For example, suppose a meter, such as the one shown in Figure 1, is used, measuring ½ cubic foot per revolution. The time for one revolution of the dial with a furnace operating is found to be 15 seconds. The gas flow rate of the furnace would be: Gas flow rate = 0.5 ∙ 3600 15 = 120 cubic feet/hour If a meter having two cubic feet per revolution of the dial were used in the above example, time for one revolution of the dial would be 60 seconds. Gas flow rate of the furnace would be: Gas flow rate = 2.0 ∙ 3600 60 = 120 cubic feet/hour Gas/Propane Basics The heat input rate is obtained by multiplying the gas flow rate by the heating value of the gas. For the above example, the furnace rate (using 1,000 BTU/ cubic foot natural gas) would be 120,000 BTU/hour (120 ∙ 1,000). Tables have been prepared so that the above equation is not needed to calculate metered gas flow rates. Table 1 lists gas input rates in cubic feet per hour for various times for one revolution of the meter dial with different size meters. One of these dial ranges appears on every meter. Note that most of the gas flow rates in Table 1 are rounded off to the nearest whole cubic foot per hour. Times for a revolution of the meter are rounded off to whole seconds. If the observed time falls between two times indicated, the rate can be estimated using the two times on either side of the observed time. If the gas used has a heating value of 1,000 BTU/ cubic foot, the input rate to the furnace is 145,200 BTU/hour (145.2 ∙ 1,000). Table 1 is just part of the entire table, which runs all the way up to 200 seconds. The complete table can be found in the latest version of NFPA 54 National Fuel Gas Code (2015): Table A.11.1.1, pages 54-121 and 54-122. ICM/Sept/Oct 2015 17
ICM September-October 2015
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