Gas/Propane Basics Flow of gas through fixed orifices Use of orifice tables • Checking the burner output without using a meter: Gauge the size of the burner orifice and determine the flow rate at sea level from Table E.1.1 (a), Utility Gases (cubic feet per hour), or from Table E.1.1 (b), LP-Gases (BTU per hour). When the specific gravity of the utility gas is a value other than 0.60, select the multiplier from Table E.1.1 (c) for the specific gravity of the utility gas served, and apply to the flow rate as determined from Table E.1.1 (a). When the altitude is above 2,000 ft (600 m), first select the equivalent orifice size at sea level using Table E.1.1 (d), then determine flow rate from Table E.1.1 (a) or Table E.1.1 (b) as directed. Having determined the flow rate (as adjusted for specific gravity and/or altitude where necessary), check the burner input at sea level with the manufacturer’s rated input. • Selecting the correct orifice size for rated burner input: The selection of a fixed orifice size for any rated burner input is affected by many variables—including orifice coefficient—and it is recommended that the appliance manufacturer be consulted for that purpose. When the correct orifice size cannot be readily determined, the orifice flow rates, as stated in the tables, can be used to select a fixed orifice size with a flow rate to approximately equal to the required rated burner input. Using Table 2, determine gas pressure, then divide the number of burners into BTU input of equipment. For example, 100,000 BTUs ÷ four burners = 25,000 BTU per burner. The gas pressure is 3.5" water column (WC). You go down the column for 3.5" until you come as close as possible to 25,000 BTUs, which would be 25 cubic feet. The closest would be 24.95. You then go to the left to the orifice size column and find that the orifice size would be a 42 drill size. The drill kit shown in Figure 3 is what would be used to determine the drill size of the orifices both for natural gas and for LP gas. This was also referred to in the July/August issue of ICM. Use the same example as above, but now apply it to propane using Table 3 (Table E.1.1b from NFPA 54 2015, next page), moving down the propane column and coming as close to 25,000 as you can. 24,630 is close enough, which would give a number 54 Drill Size. That drill size is also in the drill kit shown in Figure 3. Figure 3 Table 2 18 ICM/Sept/Oct 2015
ICM September-October 2015
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