GB2306320A

GB2306320A - Optimising fire extinguishing

Info

Publication number: GB2306320A
Application number: GB9521540A
Authority: GB
Inventors: Andrew John Coleman; David Nicholas Ball
Original assignee: Kidde Graviner Ltd
Current assignee: Kidde Graviner Ltd
Priority date: 1995-10-20
Filing date: 1995-10-20
Publication date: 1997-05-07
Anticipated expiration: 2015-10-20
Also published as: GB2306320B; GB9521540D0

Abstract

A method for optimising a fire extinguishing system comprising extinguishant discharge nozzles (14) connected by pipework to a pressurised extinguishant container for discharging dry powder into volumetric spaces within a cluttered volume (10) such as an engine bay, in which an experimental representation of the engine bay (10) is prepared and nozzles (14) are mounted in empirically selected positions. The other parameters of the extinguishing system are also initially selected empirically. A test discharge then takes place. During this test discharge, powder concentration sensors (16) sense the concentration of the discharge dry powder extinguishant in each of the volumetric spaces. If it is inadequate (or wastefully too great), one or more of the parameters of the system is adjusted and the test is repeated until the sensors (16) sense correct values. In a modification, a mathematical model is used to supplement or replace the use of the sensors (16).

Description

METHODS AND APPARATUS FOR OPTIMISING FIRE EXTINGUISHING The invention relates to and apparatus for optimising fire extinguishing. The methods and apparatus according to the invention, and to be described below by way of example, may be used for optimising the extinguishing of fires in confined spaces incorporating irregularly shaped volumes such as the engine bays of vehicles.

According to the invention there is provided a method of optimising a fire extinguishing system which system comprises a plurality of extinguishant discharge outlets connected by pipework to a pressurised extinguishant container, the outlets being intended for mounting in predetermined positions within a cluttered volume and operative when the system is activated to discharge extinguishant into respective irregularly shaped volumetric spaces within the volume, the method comprising the steps of preparing an experimental representation of the cluttered volume, mounting the outlets to discharge extinguishant into at least some of the volumetric spaces, activating the system to discharge the extinguishant, measuring the resultant concentration of discharged extinguishant in at least some of the volumetric spaces, comparing the said concentrations with predetermined values corresponding to those adequate for fire extinguishing whereby to identify any said concentration falling below the predetermined concentration by more than a predetermined amount, adjusting one or more parameters of the system in a direction tending to adjust any said identified concentration towards the predetermined concentration, activating the system again, measuring the said concentrations again, repeating the comparison step to identify any concentration falling below the predetermined concentration by more than the predetermined amount, and repeating the parameter adjusting step in respect of any such identified concentration.

According to the invention, there is also provided a method for optimising a fire extinguisher system which system comprises a plurality of extinguishant discharge outlets connected by pipework to a pressured extinguishant container, the outlets being intended for mounting in predetermined positions within a cluttered volume and operative when the system is activated to discharge extinguishant into respective irregularly shaped volumetric spaces within the volume, the method comprising the steps of selecting initial physical parameters for the pressure of the extinguishant, for the pipework and for the outlets and for the number, positioning and/or type of the outlets, using a mathematical model of the system to calculate the values of the concentration of the extinguishant which would be discharged from the outlets, comparing the concentration values with desired predetermined concentration values, adjusting the value of one or more of the said parameters if any of the calculated concentration values fall below the predetermined concentration values by more than a predetermined amount, inputting the adjusted parameter values into the mathematical model again and using the mathematical model to produce new concentration values, comparing the new concentration values with the predetermined concentration values, and repeating the process until the calculated concentration values do not fall below the predetermined concentration values by more than the predetermined amount.

According to the invention, there is further provided apparatus for optimising a fire extinguishing system which system comprises a plurality of extinguishant discharge outlets connected by pipework to a pressurised extinguishant container, the outlets being intended for mounting in predetermined positions within a cluttered volume and operative when the system is activated to discharge extinguishant into respective irregularly shaped volumetric spaces within the volume, the apparatus comprising measuring means for measuring the concentration of extinguishant discharged into at least some of the volumetric spaces by said outlets positioned to discharge extinguishant into those volumetric spaces when the system is activated, comparing means for comparing the said concentrations with predetermined values corresponding to those adequate for fire extinguishing whereby to identify any said concentration falling below the predetermined concentration by more than a predetermined amount, and means for adjusting one or more parameters of the system in a direction tending to adjust any said identified concentration towards the predetermined concentration.

