Mitigation of vapor cloud explosion by chemical inhibition

Abstract

A method to mitigate the consequences of a vapor cloud explosion due to an accidental release of a flammable gas in an open area, comprising: defining an hazardous area wherein an accidental release of flammable gas is likely to happen; receiving a signal from a detector device able to detect the presence of the flammable gas within the hazardous area, upon reception of a signal indicating the presence of the flammable gas within the hazardous area, generating a control signal to activate a release of a flame acceleration suppression product in the hazardous area, at a rate that is determined as a function of the volume of said hazardous area.

Claims

1. A device to mitigate consequences of a vapor cloud explosion due to an accidental release of a flammable gas in an open area, comprising: a memory to store an identifier of a hazardous area wherein an accidental release of flammable gas is likely to happen; an output pin or an entry port to receive a signal from a detector device able to detect presence of the flammable gas within the hazardous area; a central processing unit or a processor that are arranged to, upon reception of a signal indicating the presence of the flammable gas within the hazardous area, generate a control signal and activate a release of a flame acceleration suppression product in the hazardous area, at a rate that is determined as a function of a volume of said hazardous area, wherein the rate of the flame acceleration suppression product varies from 5 to 15 kg/s, and a ratio between the rate of the flame acceleration suppression product and the volume of the hazardous area equals a predetermined amount between 1 and 4 g.Math.s.sup.1.Math.m.sup.3 (every cubic meter with a rate of gram per second); and an output pin or an output port to transmit the control signal toward at least one nozzle connected to a vessel.

2. A method to mitigate the consequences of a vapor cloud explosion due to an accidental release of a flammable gas in an open area, comprising providing a device according to claim 1 and using said device: storing an identifier of a hazardous area wherein an accidental release of flammable gas is likely to happen; receiving a signal from the detector device able to detect the presence of the flammable gas within the hazardous area; upon reception of a signal indicating the presence of the flammable gas within the hazardous area, generating the control signal and activate a release of the flame acceleration suppression product in the hazardous area, at a rate that is determined as a function of a volume of said hazardous area, wherein the rate of the flame acceleration suppression product varies from 5 to 15 kg/s, and a ratio between the rate of the flame acceleration suppression product and the volume of the hazardous area equals a predetermined amount between 1 and 4 g.Math.s.sup.1.Math.m.sup.3 (every cubic meter with a rate of gram per second).

3. The method according to claim 2, wherein the volume of the hazardous area varies from 4500 to 5500 m.sup.3.

4. The method according to claim 2, comprising generating the control signal such that the release of the flame acceleration suppression product takes place for more than 5 minutes.

5. The method according to claim 2, wherein the release of the quantity of flame acceleration suppression product is performed continuously within a determined lapse of time.

6. The method according to claim 2, wherein the control signal to activate a release of a flame acceleration suppression product in the defined hazardous area is generated only if a number of detector devices, comprising at least two detector devices, detect the presence of the flammable gas within the hazardous area.

7. The method according to claim 6, wherein the control signal is generated only if at least two adjacent detector devices detect the presence of the flammable gas.

8. The method according to claim 2, comprising on reception on a signal indicating a slight presence of the flammable gas within the hazardous area, activating an alarm.

9. The method according to claim 8, further comprising on reception of a signal confirming the presence of the flammable gas within the hazardous area, generating a signal so as to activate pressurization of a vessel containing the flame acceleration suppression product.

10. The method according to claim 2, further comprising storing a model of flammable cloud dispersal beyond the hazardous area, upon receipt of the signal indicating the presence of the flammable gas within the hazardous area, generating an additional control signal to activate a release of some flame acceleration suppression product in an additional hazardous area that is determined as a function of the stored model.

11. The method according to claim 2, further comprising providing a computer program product comprising instructions to perform the steps of claim 1, and executing said steps with a processor.

12. A system to mitigate consequences of a vapor cloud explosion due to an accidental release of a flammable gas in an open area, comprising a device as recited in claim 1, at least one set of two vessels comprising a first vessel for storing a flame acceleration suppression product and a second vessel for storing a carrier gas, and, for each set of two vessels: at least one nozzle, a duct arranged to connect the first vessel to said at least one nozzle.

