SYSTEM AND METHOD FOR DETECTING AND SUPPRESSING DUST EXPLOSIONS

Abstract

A system (10) and method (100) for detecting and suppressing a dust explosion occurring in a process enclosure (12). A sensor (14) generates a pressure signal indicative of a pressure within the enclosure (12). A processing element (16) analyzes the signal to determine whether the dust explosion is occurring. The signal is sampled at a higher frequency, and then converted to a lower frequency by averaging, then filtered with first and intermediate filters to remove portions of the signal having rates of increase that exceed pre-established maximum magnitudes, and then filtered with a second filter having an appropriate cut-off frequency, stop band attenuation factor, and end of passband frequency. An alarm and a suppression system (18) are activated if a static pressure exceeds a limit, a rate of pressure increase exceeds a limit, or a total suppressed pressure exceeds a limit, each of which indicates occurrence of the dust explosion.

Claims

1. A method for detecting and suppressing a dust explosion occurring in a process enclosure, the method comprising the steps of: generating with a pressure sensor a pressure signal indicative of a pressure within the process enclosure; and analyzing with an electronic processing element the pressure signal to determine whether the dust explosion is occurring, wherein analyzing the pressure signal includes sampling the received pressure signal at a higher frequency, converting the sampled pressure signal from the higher frequency to a lower frequency, filtering the converted pressure signal with a first filter to remove a first portion of the pressure signal having a rate of increase that grows over one millisecond with more than a first pre-established maximum magnitude, filtering the first filtered pressure signal with an intermediate filter to remove a second portion of the pressure signal having a rate of increase exceeding a second pre-established maximum magnitude, filtering the intermediate filtered pressure signal with a second filter having a cut-off frequency, a stop band attenuation factor, and an end of passband frequency, and activating a suppression system if the first filtered pressure signal exceeds a pre-established static pressure threshold value or if the rate of increase of the second filtered pressure signal exceeds a pre-established rate-of-increase threshold value or if the total suppressed pressure predicted on the basis of the first and second filtered pressure signal exceeds a pre-established equipment strength value, each of which indicates, alternatively or additionally, the dust explosion occurring in the process enclosure.

2. The method as set forth in claim 1, wherein the process enclosure is selected from the group consisting of: dryers for drying powdered materials, mills for reducing solid materials to smaller pieces, conveyors for transporting solid bulk materials, silos for storing solid bulk materials and dust collectors for separating dust particles from an air stream.

3. The method as set forth in claim 1, wherein the higher frequency is approximately between 2 kHz and 20 kHz, and the lower frequency is approximately between 500 Hz and 1500 Hz.

4. The method as set forth in claim 3, wherein the higher frequency is approximately 16 kHz, and the lower frequency is approximately 1000 Hz.

5. The method as set forth in claim 1, wherein the first filter is a non-linear filter.

6. The method as set forth in claim 1, wherein the first pre-established maximum magnitude is approximately between 20 bar/s and 40 bar/s.

7. The method as set forth in claim 6, wherein the first pre-established maximum magnitude is approximately 30 bar/s.

8. The method as set forth in claim 1, wherein the intermediate filter is a non-linear filter.

9. The method as set forth in claim 1, wherein the second pre-established maximum magnitude is approximately between 5 bar/s and 15 bar/s.

10. The method as set forth in claim 9, wherein the second pre-established maximum magnitude is approximately 10 bar/s.

11. The method as set forth in claim 1, wherein the second filter is a digital low pass filter.

12. The method as set forth in claim 11, wherein the second filter is a finite impulse response digital low pass filter.

13. The method as set forth in claim 1, wherein the cut-off frequency is approximately between 40 Hz and 120 Hz, the stop band attenuation factor is approximately between 8 and 12, and the end of passband frequency is approximately between 0 Hz and 2 Hz.

14. The method as set forth in claim 13, wherein the cut-off frequency is approximately between 50 Hz and 120 Hz, the stop band attenuation factor is approximately 10, and the end of passband frequency is approximately 1 Hz.

15. The method as set forth in claim 1, further including the step of activating an alarm in addition to the suppression system.