Methods and apparatus according to the invention, for optimising the operation of a fire extinguishing system for extinguishing fires in engine bays of vehicles and the like, will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a side view of an engine bay incorporating apparatus according to the invention; Figure 2 is a schematic diagram of the extinguishing system; Figure 3 shows a powder concentration sensor used in one form of the methods and apparatus of the invention.

The methods and apparatus to be described are for optimising the extinguishing of fires in engine bays of vehicles using a dry powder extinguisher discharge system. Examples of such a suitable dry powder extinguishant are sodium or potassium bicarbonate.

Figure 1 is a side view, in purely diagrammatic form, of the interior of an engine bay in which any fires which may occur are to be extinguished in the manner to be described. Such an engine bay 10 will comprise a confined space partially occupied by the engine 12 itself and its many ancillary components (e.g.

12A,12B,12C...). Together, these produce, within the total volume of the engine bay 10, a number of irregularly shaped volumetric spaces between the engine and the various components and between the components themselves. In order to provide protection against fires, a number of powder discharge nozzles 14 are mounted within the engine bay 10 and connected by a pipe system (which has been omitted from Figure 1 for clarity) to a storage cylinder containing the dry powder extinguishant. In the event of a fire being detected within the engine bay, by any suitable fire or overheat detection system, the dry powder is fed from the storage cylinder (in a manner to be described in more detail below) through the pipe system and discharged by the nozzles.

A suitable fire detecting system may comprise a linear-type overheat detector responsive to local overheat. However, any other suitable fire detecting system can be used instead.

In order to provide effective extinguishing of fires, it is necessary that the nozzles are correctly positioned, having regard to the sizes, shapes and dispositions of the individual volumetric spaces within the engine bay, so that the discharged powder reaches all likely seats of fire and with the necessary concentration to provide rapid extinguishing. The positioning of the nozzles, and the selection of the other parameters of the system (pressure, flow rate), must also take into account the air flow which may be present within the engine bay. Normally, there will be a forced air flow for providing convection cooling of the engine. Obviously, this will have an effect on the distribution of the powder, and this has to be taken into account.

In view of all these factors, therefore, the nozzles 14 must be correctly positioned within or in relation to the individual volumetric spaces, and the supply of the dry powder to the nozzles 14 (via the pipework and from the storage cylinder) must be such, in relation to the discharge orifice of each nozzle 14, that the required concentration of powder is achieved in each volumetric space and in all relevant parts of it. Previously known fire extinguishing systems for engine bays and the like have used gaseous extinguishants, such as Halon. The dispersion of discharged Halon and other gaseous extinguishants within an engine bay can be assumed to take place on a bulk-mixing basis and to be substantially uniform throughout the volumetric spaces.

It is therefore relatively easy, using gaseous extinguishants such as Halon, to calculate the extinguishant concentration which is likely to be produced by a given arrangement and number of nozzles. However, gaseous extinguishants such as Halons are not acceptable now, because of their ozone depletion potention (ODP).

Suitable powders, such as those suggested above, have a zero ODP.

However, it is much more difficult to predict how ejected extinguishant powder will be distributed within an engine bay and what the resultant extinguishant concentrations will be. Unlike ejected gaseous extinguishants, ejected powder extinguishants tend to travel in a straight line. Careful position of the nozzles is therefore essential in order to provide effective extinguishing, and it is also of course necessary to be sure that adequate extinguishant concentration will be produced at all relevant parts of the engine bay. In general, the powder concentration needs to be substantially uniform, and must of course also be adequate (though not wastefully excessive). In certain places within the engine bay, though, higher concentrations of powder may be necessary.For example, a hot surface within the engine bay may need to be subjected to a higher concentration of powder in the event of a fire in order to ensure that the hot surface will not cause re-ignition after extinguishing has taken place.

The optimising methods and apparatus to be described are concerned with optimising the selection and disposition of the nozzles 14, and the flow rate of the extinguishing powder, for a particular engine bay layout, during the design stage of the extinguishing system. According to one aspect, a method of optimising the fire extinguishing system uses a plurality of powder concentration sensors 16, one of which is shown in Figure 3. Each sensor 16 is an obscuration sensor and senses the degree of obscuration in its immediate locality. One of the sensors 16 will now be described in detail with reference to Figure 3.

As shown in Figure 3, the sensor comprises a probe head indicated generally at 18 which incorporates a light emitter 20 directing light to a light detector 22.

The light emitter 20 can, for example, comprise an LED emitting diode emitting yellow light. The light detector 22 can, for example, comprise a photodiode. As shown in Figure 3, the emitter 20 and the detector 22 are separated by a sampling distance x.

In the presence of powder, the particles of the powder will cause the light from the emitter 20 to be at least partially obscured from the detector 22, the degree of obscuration being a function of the concentration of the powder (in terms of the number, N, of particles per unit volume).