13. The system according to claim 12, wherein at least one set of the at least one set of two vessels comprises, at least two nozzles, and wherein the corresponding duct is arranged to connect the first vessel of said set to both nozzles.

14. The system according to claim 12, wherein at least two nozzles are provided, and wherein two nozzles among said at least two nozzles are placed oppositely such that the flame acceleration suppression product released by one of the two nozzles placed oppositely is moved towards the other nozzle or lower, because of gravity.

15. The system according to claim 12, wherein the flame acceleration suppression product comprises a mixture of at least two compounds selected form the group consisting of potassium chloride, sodium chloride and potassium carbonate.

16. An installation comprising a plurality of systems according to claim 12, each system being associated to a block for which a corresponding hazardous area has been defined.

17. A system to mitigate consequences of a vapor cloud explosion due to an accidental release of a flammable gas in an open area, comprising a device, at least one set of two vessels comprising a first vessel for storing a flame acceleration suppression product and a second vessel for storing a carrier gas, and, for each set of two vessels: at least one nozzle, a duct arranged to connect the first vessel to said at least one nozzle, wherein the device comprises: a memory to store an identifier of a hazardous area wherein an accidental release of flammable gas is likely to happen; an output pin or an entry port to receive a signal from a detector device able to detect presence of the flammable gas within the hazardous area; a central processing unit or a processor that are arranged to, upon reception of a signal indicating the presence of the flammable gas within the hazardous area, generate a control signal and activate a release of a flame acceleration suppression product in the hazardous area, at a rate that is determined as a function of a volume of said hazardous area, wherein the rate of the flame acceleration suppression product varies from 5 to 15 kg/s, and a ratio between the rate of the flame acceleration suppression product and the volume of the hazardous area equals a predetermined amount between 1 and 4 g.Math.s.sup.1.Math.m.sup.3 (every cubic meter with a rate of gram per second); and an output pin or an output port to transmit the control signal toward at least one nozzle connected to a vessel, wherein the flame acceleration suppression product comprises a mixture of at least two compounds selected form the group consisting of potassium chloride, sodium chloride and potassium carbonate.

Description

(1) Other features and advantages shall appear more clearly from the following description of a particular embodiment given by way of a simple, illustratory and non-exhaustive example and from the appended drawings, of which:

(2) FIG. 1 and FIG. 2 are very schematic illustrations of an exemplary system according to an embodiment of the present invention.

(3) FIG. 3 represents, in a flow diagram form, the main steps of an exemplary method according to an embodiment of the present invention.

(4) Flammable gases are handled in many industrial applications, including utilities, chemical and petrochemical manufacturing plants, petroleum refineries, metallurgical industries, distilleries, paint and varnish manufacturing, marine operations, printing, semiconductor manufacturing, pharmaceutical manufacturing, and aerosol can filling operations, as a raw material, product or byproduct. In addition, combustible gases are released by leakage from above- or below-ground piping systems or spillage of flammable liquids. The invention is of high interest for the refineries and petrochemical plants.

(5) An oil refinery may comprise a number of units.

(6) The present invention may be applied for example to three units of an oil refinery, e.g., a steam cracker unit, a butadiene separation unit, an aromatics unit, etc.

(7) Every unit has been divided into process zones. There may be for example between 1 and 4 process zones per unit.

(8) Every process zone has been divided into blocks. Each block dimensions are within a predetermined range corresponding roughly to the lower limit for major VCE in naphtha cracker units [FABIG, D. Roosendans, London, Dec. 4, 2008] i.e., 5000 m.sup.3. For example, each block may have a length of 40 meters, a width of 30 meters and a height varying between 3 and 12 meters for example 4 meters (40*30*4=4800 m.sup.3).

(9) The protection of each block of the units with such size may prevent any VCE with major impact on building, material and people.

(10) For example, for the steam cracker unit, 4 zones have been defined: A furnaces zone, which comprises 2 blocks; A hot train zone, which comprises 5 blocks; A cold train and compression zone, which comprises 4 blocks; and A Separation section, which comprises 4 blocks.

(11) For the butadiene separation unit, a single zone has been defined. This single zone comprises 3 blocks.

(12) For the aromatics unit, 3 zones have been defined: a first zone with 4 blocks, a second zone with 2 blocks and a third zone with 2 blocks.

