Special Operating Conditions | Metering Orifice
TEMPERATURE CONSIDERATIONS
Example 2: From Table B2, flow through a #1670 orifice plate is 1056 scfh of propane with 3.5"w.c. drop at stp conditions.
Temperature limit of the 8697 neoprene gasket is 150°F (the disc spring about 250°F).
How many scfh of propane will flow through the same orifice at the same pressure drop with average line pressure of 10 psig?
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 (see page 9). 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
From Chart 2, flow rate factor is 1.3 for 10 psig: 1056 × 1.3 = 1373 scfh propane.
HIGHER PRESSURE DROPS For pressure drops up to 30"w.c. or so, 8697 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. Drops higher than the 8736-A Manometer's 6"w.c. can be read with a longer manometer, or with an accurate differential dial gauge (e.g., 8735-DM). 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 3: Select an 8697 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 8697-4-2000 (2" pipe size, orifice 1.020" diameter) would be a good selection.
An 8697 Metering Orifice system can be used at pressures up to 25 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 (see page 9).
Example 1: Select an orifice plate to meter 900 scfh of coke oven gas in a 5 psi line with about 3.5"w.c. pressure drop.
From Chart 2, stp flow correction factor is 1.17, which divided into 900 gives 769 equivalent cfh.
In the coke oven gas column of Table B2, nearest capacity is 826 cfh (with a #675 orifice plate).
(769/826)² x 3.5 = 3.03"w.c. drop for 900 scfh coke oven gas in a 5 psi line when using a #675 plate..
An alternate choice is a #558 plate, which passes 683 scfh coke gas at 3.5"w.c. drop. (769/683)² x 3.5 = 4.44"w.c. drop for 900 scfh coke oven gas in a 5 psi line when using a #558 plate. Having a 4.44”w.c. drop at high fire would make low fire flow rates easier to read on a manometer.
† scfh air through a #2000 plate at 3.5"wc drop (Table B2) = 1548
Bulletin 8697 Page 19
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