If I is the output signal of the light detector 22 in the presence of powder, and Io is the output signal from the detector 22 when no powder is present (and a clean atmosphere is present between the emitter 20 and detector 22), then I = Io exp (Nox) where a is the scattering cross-section of the particle.

If the wavelength of the emitted light is short, relative to the radius of the particles, then a = 2n The radii of the particles will not all be the same. However, the average radius, r, of the particles will be known and this is assumed to give an average value for a. Therefore, N can be found from the ratio I/Io. N is related to the mass density and therefore the extinguishing efficiency.

Figure 2 shows the extinguishing system in diagrammatic form.

In a practical case, the nozzles 14 would of course be physically positioned in the engine bay at the required locations to provide effective fire extinguishing action, as will be described in more detail below. The system also comprises a dry powder storage cylinder 32 which is connected to the nozzles 14 via a control valve 34 and pipework indicated generally at 36. The cylinder 32 is partially filled with the extinguishing powder and pressurised with nitrogen or air or other suitable gas. When powder discharge is to occur, in the event of detection of fire or overheat, the control valve 34 is opened automatically and the powder is forced along the pipework 36 by the pressurising gas and discharged by the nozzles 14 as a power/gas mixture.

At an initial stage in the design of a dry powder extinguisher discharge system for a particular application, that is, for a particular engine bay 10, a visual inspection is made of an experimental representation of the engine bay and an initial assessment is made of the number of nozzles 14 which will be required in order to provide adequate distribution of discharged dry powder extinguishant to all parts of the engine bay in the event of fire. The nozzles 10 are thus initially placed in their respective positions and connected by pipework 36 to the dry powder storage cylinder 32 as shown in Figure 2. Powder concentration sensors 16 (Figure 3) are then positioned at selected locations in the engine bay 10. Such sensors 16 are shown by way of example in Figure 1, though their electrical connections are omitted for clarity. With the sensors 16 in position and energised, a test is carried out by opening the control valve 34 (Figure 2), thus causing discharge of powder by the nozzles 14. The test is carried out in conditions providing a substantially clear ambient atmosphere within the engine bay 10 (until, of course, the powder discharge takes place). Each sensor 16 thus senses the local obscuration caused by the discharged powder in its locality, and produces an output to the control unit 18 (Figure 3) in terms of the local concentration of powder (in particles/unit volume, N) which it senses.

For any given type of dry powder extinguishant, the necessary concentration to provide adequate extinguishing of fires is known. Therefore, by comparing the actual local concentration sensed by each sensor 16 with the known minimum required concentration value, an assessment can be made of the adequacy of the powder discharge of each nozzle 14. Thus, an assessment can be made whether the rate of flow of the powder/gas mixture through each nozzle (volume per unit time) is sufficient to provide the required dry powder concentration in the relevant parts of each volumetric space to achieve adequate extinguishing of fires.The flow rate in each locality can be adjusted by changing the respective nozzle for a nozzle with a differently sized discharge orifice or for a nozzle of a different type (producing a differently shaped powder discharge for example), and/or by altering the diameter of the supply pipe to that nozzle. Clearly, altering the conditions at one nozzle (altering its orifice size and/or altering the diameter of the local pipework) will or may affect the discharge from at least some of the other nozzles.

After adjustment in this way, and after removing any residual powder from the engine bay so as to provide a locally clean atmosphere, the test is repeated, and the sensors 16 again measure the local powder concentration. Further adjustments are then made if necessary, and in the manner explained, to the flow rate of the powder discharged from the nozzles until optimum powder discharge is achieved from all nozzles.

Clearly, the process described is not only used to ensure that adequate powder discharge takes place at all relevant locations but also to ensure that excessive and thus wasteful powder discharge does not take place.

On completion of the optimisation process, the orifice sizes of the nozzles, their positioning relative to each other and relative to the engine and its components and parts, the dimensions of the connecting pipework, the size of the reservoir and the pressure within it, are all determined for that particular type of engine bay 10, and are thus used in subsequent production of dry powder discharge systems for that engine bay.

The optimisation method described above involves a series of repeated tests and using the outputs of the sensors 16, during each test, to adjust the physical parameters of the discharge system until optimisation is achieved. In an alternative method, the sensors 16 are used initially in the optimisation process, but the process then continues using a mathematical model to establish the physical parameters of the system. After such establishment, the system can then be tested again using the sensors 16 in order to confirm the optimisation of the system.

In this alternative method, the initial part of the process is as described above: using the experimental representation of the engine bay 10 and all its contents, the nozzles 14 are mounted in position, on the basis of the initial expert visual assessment and are connected up by the pipework 36 to the storage cylinder 32 (Fig. 2). As before, the physical parameters of the system are initially selected empirically. The control valve 34 is then opened to allow discharge to occur in the manner already explained, and the powder concentration within the locality of each nozzle 14 is measured by the respective sensor 16. On the basis of this information, therefore, the flow rate of powder from each nozzle 14 is thus known.Using the mathematical model now to be described, the method determines the required diameters of the various parts of the pipe network 36, the required volume for the storage cylinder 32 and the pressure of the gas within the storage cylinder.

Initially, particular values are assumed for these parameters.

Using these parameters, the mathematical model calculates (in the manner to be described) the resultant flow rate of the powder/gas mixture at each nozzle. If this flow rate is not at the correct value (as determined above), adjustments are made to one or more of the parameters, and the mathematical model is then used again to calculate the new value for the flow rate. This process continues iteratively until the required flow rate is achieved for each nozzle 14. In this way, the values of the physical parameters of the system are selected mathematically and without the need for repeated tests which are time-consuming.At the completion of the process, however, a practical test may be carried out, using the sensors 16 in the manner explained above, to confirm that the complete system, as designed by the mathematical model produces the required powder/gas concentration at each required location.

The mathematical model uses a time-slice scheme in which the movement of the powder/gas mixture during each one of a sequence of successive time slices is considered. The discharge process can be divided into three stages: I. Pipe filling - that is the initial stage during which the powder/gas mixture travels under pressure from the storage cylinder 32 until it has completely filled the pipework 36 (that is, has reached the nozzles 14).

II. Steady state powder/gas flow - that is, flow of the powder/gas mixture between the storage cylinder 32 and the nozzles 14, while the pipework 36 is maintained full.

III. The final stage during which the final part of the powder/gas mixture is discharged, the pipework 36 emptying during this stage.

At the beginning of stage I, the control valve 34 is opened. The powder/gas mixture moves from the storage cylinder 32 and into the pipework 36. The flow rate during this period is determined by the pressure in the storage cylinder 32. The pressure and temperature in the storage cylinder drop because of adiabatic expansion, the gas between the powder particles expanding, thus reducing the loading, and the pressure drop can thus be calculated, using the Equations listed below. Using the new (lower) pressure value for the pressure in the storage cylinder 32, the process is repeated for the next time slice. This process is repeated until the pipe network 36 is full (that is, the powder/gas mixture has reached the nozzles 14).

Stage II - see above - is now applicable. During this stage, a time slice method is again used For the first time slice, the mathematical model calculates the total pressure drop along the pipework 36 using the Equations below. The process is then repeated for the next time slice using the reduced pressure and so on until the storage cylinder 32 is empty.

Stage III - see above - is now applicable. During this stage, again the time-slice method is used. Using the Equations below, the mathematical model calculates the pressure drop along the pipework, until the pipe is empty and all the powder has been distributed.

The pressure drop within the pipework is calculated by summing the following individual pressure drops: For acceleration of gas to the carrying velocity #pa,c = GGVG/2gc [1] For acceleration of solid particles, APa,5 = Gsvs/gc [2] For friction between gas and pipe wall, APf,G = 4fGLpdGVG/2gcDt = 4fGLGGVG/2gcDt [3] For friction between particles and pipe wall, between gas and particles, and between particles, #pf,s = 4fsLpdsVs/2gcDt = 4fsLGsVs/2gcDt [4] Friction factor fs can be related to the particle drag coefficient by a force balance on the particles in the pipe as follows:: 4fs = 3PGDtC VG - 2 [5] 2psDs Vs where Ap = pressure drop C = drag coefficient, function NRe NRe = Reynolds number = Ds(Vc - VS)PC/ G D5 = diameter of solid particle Dt = diameter of pipe fG = Fanning friction factor fs = solids friction factor GG = gas mass velocity = PdGVG Q = solids mass velocity = pds5vs gc = dimensional constant L = length of pipe VG = actual velocity of gas Vs = actual velocity of solids PdG = dispersed gas density Pds = dispersed solids density PG = gas density Ps = solids density = = gas viscosity Parts of the pipework 36 may be vertical. The total pressure drop in a vertical pipe is the sum of the above terms plus the following: For support of the column of gas, tPh,G = GGgL/VGgC [6] For support of the particles, AP, s = GsgL/VsGc [7] where g = acceleration due to gravity.

Stages II and III represent the parts of the process during which powder is being discharged. The model therefore has to model the manner in which the powder is distributed into the volumetric spaces within the engine bay, taking into account any cooling air flow and other factors. This may for example be achieved using suitable standard computational fluid dynamic (CFD) software.

Claims

1. A method of optimising a fire extinguishing system which system comprises a plurality of extinguishant discharge outlets connected by pipework to a pressurised extinguishant container, the outlets being intended for mounting in predetermined positions within a cluttered volume and operative when the system is activated to discharge extinguishant into respective irregularly shaped volumetric spaces within the volume, the method comprising the steps of preparing an experimental representation of the cluttered volume, mounting the outlets to discharge extinguishant into at least some of the volumetric spaces, activating the system to discharge the extinguishant, measuring the resultant concentration of discharged extinguishant in at least some of the volumetric spaces, comparing the said concentrations with predetermined values corresponding to those adequate for fire extinguishing whereby to identify any said concentration falling below the predetermined concentration by more than a predetermined amount, adjusting one or more parameters of the system in a direction tending to adjust any said identified concentration towards the predetermined concentration, activating the system again, measuring the said concentrations again, repeating the comparison step to identify any concentration falling below the predetermined concentration by more than the predetermined amount, and repeating the parameter adjusting step in respect of any such identified concentration.

2. A method according to claim 1, in which the concentration measuring step comprises the step of locating a respective extinguishant concentration measuring sensor in relation to each volumetric space.

3. A method according to claim 1 or 2, in which the extinguishant is a dry powder extinguishant and is discharged from the container by means of gas pressure.

4. A method for optimising a fire extinguisher system which system comprises a plurality of extinguishant discharge outlets connected by pipework to a pressured extinguishant container, the outlets being intended for mounting in predetermined positions within a cluttered volume and operative when the system is activated to discharge extinguishant into respective irregularly shaped volumetric spaces within the volume, the method comprising the steps of selecting initial physical parameters for the pressure of the extinguishant, for the pipework and for the outlets and for the number, positioning and/or type of the outlets, using a mathematical model of the system to calculate the values of the concentration of the extinguishant which would be discharged from the outlets, comparing the concentration values with desired predetermined concentration values, adjusting the value of one or more of the said parameters if any of the calculated concentration values fall below the predetermined concentration values by more than a predetermined amount, inputting the adjusted parameter values into the mathematical model again and using the mathematical model to produce new concentration values, comparing the new concentration values with the predetermined concentration values, and repeating the process until the calculated concentration values do not fall below the predetermined concentration values by more than the predetermined amount.

5. A method according to claim 4, in which the step of selecting the initial parameters comprises the steps of preparing an experimental representation of the cluttered volume, mounting the system relative to the cluttered volume so that the outlets discharge extinguishant into at least some of the volumetric spaces, activating the system to discharge the extinguishant, measuring the values of the resultant concentration of discharged extinguishant in at least some of the volumetric spaces, and using the measured concentration values to establish the initial parameter values.

6. A method according to claim 4 or 5, including the step of preparing an experimental representation of the cluttered volume using the physical parameters of the system as calculated by the mathematical model1 mounting the outlets to discharge extinguishant into at least some of the volumetric spaces, activating the system to discharge the extinguishant, measuring the resultant concentration values of discharged extinguishant in at least some of the volumetric spaces, and comparing the measured concentration values with the predetermined concentration values to test the correctness of the parameter values.

7. A method according to any one of claims 4 to 6, in which the extinguishant is a dry powder extinguishant and is discharged from the container by means of gas pressure.

8. Apparatus for optimising a fire extinguishing system which system comprises a plurality of extinguishant discharge outlets connected by pipework to a pressurised extinguishant container, the outlets being intended for mounting in predetermined positions within a cluttered volume and operative when the system is activated to discharge extinguishant into respective irregularly shaped volumetric spaces within the volume, the apparatus comprising measuring means for measuring the concentration of extinguishant discharged into at least some of the volumetric spaces by said outlets positioned to discharge extinguishant into those volumetric spaces when the system is activated, comparing means for comparing the said concentrations with predetermined values corresponding to those adequate for fire extinguishing whereby to identify any said concentration falling below the predetermined concentration by more than a predetermined amount, and means for adjusting one or more parameters of the system in a direction tending to adjust any said identified concentration towards the predetermined concentration.

9. Apparatus according to claim 8, in which the extinguishant is a dry powder extinguishant and is discharged from the container by means of gas pressure.

10. A method of optimising a fire extinguishant system, substantially as described with reference to Figures 1,2 and 3 of the accompanying drawings.

11. Apparatus for optimising a fire extinguishant system, substantially as described with reference to Figures 1,2 and 3 of the accompanying drawings.