16. A method for detecting and suppressing a dust explosion occurring in a process enclosure, the method comprising the steps of: generating with a pressure sensor a pressure signal indicative of a pressure within the process enclosure; and analyzing with an electronic processing element the pressure signal to determine whether the dust explosion is occurring, wherein analyzing the pressure signal includes sampling the received pressure signal at a higher frequency of approximately between 2 kHz and 20 kHz, converting the sampled pressure signal from the higher frequency to a lower frequency of approximately between 500 Hz and 1500 Hz, filtering the converted pressure signal with a first filter to remove a first portion of the pressure signal having a rate of increase exceeding a first pre-established maximum magnitude of approximately between 20 bar/s and 40 bar/s, filtering the first filtered pressure signal with an intermediate filter between the first and second filters to remove a second portion of the pressure signal having a rate of increase exceeding a second pre-established maximum magnitude of approximately between 5 bar/s and 15 bar/s, filtering the intermediate filtered pressure signal with a second filter having a cut-off frequency of approximately between 50 Hz and 120 Hz, a stop band attenuation factor of approximately between 8 and 12, and an end of passband frequency of approximately between 0 Hz and 2 Hz, and activating a suppression system if the first filtered pressure signal exceeds a pre-established static pressure threshold value or if the rate of increase of the second filtered pressure signal exceeds a pre-established rate-of-increase threshold value or if the total suppressed pressure predicted on the basis of the first and second filtered pressure signal exceeds a pre-established equipment strength value, each of which indicates, alternatively or additionally, the dust explosion occurring in the process enclosure.

17. A system for detecting and suppressing a dust explosion occurring in a process enclosure, the method comprising the steps of: a pressure sensor configured to generate a pressure signal indicative of a pressure within the process enclosure; and an electronic processing element configured to analyze the pressure signal to determine whether the dust explosion is occurring, wherein the electronic processing element is configured to sample the received pressure signal at a higher frequency, convert the sampled pressure signal from the higher frequency to a lower frequency, filter the converted pressure signal with a first filter to remove a first portion of the pressure signal having a rate of increase that grows over one millisecond with more than a first pre-established maximum magnitude, filter the first filtered pressure signal with an intermediate filter to remove a second portion of the pressure signal having a rate of increase exceeding a second pre-established maximum magnitude, filter the intermediate filtered pressure signal with a second filter having a cut-off frequency, a stop band attenuation factor, and an end of passband frequency, and activate a suppression system if the first filtered pressure signal exceeds a pre-established static pressure threshold value or if the rate of increase of the second filtered pressure signal exceeds a pre-established rate-of-increase threshold value or if the total suppressed pressure predicted on the basis of the first and second filtered pressure signal exceeds a pre-established equipment strength value, each of which indicates, alternatively or additionally, the dust explosion occurring in the process enclosure.

18. The system as set forth in claim 17, wherein the process enclosure is selected from the group consisting of: dryers for drying powdered materials, mills for reducing solid materials to smaller pieces, conveyors for transporting solid bulk materials, silos for storing solid bulk materials and dust collectors for separating dust particles from an air stream.

19. The system as set forth in claim 17, wherein the higher frequency is approximately between 2 kHz and 20 kHz, and the lower frequency is approximately between 500 Hz and 1500 Hz.

20. The system as set forth in claim 19, wherein the higher frequency is approximately 16 kHz, and the lower frequency is approximately 1000 Hz.

21. The system as set forth in claim 17, wherein the first filter is a non-linear filter.

22. The system as set forth in claim 17, wherein the first pre-established maximum magnitude is approximately between 20 bar/s and 40 bar/s.

23. The system as set forth in claim 22, wherein the first pre-established maximum magnitude is approximately 30 bar/s.

24. The system as set forth in claim 17, wherein the intermediate filter is a non-linear filter.

25. The system as set forth in claim 17, wherein the second pre-established maximum magnitude is approximately between 5 bar/s and 15 bar/s.

26. The system as set forth in claim 25, wherein the second pre-established maximum magnitude is approximately 10 bar/s.

27. The system as set forth in claim 17, wherein the second filter is a digital low pass filter.

28. The system as set forth in claim 27, wherein the second filter is a finite impulse response digital low pass filter.

29. The system as set forth in claim 17, wherein the cut-off frequency is approximately between 40 Hz and 120 Hz, the stop band attenuation factor is approximately between 8 and 12, and the end of passband frequency is approximately between 0 Hz and 2 Hz.

30. The system as set forth in claim 29, wherein the cut-off frequency is approximately between 50 Hz and 120 Hz, the stop band attenuation factor is approximately 10, and the end of passband frequency is approximately 1 Hz.

