ASME MFC-21.1 pdf download

ASME MFC-21.1 pdf download.Measurement of Gas Flow by Means of Capillary Tube Thermal Mass Flowmeters and Mass Flow Controllers.
ASME MFC-21.1 establishes common terminology and provides guidelines for the qua1ity description, principle of operation, selection, operation, installation, and flow calibration of capillary tube thermal mass flow- meters and mass flow controllers for the measurement and control of the mass flow rate of gases. The content of this Standard applies to single-phase flows of pure gases and gas mixtures of known composition.
2 TERMINOLOGY, SYMBOLS, AND REFERENCES
2.1 DefinitIons From MFC-1M
accuracy (of rnnlsurenwiit): the extent to which a given measurement agrees with a reference for that measurement, often used by manufacturers to express the perlormance characteristics of a device.
bell prover: volumetric gaging device used for gases that consists of a stationary tank containing a sealing liquid into which is inserted a coaxial movable tank (the bell), the position of which may be determined. The volume of the gas-tight cavity produced between the movable tank and the sealing liquid may be deduced from the position of the movable tank.
calibration: the experimental determination of the relationship between the quantity being measured and the device that measures it, usually by comparison with a standard, then (typically) adjustment of the output of a device to bring it to a desired value, within a specified tolerance, for a particular value of the input.
critical flaw devices: a flowmeter in which a critical flow is created through a primary differential pressure device (fluid at sonic velocity in the throat). A knowledge of the fluid conditions upstream of the primary device and of the geometric characteristics of the device and the pipe suffice for the calculation of the flow rate.
flow conditioner: general term used to describe any one of a variety of devices intended to reduce swirl and/or regulate the velocity profile.
The sensor tube is a key component in the instrument. For this reason, the details of its design are proprietary to each manufacturer. The typical sensor tube is U-shaped and composed of a corrosion-resistant alloy. It has a total length ranging from about 10mm to 100mm (0.5 in. to 4 in.) and a relatively small internal diameter and wall thickness. The ends of the U-shaped sensor tube are embedded in a metal block with high thermal conductivity that acts as a thermal bus or ground, ensuring that both ends have the same temperature (i.e., T0 in Fig. 5.2-1). The sensor tube is located in an isothermal clamshell outer case filled with a thermally insulating material or simply dead air.
Because the capillary tube has a small internal diameter, operation is limited to clean gases. Any long-term drift in accuracy often is traced to particulates contaminating the inner wall of the sensor tube or the small flow passages in the laminar flow element. For this reason, it is common practice to install a particulate filter upstream of the instrument. For applications with gases that are not absolutely clean and those requiring very low pressure drops, some manufacturers offer straight sensor tubes with Larger internal diameters. Sensor tubes used in very high pressure applications [e.g., 340 bar (5,000 psia)I typically have larger wall thicknesses.
The typical sensor tube shown in Fig. 3.5-1 measures its internal mass flow rate, q,, by means of two windings wrapped around its outside diameter — an upstream winding and a downstream winding. The two windings are identical, adjacent to one another, and located symmetrically on either side of the center of the sensor tube’s length. Together, they cover a fraction of the total length of the sensor tube. The windings have an electrically insulating coating and are bonded to the outside surface of the sensor tube with a stable bonding compound. Since the windings (i.e., the flow sensors) are located external to the gas flow path, capillary tube thermal MFMs and MFCs have no delicate components exposed to the gas, and thus provide nonintrusive measurement of mass how rate.