OBJECTIVES To determine the critical parameters required to scale a water mist system combined with nitrogen, used as an aircraft cargo compartment fire suppression system that have met the MPS.
CONSIDERATION The scaling consideration of the water mist/N2 fire extinguishing system is for cargo compartments of volume sizes of 719 ft 3 (298 in x 125 in x 42 in) and 6252 ft 3 (590 in x 164 in x 80 in).
SCALING APPROACH Characterize the WMS system that passed the MPS tests using the NFPA 750 standard. Characterize nitrogen system. Use defined system parameters and transpose them to different scale cargo compartment, paying close attention to the vertical plane (z-axis). Adjust system as needed in scaled compartment to provide same critical parameters values.
CHARACTERIZATION OF WATER MIST (CONT.) Water collection (mass flow) on a 18” by 18” pan located on the fire zone at 36” above the cargo compartment floor in order to determine the amount of water wetting the defined combustible surface.
ADOPT NFPA 750 – STANDARD ON WATER MIST FIRE PROTECTION SYSTEMS This NFPA document was prepared by the Technical Committee on Water Mist Fire Suppression Systems and acted on by the National Fire Protection Association, Inc. on November 1999. It was issued by the Standards Council on January 2000, with an effective date of February 11, 2000 In developing this standard, the committee addressed system components and hardware, system types, installation requirements, design objectives, hazard classifications, calculations, water supplies, atomizing media, plans, documentation, acceptance criteria, and maintenance considerations NFPA 750 represents a significant advancement in water mist technology and the knowledge base associated with its application.
CHARACTERIZATION OF WATER MIST NFPA 750 Section A-5-2 (Appendix A) 1.System Flow Rate 2.System Operating Pressure 3.General Water Requirements 4.Nozzle Characteristics 5.Nozzle Spray Characteristics 6.Nozzle Installation Parameters 7.Activation Device 8.General Design Parameters
NITROGEN MEASUREMENT The measurement of the nitrogen volumetric concentration, in the scaled cargo compartment, will be achieved by determining the oxygen volumetric concentration in the hypoxic condition (using gas analyzers). The time to reach an inert condition with nitrogen will also be considered during the scaling of the system.
I. SYSTEM FIRE PERFORMANCE OBJECTIVE - Fire Suppression II. WATER MIST SYSTEM (NFPA 750 – Standard on Water Mist Fire Protection Systems) – The Environmental Engineering Concepts water mist system was a balanced high-pressure, Class I system (i.e., at least 10% of the spray is less than 200 microns). CHARACTERIZATION OF WATER MIST/N2 SYSTEM
II. WATER MIST SYSTEM (CONT.) 1. System Flow Rate (Minimum and Maximum) a. Flow rate – from 0.54 gpm (1 zone activated) to 1.84 gpm (4 zones activated) b. Flow rate per unit area – 0.343 lbs/min*ft 2 2. System Operating Pressure (Minimum and Maximum) a. Nozzle operating pressure range - from 550 psig (4 zones activated) to 1150 psig (1 zone activated). b. Pump/cylinder operating pressure range – for pump model Giant P217A the inlet pressure is up to 90 psig and the discharge pressure is up to 2500 psig c. Pump inlet and outlet pressure and flow rate requirements – the inlet pressure is from 20 (minimum) to 90 psig (maximum), the discharge pressure is up to 2500 psig (1250 psig used during the fire tests), the pump will produce a volume of up to 3.36 gpm. CHARACTERIZATION OF WATER MIST/N2 SYSTEM
II. WATER MIST SYSTEM (CONT.) 3. General Water Requirements a. Quantity/duration – 9 gallons (continuously discharging for 5 minutes) b. Quality – Special consideration should be given to the quality of water that will flow through the water mist system. Water with total dissolved solids (DS) counts of greater than 500 parts per million (ppm) or water with high calcium carbonate levels or PH levels above 7.5 may not be satisfactory and could lead to excessive nozzle blockage and nozzle maintenance. c. Temperature – Above freezing point, preferable at standard room temperature. The pump relies on the water running through it to keep it cool. d. Pressure – Correct water pressure and supply rate is essential to the pumps operation. The water supply pressure must be maintained between a minimum of 20 psi and a maximum of 90 psi to operate the pump. CHARACTERIZATION OF WATER MIST/N2 SYSTEM
II. WATER MIST SYSTEM (CONT.) 4. Nozzle Characteristics a. Type(s)/model numbers – EEC/MicroCool nozzle, nozzle orifice is 0.0245" (0.63 mm) in diameter b. Flow rate (minimum and maximum) – 0.0675 gpm to 0.23 gpm c. Operating pressure (minimum and maximum) – 1025 psig to 1150 psig 5. Nozzle Spray Characteristics a. Spray angle – 27º, nozzle pointing down vertically b. Drop size distribution – Droplet sizes range from 70 to 100 microns (Class I), provided by manufacturer. No access to PDPA system c. Momentum/velocity – 3.89 feet per second (obtained by using high speed film) CHARACTERIZATION OF WATER MIST/N2 SYSTEM