(13) FIG. 1 is a top view of an exemplary block 1, and FIG. 2 is a side view of this exemplary block 1. Both views are very schematic.

(14) Even if a single block is shown of FIG. 1 and FIG. 2 for the sake of simplicity, the skilled person will appreciate that an oil refinery comprises a number of blocks.

(15) The block 1 comprises a number of equipment and pipes (not represented), some of them carrying flammables gases.

(16) A flammable gas is any gas or vapor that can deflagrate in response to an ignition source when the flammable gas is present in sufficient concentrations by volume with oxygen. Deflagration is typically caused by the negative heat of formation of the flammable gas. Flammable gases generally deflagrate at concentrations above the lower explosive limit and below the upper explosive limit of the flammable gas. In a deflagration, the combustion of a flammable gas, or other flammable substance, initiates a chemical reaction that propagates outwards by transferring heat and/or free radicals to adjacent molecules of the flammable gas.

(17) A volume 3 is defined as a hazardous area. Its length and width may be equal to the length and width of the block, e.g., 40 meters and 30 meters respectively. The height of the volume may be of 4 meters for example. It is considered that at higher heights, the congestion by equipment and pipes is lower (lower risk of VCE) and the flammable gas is dispersed by the wind.

(18) Each block, and/or each volume 3, is protected by two pairs of nozzles 2A, 2B, 2C, 2D on each side in opposite, i.e., by a total of 4 nozzles. Each nozzle allows releasing powder into the block 1. The nozzles are placed at a height that equals the height of the hazardous area, e.g., 4 meters.

(19) The powder is a flame acceleration suppression product that acts as an inhibitor when released into a flammable gas cloud.

(20) The main action of the inhibitor is to capture chain carriers such that a chain branching rate is lowered. There will also be additional physical actions (such as cooling and adsorption) which could lower the reaction rates.

(21) After release, the flame acceleration suppression product not only dilutes the oxygen available for the combustion of the flammable gas but also impairs the ability of free radicals to propagate the deflagration.

(22) While the method of the invention can be employed to suppress deflagrations associated with flammable gases, the method is particularly applicable to suppressing deflagrations of flammable gases having combustion temperatures ranging from about 500 C. to about 2500 C.

(23) Such flammable gases may for example include ethylene, propylene, propane but also benzene, ether, methane, ethane, hydrogen, butane, propane, carbon monoxide, heptane, formaldehyde, acetylene, ethylene, hydrazine, acetone, carbon disulfide, ethyl acetate, hexane, methyl alcohol, methyl ethyl ketone, octane, pentane, toluene, xylene, and mixtures and isomers thereof.

(24) The flame acceleration suppression product may be any product which captures the free radicals and as such limits the branching reactions. Advantageously, the powder may comprise additive(s) in order to avoid caking and additive(s) to improve fluidization properties.

(25) The result is that the flame acceleration is altered and that a devastating explosion is mitigated. The flammable gas will burn more slowly and not develop in a devastating explosion in case of an ignition. The flame acceleration suppression product should not create any risk (e.g. toxic) for humans or the environment.

(26) The flame acceleration suppression product can be a gas, a liquid or a solid (advantageously in a powder form and preferably in a dry powder form).

(27) The flame acceleration suppression product may be a metal compound such as, by way of example, a salt. Several products (salts) and mixtures have been tested. The aim of the flame acceleration suppression mixture is to allow capture of different type of radicals.

(28) By way of example of flame acceleration suppression products, one can cite sodium bicarbonate (NaHCO.sub.3), potassium bicarbonate (KHCO.sub.3), sodium chloride and sodium carbonate. The flame acceleration suppression product can be mixed with primary anti-oxidants and/or secondary antioxidants.

(29) Most (e.g., 90% or more) of the particles of the flame acceleration suppression product may have a diameter varying between 20 m and 40 m, in particular when the product comprises essentially sodium bicarbonate.

(30) Arranging the nozzles in opposite, i.e., the nozzle 2A facing the nozzle 2D and the nozzle 2B facing the nozzle 2C, allows scattering the powder within the whole defined volume. As represented by the arrows 4 on FIG. 2, the released powder moves into the whole volume, or at least into a large part of this hazardous area (e.g. more than 80% of its volume). Providing nozzles on both sides of the block 1 may thus allow reaching a desired concentration of powder: in all the width of the hazardous area. in a shorter time (it is advantageous to establish as fast as possible the cloud of inhibitor with the right concentration after the gas release).

