Systems and methods for sensitive open-path gas leak and detection alarm
An apparatus for detecting leaks from a gas pipeline or storage system includes a light source configured to emit a beam with at least one spectral component capable of interacting with pipeline gas, a reflector configured to reflect a portion of the beam, an optical detector configured to detect the reflected beam, a signal processing module coupled to the output of the optical detector configured to analyze the detected beam and output a measured concentration value that characterizes the amount of target gas in the beam path, and a statistical processing module coupled to the signal processing module configured to store and analyze the measured concentration value. The statistical processing module is further configured to compute a value based on statistical information associated with the stored plurality of concentration values and compare the computed value to a first predetermined threshold, and transmit a first alarm signal if the computed value is greater than the first predetermined threshold.
1. In a gas pipeline or storage system, a method for detecting gas leaks using a tunable diode laser absorption spectroscopy transceiver, a reflective target, and an open-path laser beam, the transceiver and the reflective target aligned and maintained in a fixed spatial relationship, the method comprising the steps of:
a) measuring a gas concentration over a first time interval based on a characteristic of light reflected from the open-path laser beam;
b) repeating a plurality of times the previous step of measuring a gas concentration over a first time interval, the plurality of first time intervals being continuous during a second time interval;
c) averaging the plurality of measured gas concentration values obtained over the second time interval and storing the averaged value in a memory;
d) computing a standard deviation and a higher-order statistic of the plurality of measured gas concentration values relative to the stored average value, wherein the higher-order statistic is a function of a third or higher power of a statistical sample, including a skewness, a moment, a higher order moment, an L-moment, an L-statistic, an L-cumulant, or a kurtosis;
e) computing an alarm value based on the standard deviation, the higher-order statistic and a first predetermined threshold;
f) storing the alarm value in a memory;
g) repeating a plurality of times steps a, b, c, d, e and f;
h) computing an alarm probability based on a plurality of stored alarm values; and
i) transmitting an alarm message if the alarm probability is greater than a second predetermined threshold.
2. The method of claim 1 wherein a path length of the open-path laser beam is greater than 100 feet in length.
3. The method of claim 1 , wherein the higher-order statistic comprises a skewness.
4. The method of claim 2 further comprising measuring the gas concentration via tunable diode laser absorption spectroscopy.
5. The method of claim 2 wherein the first time interval is 100 ms.
6. The method of claim 2 wherein the second time interval is 5 minutes.
7. The method of claim 3 wherein the step of computing the alarm value comprises:
calculating a parameter by multiplying the standard deviation by the skewness; and
comparing the parameter to the first predetermined threshold.
8. The method of claim 3 wherein the step of computing the alarm value is based on a polynomial of the standard deviation multiplied by a polynomial of the skewness.
9. The method of claim 2 wherein the measured gas is one of CO 2 , methane, hydrogen fluoride, hydrogen sulfide, or ammonia.
10. The method of claim 1 further comprising comparing the measured gas concentration with a third predetermined threshold and transmitting an alternate alarm message if the measured gas concentration is greater than the third predetermined threshold.
11. In a gas pipeline or storage system, a method for detecting gas leaks using a tunable diode laser absorption spectroscopy transceiver, a reflective target, and an open-path laser beam, the transceiver and the reflective target aligned and maintained in a fixed spatial relationship, the method comprising:
a) measuring and storing a gas concentration value averaged over a first time interval based on a characteristic of light reflected from the open-path laser beam;
b) repeating a plurality of times the above step of measuring and storing a gas concentration value averaged over a first time interval, the plurality of first time intervals being continuous during a second time interval;
c) computing an average value of the plurality of stored gas concentration values obtained over the second time interval and storing the average value;
d) computing a standard deviation and a skewness of the plurality of stored gas concentration values obtained over the second time interval;
e) computing an alarm value based on the standard deviation, the skewness and a first predetermined threshold;
f) storing the alarm value in a memory;
g) repeating steps a, b, c, d, e and f a plurality of times during a third time interval;
h) computing an alarm probability based on a plurality of stored alarm values obtained over the third time interval; and
i) transmitting an alarm message if the alarm probability is greater than a second predetermined threshold.