II. WATER MIST SYSTEM (CONT.) 6. Nozzle Installation Parameters a. Distance above floor – 65" above floor b. Distance below ceiling - 1" below ceiling c. Distance above hazard – (i.) For bulk load test – 29" above hazard, (ii.) For containerized test - 1" above hazard, (iii) For surface burn test - 11" above hazard(minimum and maximum), and (iv) for aerosol can explosion test - 41" above hazard. d. Nozzle spacing - 16" between nozzles (8 nozzles per zone) e. Zone spacing - 30" between zones f. Orientation – nozzles pointing down vertically g. Minimum distance from walls and obstructions – Atomization nozzles should never be pointed towards and allowed to spray on any close object or surface otherwise the fog will condense; a distance of 18" to 24" should be preserved. Use a nozzle plug to block off the fog if this cannot be avoided. CHARACTERIZATION OF WATER MIST/N2 SYSTEM
II. WATER MIST SYSTEM (CONT.) 7. Activation Device a. Type/model number – Manual activation b. Activation, temperature – 1 minute after ceiling temperature reached 200ºF 8. General Design Parameters a. Pipe requirement i. Size – 3/8” diameter stainless steel tubing ii. Operating pressures – up to 2500 psig b. Fittings i. Type – compression fittings ii. Operating pressure – up to 2500 psig CHARACTERIZATION OF WATER MIST/N2 SYSTEM
II. WATER MIST SYSTEM (CONT.) 8. General Design Parameters (Cont.) c. Pumps i. Valves, fittings, and filters – drain valves, NPT type fittings (with swivel), twin filter components of 10 and 5-micron units ii. Power requirements – 120 Vac (Motor), 24 Vdc (Relays) iii. Operating pressure and flow rates – See Section 2.c System Operating Pressure iv. Water requirements – See Section 3. General Water Requirements CHARACTERIZATION OF WATER MIST/N2 SYSTEM
III. NITROGEN SYSTEM CRITICAL PARAMETERS 1. Cylinder valves and fittings – Two Matheson 3020 Series Regulators, CGA 580, with a maximum inlet pressure of 3500 psig, and a flow rate of 89.3 CFM (at 500 psig) per regulator. 2. Cylinder capacity – 16 High Pressure Size T cylinders, 1.73 ft 3 each 3. Cylinder operating pressures – 2500 psig 4. Volumetric Concentration – 90% N2 in air 5. Flow Rate – initial rate at 500 psig was 178.6 CFM (average = 154.7 CFM) 6. Time for Initial Protection – an average of 12.5 minutes 7. Fire/Explosion Protection Time – 180 minutes (infinite if OBIGSS system used) CHARACTERIZATION OF WATER MIST/N2 SYSTEM
TC-10 CARGO COMPARTMENT (2000 FT 3 ) WMS/N2 INSTALLATION
CHARACTERIZATION OF WATER MIST/N2 SYSTEM TC-10 13 Zones/95 Nozzles
CHARACTERIZATION OF WATER MIST/N2 SYSTEM B-737 CARGO COMPARTMENT (719 FT 3 ) WMS/N2 INSTALLATION
CHARACTERIZATION OF WATER MIST/N2 SYSTEM B-737 9 Zones/72 Nozzles
CHARACTERIZATION OF WATER MIST/N2 SYSTEM B-777 CARGO COMPARTMENT (6252 FT 3 ) WMS/N2 INSTALLATION
CHARACTERIZATION OF WATER MIST/N2 SYSTEM B-777 25 Zones/180 Nozzles
N2 INERTING OF CARGO COMPARTMENTS
WATER COLLECTION TEST RESULTS 1.Placed 18”x18” pan on scale 2.Collected weight samples every 30 seconds 3.Collected samples for 5 minutes 4.Ran tests 10x per cargo size
CHARACTERIZATION OF WATER MIST/N2 SYSTEM WATER COLLECTION TEST TC-10 1 Zone/8 Nozzles On
CHARACTERIZATION OF WATER MIST/N2 SYSTEM WATER COLLECTION TEST B-737 1 Zone/8 Nozzles On
CHARACTERIZATION OF WATER MIST/N2 SYSTEM WATER COLLECTION TEST B-777 1 Zone/8 Nozzles On
CHARACTERIZATION OF WATER MIST/N2 SYSTEM WATER COLLECTION TEST TC-10 Class E 1 Zone/8 Nozzles On
CHARACTERIZATION OF WATER MIST/N2 SYSTEM 3.8 lbs 1.5 lbs 0.9 lb 1.4 lbs
1.The NFPA 750, Standard on Water Mist Fire Protection, provides an excellent guideline to characterize an aircraft cargo compartment water mist fire suppression system. 2.The water collection test provides a simple and inexpensive method to determine the amount of water available on the combustible surface. This water, accumulated on the surface, cools the burning surface and provides liquid for oxygen dilution during the fire suppression process. This test does not provide particle size distribution, flux density, nor spray momentum, but it offers a quick and an inexpensive way to compare the WMS/N2 system in various compartment sizes.
3.After performing the water collection test on the WMS/N2 system in the B-737 and B777, it was observed that the water accumulated in the pan was the same or was more than the water accumulated during the TC-10 test. 4.The above statement can not be made for the TC-10 Class E compartment. The amount of water accumulated on the combustible surface needs to be increased; this can be achieved by reconfiguring the installation or adding more zones.