Q652. Are the results of Equations W-11A and W-11B in standard conditions or actual conditions?

A652. Actual conditions. The final results of Equations W-11A and W-11B (average flow rate in cubic feet per hour under subsonic and sonic flow, respectively) are at actual conditions, and must be converted to standard conditions.

The flow rate, FR, from Equations W-11A and W-11B is the flow rate through the restriction orifice at actual conditions (i.e., at temperature Tu and pressure P1 upstream of the restriction orifice). Prior to using the flow rate output from Equations W-11A and W-11B in Equation W-12 and subsequently W-10A to calculate emissions, it is necessary to convert FR to standard conditions. Measured and calculated FR values must be converted from actual conditions to standard conditions using Equation W-33 in 40 CFR 98.233(t), with FR substituted for Ea,n, Tu converted to degrees Fahrenheit and substituted for Ta, and P1 (upstream pressure in psia) substituted for Pa. The output of Equation W-33 will be FR at standard conditions, suitable for use in Equations W-12 and ultimately for use in W-10A to calculate emissions.

Part 98 generally specifies when a conversion is needed. In this case, that information was not included which could significantly impact the results of W-10A. EPA is considering whether a revision to Part 98 is the most appropriate approach to address this issue.