ASME MFC-6:2013 pdf download

ASME MFC-6:2013 pdf download Measurement of Fluid Flow in Pipes Using Vortex Flowmeters
5.1.2.2 digital transmitters. Digital transmitters use one or more microprocessors to process raw input signals and provide output signals and a user interface. These transmitters are often referred to as smart trans- mitters.
Their use of microprocessors provides several advantages over analog transmitters. Because the input signals are processed digitally, these transmitters can analyze the signals using mathematical algorithms to determine installation quality, external interference, and noise. Based on this analysis, the transmitters may be able to digitally flter out spurious signals. The use of microprocessors and digital processing minimizes the effect of component drift that may occur in analog transmitters.
Digital transmitters may also be able to compen- sate for changes in the meter K factor caused by changes in process temperature and pressure. Typical human interface is via a digital numeric or graphical display and buttons and optical or magnetic sensors to program parameters, or via a handheld com- municator. Digital transmitters are programmable for range and other parameters in user-selectable units and languages. This confguration is stored on the transmitter and may also be uploaded via the communication protocol to other devices. Digital transmitters may also include the ability to program a low-fow cutoff below which the instrument either emits an error signal or holds the output to zero.
Alternatively, at no-fow conditions, the digital transmit- ter may determine the low-fow cutoff by differentiating between signal and noise, and adjust the measurement threshold accordingly. Digital transmitters may be enabled with protocols that allow communication with other compatible instru- ments, communication devices, and control systems such as distributed control systems (DCS). Most digital transmitters include a programmable output range of 4 mA to 20 mA unless precluded by a communication protocol. They are also likely to include a programmable pulse or frequency output range.
5.1.2.3 Multivariable transmitters.
Multivariable transmitters are digital transmitters equipped with mul- tiple inputs to the electronics to provide the temperature and pressure of the fuid that the meter is measuring. Multivariable transmitters perform a larger number of calculations than do typical digital transmitters. Vortex shedding fowmeters measure the volumetric fow rate. In many applications, the mass fow rate of the fuid is of interest.
To calculate the mass fow rate from a vortex shedding meter’s volumetric fow reading, the fowing density of the fuid must be determined. The fowing den- sity can be calculated from the fowing temperature, pressure, and an equation of state for the specifc fuid. There are exceptions to these requirements, and in the case of a liquid, the pressure effect is typically minimal and can be neglected. In the case of saturated steam, the density can be determined from knowledge of either the pressure or temperature; however, if the steam is superheated, then both pressure and temperature are required. Multivariable transmitters may have built-in temperature and pressure sensors or may accept temperature- and, if required, pressure-measurement signals from external sensors.
The confguration of a multivariable transmitter is similar to that described for digital transmitters, but it requires additional steps to provide an accurate output. The inputs from the temperature and pressure sensors must be con- fgured in the multivariable transmitter’s electronics. The correct process fuid or equation of state must be selected in the electronics, and potentially more than one 4-mA to 20-mA output or other, digital output must be confgured.
It is extremely important that the right selection is made for the density compensation calculation.
The multivariable transmitter provides the tempera- ture and, if applicable, pressure measurements to the end user via either a digital communications protocol or multiple 4-mA to 20-mA outputs. In addition, since the multivariable transmitter computes the density of the fuid, it can provide this value as well as a number of other computed fuid parameters to the user. Consult the manufacturer’s literature to determine what vari- ables the multivariable transmitter can calculate.
The K factor is a nonlinear function of the Reynolds number (see Fig. 9.2-1), and the multivariable transmitter can compute the Reynolds number and correct the nonline- arity in the K factor. The fowing density of the fuid can be used to predict the expected strength of the vortices at a specifc fow rate, allowing more accurate fltering algorithms in the transmitter to address signal interfer- ences at low-fow rates.
5.2 equipment Markings Meters shall be marked by the manufacturer to identify the manufacturer, serial number, pressure rating, mean K factor, or meter factor, and hazardous location certifca- tion, if any. The direction of fow shall be permanently indicated by the manufacturer on the meter body. 6 aPPliCation Considerations There are several considerations related to application of vortex meters, but the three primary ones are sizing, process infuences, and safety. 6.1 sizing Size the meter according to the desired fow range rather than the nominal pipe size. The fowmeter size shall be selected such that the expected process fow rate falls between the maximum and minimum fow rates within the required uncertainty.