(31) Advantageously, the acceleration suppression product may be dispersed in the area by a carrier gas, e.g. nitrogen, originally contained in a vessel 6.

(32) The vessels 5 contain the flame acceleration suppression product.

(33) The system further comprises valves (not represented) arranged on the ducts 8 between the vessels 5 and the nozzles 2A, 2B, 2C, 2D.

(34) Processing means, e.g., a processor 7 within a control room, are in electrical communication with detector devices 8A, . . . , 8I arranged within the block 1, and with the vessels 5,6. The processor 7 is arranged to generate a control signal when 2 or 3 detectors devices are activated together, and to transmit the generated control signal to the vessels 5, 6 so as to release the powder via the four nozzles 2A, 2B, 2C and 2D of the block together.

(35) As can be seen from FIG. 1, there is provided a single vessel for the powder 5 and a single vessel for the carrier gas 6 for a couple of nozzles 2A, 2B, or 2C, 2D. The ducts 8 are arranged to connect each vessel for the powder 5 to two nozzles 2A, 2B, or 2C, 2D.

(36) This arrangement is advantageous as compared to an arrangement with a vessel 5 and a vessel 6 per nozzle, because it allows saving one vessel for the flame acceleration suppression product and one vessel for the carrier gas per couple of nozzles. The skilled person would not have combined the vessels 5, 6 with a couple of nozzles because one would have expected the flame acceleration suppression product to plug within a duct having a longer path for the product from the vessel 5 to the nozzle or to be dispersed without the same flow rate on the different nozzles. Surprisingly, it did not. In particular, the length of the path from the vessel 5 to the nozzle may reach 10 meters, or even more.

(37) In an alternative embodiment, a third nozzle may be provided on this duct 8, at a middle position between the nozzles 2A, 2B, or 2C, 2D fed by a same vessel. However, having two nozzles only per vessel 5 is advantageous because the pressure at the nozzles 2A (or 2C) is close to the pressure at the nozzle 2B (or 2D), as long as these nozzles are arranged relatively symmetrically. That is, the particle speed at the nozzles 2A and 2B, or 2C and 2D, may be substantially similar, thus allowing a better (homogeneous) scattering of the hazardous area.

(38) Each nozzle may define a simple hole having a diameter of 10-11 mm.

(39) The detector devices 8A, . . . 8I may for example comprise infrared detectors.

(40) As can be seen on FIG. 1, a plurality of infrared detectors, e.g. 9 infrared detectors 8A, . . . 8I are arranged within the block 1 or around the block 1, preferably within the hazardous area 3 of the block 1.

(41) Now referring to FIG. 3, the illustrated flow diagram corresponds to a method executed by the processor 7 of the control room of the inhibition system.

(42) On reception of signals S.sub.A, . . . , S.sub.I originating from the respective gas detectors 8A, . . . , 8I (step 300), the processor compares each received signal to a first threshold THR1. These comparison steps are not illustrated on FIG. 3. The first threshold equals 20% of a predetermined low flammable limit (LFL) value.

(43) This LFL value corresponds to a concentration of flammable gas within the air corresponding to the stoichiometric proportions of the reaction between the flammable gas and air.

(44) If two side detectors, e.g., the detectors 8A and 8B, or 8A and 8E, or 8D and 8G, have measured signals that exceed the first threshold THR1, a first detection Boolean variable 2ooN is set to 1.

(45) Otherwise, the Boolean 2ooN is maintained to a zero value. For example, if the detectors 8A, 8C, 8I have measured signals that exceed the first threshold THR1 and if the other detectors 8B, 8D, 8E, 8F, 8G and 8H have measured signals that are below the first threshold, the variable 2ooN equals zero.

(46) The generating of this variable 2ooN is represented by step 301 on FIG. 3.

(47) If the variable 2ooN equals 1 (test 302), then an alarm is activated (step 303). More precisely, during the step 303, the processor generates signals that are transmitted to an alarm system (not represented on FIG. 1). For example, activating an audible alarm and a visual alarm may allow evacuating the concerned block.

(48) Further, automatic pressurization is activated for the two powder skids pairs of every block whose each detector having measured a signal above the first threshold depend (step 304).

