ASME MFC-12M:2006 pdf download

ASME MFC-12M:2006 pdf download.Measurement of Fluid Flow in Closed Conduits Using Multiport Averaging Pitot Primary Elements
1 SCOPE
This Standard, provides information on the use of multiport averaging Pitot head-type devices used to measure liquids and gases. The Standard applies when the conduits are full and the flow (a) has a fully developed profile (b) remains subsonic throughout the measurement section (c) is steady or varies only slowly with time (d) is considered single-phase A differential pressure transmitter or other pressure measuring device, known as a secondary element, must be used with a multiport averaging Pitot primary ele- ment to produce a flow rate measurement. Although multiport averaging Pitot primary elements are sometimes used in noncircular conduits, such appli- cations are beyond the scope of this Standard.
2 TERMS AND DEFINITIONS
The terminology and symbols (Table 1) used in this Standard are in accordance with ASME MFC-1M. Some items from ASME MFC-1M are listed in para. 2.2.1 for easier reference. Terminology not defined in ASME MFC-1M, but used in this Standard, are defined in para. 2.2.2. 2.1 Symbols See Table 1. 2.2 Definitions 2.2.1 Definitions Found in ASME MFC-1M cavitation: the implosion of vapor bubbles formed after flashing when the local pressure rises above the vapor pressure of the liquid. See also flashing. differential pressure device: device inserted in a pipe to create a pressure differential whose measurement, together with a knowledge of the fluid conditions and of the geometry of the device and the pipe, enables the flow rate to be calculated.flashing: the formation of vapor bubbles in a liquid when the local pressure falls to or below the vapor pressure of the liquid, often due to local lowering of pressure because of an increase in the liquid velocity. See also cavitation. primary device (ofa differential pressure device): differential pressure device with its pressure tappings. rangeability: flowmeter rangeability is the ratio of the maximum to minimum flowrates or Reynolds number in the range over which the primary element meets a specified uncertainty (accuracy). reproducibility: the closeness of agreement between results obtained when the conditions of measurement differ; for example, with respect to different test appara- tus, operators, facilities, time intervals, etc. Reynolds number: a dimensionless parameter expressing the ratio between inertia and viscous forces.
4 OPERATING PRINCIPLES
4.1 Description of Operation The multiport averaging Pitot primary flow element or averaging Pitot tube (APT) is similar to the conven- tional single point Pitot tube in operation, but differs in construction. It is typically designed as a strut, or cylin- der (the cross section of the cylinder is not necessarily circular), that is inserted across the circular pipe or con- duit on a diameter. Some APT designs have more than one strut to achieve a more representative sample of the fluid velocity in the pipe or conduit (see Nonmandatory Appendix A). The strut has ports that sense the total velocity head (total pressure), and a reference, or low pressure. In some APT designs the reference pressure is measured at the pipe wall. Figures 1 and 2 show two commonly used methods for sensing the total pressure and reference pressure. The sensed pressure(s) are con- veyed through isolated passages, or chambers in the cylinder to the exterior of the assembly, where there are connections to the secondary device. By combining the individually sensed pressures from its sensing ports, the APT produces a differential pressure that can be related to the average fluid velocity in the pipe or conduit. 4.2 Bernoulli’s Equation As with other differential pressure-based flow pri- mary elements, the underlying principle for the APT sensor is the application of the momentum equation from basic fluid theory. Using the assumptions for steady state, inviscid, and incompressible flow along a streamline, the equation reduces to the Bernoulli equa- tion (see Nonmandatory Appendix B)