To convert from h w to a true pressure drop, the following relationship between head and pressure is used: The ASME standard provides Equation 1 to calculate mass flow of a liquid through an ASME orifice flowmeter when the orifice flowmeter diameter is at the temperature of the flowing liquid:ĮQUATION 1: Flow through ASME Orifice Flowmeter (Reference 1, page 10)įor D and D/2 taps, the equation for C (the discharge coefficient) to be used in Equation 1 is:ĮQUATION 2: Discharge Coefficient for D and D/2 Pressure Taps (Reference 1, page 30) Figure 1 from the ASME standard (Reference 1) shows the configuration of an orifice with both D and D/2 taps and flange taps. The ASME standard, then, is applicable only to systems that follow the pressure tap and orifice type, placement, and installation guidelines established by the standard. The pressure profile particular to flow through a given orifice and its immediate upstream and downstream piping will vary based on several factors that, aside from computational fluid dynamics (CFD) modeling, can only be determined with experimental data. It is crucial to note that this measured pressure differential is not equal to the irrecoverable pressure drop. In this region, the static pressure changes due to irrecoverable pressure losses from increased friction and turbulence in the flow, as well as pressure changes that are a result of the vena contracta and complex flow patterns as the fluid progresses through the orifice. The ASME standard utilizes the differential pressure measured across pressure taps located very near the orifice in areas specified by the standard.
The answer for the difference in flow rates on a basic level is that these two calculations are, fundamentally, not the same thing ASME correlates differential pressure at pressure taps for purposes of flow measurement, while AFT Fathom and Arrow calculate the irrecoverable pressure drop across the orifice for purposes of system pressure loss. He was puzzled when this mass flow rate calculation differed by approximately 100 lb m/hr between the two methods, so he reached out to AFT for help in determining the reason for the discrepancy. Recently, a customer calculated the mass flow rate of a fluid through an orifice using both the ASME standard MFC-3M-1989 and AFT Arrow.
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