(49) Again, during the step 304, the processor generates signals that are transmitted to a valve, such that nitrogen cylinders 6 pressurize the corresponding powder drum storages 5.

(50) New values of the signals S.sub.A, . . . , S.sub.I measured by the detectors 8A, . . . 8I are received during a step 305.

(51) Then, at step 306, the processor determines the value of a second detection Boolean variable 3ooN.

(52) The processor compares each received signal to a second threshold THR2. These comparison steps are not illustrated on FIG. 3. The second threshold equals 80% of the LFL value.

(53) If three side detectors, e.g. the detectors 8A, 8B and 8F, or 8A, 8E and 8I, or 8D, 8E and 8F, have measured signals that exceed the second threshold THR2, the second detection Boolean variable 3ooN is set to 1.

(54) Typically, with leak rates between 5 kg/s and 50 kg/s, it takes around 10 seconds before the variable 3ooN is set to 1.

(55) Otherwise, the Boolean 3ooN is maintained to a zero value. For example, if the detectors 8A, 8B, 8I have measured signals that exceed the second threshold THR2 and if the other detectors 8C, 8D, 8E, 8F, 8G and 8H have measured signals that are below the second threshold, the variable 3ooN equals zero.

(56) If the variable 3ooN equals 1 (test 307), then an alarm is activated (step not represented). Otherwise, the processor waits for a determined lapse of time (step 308), e.g. 1 second, before receiving new values from the detectors (step 305) and repeating the steps 306, 307.

(57) If the test 307 is positive, and if the pressure in the powder drum reaches a predetermined threshold, e.g., 16 barg, (this test not being represented), the processor generates a control signal CS(t) to activate a release of a flame acceleration suppression product in the hazardous area (step 309). This signal CS(t) is then transmitted toward a valve so as to control the release of powder (step 310).

(58) Once the variable 3ooN is set to 1, it may take 2 seconds for example before the action is launched. Then, it may take around 30 seconds for the powder cloud to be established.

(59) For example, the CS(t) signal may be generated to as to control the following sequence, during which powder is discharged several times at regular interval: A discharge valve stays open during 5 seconds. Then the valve is closed for 15 seconds. The valve is opened again for 5 seconds and so on, 15 times in succession, for a protection of 5 minutes.

(60) Alternatively, the control signal may simply allow opening the valve so as to release powder during predetermined lapse of time, e.g., 300 seconds, without any complete or partial closure of the valve during this lapse of time.

(61) Surprisingly, releasing continuously the powder allows a better reduction of the vapor cloud explosion effects.

(62) The nozzles are directed substantially horizontally, at an elevation of about 4 meters, so as to allow the cloud of released powder to spread at the first 4 meters level of the block.

(63) Having a threshold corresponding to 80% of the LFL value to open the powder discharge allows insuring that the mass of flammable gas inside the cloud is sufficient for a strong explosion to be very likely. The release will not be triggered for a quantity of hydrocarbon that is too small for a VCE, e.g. less than 50 kg.

(64) The invention is by no means limited to the use of two threshold values THR1, THR2 being equal to 20% and 80% of the LFL value. For example, one could use three threshold values, corresponding to 20%, 40% and 80% of the LFL value. When two detectors measure a concentration of flammable gas that exceeds 20% of the LFL value, an alarm is activated. When two or three detectors measure a concentration of flammable gas that exceed 40% of the LFL value, automatic pressurization is activated for the two powder skids. When three detectors measure a concentration of flammable gas that exceeds 80% of the LFL value, the powder is released into the block.

(65) Alternatively, when one detector measures a concentration of flammable gas that exceeds 20% of the LFL value, an alarm is activated. When two detectors measure a concentration of flammable gas that exceeds 20% of the LFL value, automatic pressurization is activated for the two powder skids. This pressurization thus allows saving time since the vessel should be at the proper pressure when the 3ooN variable is set to 1.

(66) When three detectors measure a concentration of flammable gas that exceeds 80% of the LFL value, the powder is released into the block.

(67) These methods, based on the monitoring of a plurality of detectors and on an initiating only after several side detectors have measured a determined concentration of flammable gas, may allow the release of the powder to take place only for possibly dangerous flammable clouds and soon enough for the ignition result in a burning of the flammable gas without explosion.