31. The system as set forth in claim 17, wherein the electronic processing element is further configured to activate an alarm in addition to the suppression system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1A is a block diagram of a system constructed in accordance with an embodiment of the present invention;

[0015] FIG. 1B is a block diagram of the system of FIG. 1A, wherein a dust explosion is occurring;

[0016] FIG. 1C is a block diagram of the system of FIG. 1A, wherein a suppression system has been activated;

[0017] FIG. 1D is a block diagram of the system of FIG. 1A, wherein the suppression system is in the process of suppressing the dust explosion;

[0018] FIG. 1E is a block diagram of the system of FIG. 1A, wherein the dust explosion has been suppressed;

[0019] FIG. 2 is a flowchart of a method practiced in accordance with an embodiment of the present invention;

[0020] FIG. 3A is a plot of exemplary raw and filtered pressure data for a short duration disturbance; and

[0021] FIG. 3B is a plot of exemplary raw and filtered pressure data during a dust explosion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Broadly characterized, the present invention provides an explosion suppression system and associated method which provide faster and more reliable activation, and which are less influenced by variations in process parameters and explosion intensity. More specifically, the present invention uses an innovative digital filter technique in dynamic detection to provide improved stability against short duration pressure reading disturbances and also uses floating detection where not preset values for static pressure threshold or dynamic rate of pressure rise thresholds are used to activate the system, but where rather the real-time measured pressure and rate of pressure rise is compared against a preset threshold for the final maximum allowed explosion pressure in the system.

[0023] Broadly, improving stability against short duration pressure reading disturbances in dynamic detection may be accomplished by filtering noise in the pressure reading with a minimum loss of system response time before the alarm condition is checked. One or more digital low-pass frequency filters with appropriate stop band attenuation and appropriate cut off frequencies (fc) may be used. Further, finite impulse response filters with minimal phase shift/time delay may be used, rather than infinite impulse response filters. The stop band attenuation factor may be chosen so as to filter only the pressure reading noise caused by reflections of explosion pressure waves against the walls of a process enclosure, which may allow for setting the dynamic alarm threshold value relatively low (e.g., 1 bar/s). This may also allow for eliminating time filtering, and may keep the phase shift/time delay introduced by the attenuation as small as possible. High frequency pressure reading disturbances with amplitudes larger than the stop band attenuation may be “topped off” before passing the signal to a low pass filter.

[0024] Stability may be improved against short duration pressure reading disturbances with frequencies above the filter's cut-off frequency. A cut-off frequency of 80 Hz may be appropriate for most applications. The system may provide response times on the order of milliseconds, which allows the suppression system to activate at low explosion pressures. The response time may be predictable and may depend on the explosion intensity, dP/dt.sub.max, the maximum explosion pressure, P.sub.max, the filter's cut-off frequency, and the dynamic alarm level, dP/dt.sub.set. Alternative cut-off frequencies may be chosen to balance stability against disturbances and system response time. If pressure reading disturbances are expected to occur with a frequency larger than the system's sampling frequency of 1000 Hz, a time filter in the order of 1 ms to 5 ms, in addition to the frequency filter, may help further increasing the stability of the system.

[0025] Floating detection may be accomplished by monitoring in real-time the static pressure (P(t)) and pressure rise (dP(t)/dt) and also calculating in real-time the predicted reduced explosion pressure if the system were activated immediately. The alarm and suppression system may activate when the calculated reduced pressure exceeds the pressure resistance of the process enclosure. Thus, the alarm condition may be adapted to the effective real-time measured explosion intensity.

[0026] The floating alarm condition may be expressed as:

[00002]$P\left(t\right)+P_{N_2}+\frac{\mathrm{dP}\left(t\right)}{\mathrm{dt}}*\mathrm{SRT}>\mathrm{TSP}$

[0027] wherein, the left side of the equation expresses the real-time calculated reduced pressure, and the right side expresses the explosion pressure resistance of the apparatus; P(t) and dP(t)/dt are real-time measured values, while P.sub.N2, SRT, and TSP are pre-established; P.sub.N2 is the pressure increase in the apparatus due to the injection of pressurized nitrogen from the suppression containers resistance of the apparatus; SRT (System Reaction Time) is the time required to extinguish the incipient explosion after activation; and TSP is the explosion pressure resistance of the apparatus.

