North American Balancing Orifices
Bulletin 8699
An 8699 Balancing Orifice is used at each burner on a gas manifold feeding multiple burners to assure equal flow to every burner. The Balancing Orifice is sized to take a pressure drop significantly larger than any other drop in the manifold or individual piping to each burner. Therefore, it compensates for variations in pipe lengths and configurations that otherwise would cause uneven flows to the burners. 8699 Orifices are used in place of individual limiting orifice valves. They cannot be used for accurate metering--8697 Meter- ing Orifices perform that function. SELECTION Pick orifice size for the largest practical pressure drop (see Table 1 on back).
Figure 1. Balancing Orifices and holders.
Note: Pipe size of selected Balancing Orifice may not match burner gas pipe size (8699 usually will be one or more sizes smaller)--use appropriate reducing fitting at burner. Balancing Orifices are not used with North American’s Tempest ® and other burners that, by design, require relatively high gas pressures during normal operation. Table 1 lists flows at pressure drops up to 16 osi. If Balancing Orifice is to be used in a high pressure gas line with significantly higher drop, consult North American or the COMBUSTION HANDBOOK for appropriate orifice sizing.
General guideline: Select pressure drop equal to about half the maximum outlet pressure from the ratio regulator.
Note in Table 1 there are two standard orifice sizes for each pipe size. With 8 osi pressure drop, the larger orifice results in about 40 fps gas velocity in the pipe, the smaller orifice about 30 fps. Example: Select Balancing Orifices for a bank of burners, each requiring 3000 cfh natural gas at high fire, with 12 osi pressure available in manifold. Drop across Balancing Orifice should be at least 6 osi (half of 12). From Table 1, choose the 8699-4-875: At 6 osi drop, its rated flow is 2985 cfh, which makes it suitable for "balancing" the 3000 cfh requirement.
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