ASME MFC-7:2016 pdf download

ASME MFC-7:2016 pdf download.Measurement of Gas Flow by Means of Critical Flow Venturis and Critical Flow Nozzles
1 SCOPE AND FIELD OF APPLICATION
This Standard applies only to the steady flow of single-phase gases through critical flow venturis (CFV) of shapes specified herein [also sometimes referred to as critical flow nozzles (CFN), sonic nozzles, or critical flow venturi nozzles]. This Standard applies to CFVs with diverging sections on the downstream side of the throat. When a CFN (no diverging section) is discussed, it is explicitly noted. This Standard specifies the method of use (installation and operating conditions) of CFVs. This Standard also gives information necessary for calculating the mass flow of the gas and its associated uncertainty. This Standard applies only to CFVs and CFNs in which the flow is critical. Critical flow exists when the mass flow through the CFV is the maximum possible for the existing upstream conditions. At critical flow or choked conditions, the average gas velocity at the CFV throat closely approximates the local sonic velocity. This Standard specifically applies to cases in which (a) it can be assumed that there is a large volume upstream of the CFV or upstream of a set of CFVs mounted in a parallel flow arrangement (in a common plenum), thereby achieving higher flow; or (b) the pipeline upstream of the CFV is of circular cross section with throat to pipe diameter ratio equal to or less than 0.25
3.2.2 Critical Flow Venturis
CFV exit plane: surface at the exit of the divergent section. CFV inlet plane: surface at the entrance of the convergent section. critical flow venturi: a flowmeter having a geometrical configuration with a constant curvature convergent section to a minimum cross-sectional area (i.e., throat) at which sonic conditions exist followed by a conical divergent section. critical (or choked) flow: maximum flow for a particular venturi that can exist for the given upstream conditions; the flow that exists when the ratio of the downstream static pressure, P 2 , to the upstream absolute pressures, P 0 , is such that the fluid velocity reaches sonic conditions at the throat. This condition is termed “choked” flow, and the flow is proportional to the inlet stagnation pressure. critical pressure ratio: the ratio of the absolute static pressure at the CFV throat to the absolute stagnation pressure for which gas mass flow through the CFV is a maximum. maximum back pressure ratio: the ratio of the highest absolute CFV exit static pressure to the absolute CFV upstream stagnation pressure at which the flow becomes critical. throat: the cross section of the CFV with minimum diameter. 3.2.3 Pressure Measurement stagnation (or total) pressure ofa gas: pressure that would exist in the gas if the flowing gas stream were brought to rest by an isentropic process. Only the value of the absolute stagnation pressure is used in this Standard. static pressure of a gas: the pressure of the flowing gas, which can be measured by connecting a pressure gauge to a wall pressure tap. Only the value of the absolute static pressure is used in this Standard. wall pressure tap: a hole drilled in the wall of a conduit, the inside edge of which is flush with the inside surface of the conduit.
3.2.4 Flow mass flow: the mass of gas per unit time passing through the CFV. In this Standard, mass flow is always the steady- state or equilibrium mass flow. steady state: the conditions under which the inlet and other measured pressures and temperatures at a CFV do not change in a transient or periodic manner by more than two times the resolution of the transducers or two times the standard uncertainty of the measurement during the period of testing or measurement.