[0028] The floating alarm condition is true for a specified consecutive amount of milliseconds, typically 3 ms, as with the static pressure detection. The delay introduced by the low pass filter and by waiting during the consecutive milliseconds that the floating alarm condition must be true, is added to the SRT used in the floating alarm condition equation. The pre-established parameters are well-controlled variables, which are independent of the properties of the process and the dust and dependent only on the properties of the process enclosure and the size and installation location of the suppressor containers.

[0029] Referring to FIGS. 1A-E and 2, embodiments of a system 10 and a method 100 for detecting and suppressing a dust explosion in a process enclosure in accordance with the present invention are described as follows. The system 10 may broadly comprise a pressure sensor 14, an electronic processing element 16, and a suppression system 18 configured to release a suppressant 20. The process enclosure 12 may be or may be part of a dryer for drying powdered material, a mill for reducing solid material to smaller pieces, a conveyor for transporting solid bulk materials, a silo for storing solid bulk materials, a dust collector for separating dust particles from an air stream, or any other process equipment in which an explosive dust-air cloud may arise during normal operation or during malfunction.

[0030] As illustrated in FIG. 1A, the pressure sensor 14 may be associated with the process enclosure 12 and configured to generate a pressure signal indicative of a pressure within the process enclosure 12, as shown in step 102. The electronic processing element 16 may be configured to analyze the pressure signal to determine whether the dust explosion is occurring, wherein analyzing the pressure signal may include the following steps. The pressure signal may be received by the processing element 16. The received pressure signal may be sampled at a first higher frequency of approximately between 2 kHz and 20 kHz, or approximately 16 kHz (i.e., once per 0.0625 ms), as shown in step 104. The time delay associated with this step may be negligible. The sampled pressure signal may be converted by averaging from the first higher frequency to a second lower frequency of approximately between 500 Hz and 1500 Hz, or approximately 1000 Hz (i.e., one data point per millisecond), as shown in step 106. The time delay associated with this step may be approximately one millisecond.

[0031] The converted pressure signal may be filtered with a first filter, which may be a non-linear filter, to remove a portion of the pressure signal where the rate of increase grows in one millisecond more than a first pre-established maximum magnitude of approximately between 10 bar/s and 40 bar/s, or approximately 30 bar/s, as shown in step 108. The time delay associated with this step may be negligible. The first filter may permanently delete the removed pressure reading data from the pressure signal. In particular, pressure increases that grow in one millisecond more than the maximum magnitude of, e.g., +/−30 bar/s, may be removed because they may occur due to a broken detector and may falsely activate the system and, if they occur in an explosion, they occur far beyond the alarm condition has occurred, whether it be static, dynamic or floating.

[0032] An exemplary algorithm for the first filter may be as follows:

TABLE-US-00001 If Abs(Abs (x.sub.0 − y.sub.−1)−Abs(y.sub.−1 − y.sub.−2)) > 30 mbarg Then  y.sub.0 = y.sub.−1 Else  y.sub.0 = x.sub.0 End If

[0033] In this example, the x-values are the digital input to the first filter, and the y-values are the digital output, and the subscript denotes the time the values are captured. So, for example, subscript 0 denotes the current pressure reading, and subscript −1 denotes the pressure reading one millisecond earlier.

[0034] The first filtered pressure signal may be used to trigger static detection protection, as shown in step 110. In one implementation, static detection protection may be activated if the first filtered pressure signal exceeds the pre-established alarm threshold, P.sub.set, for a pre-established number of milliseconds (e.g., between 1 and 5 milliseconds). The first filtered pressure signal may additionally or alternatively be used, in combination with the second filtered pressure signal as described below, to trigger floating detection protection, as shown in step 112.

[0035] In one implementation, floating detection protection may be activated if:

[00003]$P_{\mathrm{FIL}1,0}+P_{N2}+\frac{\left(P_{\mathrm{FIL}2,0}-P_{\mathrm{FIL}2,-1}\right)}{1\mathrm{ms}}\times \mathrm{SRT}>\mathrm{TSP}$

wherein P.sub.FIL1,0 is the first filtered pressure signal, P.sub.FIL2,0 and P.sub.FIL2,−1 are the second filtered pressure signal (discussed below) at times 0 and −1, and P.sub.N2, SRT, and TSP are pre-established values.

