Precautions | Metering Orifice
Most 8694A Metering Orifices perform very satisfactorily but there are occasional complaints from field engineers and servicemen of inaccurate readings, of improper location for gas pressures available, and of their presence on jobs where available gas pressures do not allow their use. Below are points to watch in applying and reading this useful tool.
1. The catalog rating of North American burners on air at 30"Hg (sea level) and 60 °F. The 8694A is rated on gas at 29.3126"Hg (Cleve- land average), 80 °F, and 0.6 gas gravity.
Result: Burner runs rich.
Correction: 3%
2. Gas pressure at 8694A above zero gauge.
Result: Burner runs rich.
Correction: at 8 osi, about 2.7%; at 16 osi, about 4.4%.
3. Gas temperature below 80 °F.
Result: Burner runs rich.
Correction: for 70 °F gas, 1%; for 60 F gas, 2%.
4. Air temperature at burner above 60 °F.
Result: Burner runs rich.
Correction: for 100 °F air, 3.7%, for 150 F air, 7.6%.
5. If one assumes 1000 Btu/cf gas, 1060 Btu/cf gas has 6% more Btu.
Result: Burner runs rich. (In some areas gas gravity may vary from 0.6.)
6. If the 8694A is to be upstream of the 7218, the estimated total pressure drop through the 7218, 8694A, 1807, etc., plus the gas pres- sure drop required across the burner, should be enough below the available gas supply pressure to allow for variables.
7. If the 8694A is to be downstream of the regulator, between the 7218 and the burner, the estimated total pressure drop through the 8694A, 1807, etc., plus gas pressure drop required across the burner, should be enough below the impulse pressure to allow for vari- ables. If a bleeder must be used, maybe an 8694A orifice meter should not be used.
Bulletin 8694A Page 11
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