Special Operating Conditions | Metering Orifice
HIGHER PRESSURE DROPS For pressure drops up to 30"w.c. or so, 8694A Metering Orifice capacities can be determined by the square root law or Table 5.42a and b in the Combustion Handbook, applying these to the "standard" capacity at 3.5"w.c. drop or the graph on page 4. For pressure drops greater than 30"w.c., an 8695 Orifice Plate should be used. Formulas in this table apply only to a gas dry enough to be considered a "perfect gas"--i.e., not near its dew point. This limits pressure drop, particularly with butane and propane. If any refrigeration effect is noticed downstream of an orifice, pressure drop is probably too high for tables or formulas based on the perfect gas law. Example 2: Select an 8694A Metering Orifice for 75 scfm (4500 scfh) of 20 psig air with 0.5 psi (13.85"w.c.) drop. Stp flow factor for 20 psig is 1.58 (Chart 2). Therefore, equiva- lent stp air flow is 4500 ÷ 1.58 = 2848 scfh. From graph on page 4, at 13.85"w.c. drop and 2848 scfh air (right hand scale), determine next larger orifice plate is #2000. Select holder size from Table B2*: Plates smaller or larger than 2000 should be available, so 8694A-3-2000 (1 ½ " pipe size, orifice 0.989" diameter) would be a good selection.
TEMPERATURE CONSIDERATIONS
Temperature limit of the 8694A meters are 200 °F.
At a given pressure drop, stp capacities decrease as temperature rises. Stp flow at 100 °F is 96.4% of what it is at 60 °F; at 150 °F, stp flow drops to 92.3%. For corrections at other temperatures, consult Chart 1. Temperature corrections can be ignored if gas and air are about the same temperature and the intent is to achieve proper air/gas ratio and/or equal firing rates at all burners. However, corrections should be considered if air and gas temperatures differ, or if intent is to measure rate of gas consumption. HIGH LINE PRESSURE An 8694A Metering Orifice system can be used at pressures up to 60 psi, but pressure drop limits must not be exceeded. Metal tubing--not rubber hose--should be used for connections between orifice holder and manometer when pressures exceed 3 psi. Since gas becomes more dense at higher pressures, stp flow rates (Table B1*) must be corrected by stp flow correction factors from Chart 2.
Example 1: From Table B2, flow through a #1670 orifice plate is 1056 scfh of propane with 3.5"w.c. drop at stp conditions.
How many scfh of propane will flow through the same orifice at the same pressure drop with average line pressure of 10 psig?
From Chart 2, flow rate factor is 1.3 for 10 psig: 1056 × 1.3 = 1373 scfh propane.
Bulletin 8694A Page 9
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