE 3 is a simplified cross-sectional view of a sensor module 20, showing the internal construction thereof; and FIGURE 4 is a graph showing simulation results (relationship between the range of fluctuations in wind direction and the expected probability of detection) at a plant yard shown in Figure 1; DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS The preferred embodiments of the present invention will next be described with reference to the accompanying drawings. It should, however, be borne in mind that the present invention is not limited to or by the following examples. The embodiment of FIGURES 1 through 4 has been applied to a liquefied ethylene production plant. As is illustrated in FIGURE 1, six module sets M 1 , M 2 , M 3 , M 4 , M 5 and M 6 , each of which is composed of a sampling module 10, a'sensor module 20, and a wind direction and velocity indicator 30 arranged 3 sets by 3 sets in parallel in a plant yard 40 of L 1 x L 2 = 60 m x 40 m. Each sampling module 10 is placed at a height 3-5 m above the ground. As is depicted in FIGURE 2, the sampling module 10 is composed of a gas pipe 11 having a length of 20 m, closed at one end and open at the other end, ten unions 12 secured at intervals of 2 m on a peripheral wall of the gas pipe 11, air inlet tubes 13 opening at both ends thereof and connected at one end thereof to the respective unions 12 in such a way that air inlet tubes 13 communicate to the interior of the gas pipe 11, and conical air samplers 14 connected to the other ends of the respective air inlet tubes 13 and having perforated discs at the free ends thereof to prevent penetration of rain water. The open end of the gas pipe 11 is connected to a vacuum pump (not shown) via the sensor module 20. The vacuum pump may be connected to two or more sampling modules 10 to draw samples of the atmosphere therethrough. Although these sampling modules 10 are placed at the height 3-5 m above the ground at various places in the plant yard 40, they may be suitably placed at a different height depending on the state of the plant yard 40. Further, the sampling modules 10 are not necessarily limited to the construction illustrated in FIGURE 2. Any modules can be used as long as they can efficiently collect samples of the atmosphere from numerous points in a three-dimensional open space and can guide the samples to their corresponding sensor module or modules. For example, pipes having numerous holes can be employed. Each sensor module 20 is, as shown in FIGURE 3, arranged on the ground by means of posts 21 and comprises a cylindrical casing 22, an air inlet nozzle 23 for introducing into the casing 22 air samples collected by the associated sampling module 10, a baffle plate 24 dividing the casing 22 into halves, a gas sensor 25 provided above the baffle plate 24, a perforated cylindrical plate 26 surrounding the gas sensor 25, a sample air outlet nozzle 27 adapted to discharge air samples therethrough subsequent to their passage through the perforated plate 26, said air outlet nozzle 27 being connected to a vacuum pump, a terminal box 28 disposed on a top part of the casing 22 and connected to the gas sensor 25, and signal lines 29. Each air sample collected by the sampling module 10 is guided through the sample air inlet nozzle 23 into the casing 22 and enters one half section of the casing 22. The air sample ascends in the space defined by the baffle plate 24 and then enters the interior of the gas sensor 25 through a plurality of holes opened in a cylindrical side wall of the perforated plate 26. The air sample flows further past the gas sensor 25 and then descends through the other half section of the casing 22. The air sample is drawn by the vacuum pump through the sample air outlet nozzle 27 and is then released into the atmosphere through an outlet of the vacuum pump. In this case, an ethylene gas sensor is provided as the gas sensor 25. When the present invention is applied to a plant, sensors for a substance handled in the plant are generally used. Sensors of the most preferable type at the present stage are semiconductor sensors, and a variety of semiconductor sensors are available on the market. Although higher detection sensitivity is preferred, the ability to detect levels of 5-10 ppm is desired. Detection signals are fed to a data processor (not shown) via the signal lines 29. One sensor module 20 is connected to one sampling module 10 in the present embodiment. It is, however, possible to arrange a plurality of sampling modules 10 in association with a single sensor module 20 and to switch over the sensor module 20 in a time-divided fashion to sequentially connect it to the respective sampling modules 10. By the combination of the constructive utilization of fluctuations in the direction and velocity of wind due to natural conditions, which idea is not previously utilized and the arrangement of sampling modules in a plant, which differs from discrete arrangement of independent detectors, unexpected, accidental leakage of gas can be discovered promptly and without failure. Such a method as to utilize time-dependent variations in the direction and the velocity of wind can also be employed in a narrow and small area. The point of leakage of gas, and time-dependent changes in the quantity of leaking gas, can be determined without relying upon human effort, by means of the wind direction and velocity indicators. Further, the sampling modules 10 can be arranged as desired, depending on the importance of each plant yard. A simulation makes it possible to determine to what extent the capacity to detect leakage changes depending on the manner of the arrangement. From the results of such a simulation, the present system can be designed as desired. The concentrations of gas, steam or the like in the individual samples of the atmosphere collected through the respective sampling modules 10, said concentrations having been determined by the corresponding sensor modules 20, as well as the data on the directions and velocities of wind determined by the respective wind direction and velocity indicators 30 are transmitted to the data processor. On a graphic screen capable of displaying all the module sets M 1 -M 6 thereon, the area probably encompassing the point of leakage is drawn for the sampling module or modules 1O which detected the leakage, on an upstream side with respect to the direction of wind measured by the wind direction and velocity indicators 30, and the area probably encompassing the point of leakage is then synthesized in accordance with time-dependent variations in the direction of wind. By tracing the time-dependent variations in the area possibly including the point of leakage, the point of leakage can be estimated. When the area possibly including the point of leakage has been substantially determined, the quantity of leakage can be calculated from the concentration measured by the sensor module 20 detecting the leakage and the wind velocity obtained from the corresponding wind direction and velocity indicator 30. Next, the method of the simulation referred to above will be outlined. In general, diffusion of gas into the atmosphere has been reported by many researchers. According to Sakagami's report [Jiro Sakagami: Koatsu Gas (High-Pressure Gas), 19 (4), 6 (1982)], when gas being continuously released at the rate of Q (m 3 /hr) from a point source at a height H (m) is caused to diffuse by wind blowing at a velocity u (m/sec) and in a direction plotted along the x-axis, the concentration of the gas C (m 3 /m 3 ) in the space coordinate (x,y,H+z) can be expressed by the following formula: C= (Q/u) {exp(-y 2 /A)/(πA) ½ }(1/B) x exp{-(H+z)/B}Io{2 x (Hz) ½ B} To simplify the explanation, only a height range several meters above the ground will be dealt with. Height directions in this range are averaged, and the term containing the height z is indicated by α, namely: exp{-(H+z)/B}Io{2 x (Hz) ½ /B} = α If this α can be empirically determined by an average, C [ppm] = α(10 6 x Q/u){(1/B)/(πA) ½ }exp(-y 2 /A) In the above formulae, A,B are functions of x alone, said functions having factors determined by the stability of the atmosphere. Accordingly, the isoconcentration curve from the point of leakage can be simplified as: Y = [Al n {1/(D (x) C)}] ½ where y: radius perpendicular to the direction of wind, and D (x) : (10 -6 u/Q){B(πA) ½ }(1/a) Needless to say, it is not always necessary to average the height directions to empirically determine α, and it is thereby possible to conduct a three-dimensional simulation including the height z. The simulation uses random numbers, i.e., it is of the type generally called the "Monte Carlo simulation". Such a simulation is simple when an average is used for the height direction as described above. When the probability of successful detection can be determined by inputting as uniform random numbers the maximum quantity of leaked gas, the point (x,y) of leakage, the maximum wind velocity, the wind direction and the stability of the atmosphere, calculating the aforementioned isoconcentration curve for the respective concentrations C (ppm) and repeatedly testing for the existence or absence of any module set detectable within the radius y, this probability can be considered to be the expected probability of detection with respect to one proposal for the arrangement of the module sets. Here, it is to be noted that the direction and velocity of wind are not constant but vary over time. Hence, the velocity of wind is fixed at the average of wind velocities within a predetermined period of time, and the usual range of fluctuations is inputted as a standard deviation above and below the average to change the wind velocity. According to a substantiation experiment conducted to substantiate the advantages of the present invention, these fluctuations may be considered to be a normal distribution whose standard deviation is about 1/2 of the average. In a plant yard, the direction of local wind fluctuates not only due to variations in weather but also due to the influence of various obstacles. Fluctuations within a certain period of time, however, show a normal distribution having a range of angular variations of a certain standard deviation in the average wind direction. In a substantiation experiment conducted to substantiate the advantages of the present invention, the standard deviation of the range of these angular variations within 3 minutes is about 44° and this value remained substantially the same even over several hours. In an analysis by such a simulation, it is therefore only necessary to consider variations in wind direction up to twice the standard deviation, namely, up to ±88° after setting the central wind direction axis (equivalent to an average wind direction) by uniform random numbers in a test. To substantiate the advantages of the present invention, a substantiation test was next conducted at an actual plant yard. The test was performed within the right-hand half of the plant yard 40 illustrated in FIGURE 1, namely, inside the area of L 2 x L 3 = 40 m x 40 m. As is illustrated in FIGURE 1, the four module sets M 2 ,M 3 ,M 5 ,M 6 are-arranged in this area. The detection performance of the system according to this invention was tested by dividing this area into 64 sections and causing ethylene to leak out in particular sections. The experiment was conducted many times at different hours of the day. The quantity of gas leakage and the height of leakage were set at 1 m 3 or 3 m 3 per hour and at 1 m or 2 m above the ground, respectively, and semiconductor sensors were employed as the sensor modules 20. The sensitivity of these sensors to ethylene gas was 5-10 ppm. Gas sensors of this type are now readily available as general commercial products. As a result of the experiment, the leakage detection probability was found to be 79.7%, the standard deviation 13.2% and the range of scattering of the data 62.5-100%, all as averages of data collected through repeated experimentation. In order to compare these results with those of the above simulation, calculation was conducted by substituting the conditions for the present substantive experiment for the trial input conditions for the above simulation. It was found that when fluctuations in the wind direction within a predetermined time period are ±80°, the expected probability of detection is 75%, which is very consistent with the results of the present substantive experiment. With respect to each arrangement pattern of module sets, it is possible to predict its results by a simulation on the basis of the above results. It is therefore possible to design the system of this invention in accordance with the nature (the scale, the properties of a substance handled, the manner of handling, etc.) of a plant yard to which the system is to be applied. The area of the plant yard to which the system of this invention was applied was enlarged to cover the entire area of the plant yard shown in FIGURE 1, and a simulation was conducted choosing leaking conditions entirely at random. The results of the simulation are shown in FIGURE 4. If the sensitivity of the sensors employed in the sensor modules 20 are of the 5-10 ppm level, the curve for 25-50 ppm can be used in the diagram. If the range of fluctuations in the direction of wind is ±88°, the expected probability of detection is 90-100%. When detectors are arranged along the boundary of a plant yard at intervals of 20 m with one detector in accordance with the statutory rule, the expected probability of detection estimated by such a simulation is only 5%. This has also been confirmed by the substantiation test described above. In general, the area in which leakage of gas can be detected-by a single detector under the conditions of constant wind direction, constant wind velocity and constant leakage rate is of an elongated elliptical shape in a height-wise cross-section of the arranged detector. On the other hand, in the case of a single sampling module equipped with a sensor module of the same sensitivity as the single detector, the detectable area is somewhat rectangular in shape and extends in parallel with the module. Even if leakage is once detected by a single detector, the location of the detectable area may soon shift outside the range of the detector if the direction of wind changes. In contrast, the use of the sampling module makes it possible to continuously output a detection signal. In the case of a single detector, the output signal is intermittent and cannot be distinguished from external noise. The present invention makes it possible to substantially increase the signal/noise ratio so that leakage of gas can be determined without failure. Moreover, in the present invention, the natural fluctuations in the direction and velocity of wind can effectively be used. If a plurality of modules are deployed, the area in which leakage of gas can be detected by the plurality of modules is in the form of overlapped rectangles due to such fluctuations, and this further increases the signal/noise ratio. In the example diagrammatically illustrated in FIGURE 4, such an arrangement results in a detection probability as high as about 5 times that (detection probability: 20%) available for a constant wind direction, namely, a detection probability of nearly 100% can be expected in a state clearly distinguishable from noise. Advantages of the present embodiment will now be summarized. In the case of leakage of 1 m 3 per hour or more, the expected probability of detection of the leakage within several minutes after the occurrence of the leakage is at least 80% when modules are arranged as in FIGURE 1. If it is sufficient to detect the leakage in several hours, it is highly likely that a wind direction fluctuation of at least ±90° would take place. This makes it possible to expect a detection probability of at least 90%, more specifically close to 100%. The conventional simple method in which detectors are arranged at intervals of 20 m with one detector at each location cannot be expected to contribute significantly to the safety of the plant. The method of this invention has, however, demonstrated that it can improve the safety to a significant extent and can be employed as a highly practical system. The shape, combination and the like of the module sets in the present invention are not limited to the examples described above. For example, it is possible to employ sampling modules in a ring-shaped, vertical, inclined or other configuration. Advantages available from such configurations are readily inferable without the need for further discussion. It is also at the user's volition to choose the type and sensitivity of sensors for use in sensor modules depending on the kind of the gas to be detected and the leakage level to be detected. Needless to say, the present invention can be used for the detection of leakage of radioactivity at nuclear power stations.

Claims: A leak detection system for gas, steam or the like in a plant yard where a flammable or noxious substance is handled in a gaseous, liquid or solid form, comprising: a plurality of sampling modules (10) arranged in the plant yard, for collecting samples of the surrounding atmosphere, each said sampling module comprising a gas pipe (11) having one open end, plural air inlet tubes (13) attached to the gas pipe in communication with the interior of the gas pipe, and a like number of air samplers (14) connected to the free ends of the respective air inlet tubes to collect samples of the surround atmosphere, at least one sensor module (20) connected to said open ends of the gas pipes of the sampling modules, for receiving the samples of the atmosphere collected through the sampling module and having a built-in sensor to detect leaked substance in the atmosphere so received if the substance is leaking from equipment or apparatus in the plant yard; at least one vacuum pump for drawing the samples of the atmosphere which exist in the sampling modules, through the sensor module; at least one wind direction and velocity indicator (30) for the real-time determination of the direction and velocity of local wind; and a data processor for the real-time analysis of wind direction and velocity data obtained from the said wind direction and velocity indicator and concentrations of the leaked substance in the atmosphere, which have been collected by the respective sampling modules, as detected by the sensor module, said data processor including a graphic screen for providing a display showing the area containing all of said modules (10), said data processor being adapted to draw on said display the area or areas extending upstream of the or each module for which leaked substance is being detected, said area or areas each probably encompassing the point of leakage; said areas changing over time as the wind direction changes, and said data processor being arranged to draw successive areas to narrow down successively the overlapped area likely to contain the point of leakage. A leak detection system according to claim 1 wherein the data processor is further arranged to determine from data of concentrations of leaked substance detected by the sensor(s) and the wind direction and velocity data the quantity of leaked substance. A leak detection system according to claim 1 or claim 2 wherein there is a respective said wind direction and velocity indicator (30) disposed in the vicinity of each sampling module (10).

Inventors: HIROSHIGE YOSHINORI, HOSONUMA AKIRA, ONODERA TERUYUKI, SAKASHITA KUNIO, TAKAHASHI SABUROH, HIROSHIGE, YOSHINORI, HOSONUMA, AKIRA, ONODERA, TERUYUKI, SAKASHITA, KUNIO, TAKAHASHI, SABUROH
Applicants: MITSUI TOATSU CHEMICALS, MITSUI TOATSU CHEMICALS, INC.
Application year: 1992
Main classification: G01N1
IPC codes: G01P13, G01N1, G01P5