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 transmitters. 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 filter 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 compensate 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 communicator. Digital transmitters are programmable for range and other parameters in user-selectable units and languages. This configuration 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-flow cutoff below which the instrument either emits an error signal or holds the output to zero. Alternatively, at no-flow conditions, the digital transmitter may determine the low-flow 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 instruments, 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 Mu Itivariable Transmitters. Multivariable transmitters are digital transmitters equipped with multiple inputs to the electronics to provide the temperature and pressure of the fluid that the meter is measuring. Multivariable transmitters perform a larger number of calculations than do typical digital transmitters. Vortex shedding flowmeters measure the volumetric flow rate. In many applications, the mass flow rate of the fluid is of interest. To calculate the mass flow rate from a vortex shedding meter’s volumetric flow reading, the flowing density of the fluid must be determined. The flowing density can be calculated from the flowing temperature, pressure, and an equation of state for the specific fluid. There are exceptions to these requirements, and in the case of a liquid, the pressure effect is typically minimal and can be neglected.
Meter manufacturers provide a series of upper velocity limits. Manufacturers may specify an upper velocity limit as a function of process fluid density. This is usually for structural integrity reasons relating to the maximum stress due to vortex shedding that the sensor or meter body are able to withstand. For liquid flows, another limit is generally set due to pressure drop. Pressure loss increases with flow rate; as a result, the meter has a maximum velocity due to cavitation for any given process pressure. Also, most meters have a maximum velocity for which an analog current output may be obtained.
6.1.2 Minimum Flow. The minimum volumetric flow rate is determined by manufacturer-recommended limits. The manufacturer specifies recommended measurement limits based on the minimum measureable and minimum accurate flow rates. Minimum measureable flow rate may be useful for start-up operations. Minimum accurate flow rate is useful for operations where accuracy is important. The transmitter provides a means of configuring the low- flow cutoff for the meter based on these recommendations or specific application requirements.
Manufacturers generally provide sizing programs or published equations to calculate minimum measurable flow rate and minimum accurate flow rates.
6.1.2.1 Minimum Measurable Flow Rate. The minimum measureable flow rate, Vmjn is the rate at which the force exerted on the sensor is too small to generate a signal strong enough for the meter to reliably differentiate between the flow signal and noise. The limit is a function of the fluid momentum and is therefore described by the following equation, where C is a manufacturer- specified value
6.1.2.2 Minimum Accurate Flow Rate. The minimum accurate flow rate is the lowest flow rate at which the meter reads at the specified accuracy. This is generally expressed in terms of Reynolds number. Generally, the minimum accurate flow rate is near 20,000. A specific meter design and size may have accuracy limits above or below that number. Some manufacturers may have recommendations for ways to use the meter at lower Reynolds numbers.
6.1.2.3 Low-Flow Cutoff. The meter will have a low-flow cutoff. This is a configurable parameter that defines the velocity, or volumetric flow rate, below which the meter will read zero, regardless of the actual flow in the pipe.