12. The method of claim 11 , wherein each time interval of the plurality of first time intervals is 100 ms.
13. The method of claim 12 , wherein the second time interval is 5 minutes.
14. The method of claim 12 , wherein the third time interval is 1 hour.
15. The method of claim 11 , wherein the step of computing an alarm value comprises:
calculating a parameter by multiplying the standard deviation by the skewness; and
comparing the parameter to the first predetermined threshold.
16. The method of claim 15 , wherein the alarm value is set to a first value if the parameter is greater than the first predetermined threshold and set to a second value if the parameter is not greater than the first predetermined threshold.
17. The method of claim 16 , wherein the step of computing an alarm probability comprises calculating the average of the plurality of stored alarm values for the third time interval.
18. The method of claim 11 , wherein the step of computing an alarm probability comprises calculating the average of the plurality of stored alarm values for the third time interval.
19. The method of claim 11 , further comprising the steps of:
comparing the average value of the plurality of stored gas concentration values obtained over the second time interval with a third predetermined threshold; and
transmitting a second alarm message if the average value of the plurality of stored gas concentration values is greater than the third predetermined threshold.
20. The method of claim 11 , wherein the gas is one of CO 2 , methane, hydrogen fluoride, hydrogen sulfide, or ammonia.
21. The method of claim 11 , wherein the skewness is obtained by the equation
g
1
=
1
N
∑
i
=
1
N
(
x
i
-
μ
)
3
σ
3
wherein g 1 is the skewness, N is the number of stored gas concentration values x i obtained over the second time interval, μ is the average value of the stored gas concentration values over the second time interval, and σ is the standard deviation of the stored gas concentration values over the second time interval.
22. In a gas pipeline or storage system, a method for detecting gas leaks using a tunable diode laser absorption spectroscopy transceiver, a reflective target, and an open-path laser beam, the transceiver and the reflective target aligned and maintained in a fixed spatial relationship, the method comprising:
a) measuring and storing a gas concentration value averaged over a first time interval based on a characteristic of light reflected from the open-path laser beam;
b) repeating a plurality of times the above step of measuring and storing a gas concentration value averaged over a first time interval, the plurality of first time intervals being continuous during a second time interval;
c) computing an average value of the plurality of stored gas concentration values obtained over the second time interval and storing the average value;
d) computing a standard deviation and a kurtosis of the plurality of stored gas concentration values obtained over the second time interval;
e) computing an alarm value based on the standard deviation, the kurtosis and a first predetermined threshold;
f) storing the alarm value in a memory;
g) repeating steps a, b, c, d, e and f a plurality of times during a third time interval;
h) computing an alarm probability based on a plurality of stored alarm values obtained over the third time interval; and
i) transmitting an alarm message if the alarm probability is greater than a second predetermined threshold.
23. The method of claim 22 , wherein the step of computing an alarm probability comprises calculating the average of the plurality of stored alarm values for the third time interval.
24. The method of claim 22 , further comprising the steps of:
comparing the average value of the plurality of stored gas concentration values obtained over the second time interval with a third predetermined threshold; and
transmitting a second alarm message if the average value of the plurality of stored gas concentration values is greater than the third predetermined threshold.
25. The method of claim 22 , wherein the gas is one of CO 2 , methane, hydrogen fluoride, hydrogen sulfide, or ammonia.
26. The method of claim 22 , wherein the kurtosis is obtained by the equation
g
2
=
1
N
∑
i
=
1
N
(
x
i
-
μ
)
4
σ
4
wherein g 2 is the kurtosis, N is the number of stored gas concentration values x i obtained over the second time interval, μ is the average value of the stored gas concentration values over the second time interval, and σ is the standard deviation of the stored gas concentration values over the second time interval.