[0036] The first filtered pressure signal may be filtered with an intermediate filter, which may be a non-linear filter, to remove a portion of the first filtered pressure signal having a rate of increase exceeding a second pre-established maximum magnitude of approximately between 5 bar/s and 15 bar/s, or approximately 10 bar/s, as shown in step 114. The time delay associated with this step may be negligible. The intermediate filter may permanently delete the removed pressure reading data from the pressure signal. In particular, pressure increases that exceed the maximum magnitude of, e.g., +/−10 bar/s, may be removed because dynamic detection alarm levels exceeding 10 bar/s may not be needed for effective protection. Furthermore, explosion pressure data may pass a region of lower rates before 10 bar/s is reached and may therefore be detected by dynamic alarm values of 10 bar/s or lower.

[0037] An exemplary algorithm for the intermediate filter may be as follows:

TABLE-US-00002 If Abs (x.sub.0 − x.sub.−1) > 1 mbarg Then <comment> Output follows input but rates are truncated <end comment>  If x.sub.0 − x.sub.−1 > 30 mbarg Then   y.sub.0 = y.sub.−1 + 30 mbarg  Elseif x.sub.0 − x.sub.−1 < − 30 mbarg Then   y.sub.0 = y.sub.−1 − 30 mbarg  Else   y.sub.0 = y.sub.−1 + x.sub.0 − x.sub.−1  End If Else <comment> After shock output shall align with input again<end comment>  If Abs (x.sub.0 − y.sub.−1) <= 1 mbarg Then   y.sub.0 = x.sub.0  Elseif x.sub.0 > y.sub.−1 Then   y.sub.0 = y.sub.−1 + 1 mbarg  Else   y.sub.0 = y.sub.−1 − 1 mbarg  End If End If

[0038] The intermediate filtered pressure signal may be filtered with a second filter, which may be a finite impulse response digital low pass filter, having a cut-off frequency of approximately between 40 Hz and 120 Hz, or approximately 50 Hz to 120 Hz, a stop band attenuation factor of approximately between 8 and 12, or approximately 10, and an end of passband frequency of approximately between 0 Hz and 2 Hz, or approximately 1 Hz, as shown in step 116. In particular, the cut-off frequency may be 50 Hz, 60 Hz, 80 Hz, or 120 Hz. The time delay associated with this step may be measured in milliseconds (e.g., 5 ms to 13 ms for a cut-off frequency of 80 Hz). As used herein, “finite impulse response” may refer to a filter in which the output of the filter is calculated as a weighted average of the current and a defined amount of historical input data. Several techniques exist to find the appropriate length, N, of the filter and the appropriate values for the weight factors H0 . . . HN-1 meeting the required frequency response.

[0039] The second filtered pressure signal may be used, in combination with the first filtered pressure signal as described above, to trigger the floating detection protection, as shown in step 112 and as described above. The second filtered pressure signal may additionally or alternatively be used to trigger dynamic detection protection, as shown in step 118. The time delay associated with this step may be on the order of a few milliseconds. In one implementation, dynamic detection protection may be activated if:

[00004]$\frac{\left(P_{\mathrm{FIL}2,0}-P_{\mathrm{FIL}2,-1}\right)}{1\mathrm{ms}}>\frac{\mathrm{dP}}{{\mathrm{dt}}_{set}}$

wherein P.sub.FIL2,0 and P.sub.FIL2,−1 are the second filtered pressure signal at times 0 and −1, and dP/dt.sub.set is a pre-established dynamic alarm threshold.

[0040] As illustrated in FIGS. 1B-E, an alarm and the suppression system 18 may be activated by the triggering of any one or more of the static alarm detection, the floating alarm detection, and/or the dynamic alarm detection. More specifically, an explosion is detected (FIG. 1B), and the suppression system 18 is caused to release the suppressant 20 to extinguish the explosion (FIGS. 1C-1D).

[0041] FIG. 3A shows a plot 200 of exemplary raw and filtered data for a short duration disturbance of amplitude 200 mbarg and frequency 100 Hz before an explosion is ignited, and FIG. 3B shows a plot 202 of exemplary raw and filtered data during the explosion. The smoothing and delay of pressure readings due to the filtering are clearly visible. The rate of the disturbance is attenuated to below 1 bar/s by the filter, and, as a result, no time filtering may be required in dynamic detection protection. The smoothing function allows for activating the suppression system 18 as soon as the filtered explosion pressure rate of rise exceeds the pre-established threshold dP/dt.sub.set.

[0042] Although the invention has been described with reference to the one or more embodiments illustrated in the figures, it is understood that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.

[0043] Having thus described one or more embodiments of the invention, what is claimed as new and desired to be protected by Letters Patent includes the following: