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Petrochem Industry Bulletin #0950 Fire System; CARBON D/OX/DE FIRE SUPPRESSION - co2 Vent Stack SnuiTing c 0 nn When it’s necessary to extinguish a fire at the end of a vent pipe, boom, or stack, C
  Fire System; co c II m U ID a 3 m C aDoX 4801 Southwick Drive Third Floor Matteson, IL 60443 Telephone: 708/748-l 503 Fax: 7081748-2847 Petrochem Industry Bulletin 0950 CARBON D/OX/DE FIRE SUPPRESSION - Vent Stack SnuiTing When it’s necessary to extinguish a fire at the end of a vent pipe, boom, or stack, CO, can be used - and it has become a popular choice. CO, is introduced into the venting pipe to flow along with the gas being vented. The introduction point is near the end of the venting pipe (within 10 feet/3 meters). The amount of CO, needed is that required to make the gas (normally methane/natural gas) non-flammable when any amount of air is added. In the past there has been some misunderstanding as to the CO2 quantities required This is not the same as a room flooding applica- tion. The design concentration given in NFPA Standard No. 12 for protecting a hazard where methane gas is the combustible does not apply. In that total flooding application, the combustible is assumed to be in the protected enclosure in the proper amount, so that when mixed with air, it is in the combustible range. Adding the proper amount of CO, will put out the fire. In this application, a specific amount of gas, which burns when it reaches air at the end of the pipe, is flowing through a vent pipe. It is necessary to add a sufficient quantity of CO, to the gas flow so that the resultant mixture is non-flammable when any amount of air is added. For this application, the reference document is Purging Princi ples and Pmctices published by the American Gas Association. From the table on the next page, you can see that good design calls for add- ing 82 CO? when venting methane. 0 1996 CHEMETRON FIRE SYSTEMS, all rights reserved. 9196)   HEmETROn Fire System; co Application Bulktin Petrochem Industry Bulletin 0950 Page 2 System Design Let’s assume that gas flow is 150 cubic meters/ hour (5300 cubic feet/hr). That is 88.3 cfm. If the mixture of CO, and gas is to be 82 CO, and 18 gas, then the 88.3 cfm of gas can be only 18 of the total flow. Total flow would be 88.3 + 0.18, or 490.7 cfm. CO, flow at 82 will be 490.7 x 0.82, or 402.4 cfm. Assuming an expansion of 1 lb. of CO, to 8.3 cubic feet, this works out to 48.5 lbs. per minute (call it 50 lbs/min). It is recommended that CO, sufficient for a 5 minute discharge be provided to allow for adequate cooling. An arrangement such as that illustrated below is recommended to assure good gas introduction, mix- ing, and nozzle projection. GAS FLOW - ORIFICE NOZZLE WITH SPRING COVER Inert Gas End Points for Pur Combustible Hydrogen Carbon Llonoxide Methane Ethane Propane Butane Iso-butane Pentane Hexane Gasoline Ethylene Propylene Benzene ing Out of Service I Purge Medium Required to render mixtures non-flammable when any amount of air is added CO, NZ 91 95 68 81 77 86 88 93 89 94 91 95 91 95 96 97 96 97 93 96 90 94 94 96 93 96 Purging end point with 20 safety factor CO, 9 1 95 93 1 96 95 1 97 95 97 ‘.- CHECK VALVE SUGGESTED NOZZLE RR NGEMENT
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